U.S. patent application number 14/738509 was filed with the patent office on 2016-01-21 for functionalized metal oxides as a stationary phase and a surface template for micro gas chromatography separation columns.
The applicant listed for this patent is Masoud Agah, Muhammad Akbar, Apoorva Garg, Leyla Nazhandali, Hazma Shakeel. Invention is credited to Masoud Agah, Muhammad Akbar, Apoorva Garg, Leyla Nazhandali, Hazma Shakeel.
Application Number | 20160018365 14/738509 |
Document ID | / |
Family ID | 55074381 |
Filed Date | 2016-01-21 |
United States Patent
Application |
20160018365 |
Kind Code |
A1 |
Agah; Masoud ; et
al. |
January 21, 2016 |
Functionalized Metal Oxides As A Stationary Phase And A Surface
Template For Micro Gas Chromatography Separation Columns
Abstract
The present invention provides a detector and method for
detecting substances in complex mixtures. The detector includes a
microfabricated preconcentrator, a separation column with an
on-chip thermal conductivity detector, a controller for controlling
flow and thermal management and a user interface. The thermal
conductivity detector includes a first resistor located at an inlet
of the separation column and a second resistor located at an outlet
of the separation column.
Inventors: |
Agah; Masoud; (Blacksburg,
VA) ; Akbar; Muhammad; (Blacksburg, VA) ;
Garg; Apoorva; (Blacksburg, VA) ; Nazhandali;
Leyla; (Blacksburg, VA) ; Shakeel; Hazma;
(Blacksburg, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Agah; Masoud
Akbar; Muhammad
Garg; Apoorva
Nazhandali; Leyla
Shakeel; Hazma |
Blacksburg
Blacksburg
Blacksburg
Blacksburg
Blacksburg |
VA
VA
VA
VA
VA |
US
US
US
US
US |
|
|
Family ID: |
55074381 |
Appl. No.: |
14/738509 |
Filed: |
June 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62011612 |
Jun 13, 2014 |
|
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62069344 |
Oct 28, 2014 |
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62114137 |
Feb 10, 2015 |
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Current U.S.
Class: |
73/23.41 ;
427/255.31; 427/255.34 |
Current CPC
Class: |
G01N 30/6052 20130101;
B01J 20/281 20130101; B01J 20/3291 20130101; B01J 20/223 20130101;
B01J 2220/86 20130101 |
International
Class: |
G01N 30/28 20060101
G01N030/28; B01J 20/281 20060101 B01J020/281; B01J 20/32 20060101
B01J020/32; B01J 20/22 20060101 B01J020/22 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &
DEVELOPMENT
[0002] This invention was made with government support under 1)
contract number ECCS1002279 awarded by the National Science
Foundation of the United States 2) by the National Science
Foundation (NSF) under CAREER Award no. ECCS-0747600 and NIOSH
Grant 5R21OH010330 and 3) under contract number 1R210H010330
awarded by the National Institutes of Health of the United States.
The government has certain rights in the invention.
Claims
1. A method of fabricating a separation column for use with a gas
chromatograph, comprising: depositing a metal oxide by atomic layer
deposition to create a stationary phase medium on said separation
column.
2. The method of claim 1 wherein said metal oxide is aluminum
oxide.
3. The method of claim 3 wherein said aluminum oxide is deposited
as a plurality of layers with each layer deposited in a cycle
comprising (a) exposure to trimethylaluminum (b) purge (c) exposure
to water (d) and purge.
4. The method of claim 3 wherein each cycle deposits a layer of
about 1-2 angstroms.
5. The method of claim 2 wherein said aluminum oxide is
functionalized by exposure to silane.
6. The method of claim 5 wherein said silane is an alkylsilane.
7. The method of claim 6 wherein said silane is
chlorodimethyloctadecylsilane.
8. The method of claim 2 wherein said aluminum oxide is
functionalized by exposure to a plurality of silanes.
9. A detector comprising: a micro-purge extractor in communication
with a micro-scale gas chromatography column for the extraction and
analysis of water organic compounds from an aqueous sample; said
micro-purge extractor having a cavity in communication with a
sample inlet port, a purge gas inlet port, a waste outlet port and
a purged water organic compound outlet port; said sample inlet port
adapted to receive an aqueous sample; said purge gas inlet port,
spaced apart from said sample inlet port, and adapted to receive
inert gas which is used to purge water organic compounds from said
cavity of said micro-purge extractor; said waste outlet opposingly
located from said purge outlet port, said waste outlet adapted for
draining water from the chip; said purge outlet in communication
with a micro-thermal preconcentrator; said micro-thermal
preconcentrator adapted to adsorb and desorb water organic
compounds; at least one resistive heating element that when
activated, causes said water organic compounds to be desorbed; and
said micro-scale gas chromatography column adapted to separate said
water organic compounds and a micro-thermal conductivity detector
for identifying said water organic compounds.
10. The detector of claim 9 wherein said separation column has an
aluminum oxide stationary phase medium.
11. The detector of claim 10 wherein said aluminum oxide is
functionalized by exposure to silane.
12. The method of claim 11 wherein said silane is an
alkylsilane.
13. The method of claim 11 wherein said silane is
chlorodimethyloctadecylsilane.
14. The method of claim 10 wherein said aluminum oxide is
functionalized by exposure to a plurality of silanes.
15. The detector of claim 9 wherein said purge outlet port and said
sample inlet port are located on a top side of said micro-purge
extractor, said purge gas inlet is located on a side of said
micro-purge extractor, and said waste outlet port is located on a
bottom side of said micro-purge extractor.
16. A detector for detecting hazardous air pollutants at
parts-per-billion concentrations in complex mixtures comprising: a
microfabricated preconcentrator; a separation column with an
on-chip thermal conductivity detector; a controller for controlling
flow and thermal management; and a user interface.
17. The detector of claim 16 wherein said thermal conductivity
detector includes a first resistor located at an inlet of said
separation column and a second resistor located at an outlet of
said separation column.
18. The detector of claim 16 wherein said separation column
includes a medium comprised of silica a nanoparticle layer with a
Tenax TA coating.
19. The detector of claim 16 wherein said separation column
includes at least one channel that linearly decreases in width.
20. The detector of claim 16 wherein said separation column
includes at least one channel that decreases in width in a stepwise
fashion.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Nos. 62/011,612 filed Jun. 13, 2014, 62/069,344 filed
Oct. 28, 2014, and 62/114,137 filed Feb. 10, 2015 herein
incorporated by reference.
INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0003] Not applicable.
BACKGROUND OF THE INVENTION
[0004] Gas chromatography (GC) is a reliable chemical analysis
technique that is used to separate and identify the constituents of
complex gas mixtures. GC has applications in a vast range of areas
such as environment monitoring, food processing, pharmaceutical
industry, biomedical science, forensic and trace analysis.
Traditional bench-top GC instruments, though widely used, are bulky
and expensive as well as time and energy intensive. The progressive
innovations in microfabrication techniques coupled with
nanotechnology have prompted a renewed interest in miniaturizing
key GC components over the last decade. The development of new
stationary phase coating techniques has been a continuous research
area and the heart of micro gas chromatography (.mu.GC) column
performance. Apart from the application of most common polymer
based gas-liquid stationary phases for .mu.GC separation columns,
there has also been research effort toward the development of
MEMS-compatible methods for the integration of solid-adsorbent
materials (monolayer-protected gold, carbon nanotubes, silica
nanoparticles, gold nanoparticles, silica and graphite) into .mu.GC
columns. Although these adsorbent/gas-solid stationary phases have
shown attractive features for the separation of complex mixtures,
the application of these coating schemes for very narrow-width
(<20 .mu.m) and deep (>150 .mu.m) rectangular microchannels
has not been straightforward as summarized in Table 1:
TABLE-US-00001 TABLE 1 Summary of microfabricated GC columns with
gas-solid stationary phases Column Phase-Deposition Authors
Geometry Method Limitations Reston [41] Rectangular Evaporation of
copper Applicable to only (1994) (30 .mu.m deep, 100 .mu.m wide,
0.9 m-long) phthaocyanine film shallow channel depth. Stadermann
[17] Rectangular Chemical vapor Incomplete coverage (2006) (100
.mu.m deep, 100 .mu.m wide, deposition of carbon of the channel and
50 cm-long) nanotubes high process temperature. Zareian-Jahromi
Rectangular Self-assembly of Extensive [18] (2010) (250 .mu.m deep,
25 .mu.m wide, 25 cm-long) thiol on electroplated gold
characterization, and Shakeel [27] Multicapillary surface
high-process (2013) (25 .mu.m wide, 200 .mu.m deep, 25 cm-long)
temperature and chip-level processing. J. Vial [22] Rectangular and
semipacked Water-level High stationary phase (2011) (100-125 .mu.m
deep, 50-150 .mu.m wide, 1 m- Sputtered silica film thickness
long). film deposition variations and applicable to low- channel
depth. Wang and Rectangular Silica nanoparticles High-process
Shakeel [21] (250 .mu.m deep, 150 .mu.m wide, 1 m-long). Using
layer-by-layer temperature and (2013) Multicapillary technique.
chip-level processing. (200 .mu.m deep, 20 .mu.m wide, 25
cm-long).
[0005] The current adsorbent based coating schemes for these
particular column designs suffer from factors such as low-yield,
chip-level coating, high processing temperatures and require
elaborate experimentation. Moreover, these methods present a major
hindrance towards the monolithic integration of different .mu.GC
components on a single chip. This warrants the need for developing
wafer-level facile complimentary metal-oxide-semiconductor (CMOS)
compatible coating techniques that produce mechanically and
thermally stable, chemically inert and selective stationary
phases.
[0006] Atomic layer deposition ("ALD") has emerged as a tool for
the growth of low-temperature thin-films on a variety of substrates
for myriad applications, generally as a gate oxide in CMOS
technology. ALD enables conformal and homogeneous coatings of flat,
very high-aspect-ratio (HAR) microchannels and even nanostructure
surfaces with precision within a few angstroms. The use of ALD
coatings for separation science has been very recently reported for
ultra-thin-layer chromatography where the enhancement of separation
capabilities of silica support media using different ALD coatings
was demonstrated. The use of ALD-based films as a stand alone
separation media for .mu.GC has not been reported.
[0007] Other applications for .mu.GC include the detection of
volatile organic compounds (VOCs) that are emitted by a wide
variety of products including solids and liquids. Prolonged
exposure to VOCs can cause serious health effects including liver,
kidney, and nervous system diseases and can even cause cancer.
Similar effects have been reported for various aquatic organisms.
Different analytical techniques for their detection have been
reported in the literature. Microscale gas chromatography provides
a solution with reduced size, low power consumption, and the
capability for handheld analysis of complex VOCs.
[0008] .mu.GCs have not been used in the detection of VOCs in
aqueous matrices due to incompatibility of the systems with aqueous
matrices. Water saturates the adsorbent in the micro-thermal
preconcentrator (.mu.TPC) by capturing available adsorption sites
and also damages most common polymer based stationary phases
resulting in changes in the retention time, selectivity and column
bleeding. Most widely used flame ionization detectors (FID) are
extinguished by water and a decrease in the sensitivity of electron
capture detectors (ECDs) has also been reported. The United States
Environmental Protection Agency (EPA) has specified a list of water
organic compounds (WOCs) with their maximum contamination level
(MCL). At levels above the specified MCL (usually in ppb), the
presence of WOCs in aqueous media poses serious threat to human and
aquatic life as shown in Table 2.
TABLE-US-00002 TABLE 2 List of water organic compounds with their
originating sources and potential health risks. Amount Potential
Contamination recovered Contaminants health effect sources (ng)
Recovery log(.sub.Kow) MCL Toluene Nervous system, Petroleum 5.7
23% 2.75 1 mg l.sup.-1 liver problems factories PCE Liver problems;
Discharge from 5.4 18% 2.57 5 .mu.g l.sup.-1 increased risk of
factories and cancer dry cleaners Chlorobenzene Liver and Discharge
from 9 25% 2.86 0.1 mg l.sup.-1 kidney problems chemical and
agricultural chemical factories Ethylbenzene Liver and Petroleum
18.7 38% 3.14 0.7 mg l.sup.-1 kidney problems refineries
[0009] The current methods for the identification of WOCs rely on
removing them from the aqueous sample prior to analysis using a
bench-top GC system. These methods include solid-phase
micro-extraction (SPME), purge and trap, and hollow fiber
membranes. They cannot be used for on-site monitoring of the
aqueous sample and rely on transporting samples to laboratories.
Currently, commercially available FROG-4000TM from Defiant
Technologies and Water Analysis Surety Prototype (WASP) from Sandia
National Laboratories are capable of performing field analysis of
water contamination. Nevertheless, the systems are large,
expensive, require a trained technician and rely on conventional
purge and trap mechanisms. Thus, there still remains a demand for
the development of a light weight, less power hungry, inexpensive
independent system capable of extracting and detecting WOCs from
aqueous media.
[0010] In addition, gas chromatography (GC) has been the
established method for assessing the presence and concentration of
VOCs in the environment, and GC coupled to mass spectrometry
(GC-MS) is one of the most accurate and widely used tools. In this
technique, samples are first collected from the field through trap
based systems such as sorbent tubes or canisters and then are
analyzed in a laboratory by trained technicians. This technique
requires manual intervention and multiple steps including sampling,
storage, and shipping before analysis, and therefore is susceptible
to higher losses and has longer measurement cycles. Most of these
drawbacks can be overcome by using portable, field-deployable, and
real-time detection systems. There have been attempts at
miniaturizing GC-MS systems, but such systems are still bulky,
expensive, and consume high amounts of power. Other real-time
detection techniques involve using a sensitive photo-ionization
detector (PD) to realize a total VOC analyzer. PD-based systems
suffer from selectivity issues and require filtering at the source,
which renders them ineffective and expensive for multi-compound
analysis. Colorimetric tubes are another inexpensive and popular
technique for VOC analysis, which rely on a color change, induced
by the irreversible reaction between the sensing material and the
analyte. However, this technique requires human intervention, and
its use is limited by slow response and large uncertainty. Several
commercial high performance portable gas chromatography systems
have been reported, but they are still bulky, energy inefficient,
and expensive for real-time environmental monitoring
applications.
BRIEF SUMMARY OF THE INVENTION
[0011] In one embodiment, the present invention provides an
approach for utilizing atomic layer deposited alumina as a
gas-solid stationary phase medium for microfabricated gas
chromatography columns. After atomic layer deposition (ALD) of
aluminum oxide, a chloroalkylsilane is utilized to functionalize
the oxide surface to improve peak symmetry and retention times.
Semipacked columns (1 m-long, 190 .mu.m-wide, 180 .mu.m-deep with
20 .mu.m-embedded circular micropillars) were utilized.
[0012] In yet other embodiments, the present invention uses the
detector disclosed in co-pending U.S. Patent Application
Publication No. US20150130473A1, the disclosure of which is
incorporated herein by reference in its entirety. In other
embodiments, the present invention combines the micro components
disclosed below with this detector in tandem or combines the
components on a single chip.
[0013] In other embodiments, the present invention uses ALD with
self-limiting gas phase chemical reactions for deposition, making
it suitable for micromachined columns having very high-aspect
ratios. The use of ALD for stationary phase coating ensures good
selectivity, separations and retention of different compounds. The
inherent properties of atomic layer deposition provide an easy
route to practically coat any microfabricated column architecture.
The metal oxide thin film provides surface properties that can
improve the binding of polymer-based stationary phases and thus can
act as a template for deposition of these polymeric phases.
Moreover, the techniques of the present invention also provide
innovative wafer-level coating approaches that are compatible with
standard CMOS and MEMS processes.
[0014] In other embodiments, ALD treated/silane functionalized
columns are used to efficiently separate a multicomponent sample
mixture and yielded 4200 plates per meter. The ALD based stationary
phase is found to be stable after multiple injections and at high
operating temperatures.
[0015] In another embodiment, the present invention provides a
method that facilitates a simple and wafer level coating scheme
that provides a highly controllable film thickness. The inherent
properties of atomic layer deposition provide an easy route to coat
very challenging microfabricated column designs.
[0016] In yet other embodiments, the present invention provides a
wafer-level application that utilizes a silane functionalized
aluminum oxide thin-film as a stationary phase medium. ALD uses
self-limiting gas phase chemical reactions for deposition, thus
affording an alumina film with very high step-coverage of
semipacked columns having aspect-ratios (depth:width) of 10:1. The
coating techniques of the present invention may also be used with
.mu.GC columns with very narrow channel widths (5 .mu.m or
less).
[0017] In another aspect, the present invention concerns a .mu.GC
architecture, based on a monolithically integrated microfabricated
separation column and thermal conductivity detector (TCD). The
architecture performs very sharp injections from the
micro-preconcentrator, which is compatible with flow sensitive gas
detectors like TCD. The embodiment has environmental monitoring
applications as well as applications for water monitoring, homeland
security, and food analysis.
[0018] In other embodiments, the present invention provides a
micro-purge extractor (.mu.PE) chip and integrates with a
micro-scale gas chromatography (.mu.GC) system for the extraction
and analysis of water organic compounds (WOCs) from aqueous
samples. The 2.times.3 cm .mu.PC chip contains inlet and outlet
ports and a sealed cavity. An aqueous sample is introduced from the
top inlet port while a pure inert gas is supplied from the side
inlet to purge WOCs from the .mu.PE chip. The outlets are assigned
for draining water from the chip and for directing purged WOCs to
the micro-thermal preconcentrator (.mu.TPC). The trapped compounds
are desorbed from the .mu.TPC by resistive heating using the
on-chip heater and temperature sensor, are separated by a 2 m long,
80 mm wide, and 250 mm deep polydimethylsiloxane (OV-1) coated
.mu.GC separation column, and are identified using a micro-thermal
conductivity detector (.mu.TCD) monolithically integrated with the
column.
[0019] In certain embodiments, the present invention is capable of
providing rapid chromatographic separation (<1.5 min) for
quaternary WOCs namely toluene, tetrachloroethylene (PCE),
chlorobenzene and ethylbenzene with a minimum detection
concentration of 500 parts-per-billion (ppb) in aqueous
samples.
[0020] In further embodiments, the present invention provides a
ready-to-deploy implementation of a microfabricated gas
chromatography (GIGC) system characterized for detecting Hazardous
Air Pollutants (HAPs) at parts-per-billion (ppb) concentrations in
complex mixtures. In one embodiment, the device includes a
microfabricated preconcentrator (.mu.PC), MEMS separation column
with on-chip thermal conductivity detector (.mu.SC-TCD), flow
controller unit, and all necessary flow and thermal management as
well as user interface circuitry to realize a fully functional
.mu.GC system. The .mu.PC and .mu.SC-TCD may be used to target
analytes such as benzene, toluene, tetrachloroethylene,
chlorobenzene, ethylbenzene, and p-xylene. A Limit of Detection
(LOD) of -1 ng was achieved, which corresponds to a sampling time
of 10 min at a flow rate of 1 mL/min for an analyte present at -25
ppbv.
[0021] In other embodiments, the present invention concerns a
method of using flow-manipulation generated by sharp injection
plugs from the .mu.PC even in the presence of a flow-sensitive
detector like a .mu.TCD.
[0022] In other embodiments, the present invention concerns a
.mu.GC architecture, which leverages monolithic integration of a
separation column with micro-thermal conductivity detectors
(.mu.SC-TCD) to minimize band broadening and chip-to-chip fluidic
interfaces. The integration eliminates the need for a reference
line and requires fewer external components. Another innovative
aspect of the architecture is a method to perform very sharp
injections from the .mu.PC even in the presence of flow-sensitive
gas detectors like TCD. The design relaxes constraints on the
design of the .mu.PCs by mitigating the effect of vapor desorption
rate on the injection-plug width. The design achieves low detection
limits suitable for environmental monitoring applications.
[0023] Additional objects and advantages of the invention will be
set forth in part in the description which follows, and in part
will be obvious from the description, or may be learned by practice
of the invention. The objects and advantages of the invention will
be realized and attained by means of the elements and combinations
particularly pointed out in the appended claims.
[0024] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0025] In the drawings, which are not necessarily drawn to scale,
like numerals may describe substantially similar components
throughout the several views. Like numerals having different letter
suffixes may represent different instances of substantially similar
components. The drawings illustrate generally, by way of example,
but not by way of limitation, a detailed description of certain
embodiments discussed in the present document.
[0026] FIG. 1: Schematic representation of ALD-based stationary
phase coating for an embodiment of the present invention. (I)
anisotropically etched semipacked column, (II) and (III) atomic
layer deposition of 10 nm alumina film and (II) 95 cycles with each
cycle having four steps (a) TMA exposure (b) purge (c) H.sub.2O
exposure (d) purge, (IV) anodic bonding of ALD-coated wafer with
glass substrate, (V) silane deactivation for 24 hours.
[0027] FIGS. 2A and 2B: Depict a 1 m-long semipacked column with
integrated 20 .mu.m circular microposts and post spacing of 40.mu.m
for an embodiment of the present invention.
[0028] FIG. 3: Effect of silane functionalization on a ALD-coated
semipacked column at 50.degree. C., 250:1 split ratio, 0.1 .mu.L
injection volume and 10 psi column pressure (compound
identification in the order of elution n-hexane (solvent), n-nonane
and n-decane).
[0029] FIG. 4: Separation performance of an ALD-coated/silane
functionalized semipacked MEMS column at 50.degree. C., 7.5 psi
inlet pressure with split ratio of 75:1. Compound identification in
order of elution 1. dichloromethane (solvent) 2. n-hexane (k'=0.23)
3. benzene (k'=0.38) 4. toluene (k'=1.01) 5. Tetrachloroethylene
(k'=1.55) 6. chlorobenzene (k'=1.90) 7. Ethylbenzene (k'=2.34) 8.
p-xylene (k'=2.75) 9. n-nonane (k'=4.00).
[0030] FIG. 5: Diagram of an embodiment of the present invention
showing the topology for the extraction and analysis of water
organic compounds. A back-side heater is utilized for thermal
desorption of analytes from the .mu.TPC for chromatographic
analysis.
[0031] FIGS. 6A, 6B and 6C: An image of an embodiment of the
present invention showing a detector fabricated (A) .mu.PE, (B)
.mu.TPC and (C) .mu.GC chip with embedded resistors utilized as the
thermal conductivity detector for aqueous analysis.
[0032] FIG. 7: Set of chromatograms indicating increase in .mu.TCD
response with increase in purge time and concentration of WOCs.
[0033] FIG. 8: Graph showing the .mu.TCD response variation with
increasing purge time for a sample containing four WOCs at 1 ppm
concentration.
[0034] FIGS. 9A, 9B, 9C and 9D: (A) Micro-devices (B) images
showing micro-posts in .mu.PC (C, D) polydirnethylsiloxane coating
on the interior wall of the column channel.
[0035] FIGS. 10A and 10B: (A) GC system block diagram of one
embodiment of the present invention, (B) Operation cycles and
timing.
[0036] FIG. 11: FM response for separation of test compounds with
integrated .mu.PC and .mu.SC. Injection performed using flow
manipulation technique with .mu.PC heated to 200.degree. C. and
desorption flow rate set to 1 mL/min.
[0037] FIGS. 12A and 12B: Chromatogram of (A) gasoline vapor and
(B) standards using Zebra GC
[0038] FIG. 13: Chromatogram of gasoline vapor sampled at ambient
pressure and temperature using sorbent tubes containing -200 mg of
Tenax TA. Gasoline vapor was analyzed by thermal desorption coupled
to GC-FID using conventional column containing (5% phenyl-, 95%
dimethyl-polysiloxane). Desorption temperature and time were
300.degree. C. and 25 min respectively. Toluene peak at -1.4 min
(16 ppmv) and p-xylene (14 ppmv) peak at -2.4 min. Benzene and
ethylbenzene not detected. Temperature programming: 35.degree. C.,
hold for 10 min, 5.degree. C./min to 150.degree. C., final hold
time 1 min.
[0039] FIG. 14: A .mu.PTGC chip of an embodiment of the present
invention.
[0040] FIG. 15: Schematic of the interface of the .mu.PTGC chip
with a Texas Instruments MSP430 series controller for automatic
operation of the system, for on-site water monitoring. Valve A and
B are inert control valves (LFYA1226032H) while the rest are
(LHLA0531111H) latching low power valves, from The Lee Co. The pump
(270 EC-LC-L) from Schwarzer Precision consumes 0.6 W. A 0.2 pm
filter (WU-81054-42) from Cole Parmer precedes the pump. A 14 oz.,
48.8 Wh, 11.1V Li-Ion battery powers the system.
DETAILED DESCRIPTION OF THE INVENTION
[0041] Detailed embodiments of the present invention are disclosed
herein; however, it is to be understood that the disclosed
embodiments are merely exemplary of the invention, which may be
embodied in various forms. Therefore, specific structural and
functional details disclosed herein are not to be interpreted as
limiting, but merely as a representative basis for teaching one
skilled in the art to variously employ the present invention in
virtually any appropriately detailed method, structure or system.
Further, the terms and phrases used herein are not intended to be
limiting, but rather to provide an understandable description of
the invention.
[0042] As shown in FIG. 1, in one preferred embodiment, the present
invention provides a method for fabricating semipacked or packed
columns that may be about 1 m-long, 190 .mu.m wide with 180 .mu.m
deep channels, with 20 .mu.m integrated circular micropillars, with
40 .mu.m post spacing. In other embodiments, the posts may have
other cross-sectional geometries and be arranged in regular
patterns and irregular patterns. In a preferred embodiment, the
posts may be rectangular and be arranged in a zigzag pattern.
[0043] The process starts with a standard RCA cleaning of 4 inch,
500 .mu.m thick n-type single side polished silicon wafers
(University Wafers). After performing wafer priming using
hexamethyldisilizane (HMDS), AZ9260 photoresist (AZ Electronic
Materials) is spin coated at 2000 rpm to acquire an 8 .mu.m thick
resist layer. This is followed by soft-baking the resist-coated
wafer at 110.degree. C. for 1 minute. The wafer is then exposed for
50 seconds using a mask aligner (Karl Suss) to transfer features
from a chrome mask on to the soft-baked resist-coated wafer.
Following exposure, the features are developed in AZ400K developer
(AZ Electronic Materials) and DI water (3:1). Afterwards, the
developed wafer is hard-baked for 3 minutes at 110.degree. C. SF6
and C4F8 are used to anisotropically etch the wafer (via a standard
Bosch process) using an Alactel deep reactive ion etcher
(DRIE).
[0044] Following etching, the photoresist is first removed using
acetone and the wafer is then placed in a piranha etch solution to
remove any organic residue left during either the etching or
photoresist removal steps (FIG. 1, step-I). Representations of a 1
m-long and 180 .mu.m-deep microfluidic channel are shown in FIG. 2
after anisotropic etching and photoresist removal. As shown, one or
more microposts 101-113 are created as described above.
[0045] The ALD coating process for aluminum oxide using
trimethylaluminum (TMA) and water as precursors is well
established. The successive reactions during aluminum oxide growth
on the silicon surface from TMA and water is described by two
sequential reactions as given below.
--S--OH+Al(CH.sub.3).sub.3.fwdarw.--S--O--Al(CH.sub.3).sub.2+CH.sub.4
(A)
--Si--O--Al(CH.sub.3).sub.2+2H.sub.2O--Si--O--AlO*(OH)+2CH.sub.4
(B)
[0046] The asterick in reaction (B) represents an oxygen shared
between adjacent aluminum atoms on the surface. During the first
reaction (A), the surface silicon hydroxyl species react with TMA
forming a dimethyl-terminated aluminum species and methane as a
byproduct. The excess TMA will not react further with the surface,
resulting in a self-limiting process. In the second reaction (B),
two water molecules react with the dimethyl-terminated aluminum
species, forming an oxide bridge between adjacent aluminum species
and a terminal hydroxide on each aluminum atom as well. The
terminal hydroxide then allows for the production of additional
aluminum oxide monolayers by the same two step cycles.
[0047] Thin alumina film deposition on the etched silicon wafer was
performed at 250.degree. C. using a thermal ALD Cambridge NanoTech
system (Savannah S100). TMA (Sigma-Aldrich) and water precursors
(deionized water used from clean room) were used to deposit a 10 nm
aluminum oxide film. Each ALD deposition cycle consists of a 15 ms
sequential precursor gas exposure of either TMA or H.sub.2O (20
sccm flow) followed by a 4 second purge (FIG. 1, step-II) to
provide a film thickness of 1.07 .ANG. per cycle. The wafer was
coated for approximately 16 minutes (95 cycles) to get the desired
film thickness of 10 nm. The surface of the ALD-coated silicon was
characterized using an atomic force microscope (Bruker Nanoscope V)
with a silicon tip in a tapping mode. A 1 .mu.m.sup.2 area was
scanned at a rate of 0.98 Hz and recorded using 512 scan lines. The
3-D surface plot of a 10 nm aluminum oxide film gave a root mean
square roughness of 0.264 nm, clearly demonstrating that the
aluminum oxide film generates a highly smooth and uniform
surface.
[0048] The thin-film deposition rate of the ALD system may be
improved further by employing a separated ALD process. It has been
shown that growth rates as high as 1.2 nm/sec may be achieved for
aluminum oxide thin films using this technique. Therefore, this
modified ALD process could be integrated into MEMS fabrication of
columns for high throughput stationary phase deposition.
[0049] After performing the ALD coating, the etched silicon wafer
was anodically bonded with a 700 .mu.m thick and 4 inch wide double
side polished Borofloat wafer (Coresix Precision Glass) at 1250V
and 400.degree. C. for 45 minutes (FIG. 1, step-III). The wafer was
then diced into individual devices for functionalization and
chromatographic testing. In order to provide an interface between
the GC injection port and detector, a 220 .mu.m outer diameter by
100 .mu.m internal diameter fused silica capillary tubing
(Polymicro Technologies) was attached to the column inlet and
outlet ports using Epoxy 907 (Miller Stephenson). The surface of
the ALD oxide film and the glass cover was finally functionalized
by filling each column with 10 mM chlorodimethyloctadecylsilane
(CDOS, Sigma Aldrich), in toluene solution for 24 hours at room
temperature (1, step-IV).
[0050] Before performing chromatographic tests, each device was
first purged for 30 minutes with nitrogen. This was followed by
column temperature conditioning in the GC oven for approximately
one hour (35.degree. C. ramped at 2.degree. C./min to 150.degree.
C.) at a constant inlet pressure of 10 psi.
[0051] A bench top 7890 series Agilent GC system equipped with a
flame ionization detector (FID), an electronic pressure controller
and an autosampler (7359A) unit was used for injection and
detection purposes. Both the injector and detector temperatures
were maintained at 280.degree. C. Ultrapure nitrogen (>99.99%)
was purchased from Air Gas Ltd, USA and used as the carrier gas.
Methane (99% grade, Matheson Trigas, Ohio) was used for calculating
the gas velocities to generate Golay plots. All chromatographic
test mixtures were prepared using standard HPLC grade chemicals
(Sigma-Aldrich).
[0052] Ideally a chromatographic peak should be symmetrical, or
Gaussian, in shape. Asymmetric peaks, usually in the form of peak
tailing, influence the column performance. A tailing factor
(T.sub.f) can be used to quantify the peak tailing and is given
by:
T f = a + b 2 a ##EQU00001##
[0053] where both a and b, measured at 5% of the peak height,
represent the front and back half-widths respectively. A T.sub.f=1
represents a perfectly symmetric peak (a=b), whereas peak tailing
(a<b) will yield values of T.sub.f>1.
[0054] Alumina as an adsorbent material has been successfully used
for conventional gas-solid chromatographic separations. Although
the ALD-based alumina coating (without any alkylsilane based
treatment) of one embodiment of the present invention clearly
demonstrates separation capabilities, significant asymmetric peak
shapes are observed as shown (FIG. 3) and tailing factors (T.sub.f)
of 5.95 and 7.15 were calculated for n-nonane and n-decane peaks
(7000 ppm diluted in n-hexane) respectively. This may be the result
of unknown changes in the alumina structure from the ALD process
that affects the equilibration of the solutes with the stationary
phase. FIG. 3 also shows the separation profile for the same
hydrocarbon mix on the atomic layer deposited alumina column after
silane functionalization. The chromatographic performance of the
alumina adsorbent is significantly enhanced using alkylsilane
functionalization performed after anodic bonding of the cover glass
wafer (tailing factor of 1).
[0055] Organochloroalkylsilanes may also be used to functionalize
the medium to enhance retention times and peak symmetry.
[0056] The primary performance criterion for GC columns is
expressed by the number of theoretical plates (N) for a certain
compound and is calculated from chromatogram using the following
formula.
N = 5.54 * ( t r w 1 2 ) 2 ##EQU00002##
[0057] where t.sub.r is the retention time and w.sub.1/2 is the
peak width at half height. Another related parameter that takes
into account the column length (L) for the comparison of different
columns is the height-equivalent-to-a-theoretical-plate (HETP),
where
HETP = L N ##EQU00003##
[0058] The ALD-coated/silane functionalized semipacked MEMS columns
provided up to 4200 plates per meter using n-decane (diluted in
dichloromethane) at 50.degree. C. at an optimum inlet pressure of
7.5 psi (linear velocity of 8.15 cm/sec). Since a meter long
microfabricated column coated with gas-solid stationary phases
(sputtered silica or carbon nanotubes) typically yields around
4000-5000 plates/meter, accordingly, in one embodiment, the present
invention provides a method for creating micro GC columns that is
comparable to other non-conventional coatings.
[0059] The separation capability of the embodiment was tested using
a mixture containing eight compounds: n-hexane, benzene, toluene,
tetrachloroethylene, chlorobenzene, ethylbenzene, p-xylene and
n-nonane diluted in dichloromethane to a concentration of 15000
ppm. An autosampler was used to inject 0.3 .mu.L of the mixture
with a split ratio of 75:1. The column pressure and oven
temperature were maintained at 7.5 psi and 50.degree. C.
respectively. The resulting chromatographic separation (FIG. 4)
provided good resolution and retention of the compounds, with the
entire separation achieved in less than a minute at isothermal
conditions. Capacity factors (k'), the ratio of the time spent in
the stationary phase relative to the mobile phase, ranged from 0.23
for hexane to 4.0 for n-nonane, with the separation driven
primarily by differences in boiling points as reflected by the
order of elution.
[0060] In one application, the present invention may be used to
separate mixtures with either straight chain alkanes or aromatic
hydrocarbons. The functionalized alumina thin film of the present
invention also shows similar separation capabilities as afforded by
conventional alumina particles.
[0061] The long-term and thermal stability of the present invention
was also evaluated. Chlorobenzene and n-nonane (diluted in
dichloromethane) were utilized as probes with multiple injections
made using an autosampler unit. The deviations in the plate numbers
(N) and the capacity factors (k') of each compound were calculated
from the 1.sup.st, 25.sup.th, 50.sup.th, and 75.sup.th injections
as presented in Table 3.
TABLE-US-00003 TABLE 3 Long-term stability of ALD based stationary
phase with multiple injections (GC testing at 50.degree. C., 7.5
psi) Chlorobenzene n-Nonane Plate Number Capacity Factor Plate
Number Capacity Injection (N) (k') (N) Factor (k') 1.sub.st 3368
1.87 4380 3.77 25.sup.th 3508 1.85 4359 3.82 50.sup.th 3319 1.85
3962 3.89 75.sup.th 3886 1.85 3948 3.90 % RSD 7.3 0.64 5.7 1.6
[0062] These results demonstrate that after multiple injections the
silane-functionalized alumina coating remains stable, with less
than 8% deviation in plate numbers and less than 3% deviations in
k' values. Similarly, using the same compounds, the thermal
stability of the ALD based stationary phase was also evaluated. The
column was subjected to thermal cycles of 100.degree. C.,
150.degree. C. and 200.degree. C. for 8 hours each under a constant
inlet nitrogen pressure of 2.5 psi. After each thermal cycle, the
column testing was carried out at 50.degree. C. and 7.5 psi. The
deviations in the N and k' values are shown in Table 4.
TABLE-US-00004 TABLE 4 Temperature stability test of ALD based
stationary phase (GC testing at 50.degree. C., 7.5 psi) Column
Chlorobenzene n-Nonane heating for 8 Plate Number Capacity Plate
Number Capacity hours @ (N) Factor (k') (N) Factor (k') Before
Heating 3990 1.80 4343 3.92 100.degree. C. 3930 1.82 4251 3.94
150.degree. C. 3808 1.80 4308 3.91 200.degree. C. 3128 1.65 4020
3.59 % RSD 10.7 4.3 3.4 4.3
[0063] The results show that the alumina based stationary phase
remains stable up to 150.degree. C. with small variations in
retention times; however, there is some degeneration at 200.degree.
C., and column performance deteriorates considerably if heated
beyond temperatures of 200.degree. C.
[0064] The effect of variations in microfabrication processes on
chromatographic efficiency was also considered. Three separation
columns were fabricated on different wafers using identical
fabrication techniques as discussed above. A 15% variation (RSD)
was observed in the plate number values (using n-decane) for the
tested columns. This high chip-to-chip variation could be
attributed to variations in the end connections.
[0065] In yet other embodiments, the present invention concerns a
device and method using atomic layer deposited alumina with
alkylsilane functionalization for use in gas chromatographic
separations. The use of ALD ensures highly conformal film
deposition inside complex column designs and affords good
selectivity, separations and retention of different compounds.
Compared to very recently reported methods for .mu.GC columns that
utilize sputtering systems for alumina deposition, the present
invention achieves very symmetric peaks and separations. Moreover,
the present invention is not limited by the column depth; thus
improving column flow rate and the sample capacity by increasing
the cross-sectional area of the microfluidic channel. In yet other
embodiments, the present invention, provides a method for tuning
the selectivity of alumina films by using silanes with different
functional groups. Since ALD is used for very thin film
depositions, in range of 5.about.15 nm, there will not be a
significant change in the separation performance.
[0066] FIGS. 5 and 6 show a preferred embodiment of the present
invention that includes detector 700 and a micro purge extractor
(".mu.PE") 704 for extraction of WOCs from an aqueous sample.
.mu.PE 704 contains two spaced apart inlets, inlet 708 for the
aqueous sample to be analyzed and inlet 710 for a pure inert gas to
purge the WOCs from cavity 705.
[0067] Spacing the inlets apart promotes uniform distribution. In
other embodiments, distribution network 712 may be provided to
uniformly spread the sample inside the chip. Similarly, network 714
for the purging gas is used to enhance the interaction between two
phases (air and water) inside the chip and to facilitate the
removal of WOCs from the streaming water. Other networks may also
be used to improve gas flow and distribution at other locations as
well. The chip also contains two outlets. Outlet 720 is used for
waste water and outlet 722 directs purged WOCs to trap .mu.TPC 730
which may be distributed by network 732. Purge outlet 722 directs
WOCs to .mu.TPC 730. It may be located at the top corner of the
.mu.PE chip. Device 700 is operated in two phases namely; (1) the
extraction phase and (2) the analysis phase.
[0068] During the extraction phase, two microfabricated chips
(.mu.PE 704 and .mu.TPC 730) may be connected in a tandem, combined
on a single chip or configured to use valve 740 while .mu.PE chip
704 is maintained vertically to prevent water from entering into
.mu.TPC 730 chip 730 via the air or purging gas outlet 710. As
shown, this locates outlet 722 above outlet 720, which permits
water to drain from outlet 720 while preventing it from entering
outlet 722.
[0069] With a source of analyte such as a vial connected to sample
inlet 708, the aqueous solution is introduced into the .mu.PE chip
704 using purified nitrogen. High purity nitrogen gas is supplied
through the air inlet 710 of the .mu.PE chip trapping WOCs on the
adsorbent surface on .mu.TPC chip 730, which may include square
shaped columns 750 or the other micro pillar configurations
described above.
[0070] The analyzed mass is calculated from the sample
concentration and the volume of water collected during the purged
time. During the analysis phase, .mu.PE 704 is taken offline and
.mu.TPC 730 is connected in series with .mu.GC column 745, which
may be constructed as was described above, with the embedded
.mu.TCD 755 detector using a six port switching valve 760. Helium
is used as a carrier gas while the outlet of the column is
connected to the FID of a commercial Agilent HP7890 GC system for
verification purposes. The sensor 762 on the backside of .mu.TPC
730 is used to monitor the temperature profile of the chip when
heated by a heater or heating element 763. A voltage applied to
heater 763 on the backside of .mu.TPC 730 heats it up from room
temperature to 150.degree. C. The desorbed WOCs are separated by
.mu.GC 745. A 40 mA current is sourced into a Wheatstone bridge
with two resistors of .mu.TCD 755 in each of its arms. The
differential voltage measured across the two resistors enables the
detection of WOCs, which is fed into a Keithley 2700 and recorded
on a LabVIEW program.
[0071] The efficiency of the coated column was evaluated with the
.mu.TCD switched to the ON condition by applying an 8.3 V DC
voltage. This voltage corresponds to a temperature of 95.degree. C.
This was measured with helium flowing at the operating pressure of
12 psi. The metric commonly used for column performance is height
equivalent to a theoretical plate (HETP) as described above.
[0072] The plate number was calculated over a range of column
pressures with the constant split injection ratio of 150:1 using
chlorobenzene diluted to 2% (v/v) in hexane.)
[0073] The separation and identification of the four WOCs using
only the column and its .mu.TCD (without the .mu.PE and .mu.TPC)
was performed by installing the chip inside the GC oven with its
inlet and outlet connected to the injector and the GC FID,
respectively. A 0.1 ml volume of the sample containing the WOCs
diluted to 2% (v/v) in hexane was injected into the .mu.GC column
for separation and identification of the four WOCs by the chip.
[0074] Similarly, for .mu.TCD response calibration, five samples
(0.5%, 10%, 20%, 30%, and 40% (v/v) in hexane) for each WOC were
prepared and tested. A 0.1 ml of each sample was injected three
times in succession using the GC autosampler module with the split
ratio maintained at 150:1. By using the density, the mass for each
WOC was calculated taking the split injection ratio into
account.
[0075] The fabrication of the .mu.TPC was performed on a standard 4
inch wafer using MEMS processing technology. First,
photolithography was performed to pattern micro-posts/fluidic
ports. The wafer was then subjected to deep reactive ion etching
(DRIE, Alcatel) to achieve a depth of .about.250 .mu.m. After
stripping the photoresist off the front-side, a 500 nm thick oxide
layer was deposited on the backside and the wafer diced into
individual chips. The chip was then filled with Tenax TA solution
(10 mg ml.sup.-1 in dichloromethane) and allowed to evaporate to
deposit a thin film (.about.200 nm) of the polymer adsorbent on the
cavity surfaces. The chip was then capped with a Borofloat wafer by
anodic bonding. Following bonding, the chips were loaded onto the
platen of an e-beam evaporator (PVD-250, Kurt Lesker) with the
backside facing the crucible. The chips were masked by a stainless
steel shadow mask patterned with the features defining the heater
and the sensor. Following this, 40 nm/100 nm/25 nm of Cr/Ni/Au was
deposited to get nominal resistances of 15 ohm and 250 ohm for the
heater and the sensor, respectively. Finally, the devices were
unloaded; the shadow masks removed off and fused capillary tubes
epoxied into the inlet/outlet ports. The fabrication process of the
.mu.PE chip followed that of the .mu.TPC but without the adsorbent
coating and backside oxide/metal deposition.
[0076] For the fabrication of the .mu.GC column with embedded TCD,
a two-step anisotropic etching of silicon was performed for hosting
the feedthroughs and the microfluidic channel by spin coating the
wafer with S1813. A shallow depth of 2-3 .mu.m was achieved which
prevented a contact between the metal inter-connects on the
Borofloat wafer and the walls of the separation column in silicon
upon bonding. A 12 .mu.m thick AZ9260 photoresist was patterned
with a mask for subsequent deep etching of the channels resulting
in 250 .mu.m deep channels for the separation. Then, TCD resistors
were fabricated on a glass substrate by utilizing a lift-off
process of a 40 nm/100 nm/25 nm Cr/Ni/Au stack in the e-beam
evaporator. After aligned anodic bonding of the diced detector on
glass and the diced separation column on silicon, capillary tubes
were epoxied into the inlet/outlet ports. The chip was static
coated with polydimethylsiloxane by filling it with a solution of
10 mg ml.sup.-1 OV-1 in pentane, followed by carefully sealing one
end with wax and pulling a vacuum at the open end. This procedure
left a thin layer of OV-1 coating (.about.250 .mu.m) on the walls
of the column channel.
[0077] To avoid changing the concentration of WOCs, a 24 ml
cylindrical vial was filled completely with deionized (DI) water
leaving no headspace. Both 1 ppm and 500 ppb solutions (v/v) were
prepared in two steps. First, 1000 ppm (v/v) solution was made by
adding 24 .mu.l of each WOC to 24 ml of DI water. Second, the
solution was further diluted 1:24 and 1:12 with DI water to achieve
concentrations of 1 ppm and 500 ppb, respectively. The solution was
analyzed immediately to avoid compromising the sample integrity.
Before processing any sample, all parts of the equipment in contact
with the sample were demonstrated to be interference free. This was
accomplished through a blank run.
[0078] Before evaluating the performance of the integrated purge
and trap .mu.GC system, the heating and sensing elements of the
microfabricated preconcentrator, separation column, and the
detector were calibrated and the separation performance of the
column was evaluated.
[0079] A 12 V DC voltage was applied to the heater and the sensor
resistance was measured until the resistance representing the
desired temperature value was reached. The sensor resistance varied
with the applied voltage due to ohmic heating. The final
temperature of 150.degree. C. was attained within 7 seconds
representing a ramp rate of 20.degree. C. This condition remained
constant during the desorption process of the WOCs trapped on the
Tenax TA polymer coating of the .mu.TPC.
[0080] The maximum plate number (optimum condition) observed for
the 2 m long column was about 6200 at 12 psi (flow rate 0.62 ml
ml.sup.-1). The column was operating at this optimum flow condition
for further investigations.
[0081] The separation and identification of the four WOCs was
performed by the method described previously. FID was used to
verify the chromatogram generated by the .mu.TCD. WOCs were
successfully separated and detected by the chip within 1.5 min.
Next, a calibration curve showing the output of the .mu.TCD as a
function of the injected WOC concentration was obtained by the
method described previously. The injected mass varied from about 3
ng to 23 ng for toluene, 5.4 ng to 43 ng for PCE, 3.7 ng to 29.3 ng
for chlorobenzene and 3 ng to 23 ng for ethylbenzene.
[0082] Following calibration and performance evaluation of each
.mu.GC unit, the .mu.PE was put in place. The ability of the
complete system comprising .mu.PE and .mu.TPC chips, separation
column, and the thermal conductivity gas detector (.mu.TCD) to
continuously monitor WOCs in the aqueous sample was realized
experimentally by the method explained earlier. The aqueous
solution was introduced into the .mu.PE chip using purified
nitrogen at 10 psi. High purity nitrogen gas was supplied through
the air inlet of the .mu.PE chip trapping WOCs on the adsorbent
surface with a flow rate maintained at 0.4 ml min.sup.-1 (5 psi)
through the .mu.TPC chip. The extraction period was varied for
three discrete periods of 7, 14 and 21 min. The set of
chromatograms in FIG. 7 was generated using 500 ppb and 1 ppm
aqueous samples for two different extraction periods. The initial
negative dip is due to the sample mixture passing under the
reference detector. At this stage, the signal detector experiences
the carrier gas and hence is constant. This results in a negative
voltage output as explained before. As the sample mixture moves
through the column, it is separated over time. When the individual
components pass under the sample detector, the reference detector
experiences the carrier gas and hence results in positive peaks
corresponding to each eluted compound. The second peak is due to
trace moisture extracted from the purge chip and is not seen on the
FID signal which is insensitive to the trace water content. The
increase in peak heights for all WOCs with the increase in
extraction time was observed which validates the design. It is also
evident that rapid chromatographic separation and detection of all
four WOCs within 1.5 min is achieved at room temperature. The
method's precision was evaluated by three repetitive analyses for
each test. After each analysis, the .mu.TCP was heated to
150.degree. C. (conditioning step) to prevent carry over from the
previous runs, following which a blank run was performed to conform
the same. The change in the detector response (area under the peak)
with the purge time was then monitored for a sample containing four
WOCs at 1 ppm concentration. The experiment was repeated thrice for
three different purging times and the average value was plotted for
each WOC.
[0083] FIG. 8 shows that the peak area increases with purging time.
The increase in the peak area was attributed to the increase in the
quantity of nitrogen (inert gas used) that bubbled through the
aqueous sample, and consequently, more quantity of WOC moved from
the liquid to the vapor phase. Additionally, in streaming mode, a
fresher sample entered the .mu.PE chip replacing the old one,
thereby increasing the amount of WOC purged over time. The results
in FIG. 8 indicate that ethylbenzene and chlorobenzene are purged
easily from the aqueous sample as compared to PCE and toluene. This
is due to their relatively high partition coefficient (K.sub.ow)
value, which is defined as the ratio of concentration of a compound
in a hydrophobic solvent (usually octanol) to its concentration in
water at equilibrium. In other words, it is a measure of
hydrophobicity and depends on size, polarity and hydrogen bond
strength of a compound. Hydrophilic compounds (with low partition
coefficient) are held by a very strong dipole-dipole interaction
and hydrogen bond in water and thus could not be easily purged from
the sample. Increasing the temperature of the sample should
increase the purged amount by supplying enough thermal energy to
the molecule to break the dipole-dipole interaction. The sample
analyzed during the purge time was collected to determine the
percent recovery of each compound. Assuming that the trap is able
to capture the entire purged amount, percent recovery is defined as
the ratio of the amount that is collected for chromatographic
analysis relative to the amount that was originally present in the
aqueous sample. Table 5 summarizes the percent recovery for each
compound at 1 ppm concentration.
TABLE-US-00005 TABLE 5 List of water organic compounds with their
originating sources and potential health risks Amount Potential
Contamination recovered Contaminants health effect sources (ng)
Recovery log(.sub.Kow) MCL Toluene Nervous system; Petroleum 5.7
23% 2.75 1 mg l.sup.-1 liver problems factories PCE Liver problems;
Discharge from 5.4 18% 2.57 5 .mu.g l.sup.-1 increased risk
factories and of cancer dry cleaners Chlorobenzene Liver and
Discharge from 9 25% 2.86 0.1 mg l.sup.-1 kidney problems chemical
and agricultural 18.7 38% 3.14 0.7 mg l.sup.-1 chemical factories
Ethylbenzene Liver and Petroleum kidney problems refineries
[0084] The percent recoveries are lower than those reported in the
literature. This can be attributed to the fact that commercial
purge and trap systems use high purging gas flow rates (normally 40
ml min.sup.-1) and also use traps consisting of a short length
micro-bore tubing packed with the granular form of the adsorbent
material. Such traps at the cost of high pressure drops and high
power consumptions can provide higher adsorption capacity. It is
notable that low recoveries have also been reported previously by
Sandia National Laboratories in their bench-top (WASP) system
described earlier due to the flow limitations in their setup. In
addition, embodiments of the present invention have achieved a
detection limit of 500 ppb, which is comparatively higher than the
commercial purge and trap systems. Part of this is attributed to
small sample volumes (in ml) analyzed by the .mu.PE chip when
compared to the commercial purge and trap systems.
[0085] The present invention, in yet other embodiments, provides a
micro-scale version of a purging device for the extraction of WOCs
from an aqueous sample. The potential application of the chip for
on-site monitoring of the aqueous sample when equipped with all
necessary .mu.GC components as described herein. In other
embodiments, to enhance the recovery of analytes may be
accomplished by modifying the design of the .mu.PE chip and
integrating temperature programming ability on this chip.
[0086] As shown in FIGS. 9A-9D and FIGS. 10A-10B, in another
embodiment, the present invention provides a detector that has two
major units: 1) sampling and preconcentration, and 2) separation
and detection. .mu.PC 1500 chip and .mu.SC-TCD 1502 chip have
integrated thin film heaters 1505 and 1506 and sensors 1510 and
1512 which are used for temperature control during desorption and
separation. The microfabricated components may be integrated with
off-the-shelf flow controllers to implement GC flow
cycles--loading, injection, analysis, and cleaning. An embedded
platform, based on an 8-bit microcontroller (ATmega640, Atmel
Corporation), is responsible for fluidic and thermal control. It
also implements a user interface, signal processing, and data
acquisition circuitry. The system is highly portable, battery
powered, and easy to operate. It can be paired with a laptop for
device control and data visualization through a Labview
application.
[0087] .mu.PC 1500 is a 13 mm.times.13 mm silicon-glass chip and
consists of an array of high aspect ratio micro-posts inside its 1
mm square cavity. Micro-posts 1520-1524 are realized by bulk
micromachining of a 4 inch silicon wafer utilizing a deep reactive
ion etching process to achieve a depth of 240 .mu.m. Micro-posts
1520-1524 may also be configured as described above.
[0088] A 1-.mu.m thick plasma enhanced chemical vapor deposition
(PECVD) oxide layer that acts as an insulator is deposited on the
backside. The wafer is then diced into individual chips. The
micro-posts are then coated with a thin film (.about.200 nm) layer
of adsorbent 1530 which may be Tenax TA followed by capping with a
Borofloat wafer via anodic bonding. A 40 nm/230 nm of Cr/Ni stack
is deposited which serves as a heater and temperature sensor on the
backside of the chip using an e-beam evaporator (PVD-250, Kurt
Lesker). The nominal resistance of the heater and sensor is around
15 ohm and 250 ohm, respectively. Finally, fused capillary tubes
are inserted and epoxied to the inlet/outlet ports.
[0089] For the .mu.SC-TCD, a two-step anisotropic etching of
silicon is performed. First, a shallow depth of 2-3 pm is achieved
which prevents the contact between the metal interconnects on the
Borofloat wafer and the walls of the separation column in silicon
upon bonding. Second, a 2 m long, 70 .mu.m wide and 240 in deep
channel is etched into the silicon wafer. TCD resistors 550 are
fabricated on a glass substrate using a lift-off process for a 40
nn/100 nm/25 mu Cr/Ni/Au stack deposited employing the e-beam
evaporator. The glass and silicon substrates are then aligned and
bonded together. The heaters and temperature sensors are fabricated
on the backside of the chip using stainless steel shadow mask.
Afterwards, the capillary tubes are epoxied into the inlet/outlet
ports. The chip is finally coated with a thin layer (.about.250 nm)
of OV-1 on the walls of the column channel.
[0090] An SEM image of the Tenax TA and OV-1 coating is shown in
FIG. 9. The optical image of all fabricated chips is shown as
well.
[0091] The microfabricated components may be integrated with a
(Parker Hannifm Co), multi-way valves (The Lee Co.), and a portable
helium cylinder. System control is through an integrated electronic
module managed by an 8-bit micro-controller. Latching valves are
selected to optimize power consumption and controlled by applying a
100 ms 5 V DC pulse through an H-Bridge. The pump flow rate is
adjusted by varying the pulse-width modulation (PWM) duty cycle,
which is an important parameter during sample collection. The
on-chip temperature sensors 1510 and 1512 are connected in a 3-wire
resistance temperature detector (RTD) configuration by using two
well-matched current sources with a high precision 24-bit ADC. The
reference voltage for the ADC is also generated using these matched
current sources through a precision resistor and applied to the
differential reference pins of the ADC. This scheme ensures that
the span of the analog input voltage remains ratio-metric to the
reference voltage and any error in the former due to temperature
drift of the excitation current is compensated by the variation of
the latter.
[0092] On-chip heaters 1505 and 1506 are controlled through PWM
channels and a digital proportional control system is implemented
as part of the embedded firmware, which generates different
profiles for temperature reference signal based on the user input
(initial temperature, step, ramp, final temperature).
[0093] The .mu.TCD is connected in a Wheatstone bridge, driven by
7.5 V DC, with low noise thin film resistors (PF1260 series, Riedon
Inc). The differential signal is conditioned and filtered prior to
feeding into an ultra-low noise 24 bit ADC (AD7793, Analog
Devices). The signal is further filtered digitally, using an
on-chip low pass modified Sinc3 filter that also provides 60 Hz
rejection. The TCD, along with the entire system, is operated at a
data rate of 10 Hz, which provides substantial resolution for the
peaks.
[0094] Microfabricated components along with the flow controllers,
integrated electronic module, and user interface circuitry are
assembled in a 30 (I).times.15 (w).times.10 (h) box, schematically
shown in FIG. 10-a. The box also houses a lithium ion battery (2200
mAh) pack and a small helium cylinder (95 uff, 2700 psi) to make
the GC highly portable (-1.8 kg). The system can be operated in a
manual or automatic mode using the LCD/Keypad based human-machine
interface, which has a menu driven system. Once the mode is
selected, the screen shows the state of the system in terms of
valve positions, temperature readings, pump duty cycle, and sensor
value. Sensor data can be visualized and recorded in the Labview
application, which receives data packets through USB or Bluetooth
interface.
[0095] FIG. 10-b shows the timing diagram for the following
automated stages: loading, injection, analysis, and cleaning. In
the loading stage, the pump applies negative pressure at the .mu.PC
outlet to load it with VOCs present in the air sample. Once a
sufficient sample is loaded in the .mu.PC, the valves are switched
to flow helium through the bypass path into the .mu.SC-TCD. In
order to ensure a sharp injection plug, the .mu.PC is heated first
at a rate of 25.degree. C./s to 200.degree. C. without flow, and
then the valve is switched to inject analytes into the .mu.SC-TCD.
The valve is switched back to the bypass path after injection and
this stage typically lasts 10-12 s. Once the analytes are injected,
they are separated and simultaneously identified in the .mu.SC-TCD.
This separation for the analytes of interest (BTEX) takes -1-2 min;
the .mu.SC can be operated at higher temperatures to reduce the
analysis time or to resolve higher boiling compounds. Once the
analysis phase is complete, the valves are switched back to flow
helium at a rate of 3 mL/min through the .mu.PC. The .mu.PC is
heated several times if necessary, to minimize residual analytes
from the previous run. Typically, one temperature cycle (10-12
seconds) is sufficient to desorb the remaining analytes because of
the high desorption efficiency of our silicon-based .mu.PC.
[0096] The integrated electronic module primarily consists of
temperature controllers, TCD interface, flow controllers, user
interface, and data acquisition circuitry. Each module was tested
and optimized separately before integration. For accurate
temperature measurement, on-chip sensors were calibrated by placing
the MEMS devices in a conventional GC (7890, Agilent, Palo Alto,
Calif.) oven to characterize sensor resistance that responded
linearly with respect to temperature with correlation coefficient
of (le) >0.99. The calibration was completed by updating the
firmware with calibration slope and offset, which were computed
from the resistance vs. temperature data. The temperature profile
required for the .mu.PC is very different from that required for
the .mu.SC; the former requires heating at a high ramp rate
(20-100.degree. C./s) to quickly desorb the analytes and generate a
sharp injection plug, whereas the .mu.SC requires heating at much
lower ramp rates (0.2-1.degree. C./s) during the analysis phase to
accelerate elution of high-molecular weight analytes. The .mu.PC
temperature reference was generated through the firmware, and a
step input was given to heat the .mu.PC to 200.degree. C., which
was sufficient to completely desorb 99% of the analytes of
interest. The heating ramp rate depends on the thermal mass, power
dissipated, and heat losses.
[0097] A maximum heating ramp rate of 25.degree. C./s was achieved
for the .mu.PC, by applying an 18 V DC across the heater resistance
(15 LI). Further increases in the voltage resulted in deterioration
of the thin film heater due to high current density. The .mu.SC was
temperature programmed for ramp rates of 20.degree. C./min and
30.degree. C./min. The power consumptions were determined to be 0.5
W and 1.2 W for isothermal operation of the .mu.SC at 45.degree. C.
and 65.degree. C., respectively.
[0098] The sensitivity of the .mu.TCD detector was improved by
increasing the signal-to-noise (S/N) ratio. The signal was
amplified (Gain 32), and the noise was reduced by filtering the
signal and packaging the detector in a small aluminum box to
mitigate the effects of ambient fluctuations. Once the measurement
circuitry was tuned, noise measurements were made under normal
operating conditions with the carrier gas flowing and the .mu.TCD
turned ON. The average peak-to-peak detector noise was 8.08 .mu.V
and measured for baseline signal captured for 10 s. The power
consumption for the .mu.TCD and pump operation was 280 mW and 165
mW, respectively. The full measurement cycle of the GC consumes an
average power of 2.75 W meaning that the battery can last up to 8
hours (-110 full cycles). It is notable that in one embodiment,
each helium refill (95 mL, 2700 psi) will last around 10,000 full
cycles, meaning that the size of the helium cylinder and
subsequently the GC can be considerably reduced.
[0099] For optimum operation, the .mu.PC was characterized in terms
of four parameters: adsorption capacity, breakthrough volume,
desorption peak width, and desorption efficiency. While evaluating
the maximum adsorption capacity of the .mu.PC, the effect of flow
rate on the adsorption process was minimized by keeping it to a low
value of 1 mL/min analytes were injected into the .mu.PC from
headspace in sealed 1 mL vials using a conventional GC autosampler
module. The split ratio and the injection volume were changed to
vary the amount of analyte introduced into the .mu.PC. Analytes not
retained by the adsorbent bed appeared as a breakthrough peak,
which was allowed to return to the baseline prior to heating the
.mu.PC. The maximum adsorption capacity was defined as the mass
retained in the .mu.PC when the injection led to -10% immediate
breakthrough. The .mu.PC can adsorb -30-400 ng of analytes
depending on their affinity to Tenax TA. The masses retained were
-30 ng, 130 ng, 240 ng, and 350 ng for benzene, toluene,
chlorobenzene, and ethylbenzene, respectively. These results
indicate that the .mu.PC can retain a sufficient amount of compound
well above the detection limit of .mu.TCD (.about.1 ng) and that it
has higher affinity to high boilers.
[0100] For breakthrough volume (BV) identification, about 4 ng of
each analyte was loaded separately on the .mu.PC at a flow rate of
1 ml/min and then 5, 10, 15, 25, and 30 mL of the carrier gas was
passed through the .mu.PC at the same flow rate. The .mu.PC was
subsequently heated and the volume of carrier gas, which resulted
in a 10% reduction in the total mass retained, was noted.
[0101] Another important parameter is the width of the desorption
peak that can directly influence the chromatographic resolution
achieved by the separation column. The initial desorption peak
width attained on the .mu.PC at a ramp rate of 25.degree. C./s and
flow rate of 1 mL/min, was 4 seconds. This was reduced by first
heating the .mu.PC without the carrier gas flowing and next,
passing the carrier gas through when the chip temperature reaches
200.degree. C. This flow-manipulation technique resulted in a
reduction of the peak width at half height (PWHH) from 4 to 8
seconds.
[0102] A minimum PWITH of 350 ms was achieved when the desorption
flow rate was increased to 2.5 mL/min. It is notable that 99%
desorption efficiency for the analyte of interest was achieved by
heating the .mu.PC to 200.degree. C. The remaining amount was
removed by subsequent heating of the .mu.PC prior to another run to
minimize carry over from the previous adsorption run.
[0103] The efficiency of the coated column was evaluated with the
.mu.TCD switched ON by applying a 7.5 V DC to the Wheatstone
bridge. This voltage corresponds to a temperature of 80.degree. C.
for the .mu.TCD and was measured with helium flowing. The heated
.mu.TCD elevated the temperature of the column to 32.degree. C. The
metric commonly used for the column performance is the
height-equivalent-to-a-theoretical-plate (HETP).
[0104] The plate number was calculated over a range of column
pressures. The maximum plate number (optimum condition) observed
was 6200 for 2-m long column at 12 psi (flow rate of 0.7
mL/min).
[0105] Further, the embodiment was tested for the separation and
identification of six VOCs using the column and its .mu.TCD. The
.mu.SC along with the interface circuitry was installed inside a
conventional GC and connected to the injection port and FD with
fused-silica capillaries. A mixture of 6 compounds (headspace),
containing benzene, toluene, tetrachloroethylene, chlorobenzene,
ethylbenzene, and p-xylene, was injected by autosampler through the
heated injection port (1 .mu.I, 50:1 split ratio). The peaks were
found to be well resolved and the separation required less than 2
minutes. Next, a calibration curve showing the output (peak area)
of the .mu.TCD as a function of the VOC injected mass was obtained.
For that purpose, a headspace sample for each VOC was prepared and
tested. The split ratio was varied from 120:1 to 50:1 based on the
vapor pressure of the VOC. Injected volumes were varied from 0.5
.mu.L to 4 .mu.L, to achieve the mass injected in the range of 1 to
5 ng.
[0106] Once individual chips were tested, the .mu.PC was connected
upstream of the .mu.SC-TCD to test the hybrid integration. The
integration was expected to both improve and compromise the system
performance of different aspects of the .mu.GC. The compact design
reduced the transfer lines, thereby reducing the formation of a
cold spot that decreases efficiency. On the other hand, the optimal
flow rate for operating the .mu.PC and MSC was different;
therefore, there was a trade-off in establishing the flow rate for
the integrated system. The inlet port of the .mu.PC was connected
to the GC injector (280.degree. C., split ratio 50:1) and it was
loaded with a mixture containing six compounds (headspace, 1
.mu.L). For initial testing, the outlet port of .mu.SC was
connected to the FID detector of a conventional GC. The flow rate
was set to 1 ml/min, for which the PWHH was measured to be 0.8
seconds from the .mu.PC and the .mu.SC exhibited well resolved
peaks for the analytes of interest. As shown in FIG. 11, the six
compounds were separated and identified in less than 2 min.
[0107] The TCD is sensitive to flow perturbation during the
switching of carrier gas into the .mu.PC; therefore, the integrated
.mu.TCD presented some challenges because of interference stemming
from the injection of the analytes by the .mu.PC. The problem was
solved by adopting an innovative system architecture. As shown in
FIG. 10-a, an alternate flow path for the .mu.PC was provided to
maintain a steady flow in the .mu.SC-TCD during the injection
cycle. The fluidic resistance of the alternate flow path was
matched closely with that of the .mu.PC path to minimize flow
perturbations while switching between the two paths. The new
architecture decreased the .mu.TCD stabilization time by one order
of magnitude (from 1-2 min to 10 seconds) and ensured continuous
flow of the carrier gas as shown in FIGS. 12A-12B.
[0108] The components of the GC were assembled as schematically
shown in FIG. 10-a. In addition to the integration of .mu.PC and
.mu.SC-TCD mentioned above, a Y-connector may be added between them
to connect the small pump through a valve. The connector isolates
the loading path of the .mu.PC from the .mu.SC-TCD, which reduced
contamination in the .mu.SC-TCD. This also permits loading at
higher flow rates to the .mu.PC since the high fluidic resistance
in the .mu.SC was avoided.
[0109] The GC was tested by loading the system with a mixture of
five compounds (headspace), containing toluene,
tetrachloroethylene, chlorobenzene, ethylbenzene, and p-xylene,
injected by the GC autosampler through the heated injection port (2
.mu.L, 40:1 split ratio) at 1 mL/min. As shown in FIG. 12-b,
calibration standards were generated with this method. The injected
mass ranged from 1-3 ng, which is equivalent to 10 mL loading of
-100 ppbv gas mixture, approximating a 10-min loading using a pump
operated at 1 mL/min The test was performed three times and
retention times were highly repeatable with an RSD less that 1.3%
for all analytes. The peak areas and peak heights had average RSDs
less than 4.7% and 8%, respectively.
[0110] The fully-assembled GC was evaluated in a simulated
environment using gasoline as the source of exposure. The test
atmosphere was generated by placing 50 mL of gasoline in a 100 mL
beaker that was placed inside a large glass chamber (-4 L). Air was
circulated inside the chamber but outside the gasoline beaker at
500 ml/min to simulate a car refueling scenario in which gasoline
vapors displaced from the tank disperse in the atmosphere, where
they may be inhaled. The top of the chamber was kept open to the
atmosphere and the chamber was allowed to be filled with vapors for
10 minutes. The vapors were sampled through the .mu.GC, by keeping
the system inlet close to the top of the chamber. Vapors were
sampled under ambient temperature and pressure for two different
sampling times, 5 and 15 minutes. To compare results with a
conventional sampling system, gasoline vapors were also sampled
using sorbent tubes packed with Tenax TA at a sampling flow rate of
69 ml/min for 3 hours. Sorbent tubes were desorbed using a thermal
desorption system coupled to a GC-FID (TD-GC-FID, Perkin-Elmer ATD
400). Toluene and p-xylene were identified at a concentration of 16
and 14 ppmv, respectively (FIG. 13). In both systems (GC and
TD-GC-F1D), benzene was not identified because it co-elutes with
the other low-boiling point components in gasoline. FIG. 12-a
illustrates that the GC detected five peaks, three of which were
identified as toluene, ethylbenzene, and p-xylene based on
retention times. Also, because Tenax TA has low affinity to
benzene, the .mu.GC system retained lesser mass of it compared to
the other analytes. The sample volume collected and analyzed in the
.mu.GC was -3 orders of magnitude lower than those collected on
sorbent tubes. These results illustrate that the present invention
is capable of detecting and separating compounds with a much
shorter sampling time and lower sample volume compared to
conventional systems to complete one full cycle of analysis.
[0111] In other embodiments, the present invention provides a
.mu.GC system suitable for environmental monitoring applications.
The system leverages micro-machined components to achieve low power
consumption (2.75 W) and fast analysis time (4.4 min). A Limit of
Detection (LOD) of -1 ng was achieved, which enables monitoring of
HAPs at sub-100 ppbv concentrations. In yet other embodiments, a
deep-etched .mu.PC, enabling high sample volume, and utilizing
semi-packed/multi-capillary columns for increased separation
efficiency may be provided.
[0112] FIGS. 14-15 show another embodiment of the present
invention. FIG. 14 shows various column patterns of designs that
may be used with .mu.Trap as well as with the other components
described above such as .mu.PC, .mu.GC, .mu.PE, .mu.TPC, .mu.TCD
and .mu.SC. Specifically, traps and concentrators are concentration
amplifiers used to improve the detection limit of an analytical
system. Preconcentration may be achieved by collecting analytes on
an adsorbent material or medium over a period of time. The
collected samples are then released via rapid thermal desorption in
the form of a highly concentrated plug for subsequent chemical
analysis. In one embodiment, micro pillars may be round 1610 or
elongated and in a zigzag pattern 1611 and/or in other
configurations as described above to increase the effective surface
area of the device and/or to ensure the enhancement of gas
molecules interaction with the adsorbent surface. The pillars may
also be coated with Tenax TA thin film using an inkjet printing
technique. In addition to its high adsorption capacity and thermal
stability (up to 450.degree. C.), one interesting feature of Tenax
TA is its low affinity to water making it a suitable adsorbent for
the extraction of VOCs from aqueous samples.
[0113] In yet another embodiment, the present invention enhances
the adsorption capability of the Tenax TA poly (2,6-diphenylene
oxide) through its deposition on a nano-structured template.
Modified Tenax TA-coated SiO.sub.2 nanoparticles (SNPs) are
incorporated as an adsorbent bed in silicon based micro-thermal
preconcentrator with an array of microposts as described above
embedded inside the cavity and sealed with a cover. The interior
surface of the chip is first modified by depositing SNP using a
layer-by-layer self-assembly technique followed by coating with
Tenax TA. The adsorption capacity of the SNP-Tenax TA medium is
enhanced by as much as a factor of three compared to thin films of
Tenax TA. The increased adsorption ability is attributed to the
higher surface area provided by the underlying porous SNP coating
and the pores between SNPs affecting the morphology of deposited
Tenax TA film by bringing nano-scale features into the polymer.
[0114] In addition, SNPs may solely be used as the adsorbent. The
medium may be created using a layer by layer coating technique. In
addition, surface functionalization may be undertaken using silanes
as described.
[0115] In one preferred embodiment, the present invention has
achieved more than 10,000 concentration factors for the disclosed
MEMS-based .mu.pTraps. Separation columns 1700 may be used to
separate a mixture of gases into its constituents. The column is
basically a long channel whose walls are coated with a stationary
phase, as described above, that is responsible for the separation
of the various gaseous species. As shown, column 1700 may be
serpentine in configuration to reduce the overall footprint of the
detector. Other channel configurations may be used. In one
alternate embodiment, the present invention provides a
linearly-variable column (LVC) and the other a step-gradient column
(SGC). In some preferred embodiments, the width of a 1 m long, 250
nm-deep LVC is gradually reduced from 120 nm to 20 nm at 1 nm/cm.
While that of a 1 m-long SGC is modulated in five steps (120 .mu.m,
95 .mu.m, 70 .mu.m, 45 .mu.m and 20 .mu.m) each with a length of 20
cm.
[0116] Microfabricated columns typically are comprised of
high-aspect-ratio rectangular channels etched in silicon and laid
out in circular or square-spiral configuration. In other
embodiments, MEMS columns including semi-packed (having embedded
micro posts 1701-1705) and multicapillary (having parallel
channels) and demonstrated columns with 12,000-20,000 number of
theoretical plates per meter, the highest reported separation
efficiencies in .mu.GCs. In other embodiments, serpentine
microfabricated semi-packed columns (.mu.SPC) with three circular
micro pillars (20 pm-wide, 20 pm-post spacing) embedded along a 1
m-long and 150 pm-wide microfluidic channel may be used.
[0117] As shown in FIG. 14, unlike conventional TCDs, in one
embodiment, the present invention provides a monolithically
integrated detector that includes .mu.Purge section 1900, .mu.Trap
section 1600, .mu.TCD section 1800 and separation column 1700.
.mu.TCD 1800 includes a first resistor 1802 located at the inlet
1812 of separation column 1700. .mu.TCD 1800 also includes a second
resistor 1803 located at the outlet 1813 of separation column 1700.
This configuration reduces reagent usage by eliminating the need
for a reference channel with a carrier gas.
[0118] In use, the overall size of the .mu.PTGC chip will be about
25 mm.times.90 mm. The chip will be connected to auxiliary
micro/mini systems including a pump, valves, and a small cylinder
of compressed helium gas as schematically shown in FIG. 15. The
chip operation has 4 stages. In the purge and trap stage, water
from sample source flows into the .mu.Purge 1900 through the water
inlet. Helium through the gas inlet purges the WOCs, through the
gas outlet, to be adsorbed in .mu.Trap 1600. An elevated
temperature in the .mu.Purge enables easier extraction of WOCs. In
the next stage, temperature of the .mu.Trap is raised to remove the
low boilers out through Aux-2. In the third stage, the temperature
of the .mu.Trap is raised further to release the target into the
separation column 1700, resulting in their separation and detection
by detector 1800. In the final stage, the .mu.Trap and .mu.SPC are
raised to a higher temperature to condition and prepare for the
next run. It should be noted that after the initial purge and trap
stage, the .mu.Purge is cleaned with DI water and methanol to
prevent microbial growth. The valves, pump and chip heating
circuitry require signal coordination to lead through the stages
with minimal computation workload. This, combined with the fact
that the unit must be portable and compact, warrants the use of a
low power processor with multiple sleep modes. By far the most
popular in this class is the Texas Instruments MSP 430 series
processors available with various cache, memory size and on-chip
peripheral specifications. A low 100 nA sleep current and a wake-up
time of few microseconds makes it well suited for on-site
deployment.
[0119] In other embodiments, the present invention provides a
method of fabricating a separation column for use with a gas
chromatograph. An oxide, such as a metal oxide or even an aluminum
oxide, is deposited by atomic layer deposition to create a
stationary phase medium on the separation column. The oxide may be
deposited as a plurality of layers with each layer deposited in a
cycle comprising (a) exposure to trimethylaluminum (b) purge (c)
exposure to water (d) and purge. In a preferred embodiment, each
cycle deposits a layer of about 1-2 angstroms. The oxide, including
the aluminum oxide, may be functionalized by exposure to one or
more silanes. In other embodiments the silane is an alkylsilane or
is chlorodimethyloctadecylsilane.
[0120] In yet other embodiments, the present invention provides a
detector having a micro-purge extractor in communication with a
micro-scale gas chromatography column for the extraction and
analysis of water organic compounds from an aqueous sample. The
micro-purge extractor has a cavity in communication with a sample
inlet port, a purge gas inlet port, a waste outlet port and a
purged water organic compound outlet port, The sample inlet port is
adapted to receive an aqueous sample. The purge gas inlet port is
spaced apart from the sample inlet port and adapted to receive
inert gas, which is used to purge water organic compounds from the
cavity of the micro-purge extractor. The waste outlet is opposingly
located from the purge outlet port, and the waste outlet is adapted
for draining water from the chip. The purge outlet in is
communication with a micro-thermal preconcentrator, which is
adapted to absorb and desorb water organic compounds. Also included
is at least one resistive heating element that when activated,
causes the water organic compounds to be desorbed. Lastly, the
micro-scale gas chromatography column is adapted to separate the
water organic compounds and a micro-thermal conductivity detector
is used to identify the water organic compounds. The separation
column may have an oxide, metal oxide or aluminum oxide stationary
phase medium, which may be functionalized by exposure to one or
more silanes. The silane may be an alkylsilane or a
chlorodimethyloctadecylsilane. The purge outlet port and the sample
inlet port are located on a top side of the micro-purge extractor
and the purge gas inlet is located on a side of the micro-purge
extractor, and the waste outlet port is located on a bottom side of
said micro-purge extractor.
[0121] In yet other embodiments, the present invention provided a
detector for detecting hazardous air pollutants at
parts-per-billion concentrations in complex mixtures. The detector
includes a microfabricated preconcentrator, a separation column
with an on-chip thermal conductivity detector, a controller for
controlling flow and thermal management and a user interface. The
thermal conductivity detector includes a first resistor located at
an inlet of the separation column and a second resistor located at
an outlet of the separation column. The separation column includes
a medium comprised of silica a nanoparticle layer with a Tenax TA
coating. In other embodiments, the separation column may include at
least one channel that linearly decreases in width and/or at least
one channel that decreases in width in a stepwise fashion.
[0122] While the foregoing written description enables one of
ordinary skill to make and use what is considered presently to be
the best mode thereof, those of ordinary skill will understand and
appreciate the existence of variations, combinations, and
equivalents of the specific embodiment, method, and examples
herein. The disclosure should therefore not be limited by the above
described embodiments, methods, and examples, but by all
embodiments and methods within the scope and spirit of the
disclosure. For example, while the terms adsorb, absorb, adsorption
and absorption are used herein, the intention of the application is
to use all term interchangeably with the broadest meaning applied
to all without any one term having a narrower meaning than the
other.
* * * * *