U.S. patent application number 10/070544 was filed with the patent office on 2003-05-29 for modulator for gas chromatography.
Invention is credited to Beens, Jan.
Application Number | 20030100124 10/070544 |
Document ID | / |
Family ID | 11133603 |
Filed Date | 2003-05-29 |
United States Patent
Application |
20030100124 |
Kind Code |
A1 |
Beens, Jan |
May 29, 2003 |
Modulator for gas chromatography
Abstract
This invention relates to a modulator for use in gas
chromatographic analysis, adopted for alternatively trapping and
releasing fractions of solutes in a length of a capillary column
within a chromatographic oven, characterized in that it comprises
at least one nozzle placed to spray at least one jet in at least
one corresponding place along said capillary column length, said
nozzle(s) being connected each to a source of liquid CO.sub.2 via a
related valve, and means for alternatively opening said valve(s)
for a predetermined time, to cause a jet of liquid CO.sub.2 to
impinge for said predetermined time on said column place and to
leave the oven atmosphere to heat said column place after said
predetermined time. The modulator can be used in a conventional GC
system or in a two dimensional GC system, for modulating the
analytes fed to the second capillary column.
Inventors: |
Beens, Jan; (Castricum,
NL) |
Correspondence
Address: |
Nixon & Vanderhye
8th Floor
1100 North Glebe Road
Arlington
VA
22201-4714
US
|
Family ID: |
11133603 |
Appl. No.: |
10/070544 |
Filed: |
April 16, 2002 |
PCT Filed: |
November 28, 2001 |
PCT NO: |
PCT/IB00/02253 |
Current U.S.
Class: |
436/161 ;
210/198.2; 210/656; 422/89; 436/133; 436/181; 73/23.35; 73/23.39;
73/23.41 |
Current CPC
Class: |
G01N 2030/025 20130101;
G01N 2030/3023 20130101; G01N 30/463 20130101; Y10T 436/204998
20150115; G01N 2030/122 20130101; Y10T 436/25875 20150115; G01N
30/465 20130101; G01N 2030/623 20130101 |
Class at
Publication: |
436/161 ;
436/181; 436/133; 422/89; 210/656; 210/198.2; 73/23.35; 73/23.39;
73/23.41 |
International
Class: |
G01N 030/02 |
Claims
1. A modulator for use in gas chromatographic analysis, adapted for
alternatively trapping and releasing fractions of solutes in a
length of a capillary column within a chromatographic oven,
characterized in that it comprises at least one nozzle placed to
spray at least one jet in at least one corresponding place along
said capillary column length, said nozzle(s) being connected each
to a source of liquid CO.sub.2 via a related valve, and means for
alternatively opening said valve(s) for a predetermined time, to
cause a jet of liquid CO.sub.2 to impinge for said predetermined
time on said column place and to leave the oven atmosphere to heat
said column place after said predetermined time.
2. A modulator according to claim 1, characterized in that said
valve(s) is (are) alternatively opened for a predetermined time
within a given cycle time and in that said column place is heated
by the oven atmosphere during the remaining cycle time.
3. A modulator according to claim 2, for trapping and releasing in
sequence fractions of solutes, characterized in that it comprises
at least two nozzles placed to spray liquid CO.sub.2 jets in at
least two corresponding separated places along said capillary
column length, and means for alternatively opening said valves each
for a predetermined time in sequence within a given cycle time, to
cause each jet of liquid CO.sub.2 to impinge for said predetermined
time on the corresponding column place and to leave the oven
atmosphere to heat said column place during the remaining cycle
time.
4. A modulator according to claim 3, wherein said predetermined
time is the same for all valves.
5. A modulator according to claim 3, wherein said predetermined
time is different for at least two of said valves.
6. A modulator according to claim 4 or 5, wherein said
predetermined time is ranging from about 0.1 seconds to about 30
seconds.
7. A modulator according to one of claims 4, 5 or 6, wherein said
cycle time is the sum of the predetermined times of all valves.
8. A modulator according to one of the claims 2 to 7, wherein said
cycle time is ranging from about 0.1 seconds to about 30
seconds.
9. A modulator according to one of the preceding claims, wherein
each said nozzles has an opening in the form of a slit parallel to
said capillary length.
10. A modulator according to claim 9, wherein said slit is about
0.04 mm wide and about 3 mm long.
11. A modulator according to one of claims 1 to 8, wherein each
said nozzle is formed by a set of capillaries aligned in parallel
to said capillary column length.
12. A modulator according to claim 11, wherein the upstream end of
said capillaries open in a common CO2 feeding duct, to which the
capillaries are glued or soldered.
13. A modulator according- to claim 12, wherein said capillaries
each have an inner diameter of the order of 0.11 mm and each set
forms a curtain having a length of about 3 mm.
14. A modulator according to one of the preceding claims, wherein
said nozzle(s) is (are) inserted in a metal socket.
15. A modulator according to claim 14, wherein said socket is in
the form of a brass tube.
16. A modulator according to one of the preceding claims, wherein
said column length is mounted in stretched conditions.
17. Use of a modulator according to one of the claims 1 to 15 for
modulating the solute fractions issued by a first chromatographic
column and to be fed to a second chromatographic column in a
comprehensive two dimensional gas chromatographic system.
18. Use of a modulator according to one of the claims 1 or 2 and 8
to 16 for modulating the injected fractions immediately downstream
the injector in a gas chromatographic system.
19. Use of a modulator according to one of the claims 1 or 2 and 8
to 16, for modulating the eluting fractions from a gas
chromatographic column immediately upstream the detector of a gas
chromatographic system.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a modulator for modulating sample
fractions in a capillary column during a gas chromatographic
analysis.
[0002] The modulator according to the present invention can be
designed for a traditional gas chromatographic apparatus in order
to enhance the sensitivity by narrowing the peaks when placed
directly in front of the detector or to focus the injected analytes
when placed directly after the injector. However, it con also
specially designed for comprehensive two dimensional gas
chromatography.
STATE OF THE ART
[0003] The comprehensive two dimensional gas chromatography, also
called comprehensive 2D GC, or GCXGC, is a gas chromatographic
technique in which the sample is first separated on a conventional
normal-bore high-resolution capillary GC column in the programmed
temperature mode. All of the effluent of this first column is then
focused in a large number of extremely narrow (<100 ms) and
adjacent fractions at regular, short intervals and subsequently
injected onto a second capillary column, which is short and narrow
to allow for very rapid separations. GCxGC can be interpreted as to
exist of two GC systems coupled in series by means of a so-called
modulation system (FIG. 1). The first GC is a conventional
capillary GC system, including a conventional injector; the second
is a fast GC, which is about 50 times faster than the first one.
This is accomplished by using a short and narrow-bore column to
provide very narrow peaks with peak widths at baseline of 100-200
ms. The modulation system provides the correspondingly narrow
injection pulses in such a way that no sample is lost during the
transfer between the chromatographic dimensions. In this way the
comprehensive GCxGC technique permits to obtain a separation power
considerably larger than that of conventional capillary gas
chromatography, together with an improved sensitivity, a better
peak identification and other advantageous features.
[0004] As previously said, in order to carry out said GCxGC it is
necessary to operate a so-called modulation system between the
first and second capillary column in order to retain and focus the
narrow fractions of the effluent of said first column and inject
the some at intervals onto said second column.
[0005] The most widely used modulators are of the thermal type,
wherein a thermal action on a column length is used to trap and
release the fractions to be injected in the second column.
[0006] The known heated modulators use an intermediate, thick film
modulation capillary to trap (parts of) the eluting analytes from
the first column by means of phase-ratio focusing. Heat is applied
to thermally desorb the analytes from the thick film stationary
phase in order to re-inject the narrow chemical pulses into the
second column. FIG. 2 presents this phase-ratio focusing and
thermally desorption process in four steps.
[0007] In the first paper describing the comprehensive GCxGC
technique, by Liu and Phillips [Z. Y. Liu, J B. Phillips, J. Chrom.
Sci., 1991, 29,227-231] and in the Phillips patent [U.S. Pat. No.
5,196,039] a dual-stage metal-coated capillary with a thick film of
stationary phase, connected with the outlet of the first column,
but placed outside the oven, was employed as a modulation system.
Sequentially the two parts of the metal coated capillary were
resistively heated to desorb the analytes trapped due to the lower
temperature of the modulation column and its thick stationary phase
film. This system appeared not to be robust enough for long use and
introduced limitations in the lower temperature of the oven housing
of the two columns (as the minimum temperature of the oven should
be in this case at least 100.degree. C. higher than the temperature
of the modulator which is kept close to the ambient one).
[0008] A more sophisticated heated desorption system was described
and made commercially available by Ledford et al. [J. B. Phillips,
R. B. Gaines, J. Blomberg, F. W. M. van der Wielen, J. M. Dimandja,
V. Green, J. Granger, D. Patterson, L. Racovalis, H. J. de Geus, J.
de Boer, P. Haglund, J. Lipsky, V. Sinha, E. B. Ledford, J. High
Resolut. Chromatogr., 1999, 22, 3-10], and Phillips and Ledford
patent [U.S. Pat. No. 6,007,602) mainly consisting of a slotted
heater moving along the thick film capillary (sweeper) within the
gas chromatographic oven.
[0009] However, this system too shows drawbacks, mainly due to the
movement of the slotted heater in the close vicinity of the tiny
capillary, which causes an easy breakage of the column and a limit
of the oven maximum temperature.
[0010] In order to render more efficient the fraction trapping and
eliminate the necessity of a special thick film capillary length,
inserted between the first and second column as well as to remove
the limitations related with the maximum oven temperature, so
called cryogenic or cooled modulators were introduced.
[0011] These modulators, consisting of a cold trap moving
sequentially forward and backwards along the inlet portion of the
second capillary column (the cooling medium sweeps an upstream
length of the second column), cryogenically trapping and focusing
(parts of) the analytes as they elute from the first column on the
first section of the second column itself [R. M. Kinghorn, P. J.
Marriott, J. High Resolut. Chromatogr., 1998, 21,620-622]. When the
cryogenic system moves away from the zone in which the analytes
were trapped, the surrounding GC-oven air quickly heats up the
trapped analytes remobilising them for re-injection in the
remaining part of the second column. This cryogenic trap, focus and
re-injection process is schematically presented in FIG. 3.
[0012] The major drawback of this system is the very frequent
breakage of the portion of the fused silica capillary column where
the cold trap is moving due to ice formation between the cold trap
and the column.
[0013] Apart from the mechanical differences between the heated and
cooled modulators, there are also some differences in their
applicability. In the heated modulators a difference in temperature
of at least 100.degree. C. is necessary between the oven and the
sweeper, to remobilize the analytes from the thick film capillary
that holds the retained fraction. The maximum temperature to which
this capillary can be heated up, i.e. the maximum allowable
temperature of its stationary phase, determines the maximum
operation temperature of the sweeper.
[0014] The maximum temperature of the column oven will be therefore
limited to 100 C below the sweeper temperature and this introduces
strong limitations in the application range covered by such
systems. This limitation does not exist with the cooled moving
modulator, the maximum operation temperature of the oven can be
much higher as it is limited only by the maximum operating
temperature of the two separation columns themselves.
[0015] The common characteristic of the thermal modulators as they
have been described, however, is the fact that both techniques use
a heating/cooling device that moves across a close distance around
a fragile fused silica capillary column. Even very accurate (and
rather tedious) tuning of these moving devices and their short
distance to the capillaries, frequently leads to breakage of the
tiny, and fragile capillaries.
[0016] Ledford [E. B. Ledford, C. Billesbach, J. High Resol.
Chromatogr., 2000, 23, 202-204] introduced a modification of its
heating sweeper, by applying a cooling jet of CO.sub.2 on the
heating arm. However, this system and the cryogenic system as
previously illustrated show all drawbacks of the modulators having
movable parts within the oven and moreover the continuous jet of
CO.sub.2 tends to create ice formations on the column which
involves, breaking possibilities and hindering of fraction
release.
[0017] Ledford (E. B. Ledford, presented on the 23.sup.rd Symposium
on Capillary Gas Chromatography, Riva del Garda, Italy, June 2000)
recently proposed a two-stage liquid nitrogen/heated air let
modulator with no moving parts. Two cooling and two heating jets
spot-cool and heat a very short section of the second column to
trap/focus and re-inject the modulated fractions. The two cooling
jets of the two-stage jet modulator alternately spray liquid
nitrogen directly onto the inlet part of the second column for
trapping/focusing. Two jets with heated gas alternately heat up
these spots to remobilize the analytes for re-injection as very
narrow pulses.
[0018] The heating jets were necessary, since the temperature of
the cooled sections of the second column could reach temperatures
as low as 190.degree. C.
[0019] Liquid nitrogen is not easily available at every laboratory
and needs bulky insulation when transported through tubes.
Moreover, the use of liquid nitrogen may create problems due to ice
formation within the oven and in particular on the jet nozzles
which may such hinder or even stop the release of liquid nitrogen.
Moreover, since the hot air jet must have a temperature at least
100.degree. C. above the oven temperature and very high air jet
temperature cannot be reached for reasons of column integrity
(maximum temperature of fused silica columns is 350.degree. C.),
this limits the maximum temperature of the oven and the range of
applications covered by such systems.
OBJECTS OF THE INVENTION
[0020] The object of the present invention is now to provide a
modulator for GC or GCxGC which optimises the analytes treatment in
a conventional GC system and overcomes the drawbacks of the
presently known modulators for GCxGC, in particular with reference
to those connected with the mobile modulators (sweepers) and with
the use of liquid nitrogen and hot air jets in the Ledford
modulator with no moving parts.
DESCRIPTION OF THE INVENTION
[0021] The main feature and further features of the modulator
according to this invention are reported in claim 1 and
respectively in the dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention will be more deeply described with reference
to the accompanying drawings, wherein:
[0023] FIG. 1 is a scheme of the GCxGC system.
[0024] FIG. 2 is a scheme of the known heating modulation system
(sweeper).
[0025] FIG. 3 is a scheme of the known cryogenic modulation
system.
[0026] FIG. 4 is a scheme of a modulator according to the present
invention.
[0027] FIG. 5 is a detail of the jet configuration of the modulator
of FIG. 5.
[0028] FIG. 6 is a chromatogram obtained by means of a GCxGC
separation of C.sub.8 through C.sub.18 with a modulator according
to the present invention.
[0029] FIG. 7 is a chromatogram obtained by means of a GCxGC
separation with a modulator according to the invention and showing
the shape of the modulated n-C.sub.14 peaks.
[0030] FIG. 8 is a scheme of a modulator according to the present
invention when applied to a conventional GC system.
[0031] FIG. 9 represents two chromatograms showing the effect of
peak sensitivity enhancement.
[0032] FIGS. 10a and 10b are diagrammatic representations of an
alternative embodiment of the jet configuration, respectively in
front view and side view.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] Referring to the drawings, FIG. 1 diagrammatically shows the
components of a known GCxGC system, preferably housed in a single
oven. FIG. 2 shows a scheme of the heating modulation process, in
which the fraction eluting from the first column is trapped at the
upstream end of the thick film of the modulation capillary (phase
trapping) (step 1); when the heating sweeper comes in
correspondence of this capillary upstream end, the heat effect
releases the fraction (step 2) and transports the same along the
thick film capillary, while a further fraction is trapped at the
capillary upstream end (step 3).
[0034] When the sweeper reaches the second column, the first
fraction is released on the same, while the further fraction is
still trapped at the modulation capillary upstream end (step
4).
[0035] FIG. 3 schematically shows the cryogenic modulation process,
wherein a cooling medium cools an upstream length of the second
column. In correspondence of the cooling medium the fraction is
trapped by thermal action and then released when the cooling medium
is removed.
[0036] FIG. 4 is a scheme of a GCxGC system with a modulator
according to this invention. The system comprises, in a GC oven 8,
an injector 1, a first column 2 and a second column 6 which are
connected at 3 according to a well known technique. The second
column 6 ends in a detector 7.
[0037] On an upstream length 9 of the second column 6 two jets 4A
and 4B operate alternatively and at a suitable frequence, which are
fed, through corresponding valves 5A and 5B, by a source of liquid
CO.sub.2 10 so that two parts of the capillary length 9 are
directly cooled alternating in order to trap and focus the
fraction, whereafter they are remobilized by the heat of the
surrounding oven air. The opening time of each valve is preferably
the same for all valves and half the cycle time, while the opening
and closure of the valves are carried out in sequence to cover a
cycle time in the order of 0.1 to 30 seconds. It is to be noted
that the opening time of said valves could also be different and
that this opening time may vary from about 0.1 to about 30
seconds.
[0038] The CO.sub.2 jets in FIG. 5 consist of two electrical-driven
two-way valves 5A, 5B that open and close the liquid-CO.sub.2 line
alternating through two pieces of 40 mm long, 0.8 mm ID capillaries
11A/11B, coupled to the nozzles (12A, 12B), 50 mm long 0.5 mm ID
capillaries. In order to force as much CO.sub.2 from the outlet of
the jets to touch the column, the outlets have been modified to
form a slit, 0.04 mm wide and 3 mm long, in parallel above the
capillary. To prevent ice formation onto the outside of the jets at
oven temperatures below about 100.degree. C., they have been
inserted in a 12 mm diameter brass socket to increase the heat
capacity.
[0039] An alternative embodiment of the jet configuration is shown
in FIGS. 10a, 10b and 11, wherein, instead of the slit, the outlet
is constructed by inserting a series of seven capillaries in a row
between the same brass half blocks. More detailedly, as shown in
FIGS. 10a and 10b, each brass block 20 houses a stainless steel
capillary 21, for instance having {fraction (1/16)}" OD and 0.7 mm
ID, said capillary 21 being connected through a related valve 15,
to the CO.sub.2 source 10. Within the end of capillary 21 are
inserted for instance seven capillaries 22 placed according to what
is shown in FIG. 11 and fixed preferably by a ceramic glue or
soldering 23, which is able to withstand temperatures of up to
400.degree. C. In the shown example the capillaries have the
following dimensions: length 35 mm, OD 0.23 mm, ID 0.11 mm and
their free portions are aligned so to run in parallel with the
secondary GC column 9 so that an optimum heat exchange is enabled
by generating a "curtain" of expanding CO.sub.2.
[0040] The axes of the outlet openings of the capillaries 22 are
placed 0.4 mm apart, so that the total length of the nozzle again
is 3 mm. Of course, the above stated number and dimensions of
capillaries can be changed at will.
[0041] The above stated construction allows to decrease the
consumption of CO.sub.2 and optimize the effectiveness of the
throttling process at the nozzle outlet of the cryogenic jets.
[0042] As the liquid CO.sub.2 expands at the outlet of the nozzles,
the throttling process cools the departing gas through the
Joule-Thompson effect. Since this gas is sprayed directly onto the
second column length 9 at the prevailing flow, the column quickly
cools down to about 100.degree. C. below the oven temperature.
Closing the valve will immediately stop the cooling process and the
surrounding air from the stirred oven will heat up the short cooled
section of capillary (about 10 mm) momentarily to oven temperature.
The time required to heat the capillary column from cryogenic to
oven temperature is only 13 ms for a normal 100 .mu.m column (15
.mu.m polyimide and 80 .mu.m fused silica walls).
[0043] The length 9 of the second column in which the modulation
takes place, is stretched and secured between two Valco unions 13
mounted on a bracket 14. The stretching is necessary in order to
avoid vibration of the column caused by the rather intense flow of
cold CO.sub.2 that is sprayed onto the column. The unions are
mounted onto two bonds of 1 mm thick, resilient steel in order to
compensate for the difference in thermal expansion of the steel
bracket and the fused silica column.
[0044] A simple timing device that generates the 24 DC voltages for
valve switching controls the modulation process. Modulation times
shorter than 0.3 seconds can be established.
[0045] In order to test the performance of the modulator according
to the invention, a gas chromatograph was used with a
split/splitless injector and a Flame Ionisation Detector capable to
produce a digital signal sampled at 200 Hz rate. The first
dimension column 30 m.times.0.32 mm ID was coated with
methylsilicon polymer, 0.25 microns film thickness. It was coupled
through a press-fit connector to the second is column 1.5
m.times.0.10 mm ID, which was coated, with 0.1 .mu.m BPX50 (SGE
International, Ringwood, Australia). The flow was set to 1.0 mL/min
through a column head pressure of 170 kPa helium. The columns were
temperature programmed from 50.degree. C., 4 min isothermal,
2.degree. C./min to 300.degree. C.
[0046] The main functions of the modulator are twofold: focusing
small fractions from the effluents of the first column into narrow
pulses and re-injection of these pulses into the remaining part of
the second column. To judge the performance of the modulator, it is
sufficient to measure or calculate the bandwidth of the injected
pulses. To judge the performance of the dual jet modulator, a
series of n-alkanes (C.sub.8 through C.sub.18, see FIG. 6) was
separated. From calculations of the peaks modulated from n-C14 (see
FIG. 7), the peak widths are .sigma.=30 ms, which is better than
second dimension peaks previously reported in the literature for
known modulation systems (sweeper and cryomodulators). The
injection bandwidth appeared to be .sigma..sub.i<10 ms, which is
also better than the injection bandwidths of the known sweeper and
cryo modulators.
[0047] According to what stated above, the jet modulator of this
invention is very simple in construction and easy to install and
maintain. Its control is performed by simply switching one, two or
more valves, so that no movable part are foreseen within the oven,
thus preventing any column breakage due to movement of the
previously known movable modulators.
[0048] Moreover, it has been ascertained that the ability of the
modulator according to the invention to focus the trapped first
dimension fractions into narrow pulses is superior to that of the
modulators known, tested and described in the prior art.
[0049] It is to be finally noted that the present modulator, when
designed with one liquid CO.sub.2 jet only, can act as an injection
focusing device and/or as a peck narrowing and then a detector
sensitivity enhancing device in a conventional one-dimensional GC
system. This configuration is depicted in FIG. 8, where a capillary
column 2 is conventionally housed in an oven and connected with an
injector 2 and a detector 3. A jet of liquid CO.sub.2 issued by a
source outside the oven and controlled by a valve, placed outside
the oven, con be foreseen to impinge on a column portion
respectively directly after the injector (position A) and/or
immediately before the injector (position B).
[0050] When in position A, the CO.sub.2 jet allows to focus the
injected analytes, while when in position B the jet enhances the
sensitivity of the detector by narrowing the peaks.
[0051] This is confirmed by the chromatograms of FIG. 9, comparing
the detector response under the some conditions respectively
without sensitivity enhancement (CO.sub.2 jets in position A and B
not operative) and with sensitivity enhancement (CO.sub.2 jet in
positron A not operative and CO.sub.2 jet in position B operative).
A series of low concentration impurities in a main component are
shown in the chromatograms of FIG. 9, wherein the upper
chromatogram shows the main peak together with a series of low
concentration impurities in the conventional way, where the lower
chromatogram shows how these impurities are collected by means by
the single liquid CO.sub.2 jet in position B (at the time of valve
on) and released as a series of sharp peaks (at the time of valve
off) at increased peak intensities.
* * * * *