U.S. patent application number 10/273123 was filed with the patent office on 2003-03-20 for method and apparatus for determining concentration of nh-containing species.
Invention is credited to Gelernt, Barry.
Application Number | 20030054561 10/273123 |
Document ID | / |
Family ID | 26844509 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030054561 |
Kind Code |
A1 |
Gelernt, Barry |
March 20, 2003 |
Method and apparatus for determining concentration of NH-containing
species
Abstract
A method and apparatus for determining ammoniacal species
concentration in a gas sample. In one embodiment, trace
concentration of ammonia in an air sample is determined by
monitoring emission intensity from an excited radical species
(NH*), which is produced in a reaction between ammonia and
fluorine. The observed emission intensity is compared with
calibration data obtained from previously analyzed gas samples
containing ammonia. The method and apparatus can also be adapted to
detect ammoniacal species concentration in other NH-containing gas
samples.
Inventors: |
Gelernt, Barry; (Oceanside,
CA) |
Correspondence
Address: |
MOSER, PATTERSON & SHERIDAN L.L.P.
595 SHREWSBURY AVE
FIRST FLOOR
SHREWSBURY
NJ
07702
US
|
Family ID: |
26844509 |
Appl. No.: |
10/273123 |
Filed: |
October 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10273123 |
Oct 17, 2002 |
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09624118 |
Jul 24, 2000 |
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6509194 |
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60147017 |
Aug 3, 1999 |
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Current U.S.
Class: |
436/113 ;
422/82.05; 422/83; 436/124; 436/181 |
Current CPC
Class: |
Y02A 50/246 20180101;
G01N 33/0054 20130101; Y10T 436/25875 20150115; Y10T 436/17
20150115; Y10T 436/175383 20150115; Y10T 436/19 20150115; Y02A
50/20 20180101 |
Class at
Publication: |
436/113 ;
436/124; 436/181; 422/82.05; 422/83 |
International
Class: |
G01N 033/00; G01N
021/00 |
Claims
What is claimed is:
1. An apparatus for determining concentration of an ammoniacal
species in a first gas sample, the apparatus comprising: a reactor
having a first inlet for introducing said first gas sample into a
reaction zone inside said reactor; an optical detection-system to
detect radiation arising from said first gas sample inside said
reactor; a second inlet for introducing a second gas sample
containing fluorine into said reactor, wherein said radiation
arising from said first gas sample is generated from a reaction
between said first gas sample and said second gas sample within the
reaction zone inside said reactor; and a data acquisition and
storage system to convert said detected radiation into a radiation
intensity parameter, wherein said radiation intensity parameter is
used to determine concentration of said ammoniacal species in said
first gas sample.
2. The apparatus of claim 1, wherein said reactor further comprises
a pressure measuring device, first and second gas flow controllers
to control said first and second gas sample flows into said reactor
and a vacuum pump, wherein said pressure measuring device, said
first and second gas flow controllers and said vacuum pump
cooperate with each other to maximize said radiation generated from
said reaction between said first and second gas samples.
3. The apparatus of claim 2, wherein a reaction between said first
and second gas samples is performed in a pressure range of about
0.1 to about 50 mbar.
4. The apparatus of claim 2, wherein said optical detection system
comprises an optical device which selectively transmits radiation
originating from said reaction between said first and second gas
samples and a photodetector capable of detecting said transmitted
radiation.
5. The apparatus of claim 4, wherein said optical device transmits
radiation with a full-width half-maximum bandpass of between about
331 nm and about 341 nm.
6. The apparatus of claim 1, wherein said first gas sample
comprises ammonia.
7. The apparatus of claim 5, wherein said radiation arising from
said first gas sample originates from NH* radical.
8. The apparatus of claim 6, wherein said concentration of ammonia
in said first gas sample is determined by comparing said radiation
intensity parameter with at least one provided set of calibration
data regarding ammoniacal species concentration or a functional
relationship correlating detected radiation from excited imidogen
radicals with ammoniacal species concentration.
9. The apparatus of claim 8, wherein said calibration procedure is
performed inside said reactor by reacting each one of a plurality
of calibration gas samples comprising known concentrations of
ammonia with a reactant gas comprising fluorine, detecting
radiation from NH* radicals generated from each reaction,
converting said detected radiation into a calibrated radiation
intensity parameter for each of said plurality of calibration gas
samples, and forming calibration data associating said calibrated
radiation intensity parameter with its corresponding said known
concentration of ammonia.
Description
CROSS REFERENCES AND RELATED APPLICATION
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 09/624,118, filed Jul. 24, 2000, which also
claims priority to U.S. provisional patent application Serial No.
60/147,017, entitled "Method and Apparatus for Determining
Concentration of NH-Containing Species," filed on Aug. 3, 1999,
which are herein incorporated by reference.
BACKGROUND OF THE DISCLOSURE
[0002] 1. Field of the Invention
[0003] The invention relates generally to a method and apparatus of
determining gas phase species concentration, and more particularly,
to a method and apparatus for detecting concentration of
NH-containing species.
[0004] 2. Description of the Background Art
[0005] The use of ammonia (NH.sub.3), a corrosive and toxic gas, in
industrial processes is wide spread. Trace amount of NH.sub.3 has
also been shown to adversely impact the use of chemically activated
deep ultraviolet photoresists in advanced semiconductor
fabrication. The need for worker protection, from either acute
exposure to high NH.sub.3 concentrations or long term exposure to
very low concentration levels, has resulted in the development of
sampling methods for the detection and quantitative measurement of
NH.sub.3 in ambient air. Some existing analytical techniques for
NH.sub.3 detection are briefly described below.
[0006] a. Electrochemical Method
[0007] In this method, gaseous NH.sub.3 is absorbed into an
electrochemical sensor assembly with a resultant change in the
electrical conductivity of the sensor cell. The increased current
flow allowed by the sensor is fairly linear over the concentration
range of 1-50 ppm. A lower detection limit is about 500 ppb, but
reproducibility of the sensor to periodic exposure of NH.sub.3 is
only fair.
[0008] b. Ozone Method
[0009] This method uses a reaction between ozone (O.sub.3) and
ammonia, in which NH.sub.3 is first converted to NO.sub.2, followed
by a chemiluminescent reaction between NO.sub.2 and O.sub.3. The
reaction with O.sub.3 results in the formation of excited state
NO.sub.2 molecules, denoted as NO.sub.2*, and the intensity of
emission from NO.sub.2* is used to determine the original NH.sub.3
concentration. However, difficulties in quantitative measurement
result from side reactions during the conversion from NH.sub.3 to
oxides of nitrogen (forming NO and, perhaps, NO.sub.3 or HNO), and
also from non-stoichiometric side reactions between NO.sub.2 and
O.sub.3. In addition, the emission from excited NO.sub.2 species
(NO.sub.2*--the asterisk "*" is used in this disclosure to
designate an excited state of a species) extends from the near UV
into the yellow-green region of the visible spectrum (this emission
is the well known "air afterglow" in the night sky, and results
from the reaction: NO+O.sub.2.fwdarw.NO.sub.2*+O). Detection of
this very diffuse emission over a broad spectral region is
susceptible to interference from other emitting species, and may
pose difficulties in accurate concentration determination.
[0010] c. Air Sampling Method
[0011] In this method, air samples are collected via a carefully
prepared evacuated sampling ampoule and injected into a gas
chromatograph (GC) for comparison against analyzed standards by
well known methods. Careful selection of the GC column and
temperature settings are necessary in order to obtain reliable
results. A number of detectors are available for this method. One
very sensitive detection method is mass spectrometry, but
calibration for quantitative work is very difficult. Additionally,
the GC/MS method is very expensive, and it is difficult to
configure in a continuous sampling mode.
[0012] d. Laser Induced Emission
[0013] This method has the potential for great sensitivity, but
requires great expertise and expense due to its sophistication.
NH.sub.3, or a fragment thereof, is electronically (or
vibrationally) excited by a pulsed, tunable dye laser, thereby
creating observable fluorescence. However, non-linear optical
effects and saturation effects tend to make quantitative
measurements extremely complex, if at all possible.
[0014] Each of these prior art techniques has its own limitation
and varying degrees of experimental complexities. Therefore, a need
exists in the art for alternative analytical methods that allow
continuous on-line determination of low level of ammonia in ambient
air or gas samples.
SUMMARY OF THE INVENTION
[0015] Embodiments of the invention generally provide a method and
apparatus for determining the concentration of an NH-containing
species in a gas sample. The method comprises detecting radiation
from excited imidogen radicals (NH*) generated from the gas sample,
and determining the concentration of the NH-containing species from
calibration data correlating detected NH* radiation intensity with
concentration of the NH-containing species. In one embodiment, the
NH-containing species is ammonia (NH.sub.3), and the NH* radiation
is generated by reacting NH.sub.3 with a gas sample containing
fluorine. Using a bandpass optical device, NH* radiation around
336.degree. nm can be selectively transmitted and detected, with
minimal interference from other emitting species.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The teachings of the present invention can readily be
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0017] FIG. 1 depicts a schematic diagram of an apparatus for
determining ammonia concentration according to one embodiment of
the invention; and
[0018] FIG. 2 is an illustration of a calibration plot that can be
used for determining concentration of ammoniacal species.
[0019] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures.
DETAILED DESCRIPTION
[0020] The present invention generally provides a method and
apparatus for determining concentration of an ammoniacal
(ammonia-like, or NH-containing) species in a gas sample. In one
embodiment, the ammoniacal species is ammonia (NH.sub.3). It is
known to those skilled in the art of molecular spectroscopy that
gaseous NH.sub.3 and molecular fluorine (F.sub.2) will
spontaneously react, typically at sub-atmospheric pressures. As is
the case with many gas phase reactions, several chemical reaction
pathways are possible, giving rise to different reactive or
non-reactive intermediate or product species. It is also known that
light emission accompanies this spontaneous reaction, and that the
emission is characteristic of energetic, or excited state, species
generated in the reaction. Among these electronically excited
species is the diatomic imidogen free radical (NH*), which has a
spectral emission in an unusually narrow wavelength region around
336.degree. nm (due to the NH A.sup.3.PI.-X.sup.3.SIGMA..sup.-
transition). This light emission, also known as fluorescence, is
the predominant emission in the visible and ultra-violet (UV)
region of the optical spectrum from the spontaneous reaction
between NH.sub.3 and F.sub.2. When the emission is generated from a
chemical reaction, it is sometimes referred generally as
chemiluminescence.
[0021] Embodiments of the invention provide a method and apparatus
by which a trace concentration of NH.sub.3 can be determined from a
functional relationship between the NH.sub.3 concentration and the
observed NH* emission intensity, where NH* is used to denote
generally an excited state of the NH species. In particular, the
method relies on two assumptions: 1) that the detected NH* emission
intensity (I.sub.NH*) is proportional to the concentration of NH*
species; and 2) that the concentration of NH* is in turn correlated
with the initial NH.sub.3 concentration prior to the reaction with
F.sub.2.
[0022] The first assumption can be expressed as:
I.sub.NH*.degree.=.degree.k*[NH*] Eq.(1);
[0023] where k* is a proportionality constant related to a variety
of factors specific to the experimental setup, including light
collection efficiency, detector sensitivity, and the like; and
[NH*] is the concentration of excited NH species present in the
detection volume.
[0024] The second assumption can be expressed as:
[NH*].degree.=.degree.k.sub.1f([NH.sub.3]) Eq.(2);
[0025] where k.sub.1 is a proportionality constant, and
f([NH.sub.3]) denotes generally a function of the concentration of
NH.sub.3. Again, k.sub.1 is an experimental constant which depends
on a variety of factors related to the reaction kinetics between
NH.sub.3 and F.sub.2. This, along with Eq. (1) above, leads to:
I.sub.NH*=kf([NH.sub.3]) Eq.(3);
[0026] where k=k*k.sub.1.
[0027] According to the method of the invention, the concentration
of NH.sub.3 present in a gaseous sample can be determined by
experimentally measuring the intensity of emission from NH*, and
determining the NH.sub.3 concentration [NH.sub.3] from the
functional relationship of Eq.(3). The exact functional
relationship f([NH.sub.3]) can be obtained by a calibration
procedure to be described below. The method is particularly suited
to the determination of trace level of NH.sub.3 in a gas
sample.
[0028] In general, the concentration of an intermediate species in
a reaction, such as an excited state of a reactive radical (NH*),
is very low, and one may encounter difficulties in detecting
emission from such a species. However, one can take advantage of
the fact that the predominant visible and UV emission from the
NH.sub.3+F.sub.2 reaction originates from NH*. By using a suitable
bandpass optical device, such as an optical interference filter or
monochromator, one can selectively transmit and detect the NH*
emission around 336.degree. nm to the exclusion of background
signal from other emitting species. Any background emission, if not
properly excluded, may interfere with (i.e., contribute to) the
observed light emission intensity and thus affect the accuracy of
the determination of NH.sub.3 concentration.
[0029] Since the reaction between NH.sub.3 and F.sub.2 occurs in
the absence of heating or other external energy sources (i.e., as a
"dark reaction"), the resulting fluorescence can be measured
against a dark background. This allows the use of extremely
sensitive light detection methods, such as photon-counting, to
detect and quantify trace amounts of NH.sub.3 present in a gas
sample, such as an ambient air sample containing NH.sub.3. Hence,
the invention has superior sensitivity over existing methods.
[0030] An apparatus suitable for practicing the present invention
is illustrated schematically in FIG. 1. The apparatus 10 comprises
a vacuum system 100 and an optical detection system 160. The vacuum
system 100 further comprises a reaction vessel 102 (or reactor)
connected to a pressure-reducing device such as a vacuum pump 180
and other gas flow and pressure regulating components. As shown in
FIG. 1, the gas flow and pressure regulating components may
illustratively comprise vacuum valves 104, 106, 108 and 110. The
valve 104 controls gas flow at the inlet 124, while the valve 108
controls gas flow at the outlet 128. At least one of these valves
104, 108 should have an adjustable orifice for variable gas flow
control, such as that provided by a needle valve. Different needle
valves with varying sizes of orifices can result in a fine control
of the gas flow up to a range of, for example, 500.degree. sccm.
The exact flow range, however, is not critical to the practice of
the present invention.
[0031] The valve 110 is a throttle valve connecting the outlet 128
to the vacuum pump 180 via a vacuum line 184. For example, the
vacuum pump 180 may be a mechanical pump with inert fluorocarbon
oil having a 2 CFM pumping capacity. The exhaust gases, including
carrier gas, reactant and product gases, are evacuated through the
vacuum line 184. The pumping capacity (or speed) provided to the
vessel 102 can be varied by adjusting the throttle valve 110. The
adjustment of valves 104 and 108, in conjunction with the throttle
valve 110, allows control of the gas flows through the vessel 102.
Thus, a partial vacuum in the range of about 0.1.degree. mbar to
about 50.degree. mbar can be achieved inside the vessel 102. A
pressure transducer 182 is also provided for pressure measurement.
It is preferable that more than one pressure gauge be used for
pressure monitoring at different pressure ranges. For example,
capacitance manometers available from MKS Instruments, Inc.,
Andover, Mass., are suitable for this purpose.
[0032] The reaction vessel 102 also comprises a second inlet 126. A
valve 106 is used to control the gas flow through the inlet 126,
which extends into the interior 102I of the vessel 102, and
terminates in an inlet tip 127. An optical window 152 is provided
on one side 103 of the vessel 102 at close proximity to the inlet
tip 127. The reaction vessel 102 is preferably made of glass or
quartz, but other materials such as stainless steel are also
acceptable, as long as it is compatible with the chemicals or gases
used. The optical window 152 should be made of a material which can
transmit radiation around 336.degree. nm. In general, any
ultra-violet (UV) transmitting materials such as different grades
of quartz will suffice.
[0033] To perform the NH.sub.3 concentration measurement according
to embodiments of the invention, the vessel 102 is evacuated with
the throttle valve 110 and valve 108 fully open. After a base
pressure of about 0.1.degree. mbar or below is reached, the valve
108 is closed to some appropriate intermediate position while a gas
sample to be analyzed, e.g., air containing an unknown
concentration of NH.sub.3, is introduced into the vessel 102 via
the inlet 124. The flow rate of this NH.sub.3/air sample through
the vessel 102 can be controlled by adjusting the valves 104 and
108. Asteady flow of the gas sample may be established within a
range of about 100-500.degree. sccm, preferably at about
300.degree. sccm. An operating pressure in the range of about
0.1.degree. mbar to about 50.degree. mbar, preferably at about
10.degree. mbar, may be used.
[0034] With the NH.sub.3/air flow rate and pressure established, a
second gas sample containing fluorine--e.g., a dilute mixture of
F.sub.2 in a carrier gas such as argon (Ar) or helium (He), is then
is introduced into the vessel 102 through the inlet 126 by the
valve 106. This fluorine-containing gas sample, also referred to as
a reactant gas, is used to initiate a reaction between NH.sub.3 and
F.sub.2. The reactant gas is preferably a highly diluted mixture of
F.sub.2 in a non-reactive carrier gas such as ultra-high purity
(UHP) Ar or UHP He. Of course, other similarly non-reactive or
inert gases, e.g., nitrogen (N.sub.2), may also be used as a
carrier gas, provided that they do not substantially interfere
either with the NH.sub.3+F.sub.2 reaction or the detection of the
NH* emission. The F.sub.2/carrier gas mixes with and reacts with
the flow of air containing NH.sub.3 (or generally, the gas sample
to be analyzed) just down-stream of the gas inlet tip 127. This
counter-flow reaction method and apparatus design is well known in
experimental gas kinetics.
[0035] A reaction zone 150, where F.sub.2 and NH.sub.3 reaction
occurs, is generally defined in the vicinity of the reactant gas
inlet tip 127 inside the vessel 102. By controlling the flow rate
of the F.sub.2 gas into the air/NH.sub.3 flow stream, one can
confine the reaction zone 150 to within a relatively small,
well-defined volume. A better defined reaction zone 150 is
preferable because it allows an efficient collection and detection
of the chemiluminescence.
[0036] As the reactant gas reacts with NH.sub.3 in the NH.sub.3/air
sample, emission from excited NH* species is detected using the
optical detection system 160 to be described below. The reactant
gas flow should be adjusted so as to maximize the NH* emission
intensity I.sub.NH* detected by the optical detection system 160.
That is, at a fixed flow rate of NH.sub.3/air, the reactant gas
flow should be sufficiently high such that additional F.sub.2 (or
reactant gas) will not result in an increase of detected I.sub.NH*
signal for a given configuration of the optical detection system
160.
[0037] It is understood that the process parameters disclosed
herein are meant to be illustrative, and other gas flow rates and
operating pressures may be adjusted as appropriate to different
reaction vessels. In general, the choice of the operating pressure
may be based on several considerations--e.g., a higher operating
pressure tends to favor a higher reaction rate between NH.sub.3 and
F.sub.2. However, a higher pressure also results in increased
collisions between the excited NH* and other gas molecules. These
collisions may lead to "quenching" of the NH* emission, and thereby
reduce the amount of detectable optical signal. Therefore, an
optimal operating pressure may involve balancing these competing
considerations, and one can experimentally arrive at the desired
operating pressure by establishing initial flows of the NH.sub.3
and F.sub.2 gases, and adjusting valves 104, 106, and 108 to
maximize the NH* signal. Such optimization technique is well-known
to one skilled in the art of chemical kinetics.
[0038] If the gas sample to be analyzed is being used as a process
gas in a certain process application, the apparatus 10 may also be
used for continuous on-line measurement of NH.sub.3 concentration
in the process gas. For example, the apparatus 10 may be connected
(e.g., at its inlet 124) to a reactor (not shown) used for the
particular process application, and a relatively small flow of the
process gas may be diverted from the reactor into the reaction
vessel 102 via the inlet 124. The NH.sub.3 concentration may then
be continuously monitored according to embodiments of the
invention, without interfering with the particular process
application.
[0039] Optical Detection System
[0040] The light emission from the reaction of NH.sub.3 and F.sub.2
(due to the NH A.sup.3.PI.-X.sup.3.SIGMA..sup.-. transition), is
transmitted through a suitable optical window 152 and a bandpass
optical device 162, and detected by a detector 164. A lens 161, or
similar imaging optics, may also be used to facilitate the
collection and imaging of light emission from a sample volume
(e.g., the reaction zone 150) onto the detector 164.
[0041] The bandpass optical device 162 preferably has a bandpass
that is sufficiently narrow as to transmit the NH* emission near
336.degree. nm, while substantially rejecting emissions from other
species that may interfere with the detection of the NH* emission
(i.e., selectively transmitting the desired NH* emission). In one
embodiment of the invention, a narrow bandpass filter 162, e.g., an
interference filter having a bandpass of about 10.degree. nm (i.e.,
full-width bandpass at half-maximum intensity, or FWHM), with a
peak transmission of about 10-50% around 336.degree. nm may be
used. Due to the "piling-up" of the Q-branch of the NH
A.sup.3.PI.-X.sup.3.SIGMA..sup.- electronic transition, most of the
NH* chemiluminescence can be transmitted through the interference
filter 162, which also effectively blocks other undesirable or
background emissions, thus facilitating the detection of NH*
emission. Such an interference filter is available from commercial
optics supply vendors. The optical characteristics of the
interference filter cited herein are meant to be illustrative. It
is understood that filters with different optical characteristics
(i.e., FWHM bandpass, peak transmission percent and peak
wavelength) may also be used to transmit the NH* emission for
practicing embodiments of the invention. For example, if measures
are taken to eliminate interfering emissions (e.g., by eliminating
species having interfering emissions), a wider bandpass filter may
be tolerated.
[0042] In other embodiments, the bandpass optical device 162 may
comprise a combination of different filters that is effective for
selectively transmitting the desired NH* emission, while blocking
interfering emissions from other species. For example, the
combination may include a longpass filter and a shortpass filter
with appropriate cut-off wavelengths, or a bandpass filter having a
FWHM bandpass larger than about 10.degree. nm and a suitable
cut-off filter. One example of a possible interfering emission
originates from OH radicals, which may arise from the presence of
moisture or other reactions in the reactor. It is known that an
excited state of the OH radical has a strong emission around
306.degree. nm. If a short wavelength cut-off filter (or longpass
filter) is used to block the 306 emission from excited OH radicals,
then a filter having a FWHM bandpass larger than about 10.degree.
nm may be used. Other bandpass optical devices such as a
monochromator or similar equipment with wavelength selection
capabilities can also be used in place of an interference
filter.
[0043] The light emission that passes through the bandpass device
or filter 162 is incident upon the detector 164, which is selected
to be sensitive to the transmitted emission. For example, the
detector 164 may be a RCA 1P28 photomultiplier tube operating at
about 800V DC. The photocurrent generated by the emission can be
detected using commercially available detection and amplification
equipment 166. Suitable detection and amplification equipment 166
may include picoammeters or photon-counting devices with dynode
pulse discrimination electronics, among others. In general, various
combinations of detectors and amplification equipment may be used
to detect the emission through the optical device 162 and convert
it to a radiation intensity parameter that correlates with the
intensity of the NH* emission. The apparatus 10 should preferably
include a device 168 for monitoring and/or recording of the
amplified optical signal, or more generally, the radiation
intensity parameter. The device 168 may illustratively be a
computer that interfaces with the detection and amplification
equipment 166 and provides for data storage and retrieval.
[0044] Calibration of the apparatus 10 is accomplished with dilute,
analyzed samples containing NH.sub.3--e.g., NH.sub.3 in N.sub.2 or
in air, or other suitable carrier gases. A calibration plot, for
example, is constructed by plotting the chemiluminescent intensity
during reaction with excess F.sub.2 against known NH.sub.3
concentrations [NH.sub.3] from analyzed, calibration gas
samples.
[0045] Calibration Procedure
[0046] In order to determine the concentration of NH.sub.3 in an
unknown gas sample, a calibration procedure is performed in the
reaction vessel 102 to generate calibration data which correlate
detected NH* emission intensities with known NH.sub.3
concentrations in calibration gas samples. The calibration gas
samples, e.g., NH.sub.3/air mixtures, can be analyzed to obtain
known NH.sub.3 concentrations by conventional analytical methods,
or prepared by successive dilutions from more concentrated mixtures
that are amenable to conventional analytical techniques, or
procured as analyzed mixtures from any number of industrial gas
suppliers.
[0047] The calibration procedure involves experimentally measuring
the NH* emission intensities (I.sub.NH*) from reaction with F.sub.2
for several calibration gas samples with known NH.sub.3
concentrations [NH.sub.3], using the procedure previously
described. For example, NH* emission measurements can be performed
for each of several calibration gas samples containing NH.sub.3
concentrations between a few hundred to a few thousand parts per
billion (ppb) by mixing each of the calibration gas samples with a
reactant gas containing fluorine. Although the fluorine-containing
reactant gas used in the calibration procedure may be different
from that used in the reaction with the gas sample having the
unknown NH.sub.3 concentration, it is preferable and more
convenient to use the same reactant gas (e.g., similar F.sub.2
concentration and/or carrier gas).
[0048] The calibration data comprising detected emission intensity
I.sub.NH* (or a calibrated radiation intensity parameter
correlating with I.sub.NH*) and its corresponding NH.sub.3
concentration [NH.sub.3] may be represented in a calibration plot,
such as that illustrated in FIG. 2, which can be extrapolated to
lower concentrations. More generally, a functional relationship
between I.sub.NH* and [NH.sub.3] can be derived from the
calibration data. The NH.sub.3 concentration in a gas sample with
an unknown [NH.sub.3] can then be determined by comparing the
observed NH* emission intensity (from a reaction between the
NH.sub.3-containing gas sample and the reactant gas containing
fluorine) with the calibration data, or from the functional
relationship correlating NH* emission intensity I.sub.NH* with
NH.sub.3 concentration.
[0049] In one illustrative embodiment, a mixture of 0.5% F.sub.2 in
UHP Ar can be used as a reactant gas for calibration. In practice,
it is not possible to maintain a stable concentration of highly
diluted F.sub.2 gas in a vessel (gas cylinder) for an extended
period of time due to the corrosive or reactive nature of F.sub.2.
However, this will not affect the calibration procedure because
F.sub.2 is introduced in "excess" to produce a maximum detected
I.sub.NH* for the given detection system 160. The reaction vessel
102 is first evacuated to a pressure of about 0.1.degree. mbar.
After a steady flow of one calibration NH.sub.3/air sample (i.e.,
previously analyzed, with known NH.sub.3 concentration) is
established, e.g., at a pressure of about 10.degree. mbar and a
flow rate of about 300.degree. sccm (or generally, the same
pressure and flow rate as used for the reaction with the gas sample
having unknown NH.sub.3 concentration), the F.sub.2/Ar reactant
mixture is introduced into the vessel 102 at the inlet tip 127. To
ensure that F.sub.2 is in "excess", the F.sub.2/Ar reactant gas is
introduced until the observed NH* emission intensity no longer
increases with additional F.sub.2/Ar reactant mixture. The NH*
emission from the reaction zone 150 is detected as described above
using detector 164.
[0050] The observed intensity I.sub.NH*, along with the known
concentration of the previously analyzed NH.sub.3/air sample, may
be recorded or stored in a suitable medium, e.g., a computer. The
calibration procedure is repeated for the remainder of the
calibration gas samples (that have previously been analyzed to
obtain known NH.sub.3 concentrations). A calibration plot such as
that shown in FIG. 2 can be recorded, showing the observed
intensity I.sub.NH* vs. the known NH.sub.3 concentration. In
general, a functional relationship derived from the calibration
data will be used for the determination of NH.sub.3 concentration
in gas samples. The calibration results should ideally be recorded
by electronic means, for example, a computer or processor having a
storage device, to facilitate data storage and retrieval.
[0051] Other NH-Containing Samples
[0052] Although the present embodiment focuses on the measurement
of NH.sub.3 in a gas sample, this invention can be extended to the
determination of other ammoniacal, or ammonia-like species, e.g.,
molecular species with a N-H bond, such as organic amines, imines
and other NH-containing species. Although the detailed chemical
reactions may differ, it is anticipated that reactions between
different ammoniacal species and fluorine (contained in the
reactant gas sample) will lead to the formation of excited NH
species, or NH*. Depending on the specific ammoniacal species, the
reaction may not be "dark", as previously explained for the case of
NH.sub.3. However, the use of a narrowband interference filter
should still suffice to isolate the emitted radiation from NH*, for
example, around 336.degree. nm, to allow for a determination of the
ammoniacal species concentration. Of course, a separate calibration
procedure has to be performed as previously described for a number
of the gas samples containing known concentrations of the species
of interest. The choice of pressure and flow parameters used in the
F.sub.2 reaction can readily be arrived at through experimentation
which is known to those skilled in the art of chemical
kinetics.
[0053] Although one embodiment which incorporate the teachings of
the present invention has been shown and described in detail
herein, those skilled in the art can readily devise many other
varied embodiments that still incorporate these teachings.
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