BACKGROUND OF THE DISCLOSURE
This disclosure is directed toward the measurement of concentrations
of compounds in a gas sample. More particularly, the disclosure is directed toward
methods and apparatus for measuring concentrations of compounds containing chlorine
within a mixture of noble gases and, in particular, within a helium carrier gas
which has been doped with relatively small concentrations of krypton. The disclosed
system has also been used for measuring concentrations of other halogens such
as fluorine. Other noble gas dopants such as xenon and argon have a!so been employed.
Spark discharges are used to initiate a series of reactions within the carrier
gas and sample mixture which results in the formation of an excited chlorine-krypton
molecule which, in turn, emits characteristic photon radiation. The concentration
of chlorine within the sample is then determined from the measured intensity of
the characteristic photon radiation from the decay of excited chlorine-krypton.
It is emphasized that the intensity of the characteristic photon radiation is proportional
to the mass of chlorine within the sample. The chlorine content is, therefore,
determined independently of the structure of the compound in which it is contained.
For many years, there has been a strong desire to develop gas chromatographic
detectors that detect only a specific element. It is well known in the art that
a gas chromatographic (GC) column is able to separate very similar compounds into
separate peaks output as a function of time. This time-based separation is especially
useful in delineation of adjacent peaks. Once the peaks are separated, it is necessary
to identify the constituents of the peaks, and then to quantify these constituents
so that concentrations of compounds or elements within the sample gas input into
the GC can be determined.
Detectors which respond to chlorine are especially needed in the
field of pollution monitoring and control. Chlorine pollutants in air or in water
are generally in the form of organic compounds. Pollutants, however, also include
a far greater variety of compounds which have chlorine in them. Common, and potentially
dangerous, water and air pollutants include gaseous freons, pesticide residue in
the soil, polychlorobiphenyl (often called PCB) and many others. While all of
these can be extremely valuable compounds in one aspect, they can also be detrimental
in trace quantities where they are not wanted. They are especially difficult to
isolate, quantify and remove in trace quantities. Even trace quantities of these
materials can pose significant environmental hazards in air, water, and soil.
Detectors which respond to trace amounts of chlorine are also needed
in many chemical processes which utilize catalysts. Even trace amounts of chlorine
can deteriorate or even destroy catalysts. As an example, in the crude oil refining
industry, concentrations of chlorine as low as ten parts per million can essentially
destroy or "poison" the catalyst used in refineries which produce gasoline.
SUMMARY OF KNOWN PRIOR ART
Various techniques have been used to measure trace quantities of
materials, and in particular, traces of the element chlorine or chlorine compounds
in air, water and soil samples.
There are numerous "wet chemistry" techniques known in the art for
detecting chlorine. As a group, these techniques are time consuming, relatively
expensive, and certainly not suited for on-line, real-time monitoring of trace
U. S Patent No. 5,019,517 to Dale M. Coulson discloses a detector
system and method for detecting trace gases. The detection system is especially
suited for detecting a halogen containing component in a gas stream, and includes
a gas chromatograph which is connected by a gas line to a pyrolysis chamber. Sources
of additional gas streams are connected by a second gas line to the pyrolysis chamber.
The system features a temperature control feedback system which maintains the
temperature of the pyrolysis chamber independent of the detector electrodes and
at a temperature between about 700 and 1,000 degrees Centigrade (°C). U. S. Patent
No. 4,440,726 to Dale M. Coulson discloses an electrochemical detector cell which
is sensitive to trace elements including chlorine and chlorine compounds. These
techniques are usually slow, require apparatus which is relatively expensive to
fabricate, operate and maintain. In addition, the apparatus associated with these
techniques are relatively large physically, and the actual measurements are compound-specific.
Test standards of the EPA are similar to the Coulson process mentioned above.
Other techniques have been used to monitor crude oil for trace amounts
of chlorine prior to feeding the crude stock to a refinery. A particular nuclear
technique is described in U.S. Patent Nos. 4,200,789 and 4,209,695 to Dan M. Arnold,
et al wherein thermal neutron capture is used to detect elemental chlorine concentrations
as low as 5 parts per million (ppm). The apparatus required to make this measurement
is quite large, and is also expensive to fabricate. In addition, the isotopic
source of neutrons requires significant radiation control and safety procedures.
An electrolytic conductivity detector (ELCD), known in the art as
the Hall detector, has been used to detect very small quantities of compounds containing
elements such as sulfur and nitrogen. This technology is taught in U.S. Patents
Nos. 3,934,193 and 4,555,383 to R. C. Hall. Hall's teachings of chlorine compound
detection is limited to the detection of chlorinated hydrocarbons such as pesticides.
Detection devices consisting of a pulse discharge chamber have been
used to detect very small elemental concentrations, including chlorine. This type
of detection device is exemplified by the disclosures of U.S. Patent Nos. 5,153,519
and 5,317,271 to Stanley D. Stearns and Wayne E. Wentworth, both of which are assigned
to the assignee of the present disclosure. Selective sample ionization and element-specific
photon radiation, both generated by continuous or pulsed spark discharges, are
used to determine quantitatively elements and compounds of interest within a sample.
As an example, chlorine can be detected by the direct excitation of chlorine which
decays by the emission of relatively intense radiation at a characteristic wavelength
of approximately 138 nanometers (nm). This emission is resolved from other radiations
generated within the pulse discharge chamber by means of a monochromator or other
suitable spectrographic device. Since air is not transparent to radiation of wavelength
less than approximately 200 nm, the monochromator of other spectrographic devices
must be operated under vacuum conditions. Likewise, the "window" within the pulse
discharge chamber through which the 138 nm emerges and impinges upon the monochromator
must also be transparent to this radiation of interest. Relatively inexpensive
quartz can not be used as a window material since it is only transparent to radiation
above 200 nm. Instead, generally more expensive materials, such as magnesium fluoride,
must be used as the chamber window material.
SUMMARY OF THE INVENTION
One objective of the invention is to provide a chromatographic detector
system which responds specifically to the element chlorine, or to any compounds
containing the element chlorine. A further objective of the invention is to provide
a detection system which can detect chlorine in amounts as small as 1 picogram
(pg), and which responds linearly to the amount of chlorine within a sample over
at least three orders of magnitude. A still further objective of the invention
is to provide a system which can measure concentrations of other halogens. A still
further goal of the invention is to provide a chlorine specific detection system
which is relatively inexpensive to fabricate and maintain, physically compact
and rugged, and relatively easy to operate. This objective includes a detection
system which utilizes a spectrographic system such as a monochromator which can
be operated in air at atmospheric pressure.
The present disclosure sets forth a chlorine specific detection system
which meets the foregoing objectives, and which provides additional advantages
that will become apparent in the following disclosure. The system can also be
used to measure concentrations of other halogens. The system is especially successful
in detecting and quantifying chlorine in the form of both volatile organic and
inorganic chlorine compounds, as well as chlorine gas. The sample is commingled
in a mixture of noble gases. More specifically, a small concentration of krypton
(Kr) gas, referred to as a "dopant", is added to the output from a GC, which contains
the suspected chlorine sample of interest dispersed in a helium (He) carrier gas.
Alternately the Kr dopant can be added to the carrier gas to the GC. The carrier
gas is typically "doped" with 1.0% to 5.0% Kr, and preferably with approximately
1.0% Kr. Tests have shown that the concentration of Kr dopant does not affect the
quantitative Cl measurement if the concentration exceeds 0.2% threshold level.
This aspect will be discussed further in a subsequent section of this disclosure.
The mixture of Kr doped helium carrier gas, which also contains the chlorine sample
to be detected and quantified, is next passed into a spark discharge chamber in
which various components of the gas are ionized and excited with an electrical
spark discharge. The spark discharge can be either continuous or pulsed. Specifically,
the spark discharge produces He+ and He2+ ions.
These He+ and He2+ ions then react with the Kr
dopant atoms to produce krypton ions, Kr+, and one or two He. Defining
a generic molecule containing chlorine as "RCl" where "R" represents one or more
additional elements, the Kr+ ion then reacts with the RCl molecule to
form the excited molecule KrCl* and the ion R+. The excited molecule
KrCl* decays to the ground state by the emission of characteristic photon radiation
in a band centered about a wavelength of 222 nm. Throughout the remainder of this
disclosure, the emission of photon radiation will be referred to as an emission
at 222 nm, but it should be understood that the emission is encompassed by a narrow
band of wavelengths which is centered at 222 nm. This particular reaction is discussed
in the publication "Comparison of the Rg+(2P1/2)/Cl-/He
and Rg+(2P3/2)/Cl-/He three-body ionic-recombination
reactions for the formation of RgCl*, Rg* and Cl*", Masaharu Tsuji et al,
Chem. Phys., 94(6), 4291 (1991).
Assuming that the Kr dopant concentration exceeds the 0.2% threshold,
the intensity of the emitted 222 nm photon radiation is proportional to the amount
of compound RCl in the sample, and more specifically, proportional to the amount
of elemental chlorine contained as sample in the Kr doped, helium carrier gas
output from the GC column. The pulse discharge chamber incorporates a window or
port which is transparent to photon radiation at 222 nm. Quartz is a suitable window
material which meets this transparency requirement for the radiation of interest
at 222 nm. If the detection system is used to detect emissions below 200 nm, window
material transparent to these emissions, such as magnesium fluoride (MgF2),
must be used. Photon radiation emerges from the ionization chamber through the
window, and into a monochromator which disperses the photon radiation generated
within the ionisation chamber. The monochromator therefore provides a means for
isolating the 222 nm photon radiation resulting from the decay of excited KrCl
from radiation of other wavelengths which might be generated within the pulse
discharge chamber. The intensity of 222 nm photon radiation delineated or resolved
in the monochromator is then measured with a photomultiplier detection system
featuring a photomultiplier tube and associated amplification and power circuitry.
The output of the photomultiplier detection system is, therefore, proportional
to the amount of chlorine in the gas which is input into the pulse discharge chamber.
The photomultiplier detection system output, which is typically an electrical current,
can be converted to quantitative measures of sample chlorine content by using a
calibration conversion constant. This calibration conversion constant is determined
by measuring the output current of the photomultiplier detector system using samples
containing known amounts of chlorine, again assuming that the Kr dopant concentration
exceeds the previously mentioned 0.2% threshold concentration.
The present detection system offers many advantages over prior art
systems. The photon radiation centered about 222 nm from excited KrCl is easily
measured using the previously described photomultiplier detection system. Furthermore,
air is transparent to the 222 radiation thereby allowing the monochromator used
to isolate the photon radiation from chlorine to be operated with air at atmospheric
pressure. Furthermore, relatively inexpensive quartz can be used as window material
in the pulse discharge chamber. If however, the system is used to detect photon
radiations below 200 nm resulting for other emissions, air is no longer transparent
to radiations of these wavelengths, therefore, the monochromator must be operated
in a vacuum. Previously referenced detectors, which also employ pulse discharge
chambers as disclosed U.S. Patent Nos. 5,153,519 and 5,317,271, can excite elemental
chlorine. The emission from excited chlorine occurs at wavelengths ranging from
approximately 130 nm and into the UV-visible region. The atomic emission at the
low wavelength of approximately 138 nm is intensive, and elemental chlorine concentration
can be determined from a measure of this photon radiation. In such a measurement,
however, the monochromators required to disperse radiation at this wavelength must
be devoid of air, since air absorbs radiation at this wavelength. Commonly, these
monochromators are put under vacuum using vacuum pumping in order to eliminate
the air. This adds significantly to the cost, size, complexity and operating expense
of such a prior art detection system. Furthermore, quartz is not transparent to
radiation at this wavelength. Windows for the pulse discharge chamber made of material
transparent at 138 nm, such as magnesium fluoride, must be used in the spark discharge
chamber. Magnesium fluoride is considerably more expensive to obtain and to fabricate
than quartz, thereby further increasing the cost of such an elemental chlorine
Turning again to the present detection system, studies have shown
that, for a given Kr dopant level exceeding the threshold concentration of 0.2%,
the sensitivity of the device is as low as 1 to 2 pg of chlorine. Studies have
further shown that the output of the photomultiplier detection system is linear
with sample chlorine content over at least three orders of magnitude. Many chlorine
compounds of interest also contain carbon, such as carbon tetrachloride (CCl4).
Furthermore, many of the chlorine compounds of interest are found in hydrocarbons,
thus much carbon is present, from other sources such as gasoline, in the sample
to be analyzed. Carbon will therefore also be excited within the pulse discharge
chamber in addition to excited chlorine as KrCl*. Excited atomic carbon emits
photon radiation at wavelengths of 193.1 and 247.9 nm. The monochromator and associated
photomultiplier detector system can easily resolve the 222 nm radiation of interest
from the "interfering" photon radiation from carbon.
To briefly summarize, the present chlorine specific detection system
is very sensitive and is a simple system to operate once the various components
have been assembled. It utilizes two sources of noble gas (as a carrier gas system)
in which the sample gas containing chlorine is dispersed. The primary noble gas
is helium which is the carrier gas for a GC column and contains the chlorine compound.
The trace or dopant gas is krypton which is added to the mixture of helium carrier
and sample gas. This mixture is then input into the pulse discharge chamber. The
spark discharge can be either pulsed or continuous. The discharge creates excited
KrCl through the reaction sequence previously discussed. Radiation from the decay
of excited KrCl is detected with a photomultiplier detection system which utilizes
a monochromator operating in air at atmospheric pressure. The output of this detection
system is used to determine the concentration of chlorine within the sample. It
is emphasized that the detection system responds to the mass of chlorine in the
sample independent of the structure of a compound. As an example, CCl4
induces four times the detector response than does CH3Cl.
While the foregoing summarizes a number of aspects of the present
disclosure, the detailed description will set forth the preferred embodiment. That
will be understood in conjunction with or in reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWING
So that the manner in which the above recited features, advantages
and objects of the present invention are attained and can be understood in detail,
more particular description of the invention, briefly summarized above, may be
had by reference to be embodiments thereof which are illustrated in the appended
The single drawing is a schematic block diagram of the detection
apparatus of the present disclosure, and sets forth a pulse discharge chamber which
has input from a source of an unknown sample, typically a gas chromatographic
column, and which exposes a Kr trace gas and He carrier gas to a pulse discharge
to thereby create metastable molecules which emit a photon at a particular frequency
for quantification of any chlorine which may be present in the sample.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiments of the system will be presented in two
sections. The first section will disclose in detail the detection system apparatus
and further, the key elements of the detection system apparatus. The second section
will present in detail the series of reactions which occur within the pulse discharge
chamber which eventually lead to the emission of photon radiation characteristic
of the decay of KrCl excitement. The detection of this radiation is, as outlined
previously, the means by which the chlorine content of the sample is determined.
Attention is directed to the only view which identifies a test system
in block diagram form with the numeral 10. The test system utilizing a pulse discharge
chamber 12. The pulse discharge chamber is a device of the sort described, for
example, in Fig. 2 of U.S. Patent No. 5,153,519 and also exemplified in U.S. Patent
No. 5,317,271, both of which are assigned to the assignee of the present disclosure.
The pulse discharge chamber utilizes a current pulsing system 14 which provides
a spark across a pair of spaced, separated terminals or electrodes (not shown)
in the pulse discharge chamber. Pulsed excitation as commonly used by steady state
excitation of the gas flow is acceptable.
A mixture of gas is input into the pulse discharge chamber 12. There
are several embodiments of the gas input system that will produce acceptable results
from the disclosed chlorine detection system. The schematic illustrates helium
carrier gas from a reservoir 24 being input into the gas chromatograph (GC) column
16 as a carrier gas. Sample containing the chlorine compound or compounds to be
detected and quantified is input into the GC column 16 from a sample source 25.
Gas eluted from the GC 16 is then flowed through a mixing valve 28. Krypton dopant
gas from the reservoir 26 is also flowed into the mixing valve 28 where it is commingled
with the helium carrier gas containing the unknown sample or samples of chlorine.
Alternately, the dopant gas can be added to the carrier gas to the GC. It is also
possible to buy helium in a large tank, either pure or mixed with trace amounts
of other noble gases. If desired, the correct concentration of dopant can be mixed
in the helium to reduce valving complexity. Typically, this gas mixture is then
flowed into the inlet port 13 of the pulse discharge chamber 12 where it passes
between the terminals of the pulse discharge chamber and exposed to a pulse discharge
spark. The gas eventually exits the pulse discharge chamber 12 through the exit
port 15 thereby giving a net flow of gas through the chamber from right to left
as illustrated in the block diagram of the system. The gas which is exposed to
the spark is excited. When that occurs, and long after the spark has ended, there
remains a number of excited gas molecules as described in detail in the previously
referenced U.S. patents Nos. 5,153,519 and 5,317,271, the descriptions which are
entered herewithin by reference.
As discussed previously, the system produces, through a series of
reactions within the pulse discharge chamber 12, photons from the decay of excited
KrCl (KrCl*) which are used to identify and to quantify the chlorine in the sample
material. The characteristic photon radiation from the decay of KrCl* is a band
centered about a wavelength of 222 nm. Details of the reactions occurring within
the pulse discharge chamber 12 will be presented in the next section of this disclosure.
The pulse discharge chamber 12 contains an optical window which is
transparent to the 222 nm photon radiation emitted in the decay of KrCl excitement.
Quartz is a material which meets this requirement. This window is identified by
the numeral 11 and is depicted conceptually as being physically located in the
side of the pulse discharge chamber 12. It should be understood that the optical
window can be placed at a variety of locations in the pulse discharge chamber.
As an example, in the previously referenced U.S. patents Nos. 5,153,519 and 5,317,271,
it is disclosed that the optical window can even be located at the input orifice
13 of the pulse discharge chamber 12, or at any other position on the chamber
where photon emission within the chamber 12 can be "viewed".
A monochromator 18 is positioned near the optical window 11 to receive
photon emissions from the chamber. Air at atmospheric pressure is transparent to
the 222 nm radiation from chlorine. The monochromator can, therefore, be operated
in air at atmospheric pressure, and does not require the vacuum operation or controlled
gas environment operation of prior art devices. The monochromator can be of the
prism type wherein the angle of the prism is adjusted with respect to the incident
photon radiation to resolve radiation of a specific energy which, in the chlorine
detection system, is 222 nm. The 222 nm radiation exits the monochromator 18 and
then impinges upon a photomultiplier tube (PMT) 20 which converts the photon signal
to a corresponding electrical signal in a manner well known in the art. High voltage
and B+ power supplies are shown conceptually as a power supply element 29. The
current output from the PMT 20 is then amplified by means of the amplifier circuit
30 and subsequently recorded by the recorder 22. The intensity of the recorded
current is proportional to the chlorine content within the pulse discharge chamber
12. By measuring the current using samples of known chlorine content, the current
calibration constant for the photomultiplier detection system can be determined
thereby allowing the measured current to be converted to corresponding absolute
measures of chlorine in the sample.
Alternate means can be used to resolve the characteristic 222 nm
photon radiation resulting from the KrCl excitement. In one alternate embodiment,
the monochromator 18 contains a grating which is adjusted to pass only the characteristic
KrCl radiation. This radiation is again detected by the PMT 20 and processed by
the photomultiplier detection system as described previously. In a second alternate
embodiment, the monochromator is replaced with an interference filter (not shown)
which is again set to pass a band of photon radiation centered at 222 nm with a
band width of, perhaps, +/- 5 nm. Again the PMT detects the passed photon radiation
and the photomultiplier detection system converts this signal to a corresponding
optical signal in a manner previously discussed.
If the detection system is embodied such that additional photon radiation
below approximately 200 nm is detected, then quartz is no longer transparent to
the emitted radiation. Magnesium fluoride (MgF2) is a suitable window
material which is transparent to radiation below 200 nm. Likewise, the monochromator
can no longer be operated in air at atmospheric pressure since air is not transparent
to photon radiation below approximately 200 nm. The monochromator or any alternate
spectrographic system must be operated under vacuum conditions.
PULSE DISCHARGE CHAMBER REACTIONS
Attention is now directed toward reactions which occur within the
pulse discharge chamber and which eventually lead to the emission of radiation
characteristic of the decay of excited KrCl to the ground state.
Krypton doped helium carrier gas passed through the electrical pulse
discharge or spark produces Kr+ by means of the reaction
He2+ + Kr = Kr+ + 2He .
A generic chlorine compound will be designated as "RCl" where "R" represents one
or more elements forming the chlorine molecule. As an example, "R" would represent
CH3 in CH3Cl. The Kr+ ions then react with any
chlorine compound, RCl, within the chamber yielding KrCl* through the reaction
Kr+ + RCl = KrCl* + R+.
The excited molecule KrCl* then decays yielding
KrCl* → KrCl + hν222
where hν222 represents a narrow band of photon radiation centered
about a wavelength of 222 nm.
Attention is again directed to the Kr dopant gas, and more specifically
to the concentration of Kr required in the carrier gas to eventually yield the
reaction of equation (3) wherein the intensity of hν222 is truly
a linear function of the concentration of RCl. For a given concentration of RCl,
it has been found that hν222 increases linearly with Kr concentration
up to a Kr concentration of approximately 0.2%. Above 0.2%, the concentration
of Kr is sufficient to deplete the He2+ of equation (1) and
therefore the intensity of hν222 ceases to be a function of increased
Kr concentration. The desired concentration of dopant is greater than 0.2%, but
perhaps less than 1.0% to minimize the operating cost of supplying Kr. Stated
another way, if the Kr dopant concentration is above 0.2%, the yield of 222 nm
photon radiation will be independent of the Kr concentration and vary only with
the concentration of RCl, as desired. An oversupply of krypton above the threshold
0.2% level does not, however, seem to handicap the operation of the system.
Sensitivity of the chlorine detection device has been measured and
has been found to be approximately 1 to 2 picograms of chlorine. The reason for
this exceptionally high sensitivity apparently arises from the inherent high rate
constant for an ion-molecule reaction such as the one depicted in equation (2).
Furthermore, at concentrations of Kr dopant above the "saturation" level of 0.2%,
the response of the system to concentrations of Cl has been found to be linear
over at least three orders of magnitude which allows the previously discussed
calibration constant, for converting output current from the photomultiplier detection
system to absolute chlorine content, to be a simple multiplicative constant.
The reactions of equations (1) through (3) are certainly not representative
of all of the reactions that can, and do, occur within the pulse discharge chamber
12. Many general classes of reactions that can occur have been tabulated in the
previously referenced U.S. Patents Nos. 5,153,519 and 5,317,271.
Attention will be directed to another reaction which has practical
and commercial bearing on the operation of the chlorine detection system. Many
chlorine compounds of interest also contain carbon, such as carbon tetrachloride
(CCl4). Furthermore, many of the chlorine compounds of interest are
found in hydrocarbons, thus carbon is present, from other sources such as gasoline,
in the sample to be analyzed. Carbon will therefore also be excited within the
pulse discharge chamber 12, in addition to excited chlorine as KrCl*. Excited
atomic carbon emits photon radiation at wavelengths of 193.1 and 247.9 nm. The
monochromator 18 and associated photomultiplier detector system can easily resolve
the 222 nm radiation of interest form the "interfering" photon radiation from
carbon. It should be noted that emission from C2* does fall very close
the 222 nm wavelength and poses a potential interference to the chlorine emission.
At low concentrations, however, C2* emission should be weak since C2*
is formed by a second order reaction of carbon.
The response of chlorine to krypton initiated excitation is observed
at 222 nanometers. The table below lists the response of both chlorine and fluorine
to three noble gases (dopants) in the helium carrier gas. Each response is an
observed emission wavelength in nanometers:
The intensity for these responses is:
In addition, xenon dopant reacts with fluorine to yield a moderate
response at 351 nm.
In the tabled data, xenon is not able to discriminate between the
two halogens so it is less desirable.
The foregoing wavelength data shows that argon responses are below
200 nm in wavelength, and therefore are in a range at which air absorption is more
likely to handicap intensity measurement. The `tabled data therefore suggests
preferred use of krypton provided sharp discrimination of the fluorine and chlorine
emissions is implemented to separate signals at 222 and 248 nm. As a generalization,
the krypton is the preferred dopant.
To briefly summarize, the present chlorine specific detection system
is a simple system to operate once the various components have been assembled.
It utilizes two sources of gas in which the sample gas containing chlorine is dispersed.
The primary noble gas is helium which is the carrier gas from a GC column and
contains the chlorine compound. The trace or dopant gas is krypton which is added
to the mixture of helium carrier and sample gas. This mixture is then input into
the pulse discharge chamber. The discharge creates excited KrCl through the reaction
sequence previously discussed. Radiation from the decay of excited KrCl is detected
with a photomultiplier and monochromator detection system which can be operated
in air at atmospheric pressure, and the output of this system is used to determine
the concentration of chlorine within the sample.
The system has been found to be extremely sensitive to chlorine,
and that the response of the system is linear with sample chlorine content over
at least three orders of magnitude. Furthermore, photon radiation from most common
"interfering" reactions that can occur within the pulse discharge chamber can be
resolved from the desired radiation from the decay of excited KrCl using a monochromator,
or alternately, by using a grating or an interference filter.
It has been found that the system can also be used to measure concentrations
of other halogens. These concentrations are detected in the same methodology as
is used to measure chlorine concentration, namely, by measuring radiation characteristic
of the decay of a particular halogen compound. It has also been found that noble
gases other than Kr can be used as a dopant.
While the foregoing is directed to the preferred embodiment, the
scope thereof is determined by the claims which follow.