Gas analyzer and a source of IR radiation therefor

A source of IR radiation for use with a gas analyzer including a sealed-off enclosure containing at least one molecular, IR-active gas which, upon excitation, is capable of emitting IR-radiation of a known, discrete spectral distribution. The excitation is effected by electrical discharges taking place in a limited portion only of the sealed-off enclosure, the rest of the enclosure serving as reservoir of the gas. The electrodes producing the discharges are disposed outside of the enclosure. A gas analyzer incorporating the source is also described.

The present invention relates to an infrared gas analyzer, that is, a 
device using infrared radiation to determine, in a gas sample, the 
presence and concentration of a selected gas. 
Infrared gas analysis is based on the absorption, by infrared-active gas 
molecules undergoing transitions between roto-vibrational levels, of 
radiation in this particular region of the spectrum. Each of these gases 
has its own very specific infrared absorption band, which can be regarded 
as its infrared "signature". If the gas to be analyzed is placed between 
an infrared source and a detector, its concentration can be determined by 
measuring the absorption at wavelengths corresponding to this "signature". 
Gas analyzers based on the IR-absorption principle are well known in the 
prior art, and while they differ in their respective designs, they have 
several features in common, the most important one of which is the 
IR-source which, with almost all of them is a "black body" (BB) thermal 
radiator in the form of a solid heated to incandescence. Such radiators 
produce a continuous spectrum covering the entire range from the far IR 
(about 20.mu.) into the visible region (about 0.5.mu.) and are used 
generally in conjunction with band pass filters which reduce this 
extensive spectral range to that where most of the "signature" band lines 
of the target gas are located. In order to detect the amount of 
absorption, most BB-source equipped instruments incorporate also a 
mechanical "chopper" to modulate the radiation reaching the detector, as 
the source itself (the incandescent solid) cannot be modulated directly, 
at a rate fast enough for convenient electronic processing, because of its 
high thermal capacity and, consequently, thermal inertia. The power 
consumption of a gas analyzer incorporating a BB-source is relatively high 
(up to 50 W) and, considering the added complexities introduced by the 
need for filtering, mechanical "chopping" of the radiation and the 
elaborate associated electronic circuitry, so is their price. With even 
the large, stationary, laboratory type of these instruments occasionally 
showing a less than satisfactory selectivity and sensitivity, portability 
has in the past been achieved only at the expense of further reduction of 
these qualities. 
IR-sources other than BB-radiators were disclosed by Webley (U.K. No. 
1591709) and Javan (U.S. Pat. No. 4,274,063); both of whom proposed 
IR-sources in the form of gas discharge tubes. Webley, however, explicitly 
stated that sealed-off gas discharge tubes would have a short useful 
lifetime due to the discharge-caused dissociation of the gases, to 
counteract which he provides, inside the tubes, a carbon filament that, 
when heated by an electric current, is expected to regenerate the CO or 
CO.sub.2 concentration level in the tube. Javan, on the other hand, solves 
the dissociation by continuously replenishing the gas in the non-sealed 
chamber by continuous flow of an unused gas mixture through the chamber 
via an inlet and an outlet tube. Both, Webley and Javan use internal 
electrodes which often shorten the useful life of the tubes. 
It is one of the objects of the present invention to overcome the 
limitations and shortcomings of the prior-art IR gas analyzers, and to 
provide an infrared gas analyzer which is equipped with an IR-source that 
produces a noncontinuous spectrum comprising specific, discrete 
wavelengths only, selected to be essentially identical with the absorbable 
wavelengths forming the spectral "signature" of the target gas, is 
therefore highly specific, yields a very high signal to noise ratio 
compared to conventional BB-source equipped analyzers, has practically no 
thermal inertia and can therefore be modulated electronically rather than 
mechanically, has a sealed-off source with external electrodes, can be 
battery-operatable, and is compact, portable, highly selective and 
sensitive, yet very much cheaper than comparable prior-art IR-analyzers. 
This the invention achieves by providing an infrared gas analyzer 
comprising: 
a source of IR radiation containing at least one molecular, IR-active gas 
which, upon excitation, is capable of emitting IR radiation of a known, 
discrete spectral distribution; 
a driver for providing energy for said excitation; 
at least one detector placed at a distance from said source of IR 
radiation, which distance defines an analytical space wherein the gas to 
be analyzed is exposed to, and can absorb at least part of, said IR 
radiation, which detector serves for determining the absorption of said IR 
radiation by said gas in said space, and 
means responsive to the output of said detector, 
characterized in that said source of IR radiation is of the kind that 
produces a non-continuous spectrum comprising specific, discrete 
wavelengths only, being substantially those wavelengths that are 
characteristically absorbed by the gas the presence and concentration of 
which are to be established: 
said gas is contained in a sealed-off enclosure; 
said excitation is effected by electrical discharges taking place in a 
limited portion only of said sealed-off enclosure, the rest of said 
enclosure serving as a reservoir of said gas, and 
that electrodes producing said discharges are located outside of said 
enclosure. 
The invention further provides an infrared gas analyzer comprising: 
a source of IR radiation containing at least two IR-active gases, each of 
which, upon excitation, is capable of emitting IR radiation, the IR 
radiation of at least the second of said gases being of a known, discrete 
spectral distribution, 
a driver for providing energy for said excitation; 
at least one detector placed at a distance from said source of IR 
radiation, which distance defines an analytical space wherein the gas to 
be analyzed is exposed to, and can absorb at least part of the IR 
radiation of said second gas, which detector serves for determining the 
absorption of said IR radiation by said gas in said space, and 
means responsive to the output of said detector, 
characterized in that the IR radiation of said second gas is of the kind 
that produces a non-continuous spectrum comprising specific, discrete 
wavelengths only, being substantially those wavelengths that are 
characteristically absorbed by the gas the presence and concentration of 
which is to be established; 
said two gases are contained in a sealed-off enclosure subdivided by an 
IR-transparent partition wall into a first chamber containing at least 
said first gas and a second chamber containing at least said second gas; 
said excitation is effected by electrical discharges taking place in a 
limited portion only of said first chamber, the rest thereof serving as 
reservoir of said first gas: 
electrodes producing said discharges are located outside of said first 
chamber, and 
that said fragile, second gas in said chamber is excitable by IR radiation 
emitted from said first chamber through said partition wall. 
A further drawback of all prior-art IR analyzers with BB-source resides in 
the fact that they are inherently incapable of detecting small shifts in 
absorbed wavelengths, as will occur when, for instance, an ordinary 
molecule is substituted by its rare isotope. Such capability can be 
achieved by using IR-sources with discrete emission spectra, in which the 
molecular gas in the source has been substituted by a chemically identical 
gas, but composed of molecules where at least one of its constituent atoms 
is replaced by its rare isotopes. The emission spectrum of such an 
isotope-substituted gas will show a slight shift as compared to that of 
molecules composed of the abundant isotopes, and will be absorbed mainly 
by molecules with the same rare isotope constitution, but not by the 
regular molecules. Typical cases are, e.g., the rare-isotope variants of 
regular CO.sub.2, (.sup.12 C.sup.16 O.sub.2) namely .sup.13 C.sup.16 
O.sub.2, .sup.12 C.sup.18 O.sub.2, .sup.12 C.sup.18 O.sup.16 O, or the 
rare-isotope variants of regular H.sub.2 O: namely D.sub.2 O, HDO. 
Being able to make use of such specific IR-sources, the IR-analyzer 
according to the invention is thus capable of identifying, and measuring 
the concentration of, isotopically substituted "marker" molecules. 
It is also capable of producing from a single source two different, 
discriminable, specific radiations that, being chemically identical, will 
change identically with time, one of which radiations can be used as 
reference to the other to account for drifts in the system. 
The invention will now be described in connection with certain preferred 
embodiments with reference to the following illustrative figures so that 
it may be more fully understood. 
With specific reference now to the figures in detail, it is stressed that 
the particulars shown are by way of example and for purposes of 
illustrative discussion of the preferred embodiments of the present 
invention only and are presented in the cause of providing what is 
believed to be the most useful and readily understood description of the 
principles and conceptual aspects of the invention. In this regard, no 
attempt is made to show structural details of the invention in more detail 
than is necessary for a fundamental understanding of the invention, the 
description taken with the drawings making apparent to those skilled in 
the art how the several forms of the invention may be embodied in practice 
.

Referring now to the drawings, there is seen in the block diagram of FIG. 1 
a driver 2 powering and controlling an IR-source 4. The latter emits 
infrared radiation which passes through an analytical space 6 in which is 
located the gas to be analyzed. A detector 8 mounted downstream of the 
space 6 senses if and how much of the IR-radiation was absorbed by the 
gas. Signals from the detector 8 are amplified in the amplifier 10 and fed 
to the display unit 12 which indicates the concentration of the target gas 
in the analyzed sample. 
The "heart" of the gas analyzer according to the invention is its IR-source 
which consists of a hermetically sealed-off vial or tube 4 containing a 
molecular, IR-active gas or a mixture of gases at, generally, 
subatmospheric pressure. When excited by electromagnetic waves in the RF 
(KHz, MHz) or microwave region, these vials act as electric discharge 
lamps, emitting IR radiation over a spectrum that, as already mentioned, 
is noncontinuous and consists of a band of discrete, well-defined lines. 
For every target gas, an IR-source is selected that will produce radiation 
of a spectrum substantially identical to the absorption band of that 
particular gas. In some cases, only the IR-active gas is introduced into 
the source vial 4. Others require additive gases that exhibit no 
roto-vibrational transitions, such as noble gases, or homonuclear 
diatomics such as N.sub.2, O.sub.2 or H.sub.2 to enhance IR-emission and 
to reduce molecular dissociation due to the electrical discharge. The 
useful life of these IR-sources is at least several thousand hours of 
continuous operation. 
The remarkable service life of these sources is achieved by several 
measures: 
(1) Discharge takes place in a portion only of the vial 4, the rest of the 
vial serving as reservoir essential for maintaining proper gas composition 
in the discharge volume, which is several times smaller than the reservoir 
volume; 
(2) Electrodes are disposed outside of the vial, and are therefore not 
liable to deterioration thus do not interfere with the critical purity of 
the gas contents. Also, sputtering of the electrode and its deposition on 
the transparent walls of the gas enclosure are stopped. Excitation is 
effected either by capacitive, inductive or radiative coupling. The 
electrodes are in the first case flat metal rings, or parts of such rings, 
surrounding the vial 4, preferrably contacting the vial surface and, in 
the second case, wire coils analogously positioned. 
The IR-active molecular gases as well as the atomic or molecular buffer 
gases are maintained at pressures not exceeding several tenths of a Torr 
for low-power excitation. 
In some cases it is advantageous to provide the IR-source with spectral 
filters consisting of absorption cells filled with a gas the specific 
absorbable radiation of which is involuntarily emitted from the discharge 
zone due to the presence, in this zone, of IR-active molecules or radicals 
different from those of the target gas. 
Similar absorbing means can also be provided for when the presence is 
likely, in the tested gas mixture, of a certain gas with an absorption 
band liable to be superposed upon the target gas band. 
The vials can be made of any suitable material, but must have at least one 
region, serving as outlet "window", capable of transmitting an amount of 
radiation specific to the target gas, significant enough to permit 
detection of the radiation and of its absorption. Different target gases 
will make necessary the choice of different window materials, e.g. soda 
glass, pyrex, sapphire, barium fluoride, etc. A source can be provided for 
emitting radiation for more than one gas with filters used to select a 
given radiation at a given time. 
The power required to drive these IR-tubes is exceedingly small, varying 
from fractions of a watt to a few watts and, for a given emitted power 
level of the relevant radiation absorbed by the target gas, is lower by up 
to two orders of magnitude than that required for conventional 
BB-radiators. 
In FIG. 2 the narrow, distinct and discrete line spectrum A of the 
IR-source of the analyzer according to the invention is compared with the 
broad, continuous spectrum B of a BB-radiator at 1200.degree. C. The 
A-spectrum shown matches the CO.sub.2 absorption band. 
A basic embodiment of the IR-gas analyzer according to the invention is 
illustrated in FIG. 3. There is seen the driver 2, which comprises a power 
source 14, a modulator 16 which serves as an electronic "chopper" 
producing, e.g., a square-wave like pulse of selectable duty cycle and 
rate, and an oscillator 18 acting as an RF source. The IR-tube 4 is 
capacitatively coupled to the RF-source 18 by means of metal rings 20 
which serve a capacitor plates, and a coaxial cable 22. Optical means can 
be used to direct the radiation into the analytical cell. 
The analytical cell 6 has an inlet 24 and an outlet 26, as well as two 
windows 28 which, obviously, must be at least partially transparent to the 
specific IR-radiation emitted by the source 4. 
For many applications, however, the gas sample need not be confined in a 
cell. With the IR-radiation suitably concentrated or collimated by optical 
means per se known, measurements can be taken also in free space over 
relatively large distances intervening between the source 4 and the 
detector 8. It is thus possible to measure or monitor CO levels in 
vehicular tunnels, or in chimneys, or the like. 
The IR-detector 8 is of the commercially available type, e.g., a lead 
selenide detector such as OE-15-54 manufactured by Optoelectronics. It 
could also be an Eltec 408 pyroelectric type detector, or a photoacoustic 
detector. A detector working on a different principle consists of a cell 
having an IR-permeable window and filled with an IR-absorbing gas which, 
in dependence of the amount of radiation absorbed, heats up, temperature 
variations being measured with the aid of a thermocouple. 
In some cases the detector is arranged to process test and reference 
signals in sequence, at different and specific times, switching over being 
effected by an "information" link between the IR-source and the detector. 
The output of the detector 8 is processed and amplified in the amplifier 10 
and eventually reaches the display unit 12. The latter can have many 
forms, analog or digital, giving the concentration in %, ppm, etc. Where 
absolute values or great accuracy are not required, concentrations may be 
indicated by a number of LED's, with more LED's lighting up the higher the 
concentration determined. Other indicating means may include optical or 
acoustical or speech warning devices. 
FIG. 4 schematically illustrates a further embodiment, in which use is made 
of a reference cell 6', filled with a known concentration of the target 
gas, say CO.sub.2 or with a "transparent" solid or gas like N.sub.2, and 
having its own detector 8'. The outputs from the two detectors 8 and 8' 
are fed to an electronic unit 30, where they are compared and the thus 
processed signal amplified and transmitted to the display unit 12 and/or 
to a control unit 13 used for controlling equipment such as blowers, 
exhausters, humidifiers, etc., to maintain target-gas concentrations 
within presettable limits. 
Yet another embodiment is illustrated in the schematic drawing of FIG. 5. A 
gas analyzer of this type is used for clinical purposes in the 
determination of the CO.sub.2 -content of exhalation air. The patient 
inhales and exhales through the tubular cell 6 which, during the 
inhalation stroke I, acts as reference cell, passing as it does the room 
air with its known CO.sub.2 content. During the exhalation stroke E, 
CO.sub.2 concentration in the tubular cell 6--now acting as analytical 
cell--increases, causing absorption to increase, and the detector will 
consequently receive less radiation. Detector signals after each stroke 
are compared in the comparator and amplifier unit 30, and the exhalation 
value fed to the display unit 12. 
FIG. 6 represents an IR-source according to the invention. There is seen 
the vial 4, the electrodes 20 which in this embodiment consist of rings of 
metal foil attached to the vial and connected to the driver 2 (FIG. 1) by 
means of a coaxial cable 22. While in this embodiment the vial 4 is 
capacitively coupled with the RF-source, inductive coupling is also 
possible, as has already been mentioned, by replacing the two electrodes 
20 by a wire coil. 
Electrical discharge takes place only in the zone 32 delimited by the 
electrodes 20, the rest of the vial volume serving as reservoir 34 used to 
maintain the proper gas composition of the discharge zone 32. In many 
applications, the IR-radiation would be emitted in direction of arrow A. 
However, by appropriate choise of window material vs. envelope material, 
radiation can also be emitted in direction of arrow B. 
FIG. 7 schematically represents another embodiment, in which the 
IR-radiation is emitted over a relatively wide front, as indicated by the 
arrows. Here, the extent, in depth, of the discharge zone 32 is defined by 
the circumferential reach of the electrodes 20. The vial volume below that 
reach constitutes the reservoir 34. 
FIG. 8 shows an embodiment of the IR-source that simultaneously emits test 
as well as reference signals. The vial 4 in this embodiment is U-shaped, 
each limb of the U having a set of electrodes 20 and a window 36. The gas 
filling of the vial is such as to produce two different radiations, one of 
which is the test radiation T which is to be absorbed by the target gas, 
the other is the reference radiation R, which is not absorbed by the gas. 
Further provided are two filters 38, 40, the first one of which filters 
out the test radiation T, leaving only the reference radiation R, the 
other one filtering out the reference radiation R, leaving only the test 
radiation T. The relative intensities of T and R are at a known and fixed 
ratio that will not change with time, even if vial output should vary due 
to aging, surges, or the like. The vial is connected to a driver which 
alternatingly excites one pair of electrodes 20 at a time, and so the 
source alternatingly emits radiations of different spectral composition 
from different portions of the source. 
The embodiment shown in FIG. 9 provides a solution to the problem of 
molecular gases P which are fragmented in the presence of energetic 
electrons such as those prevailing in an electrical discharge and for 
which the embodiments discussed so far do not provide a satisfactory rate 
of recombination. 
As can be seen in FIG. 9, the vial 4 is subdivided by an IR-transparent 
partition wall 4 into two chambers, a first chamber 44 and a second 
chamber 46. Chamber 44 acts like any of the sources described above which 
contain an IR-active gas A, and chamber 46 contains a mixture of at least 
the gases A and P in an appropriate ratio. 
Resonant IR-radiation emitted by gas A from chamber 44 into chamber 46 is 
absorbed by the gas component A in chamber 46, producing excited 
vibrational states of molecules A. By v--v transfer, energy is transferred 
from molecules A to molecules P, which now radiate their specific 
IR-radiation when decaying to the ground state. This type of activation is 
known as optical pumping. 
To have an efficient v--v transfer between A and P, A has to be chosen so 
as to have a close energy match with the relevant energy levels of P. 
It will be evident to those skilled in the art that the invention is not 
limited to the details of the foregoing illustrative embodiments and that 
the present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.