Method and apparatus for detecting and measuring trace impurities in flowing gases
This is a continuation of application Ser. No. 659,439 (now abandoned), filed Feb. 19, 1976. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention described herein relates to method and apparatus for quantitatively monitoring trace impurities in flowing gases through the reaction of the impurities with electronically metastable atoms or molecular species produced in the afterglow of a flowing plasma. 
     There are many situations in which it is necessary to rapidly and effectively monitor the content of trace impurities in flowing gases. The high temperature gas-cooled reactor (HTGR) is an example of a system in which such monitoring is imperative. The coolant gas flowing through the primary loop of the HTGR is helium. The helium in the primary loop may be contaminated with water vapor from the secondary side of the heat exchangers or from the helium circulator bearings. Reactions of water with the hot graphite fuel blocks or core support structure of the reactor through which the coolant helium flows are highly deleterious. As a consequence, it is imperative that the moisture level in the primary loop be measured accurately on a continuous basis. Preferably, the monitoring method and apparatus should be capable also of quantitatively measuring reaction products of the water and the hot graphite as, e.g., CO, CO 2 , and CH 4 . 
     SUMMARY OF THE INVENTION 
     In its broad sense, the invention encompasses method and apparatus for detecting and quantitatively measuring trace impurities in a flowing gas by (a) forming a metastable species which maintains a stable afterglow and which reacts with said trace impurities to ultimately produce radiating species particular to the impurities, (b) mixing the metastable species with the gas containing the trace impurities at known flow rates to produce the radiating species, (c) detecting and measuring the intensity of the radiation emitted by the radiating species, and (d) for each radiation corresponding to each impurity, comparing the measured intensity to a predetermined calibration curve at the same flow rates to determine the quantitative amount of each impurity in the flowing gas. 
     The invention is particularly applicable to the detection and measurement of trace impurities such as H 2  O, CO, CO 2 , and CH 4  which may exist in the helium used in the primary coolant loop of the HTGR. Thus, e.g., using the apparatus and method of the invention, the presence of trace amounts of H 2  O in flowing helium may readily be detected by (a) forming He(2 3  S) metastable atoms from pure He, (b) intermixing a flow of these metastable atoms with a flow of the He in which H 2  O is to be detected, and (c) measuring the intensity of any resultant band emission in the 306.4 nm OH(A 2  Σ + )→OH(X 2  π) system resulting from the He(2 3  S)+H 2  O reactions. 
     Particularly advantageous features of the invention are that it permits continuous monitoring of the flowing gas, simultaneous detection of multiple impurities, fast response times to concentration fluctuations of the trace impurities, extremely high sensitivity, and economic monitoring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of apparatus useful in practicing the invention. 
     FIG. 2 is a calibration curve for a single set of helium flow conditions over a moisture content range of 0.22 to 1400 ppm. 
    
    
     DYNAMIC ATOMIC-MOLECULAR EMISSION SPECTROGRAPHY 
     The apparatus of the invention is essentially a dynamic atomic-molecular emission spectrograph, utilizing as its energy source the energy transfer reactions of metastable species with the impurities in a flowing gas. Quantitative determination of impurity levels is accomplished by the measurement of line and/or band intensity as a function of concentration employing emission spectroscopic techniques. The electronically metastable species (usually rare gas atoms, but occasionally molecular species) are produced in the afterglow of a flowing plasma, which may be initiated by virtually any energy source, e.g., ionizing radiation, electric fields, etc. 
     A versatile array of noble gas metastable atoms, e.g., He(2 1  S, 2 3  S), Ne, Ar, Kr, and Xe( 3  P 0 ,2), may be used as well as molecular metastable sources, such as N 2  (A 3  Σ u   + ) or CO(a 3  π). In the simplest sense, the metastable atom and/or molecule sources are produced using a fast flowing plasma, with the metastable species formed directly from the plasma as with the rare gas atoms or from the secondary afterglow reactions as in the formation of molecular metastable species. An example of the latter case is the use of Ar( 3  P 0 ,2) to form N 2  (A 3  Σ u   + ). Molecular metastable species may also be formed from microwave induced plasmas of active gases, such as &#34;active nitrogen,&#34; or ion-electron recombinations, such as CO 2   +  +e -  →CO(a 3  π)+O. The apparatus and technique are amenable without significant modification to producing detectable emissions from exothermic chemiluminescent sources, such as H-atom, F-atom, O-atom, etc. reactions with impurities. The flowing plasma may be sustained by any ionizing energy source or resonance light absorption. 
     Using the apparatus and method of the invention, the presence and quantitative amount of H 2  O in the primary loop of the HTGR may be rapidly and continuously monitored using the flowing He(2 3  S) afterglow technique. The energy transfer from the metastable He atoms to H 2  O and the resultant spectroscopic emissions result from the following reactions: 
     
         He[(2.sup.3 S), 19.7 eV]+H.sub.2 O→He(.sup.1 S.sub.0)+H.sub.2 O.sup.+ +e.sup.-                                          (1) 
    
     
         H.sub.2 O.sup.+ +e.sup.- →H.sub.2 O.sup.* (˜12.6 eV) (2) 
    
     
         h.sub.2 o.sup.* →oh(a.sup.2 Σ.sup.+)+h(.sup.2 s) (3) 
    
     
         oh(a.sup.2 Σ.sup.+)→oh(x.sup.2 π)+hν    (4) 
    
     Reaction (1) is a Penning ionization in which the excess energy, E[He(2 3  S)]-IP(H 2  O)≃7.2 eV, is partitioned between the relative kinetic energy and intramolecular excitation of H 2  O + . Part of the radiation emitted by reaction (4) is at 306.4 nm and is readily measured spectroscopically. By comparing the intensity of this emission against predetermined calibration curves, the quantitative amount of H 2  O present in the flowing gas is readily determined. 
     Simultaneously, the presence and amount of CO, CO 2 , and CH 4   may be determined through the use of the following reactions: 
     
         He[(2.sup.3 S), 19.7 eV]+CO→CO.sup.+ (B.sup.2 Σ.sup.+ →X.sup.2 Σ.sup.+), 219.0 nm+He(.sup.1 S.sub.O) (5) 
    
     
         he[(2.sup.3 S), 19.7 eV]+CO.sub.2 →CO.sub.2.sup.+ (B.sup.2 Σ.sub.u.sup.+ →X.sup.2 π.sub.g), 288.3-289.6 nm+He(.sup.1 S.sub.O)                                                  (6) 
    
     
         he[(2.sup.3 S), 19.7 eV]+CH.sub.4 →CH(A.sup.2 Δ→X.sup.2 π), 430.0 nm+3H(.sup.2 S)+He(.sup.1 S.sub.O)           (7) 
    
     note: The detectable bands described in these reactions are not the only usable emission systems available from these reactions. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Apparatus for the detection and quantitative measurements of trace impurities in a flowing gas in accordance with the method of the invention is shown schematically in FIG. 1. The major components of the apparatus of FIG. 1 are excitation source 30, rare gas atom source 31, pumping system 32, reaction chamber 13, detection system 34, signal system 35, recording system 36, calibration system 37, and gas input metering unit 38. 
     Excitation source 30 is the means for producing the metastable rare gas atoms. It may be any means for producing ionization, e.g., ionizing nuclear radiation (α, β, or γ), x-radiation, cold-cathode discharges, and microwave excitation. Alternatively, excitation source 30 may be an ultraviolet laser, which produces resonance absorption into states above the metastable states and subsequent radiative cascading into the metastable states. In a preferred embodiment, excitation source 30 is a cold-cathode discharge produced in flowing rare gas atoms 39 by means of 0-500 Vdc supply 14. 
     The source 31 of rare gas atoms 39 may be a direct or purified tank supply of the rare gas or a dilution of the rare gas atoms of choice in a carrier gas stream. As shown in the embodiment of FIG. 1, the rare gas may be obtained from calibration system 37 through valve 27 or from an independent source 26. If obtained from source 26, the gas passes through pressure regulator 25 into a purification system, such as baffled liquid nitrogen trap 23, with flow controlled by needle valve 22 and floating ball flowmeter 21 into liquid nitrogen trap 16. From trap 16, the gas passes through pressure-vacuum gauge 15 and into excitation source 30. The metastable atoms produced therein then flow into reaction chamber 13. 
     To extend the collision lifetime of the metastable rare gas atoms which would be extremely short at high pressures, gas pressure downstream of the metastable rare gas production region 41 is maintained at a low level (≦10 torr) by means of high speed pumping system 32. Means for producing and maintaining the requisite low pressure include all the standard vacuum pumping systems such as mechanical forepumps, ion diffusion pumps, or turbo-molecular pumps. In the preferred embodiment, pumping system 32 consists of a 500 liter per minute vacuum pump 9 connected by means of bellows vibration damper 8 to 24-liter ballast volume 7 which is in turn connected to reaction chamber 13. 
     Reaction of the metastable rare gas atoms with the impurities in the gas stream under analysis occurs in reaction chamber 13 which comprises a simple mixing region 33 which may be constructed of any materials suitable for moderate vacuum service. Chamber 13 contains a window 42 having a high transmission factor for the radiation of interest. Pressure within chamber 13 is measured by means of silicone oil manometer 6, or other suitable device. 
     Radiation passing through window 42 is detected by detection system 34 which may consist of a scanning monochromator with an associated photosensitive device such as a photomultiplier tube or photosensitive semiconductor detector. Alternatively, a spectrometer with a multiplicity of photosensitive devices may be employed. In the preferred embodiment, detection system 34 consists of a scanning 0.45 m monochromator 2, a photomultiplier tube 3, and a 0-1000 Vdc power supply 1. 
     Either currents or voltage pulses from the photosensitive devices may be sensed in signal sensing system 35 using either electrometers or photon-counting systems. In the preferred embodiment, an electrometer 4 is used. Recording of the sensed signal may be done with standard chart recorders for the scanning systems or with multichannel pulse height analyzers if multiple detector systems are used. In the preferred embodiment, recording system 35 consists of a strip chart recorder 5. Any of the various detection systems may be made process computer compatible and controlled for various signal enhancement schemes obtainable through software. 
     It is highly desirable that the device incorporate a calibration system 37 which consists of any method of introducing a known quantity of the impurity gas into reaction chamber 13. This may be accomplished using direct dilution in a carrier gas or any of various standard &#34;leak&#34; techniques well known in the sampling and detecting arts. In the preferred embodiment, calibration system 37 consists of a 24-liter standard volume tank 19 having a pressure-vacuum gauge 17 and a system 20 for introducing a sample or a calibration gas. Baffled liquid nitrogen trap 29 may be introduced into the calibration system or isolated therefrom by means of valves 28, 43, and 44. If trap 29 is used, gas is circulated therethrough by means of circulating bellows pump 18. 
     The calibration gas or the gas under analysis is introduced into reaction chamber 13 by means of gas input metering unit 38 which consists of needle valve 11 and calibrated flowmeter 12. From the known flow rates of the metastable rare gas atom bearing stream and the gas under analysis and previous calibrations, the quantity of impurities in the gas under analysis may easily be determined. 
     Using the apparatus of FIG. 1, the calibration data of FIG. 2 were obtained for a single set of flow conditions for helium having a moisture content range of 0.22 to 1400 ppm. Flow rates of 2000 μmols per second for the metastable atom bearing stream and 175 μmols per second for the moisture laden stream, which are typical, were used. It will be understood that variations from these flow rates may be desirable to displace the dynamic range from that displayed in FIG. 2. The full curve of FIG. 2 represents a least squares linear fit to an apparent power law behavior. The least squares fit equation is 
     
         M=0.516 I.sup.1.23 
    
     where M is the moisture content in ppm, and I is the photomultiplier tube current in nA. 
     Although the preferred embodiment is directed to the determination of trace impurities in a flowing rare gas, as, e.g., moisture in the flowing helium in the primary coolant loop of the HTGR, neither the apparatus nor the method of the invention is limited to this example. Instead, the method and apparatus are applicable for use with any metastable species which can be made to form a stable afterglow and then react with trace impurities in a flowing gas to ultimately form excited products which radiate sufficiently for the radiation to be detected.