ENGINEERING. DESIGN. A N D PROCESS DEVELOPMENT system during the previous 45 minutes of operation. I n addit,ion, the small step on the record caused hy the drift correction can be used as a key to correlating the chart record with the various samples. Details of the measuring and standardizing circuits are shown in Figure 4. A typical chart as obtained on the recorder is shown in Figure 5.
Accuracy of Measurement The range for n-hich the analyzer is calibrated is 0 to 207, isobutane. Sensitization has resulted in suppressing the srnsithat, of isobutanc. tivity of the analyzer to isopentane to about With this degree of suppression and the relatively small quantit'y of isopentane preeent, a concentration change of f1.0% isopentane result's in less than 0.2% error in isobutane. Comparison of the analyzer results with laboratory spectroscopically analyzed spot sampled indicates satisfactory agreement (usuallvr-ithin 0.2 to 0.3% isobutane). During periods of rapid changes in t'he toxers comparisons are difficult to obtain because of uneven h e lags arid nonrepresentativr samples.
Maintenance and Safety Routine maintenance of the analyzer consists primarily of daily observation for signs of difficuky and refilling of the reference sample drum as required (usually every 3 weeks). Spot samples from each of the columns are sent to the laboratory for analysis each shift. These samples are compared with the analj-zer as a furt,her check on accuracy. This maintenance of the analyzer is considered a function of the instrument man assigned to the unit who, by training and experience, is familiar v i t h the entire operation of the analyzer as well as the trouble spots to be anticipated. The routine maintenance required by the analyzer is approximately 15 man-hours per month. The service factor is estimated to be 96%. The most troublesome maintenance items are failure of the thermostatic temperature control, moisture in the Eample, and leakage of the solenoid valves. As mentioned previously. all equipment, in the analyzer house is of explosionproof design. A vent,ed manometer is attached t o the analyzer sample line as a safety to prevent overpressuring the cells. Precautions are also taken to prevcnt liquid from accidentally entering the analyzer. An electronic relay is installed t o det,ect,the presence of liquid in a trap placed in the sample
line. I n the event liquid is detected at this point a solenoid valve closes in the sample line.
Advantages of Continuous Analyzer Prior to the installation of the continuous analyzer the operation of each of the columns was adjusted on the basis of laborutory analyzed spot samples. The data available a t the units lagged 6 to 8 hours behind actual process conditions because of t,he time required to transport and analyze the samples. As a result, over-all operation of the units x a s erratic and resulted in substantial loss of isobutane through t8hetax-er bottoms streanis as shown in Figure 1. Installation of the continuous analyzer resulted in reducing the time lag to 5 minutes IThich allowed greatly improved control of the operation as also shown in Figure 1. The additional isobutane recovery resulting from this improvement in operations has been estimated t,o be worth $10,000 per year on this particular application. No credit is taken for t'he reduction in the number of laboratory analyses brought about through the use of the continuous analyzer. The possibility of utilizing the analyzer to control automatically the process operations is not considered feasible because of the technical complications involved. T o do so vould require the installation of two additional analyzers (one for each column) as intermittent automatic controls cannot be made to operate satisfactorily. This would also require elimination of the drift compensating automatic standardizing system vhich would reduce t'he analyzer accuracy. And last, it is believed that lit& addit,ional improvement in process operations would be obtained.
Literature Citad ( I ) Ijerger, D. E.. Inatruments, 26,872 (1953). ( 2 ) Devine, ,J. ll.,Ihid.. 24, 1296 (1951). (3) Fastic, W. G., and Pfund, A. H., J . O p t . SOC.Amer., 37, 702 (1947). (4) Herscher, L. W., and Wright, N., I b i d . . 43, 980 (1953). ( 5 ) Kratochvil. K. V.,and Berger, D. E.. Proc. Instrument Soc. Am., 7, 25 (1952). , I n s t r u m e n t $ , 26, 421 (1953). ( 7 ) Thomas, 13. W., Petroleum Refiner, 30, 81 (February 1951). (8) Wright, S . . and Herscher, L. TV., J . O p t . SOC.Amer., 36, 195 (1946). RECEIVED f o r r e r i e m September 7, 1953.
Sensitizing ELLIOT H. WOODHULL,
ACCEPTEDMay 3 , 1934.
lyzer E. H.
SIEGLER, AND HAROLD SOBCOV
The Perkin-Elmer Corp., Norwulk, Conn.
The operating principle of a specific nondispersive infrared-type instrument, the Tri-Non analyzer, i s described briefly. A method which permits this instrument to be calibrated in per cent concentration of one component in a multicomponent gas stream i s described. This procedure i s general and can be applied to nondispersive analyzers using either selective or total radiation-type detectors. Data taken for a typical four component stream are used to illustrate the method.
A
LTHOUGH nondispersive infrared analyzers havc been in plant
use for a number of years, the methods of sensitizing these instruments t o the gas of interest and of maliing them insensitive to the background components have seldom been described, partly as a result of their trial and error nature ( 1 , 2 ) . This paper presents a general systematic method for sensitizing a nondis-
1396
persive analyzer for a multicomponent stream analysis in which there is spectral overlap with the gas of interest. The method chosen enables one to determine t,he effcctJivcness of filtering and compensation and permits a rapid and rational determination of a sat'isfactory mixture for filling the filter cell. Also, the approach indicates the order in which an iterative
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46, No. 7
PROCESS INSTRUMENTATION instrument a t a true radiation null-Le., when any signal exists a t the output of the detector, this signal operates a radiation attenuator in path PBuntil this path transmits to the detector an amount of radiation equal to the difference between that transmitted by PI and P p . Since this difference after proper sensitization is a function only of the concentration of the gas of interest in the sample, the position of the radiation attenuator can be calibrated in terms of this concentration.
T
Sensitizing Procedures
II
General. Filling the detector D with the gas of interest renders the analyzer selective to that gas since the detector responds only to radiation in regions where the gas of interest has Figure 1. Schematic Drawing of Tri-Non Analyzer absorption bands. Calculations performed on typical hydrocarbon spectra show that only a S. Source Sa. Sample cells few per cent of the total energy emitted by the CH. Radiation chopper N. Nulling tube PI, Pp, Pa. Radiation paths F. Filter cell source is absorbed by the selective detector when D. Radiation detector no absorbing gases are between the source and TI,Jz, J 3 . Optical trimmers A. Radiation attenuator detector. Hence the selective detector eliminates TS. Transmitting slidewire Se. Sensitizer cell the large radiation background that would be Co. Compensation cell G. Reference generator absorbed by a black detector and which contains no useful information. A rather complete comparison of selective and nonselective detectors has been given filling procedure should be followed to obtain the correct comby Smith (5),a?d an analysis with applications of a nondispersive pensator cell filling. analyzer with a nonselective detector has been published by Koppius ( 2 ) . Background gases in the sample stream, however, Operating Principle of Tri-Non Analyzer can produce false signals a t the detector if they have absorption bands which overlap those of the gas of interest. (If a backThe sensitizing technique which will be described has been apground gas has an overlapping absorption region, it will be replied to a specific instrumenl, the Tri-Xon (trade-mark of The ferred to as an interfering gas.) Perkin-Elmer Corp.) analyzer, which will be described briefly. Figure 1 is a schematic drawing of the Tri-Non analyzer showing a source S, a selective detector D, and three channels of I radiation between source and detector. The mechanical chopper, C H , is arranged so that the detector alternately receives the radiation transmitted by channel PZ and then the sum of the radiations from channels P1 and P J . If these two radiation signals are of equal intensity, there will be no alternating output from the radiation detector D. To sensitize the instrument so that it selectively measures the concentration of the gas of interest in the presence of the background components in the sample stream, the following general bteps are taken: 1. The detector is filled with the gas of interest which renders the detector primarily sensitive to this gas. 2 The filter cell, F , is filled with a mixture of those background gases which have spectral overlap with the gas of interest. This reduces or eliminates the unwanted signal from background gases by absorbing overlapping wave lengths before they reach the detector. 3. The gas of interest is placed in the sensitizer cell, Se, in path P,. This removes a large percentage of the radiation in PI in wave length regions where the detector absorbs. An energy unbalance between PI and Pz now exists a t the detector. When the gas of interest is floJwd through sample cells, Sa,the unbalance will decrease since the sample removes more energy from Pz than from P,. 4. The compensator cell, Co, in path P1 is filled with an appropriate mixture of the interfering gases. A background component will interfere if after filtering there remain absorption regions common to those of the gas of interest and will produce a false signal proportional to the degree of spectral overlap. If, however, this background gas is placed in the compensator cell in the proper concentration, it is possible to reduce the false signal substantially to zero. This procedure is not peculiar to the Tri-Eon analyzer but is a typical sensitizing procedure for a nondispersive analyzer. The third path, PB,of the Tri-Non analyzer is used to operate the July 1954
20I I I I 20 40 60 80 ABSOLUTE PRESSURE IN I O GM. FiLTER CELL ( G M . H G )
0'
Figure 2.
Absorption Data for Gas of Interest and interfering Gases
Sample cell with filter cell filled with 20 cm. Hg CHI, and 10 cm. Hg CO?
2 0 cm. Hg CzHs,
Before one attempts to sensitize a nondispersive analyzer for the gas of interest in a multicomponent plant stream it is desirable to obtain infrared spectra of each component recorded at the concentration extremes which occur in the stream. 4 brief review of these spectra will indicate which regions of absorption of the gas of interest are unique or most nearly unique with respect to the background components. This information enables one to choose suitable infrared windows for the analyzer cells and to estimate very roughly the problem difficulty. Choice of window material and the selective detector filling are not, in general, sufficient to eliminate false signal from inter-
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
1397
ENGINEERING. DESIGN, AND PROCESS DEVELOPMENT
Table I. Component Ci2Hda CzHe C Ha
con a
Stream Component Spans Concentration Span Partial pressure, Percentage ern Hg 40-80
10-30 10-20 0-10
32.4-60.8 , .6-22,8 7.6-15.2 0-7.G
Gas of interest.
fering gases. The complementary processes of filtering and conipensation are used greatly to reduce thc false signal reeulting from overlap regions. Filtering consists of introducing t,he interfering background gases in the filter cell F Thich is placed across all radiation paths. The absorption of radiation by the filter gases reduces the intensity of the radiation in the overlap regions mhich reach the detector and hence reduces the effect of inkrfering gases. Filtering is most effective in those portions of the overlap regions where the int,erfering gas has a high absorption coefficient, Conipensat,ion is achieved by filling the sensitizer cell n-ith the gas of interest and the compensator cell with a blend of the jnt,erfering gases in such proportions that ivhen an interfering gas is flowed through the sample cells an equal amount of energ>is absorbed from paths PI and Pz and no net radiat,ion signal reaches the detector. Since the sensitizer and compensator cells make pat'hs PI and P1 selective to different portions of the Tyaw lengt,h regions that the detect'or absorbs and since the integrated absorption of these paths as a function of any $ample gas concentration is a nonlinear phenomena, it is, in general, impossi')le t,o subtract these two nonlinear absorptions in paths Pl and of and exactly nullify the difference over any appreciable span P interfering gas eoncentration. If, hoviever, the absorption in the t\To paths can he made linear with the concentration of each interfering gas, exact cancellation is possible and exact conipensation possible. Therefore, to ohtain good conipensat,ion for interfering gases, it is, in general, necessary t o fill the filter cell in such a n-ay that all interfering gases in the sample cella remove radiation from both paths Pl and P2 as a nearly linear function of gas concentration. Sensitization for Ethylene in Four Component Stream. The problem chosen t o illustrate the sensitizing procedure is the determination of ethylene in the presence of et'hane, methane, and carbon dioxide in a sample stream a t atmospheric pressure. The specific spans of the stream components are listed in Table I. Detector Filling and Optical Balancing. For the Tri-Son analyzer detector atmospheric pressure of a blend consisting of 5 to 10% of the gas of interest in argon has proved satisfactory. Argon which is monatomic and not infrared active has a specific heat ratio of 1.66 and increases the heat engine efficiency of the detector (3). Gas of the highest purity obtainable is recommended since the impurities most commonly encounteredprobably are present in the plant stream or thesa impurities may have pa,rtial spectral overlap with the background gases in the stream arid hence make sensitization and compensation more difficult. 811 cells are now evacuated, path Pi is blanked off by trimmer !PSIand trimmer T I or T?is adjusted until the energy transmitt,ed through path P1 equals that through path Pa and results i n no a.c. signal a t the output of the selective detector. Wit,h this condition achieved, path P I (or P ? ) is blanked off and the large alternating signal a t the detector output mliich results from transmitting alternating radiation t,hrough path Pl (or P I ) ,when! this path is clear of absorbing gases, is defined as the full beam signal (f.b.s.). Filtering. The relative absorption of the gas of interest arid each background component in the detector absorption regions is determined by measuring t,he per cent of the f.b.s. transmitted through channel Pu as a function of partial pressure of each gas as it is separately placed in the filter cell. Vnless tho 1398
absorption bands of the gas under consideration broaden markedly with pressure, this procedure produces sufficiently accurate data for this st'udy. The data in Figure 2 for the specific problem show t8hevarying degrees of interference of the background components. Examination of the curves of Figure 2 reveals: 1. The addition of 10 em. Hg of carbon dioxide in the filt,er cell reduces t'he f.b.s. by 3%, but changing t'he concentration from 10 t o 76 cm. H g increases the overlap absorption by only 0.8%. This clearly indicates that if 10 cm. Hg of carbon dioxide or more is placed in the fdter cell, the unbalance signal which will result from a change in carbon dioxide concentration in the sample cells is negligible. Thus, filtering alone removes the carbon dioxide interference. 2. Examination of the curve for methane shows that it absorbs energy rapidly up to a concentration of 20 em. Hg and then more slowly a t a nearly constant rate in the concent,ration range 20 to 76 em. H g This indicates that it is desirable to add 20 em. Hg of methane to the filter cell so that as methane passes through the sample cells the energy change which occurs in each channel is characterized by the region of gradual and nearly constant slope. The instrument' mill still respond to a change of methane in the sample since different levels of the radiation that is detected are transmitted in the two paths. This response t o methane, however, is noiv nearly linear and permits the elimination of interference by compensation. The addition of more than 20 cm. Hg of methane to the filter cell is unwarranted since it would reduce the response to met,hane relatively litt,le, but would lower the radiation level a t the detector and thus reduce the signal-to-noise ratio. 3. The interference curve of ethane in Figure 2 is generally similar to that for methane except that it absorbs more strongly and does not exhibit a nearly constant slope until 40 em. Hg of the gas have been placed in the filter cell. The same filtering considerations apply that LTere discussed for methane, but a conipromise between energy level and filter effectiveness must be made; hence, 20 em. Hg of et'hane is selected for the filter cell.
Sample Cell Selection. Assuming that Beer's law approximates the abeorption of entire bands, the ethylene curve in Figure 2 provides the necessary information to estimate the sample cell length. The aim is to obtain the maximum signal change for ethylene varying from 40 to 80% and simultaneously to have good calibration linearity. For the specific problem considered, an ethylene concentration change from 30.4 t,o 60.& em. Hg gives 8% change of f.b.3. for a cell 10 em. long. Since partial pressure and cell length appear as a product, in the exponent of Beer's law, t'he per cent of f.b.8. change as a functiorr of ethylene pressure for a cell 0.1 the length of the filter cell can be predicted from the data of Figure 2 . For the same concentration span (30 to 60 cm. Hg) t h k per cent of f.b.s. change becomes the ethylene signal variation observed over 0.1 the pressure span (3 to 6 em. Hg). Thus, if a 1-cm. sample cell length is used, a 15% change of f.13.s. results which is close t o the maximum change obt'ainable. The filter cell is filled with a mixture of 20 cm. Hg of methane, 20 cm. Hg of ethane, and 10 em. Hg of carbon dioxide, and the interference measurements are repeated by flowing each gas into sample cells 1 em. in lengt'h. The results obtained are shown ic Table IL
Blends of Interfering Gases Coinr,osition
Blend No. Group I I 2
3 4
5 0 7
8
Group Ill
9 10 II
80'4 CzH4, 20'5 Nr 80% carla, 20% C ~ H ~ 80% CzHi, 15% CH4, 5% Nz
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
V d . 46, No 7
PROCESS INSTRUMENTATION lures. The addition of methane to the compensator cell may upset the ethane compensation slightly. Therefore, blends KO.7 and KO.8 are rerun, and the necessary correction is made to the ethane concentration of the compensator cell. Primary compensation for the worst interferent simplifies the iteration since this interferent also partially compensates for the remaining * interferents. When the chart readings of all blends of Group I1 converge closely about a common point, Group I and Group I11 blends are run through the sample cells. A study of these data indicates the direction of slight modification in the compensator cell filling. The final calibration obtained is shown by Figure 4.
0
20
40
60
60
PARTIAL PRESSURE IN I CM. SAMPLE CELL (CM HG)
Figure 3. Absorption Data for Gas o f Interest and Interfering Gases in Filter Cell with Remainder of Path Evacuated
Figure 3, each plot is linear to the accuracy of measurement. With sample cells evacuated, the total beam energy has been reduced to approximately 44% of the f.b.s. If this transmission were predicted from the interference data of Figure 2, it would be (0.96 X 0.81 X .50) = 39%. The discrepancy between the observed signal reduction and the calculation is due to the spectral regions which the background gases have in common. For the concentration changes shown in Table I, the following changes in f.b.s. are observed from Figure 3: ethylene-5.5’%, ethane-l%, methane < O.,i%, and carbon dioxide-0%. Compensation for Interfering Gases. Having obtained the above selectivity by filtering, additional background gas interference is now removed by appropriate filling of sensitizer and compensator cells. Before final compensation and calibration were done, three groups of carchfully mixed blends shown in Table I1 were made. Groups I and I11 comprise blends of the average values of the interfering gases in the span extremes of the gas of interest, whereas Group I1 blends encompass the span extremes of the interfering gases and the average value of the gas of interest. Xo blends including carbon dioxide were made since the instrument is insensitive to this component. Since the instrument is sensitized for ethylene, a high relative accuracy of the ethylene content in the blends of each group is necessary. Therefore, all the tanks for each group of blends weie evacuated on a common manifold and filled simultaneously to the same pressure with ethylene. Background components were added to each tank on a pressure basis and the total pressure of each tank brought to the same value. The method used in filling the sensitizer and compensator cells is an iterative one. Ethylene is introduced in the sensitizer cell until the radiation transmitted through path PI is reduced by about 70%. A trial pressure of the worst interferent, ethylene, is then placed in the compensator cell. Blend No. 7 is now placed in the sample cells, the paths PI and Ps unmasked and trimmer T S adjusted until the pen is approximately mid-scale on the recorder chart. The other extreme of ethane interference, blend No. 8, is next introduced in the sample cells. If the pen moves upscale in the direction of increasing ethylene concentration as blend No. 8 replaces blend No. 7, more energy is being absorbed by ethane in path PI than in path Pz,hence ethane must be added to the compensator cell. The ethane concentration of the compensator cell is thus altered stepwise until the minimum change a t the recorder pen is noticed as blends S o . 7 and No. 8 are successively placed in the sample cells. The procedure is repeated for methane interference. Blends KO.5 and No. 6 are alternately placed in the sample cells and methane is successively added to the ethane content of the compensator cell until compensation is achieved for these two mixJuly 1954
loor
80 -
t
l
: /
BLENDS #I-3 ( 4 6 - 50)
40
% C,H,
I
60 IN SAMPLE
1
-
80 STREAM
Figure 4. Instrument Calibration Showing Selectivity to C2H4and Insensitivity to Interfering Gases Gas blends those of Toble II. Calcium fluoride windows used throughout. Cell data: Detector. 3 cm. Hg CzH4 and 27 cm. Hg Argon Sensitizer cell, 40 cm. Hg CzH4 Compensator cell, 20 cm. Hg CzHe and 15 cm. Hg CHa Filter cell, 20 cm. Hg &He, 20 cm. Hg CHa, ond 10 cm. Hg COz
The blend numbers are those of Table 11 and the numbers in parentheses are the calibration extremes for the blends indicated. 911 calibration data are to an accuracy greater than i.1% which is considered the accuracy of the blends. A volume or mass method would be required for greater precision of the blends. Sfter filtering and compensation, the ethylene span of Figure 4 produces a 3.9% change of the initial f.b.5. This illustrates clearly that the usable fraction of the integrated absorption of the gas of interest is strongly dependent upon the degree of spectral overlap and the relative concentration spans of the components in the sample stream. Second Order Source Temperature Compensation. When the sensitizer and compensator cells are filled, paths PI and Pz are made selective to different wave length absorption bands of the gas of interest to which the detector responds. As the temperature of a reasonably black body source changes by a small amount, the resulting intensity change is comprised of a proportional change a t all wave lengths and a nonlinear part which
INDUSTRIAL AND ENGINEERING CHEMISTRY
1399
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT produces relative changes between different spectral regions. Since the Tri-Non analyzer photometer system operates at a true radiation null, the proportional intensity changes affect' all paths equally. Hoiuever, if the radiat,ion intensities of pat,hs P I ,P2,and Ps experience relat,ive changes, small zero shifts result. This effect can be reduced if the nulling path which is chopped in phase with PI if filled with a mixture of int,erfering components in the approximate ratio that' they appear in the compensator cell. Then, the wave length distribution of t,he sum of the radiations transmitted through paths PI and P, more closely approximates t,he radiation transmitted by path P2.
1
Literature Cited (1) Hasegawa, I., and S h a r d , 12. G., PTOC. A m . Petroleum Inst., 28 (111) (1948). (2) Koppius, 0. G., AmZ. Chew?.,2 3 , 5 5 4 (1951). (3) Smith, V. N., Insfrunze?zis, 26, 421 (1953).
Conclusion A method of sensitizing a particular infrared nondispersive
nitor J. K. WALKER, A. P. GIFFORD,
analyzer for a particular gas analysis has been presented; this method is generally applicable to nondispersive analyzers. This procedure separates the effect of filtering and compensation and allows optimum cell fillings for a particular problem to be approached. The illustration given shows that for a problem with typical spectral interference the energy actually used is a small portion of the initial full beam signal.
RECEIVED for review Septeniber 7 , 1953.
ACCEPTEDJanuftry 2 5 , 1934,
ss AND
R. H. NELSON
Consolidafed Engineering Corp., Pasadena, Calif.
a small,
portable mass spectrometer i s described that i s a conventional 180" type unique in compactness and simplicity. The instrument i s usable for batch analyses of light gas mixtures but i s primarily designed for the continuous monitoring of one or more specific components in CI process gas stream. A description i s presented of a flexible building-block concept for designing process monitoring systems for given requirements. System components include an automatic programming device for monitoring numerous gases in one or more streams alternately on a preset cycle. A record of the concentrations can be presented directly in terms of mole per cent. Examples are given of analyses of known multicomponent gas mixtures and applications of mass spectrometer monitoring systems. Automatic, periodic standardization of the mass spectrometer can be made against normal stream composition or stored samples of the pure gases. The final step of closing the control loop on the basis of product analysis i s also shown to be within reach.
SMALL, portable, inexpensive mass spectrometer is nom available for the continuous monitoring of stream compoeition or laboratory batch analyses of light gas mixtures. This instrument is an outgrowth of several forerunners in the field of monitor mass spectrometers including a large scale model developed in 1949 (3,4),t'he helium leak detector, and several models developed for use by the AEC ( 2 ) . This instrument is in effect a coinpromise between various design features of the first two instruments mentioned. Since a comparison of these instruments is covered in a report by Slarr et al. (4),the scope of this paper is limited t o an instrument description, performance characterist'ics, and applications of the instrument both as a laboratory bat'ch analyzer and a continuous stream recorder as adapted t o an acetylene plant. By monitoring vital component's of t,he lean cracked gas for maximum production versus operating parameters, the final step of closing the control loop in the automat'ic plant is shown to he within reach in the near futurc.
lnotrument Description The compactness and four-wheeled portability of the instrument is shown in Figure 1. T h e instrunient is provided with a floor lock and requires only 110-volt power and cooling rvater for operation. The small 40 X 24 X 38 inch cabinet can be placed in an out-of-the-way, on-the-site location in a plant for remote recording a t a master control panel.
1400
The instrunlent is designed t o be one of a series of building blocks necessary for a closed loop automatic plant control With this principle in mind the instrunlent is compos group of easily removed, replaceable units as illustrated in Figure 2. K i t h all of the side panels removed, the angle iron frame and track containing the magnet and analyzer assembly can be seen on the right' half of t,he case. The electronic cont,rols are locat,ed in the extended drawer-type chassis, which can bc operated either in place or removed from the instrument. The vacuum pumps are located on floating trays directly beneath the dravver. The standard unit includes a continuous gas sampler, automatic scanning circuit, and cyclic scan mechanism for repeated scanning of a single mass peak. The sampler requires only that the sample be clean, dr?, and a t atmospheric pressure. -111 coniponents of the design were chosen to withstand maximum conditions of temperature, humidity, and vibrat,ion. Accessory units include the batch inlet system and recorder contained in the top cabinet'. Space is provided above the pumps for a stream programmer and automatic peak selector. These units xi11 permit scanning of units of G selected peaks of the spectrum in one or more gas streams a t discrete time intervals.
Performance ~
~
~
This mass spectiometer by necessity of its reduced sme saciifices some of the iesolution previously enjo\red in certain laboi atory-type instruments. The resolution as implied in the name of the ionization chamber, Diatron-20, provides complete scparation of adjacent peaks a t mass 20 as shown in Figure 3. S o signifi-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46, No. 1
~
