Infrared Spectrometric Method for Monitoring Gaseous Organic Substances in Atmosphere FRED E. LITTMAN and JAMES 0.DENTON Stanford Research Institute, Air Rerearch Laboratories, Pasadena, Cali/.
collimated in m y way. After passing through the filter and sample cells the light is received by the two detectors. Each detector eodsists of B ~aefilledchamber. which is divided in two
Because of the importance ascribed to organic suhstances in the formation of smog in Los Angeles, a continuously recording infrared gas analyzer oapable of following changes in their concentration in the atmosphere was developed. Of the instruments commereially available, a nondispersive infrared gas analyzer initially designed by Shell Development Co. was judged to hold the most promise. The instrument was modified to improve its performance; a full-scale sensitivity of 14 p.p.m. of butane was eventually achieved. The modified instrument appears to he suitable for continuous monitoring of organic concentrations in the range from 0.1 to 10 p.p.m., without prior treatment of the atmosphere. T h e concentration of organic substances in the Los Angeles atmosphere has been successfully monitored for se.ieral months.
B
ECAUSE of the importance of changes in the level of organic substances in the atmosphere and their r&tionship t o smog s, continuous method of anslysis was needed which would produce records that could he compared directly with those ohtained for the concentration of other important constituents of the atmosphere. Such B continuous method would be more diffioult to raohieve than a method based on spot tests, hut i t was believed that &dam from the errors inherent in spot sampling procedures would more than compensate for the anticipated difficulties. A literature search revealed very little information on the determination of organic compound-particularly those without functional grou-in the concentrations of interest in the study of smog. Shepherd and others (8)had successfully used the mass specbameter ta identify some of the organic constituents of the Los Angeles atmosphere. Stitt, Tjensvold, and Tomimatsu (S)determined low concentrations of ethylene by oxidation with mercuric oxide. A colorimetric method for the determination of aromatic compounds is also available (MSA aromatic hydrocarbon detector, Mine Safety Appliances Co., Pittsburgh, Pa.). Mader and others ( 1 ) described a. technique for using an infrared spectrophotometer to determine low concentrations of hydrocarbons collected and concentrated in a freeze-out trap. Of the instrumental methods, those utilieing the infrared absorption common to all organic meterials appeared the most promising. When it wm decided to use an infrared technique for the determination of organic materials, a survey of existing equipment was made. Beoause of their high sensitivity, the nondispersive gas analyzers seemed preferable to the conventional grating or prism spectrographs. Of the several instruments then on the market, the Model 70 spectrometer developed by Shell Development Co. and manuisctured by Applied Physics Carp., Pasadena, Calif., w&s chosen because of its high sensitivity, stability, and unusual flexibility.
raatatlon. Also supplied with the Model 70 analyzer is n modified Brown
Figure 1. Infrared analyzer assembly
111 I
/I'
, I
EXPERIMENTAL
I
I I , " I O Y O L I l l i r /*/GnT/"EJ
I
E.
Figure 2. Schematic diagram, Model 70 infrared analyzer
and then chopped by a-rotating shutter at-20 c.p.s.,but is not 945
ANALYTICAL CHEMISTRY
946
Figure 3.
Modified infrared analyzer
suip-chart Electronik rocorder. This recorder measures the ratio of the two mierophane-polarising voltages, and comequently does not have a dry cell, standard cell, or selfhtanditrdizing mechanism. Despite the fact that the energy of the whole infrared spectrum is used, a relatively high degree of specificity can he obtained because the detector cell a n he filled with a gas having a suitable absorption spectrum. Consequently, relittively small amounts of the same gas in the sample cell block out exactly the 8ame radiation to which the detector cell is sensitive, whereas another gas, although i t might show a strong ahsorption of infrared radiation, interferes only to the extent that its spectrum overlaps that of the gas in the detector cells. By choosing the gases in the detector and filter cells, as well a8 using the compensation afforded by a double-beam instrument, i t is possible to obtain it surprising degree of specificity a t very low concentrations. Because the analyzer just described did not have the required sensitivity, i t was modified in several respects. The modifications are discussed in detail later in this paper, hut are summarised here for convenience. The operating temperature of the glower unit wag increased by doubling the operating voltage from 8 to 16 volts, resulting in an increased output of the detector cells. A water-cooled radiator was substituted for the air-oooled one to protect the Source unit from overheating. The metal shutter was replaced hy one of polyethylene; this resulted in a twofold increase in recorder deflection. The 27-cm. sample cells were replaced by 290-om. (approximately 10-foot) lengths of 1-inch borosilicate glass pipe, which were vacuum aluminiaed on the inside. An optioal bench was built to accommodate the new cells a8 well as the 6oume and detector unit (Figure 3). Operation of Infrared Analyzer. The gas inlet system is shown in Figure 4. Incoming sir was drawn through the system by a pump, the flow being controlled by solenoid valves activated by a timer. Particulate matter was removed by a Millipore filter (Millipore Filter Corp., Watertown 72, Mass.). A t predetermined intervals a blank was obtained by passing t,he air through a Combustion tube filled with a catalytic element and maintained a t 800' C. Attempts to remove trace amounts of organic material from synthetic mixtures by the use of a standard copper oxide catalyst a t 800" C. indicated incomplete Combustion, as shown in Table I.
Table I.
Combustion of Traces of Hydrocarbons over Various Catalysts (Sample,.6O P . P . ~ .01 butane) Treatment
None Combustion over copper oxide a t 800' C. Combustion over preoiaua metal Oatalygt at 800' C.
Reoorder Deflections (Scale Divisions) 60.5
8.3 0
Figure 5. Effeot of increased source voltage
Figure 6. Hydrocarbon and water sensitivity as function of deteotor cell filling 27-om. %ampleoells, Quarts filters
The precious metal catalyst, which is mitnufactured by t h e Catalytic Combustion Corp., Detroit, Mich., appears to he capable of completely removing traces of organic materials. This method was adopted in preference to adsorption techniques, heemm it, i s ivreversihle and does not, denend an the concentration of the organic material. outsidc air is used in the reference cell, the When ~~~~~
~~
~~
is much too small t o be ;ecorded by the detect& DISCUSSION AND RESULTS
Modifications of Instrument. The instrument as supplied b y the manufacturer had a full-scde sensitivity of about 500 p.p.m. of propane, which was about one order of magnitude above t h e desired sensitivity. The most obvious approach t o heightening
947
V O L U M E 28, NO. 6, J U N E 1 9 5 6 the sensitivity was to increase the length of the sample cells, because the sensitivity is roughly proportional to the length of the light path. However, several changes were made prior to such a major modification. A necessary preliminary t o the use of longer cells was t o increase the output of the infrared gloner. This R-as achieved by increasing the operating voltage, with the results shown in Figure 5 The construction of the glomer, iThich consisted of a plane helix of Xichrome n-ire, the adjacent turns of which were insulated from each other only by an oxide coating, did not permit n further increase in the operating temperature. This change in output of the glon-er resulted primalily in an increase of the
\
i
I
0
I
I
0 20 DETECTOR CEL.
I
I
I
30 40 50 65 F I L L I Y G (-1. P R O P I N E I N A R C G N I
70
Figure 7. Analyzer sensitivity as function of concentration of propane in detector cells 27-cm. cells, quartz filters
I
I
i I
I
!
I
I
signal-to-noise ratio, which is the ultimate factor limiting the useful sensitivity. This, in turn, permitted the sensitivity of the Brown recorder to be increased by shunting the slide-wire. Shunting with a 16-ohm resistor resulted in an increase t o a point where a useful sensitivity of the order of 2 p.p.m. of butane per scale division &-as obtained. T o determine the composition of the gases in the detector cell which would be most suitable for the determination of trace amounts of organic materials in the atmosphere, several series of experiments were run. Figure 6 shows the results using variOLIS hydrocarbons at a concentration of 57, in argon. (.krgoii was used because it has no infrared absorption and a lorn- specific. heat.) Although ethane appeared t o have the highest sensitivity: propane showed a better hydrocarbon-to-water ratio and IWF therefore adopted for further work. There n'as reason to believe that the sensitivity of the instrument (as expressed in scale divisions per part per million of a given hydrocarbon) >vas also a function of the concentration of the sensitizing gas in the detector cell; therefore, a series of experiments was run using different concent,rations of propane in argon. The results are shown in Figure 7 . They indicated that a lo^ concentration of sensitizing gas n-as drsirable, but that below 57, the advantages mwe offset by rapidly decreasing Pignal-to-noise ratio. A concentration of 5
