Infrared Analysis of Gases : A New Method Mark M. Rochkind Bell Telephone Laboratories,
Murray Hill, N. J.
A sensitive method for infrared chemical analysis based on a cryogenic sampling technique has been proposed. Diluted gas mixtures are deposited in pulses onto a cooled alkali halide substrate; sample preparation and deposition need require only seconds. Extraordinarily simple spectra superbly suited for digital analysis result. Computer analysis of the raw spectral data suggests applications in the areas of air pollution control and atmospheric research. Spectral data for hundreds of gases might be stored within a central computer linked via telephone to numerous experimental stations. Transmitted dsta could be analyzed in seconds and a report communicated to a local station within minutes. Modifications of the sampling technique to accommodate liquids and solids seem pcssible.
W
hereas low temperature optical spectroscopy has found widespread use as a research tool, no application of cryogenic sampling as an instrument for chemical analysis has been demonstrated. The recent advances in cryogenic technology which have made Joule-Thomsen refrigerators commercially available provide impetus in the area of low temperature research and bring cryogenics within the reach of most chemical laboratories. This being the case, exploitation of low temperature techniques for spectrochemical analysis is warranted. We have developed a new method for infrared chemical analysis of gas mixtures which involves an atypical method of cryogenic sampling which is highly selective, highly sensitive, and rapidly executed, yet which requires only conventional infrared instrumentation (Rochkind, 1967). This method provides the most sensitive means of infrared gas analysis demonstrated to date. Micromole quantities of most chemical species are sufficient for positive qualitative identifications, and in many instances fractions of a micromole suffice. The method is suitable for impurity analysis. It provides a means of spectrochemical analysis for all gaseous species excepting homonuclear diatomic molecules, and modifications of the sampling procedure to encompass liquids and solids are being studied. In addition to providing a sensitive means for differentiating among structurally similar molecules, the method is capable of differentiating among isotopic species of relatively light polyatomic molecules. In range of application the proposed method is most comparable to mass spectrometry. Although its sensitivity is not so great, analysis of infrared data is considerably simpler than the multidimensional analysis required for interpretation of mass spectra. Capital investment provides another contrast; it proves sizably greater for a mass spectrometric laboratory. Feasibility experiments recently conducted have demonstrated application of this infrared method for a series of 13 structurally similar, simple (C, to C,)hydrocarbons (Rochkind, 1967). We are currently expanding our experimentation to larger hydrocarbons and molecules of more varied atomic composition. Our choice to demonstrate feasibility via analysis of simple hydrocarbons was motivated by the relatively small 434 Environmental Science and Technology
extinction coefficients and great similarities in normal vibrations exhibited by members of this series. Thus, success with an unadvantageous system assures credibility for the method. Although serious efforts liave not yet been made to define the technique’s quantitative capabilities, preliminary results indicate that quantitative methods employing cryogenic sampling can be developed. The merit of the proposed method derives in large part from the extraordinary spectral simplifications which result from low temperature simpling. Removal of rotational degrees of freedom yields a great enhancement of spectral intensity as bandwidths are reduced from 100 to 3 or 4 cm.-l; interspersing inert molecules in the condensed solid partially to surround and thus separate molecules of gas mixture being analyzed reduces molecular interactions to yield vibrational spectra characterized by narrow lines, well defined frequencies, and high reproducibility. Such would not be the case for spectra of simply condensed reagent gas mixtures. Workers in the field of molecular spectroscopy will recognize the proposed technique as an adaptation of the matrix isolation method which was developed to provide controlled environments for the in situ generation and trapping of unstable radical species. To achieve good “matrix isolation” a reagent gas is dissolved in very high dilution in an inert gas and the resulting solution is slowly condensed onto a cooled substrate. The expectation that high dilution coupled with slow deposition will yield an inert matrix within which are dispersed randomly distributed reagent molecules is well borne out. While it is a very powerful spectroscopic technique, the matrix isolation method is time-consuming and thus not suitable for chemical analysis. Our adaptation employs an inert matrix gas but a very rapid, pulselike deposition which proves a reproducible manner of sample preparation and provides a degree of “isolation” sufficient to suppress frequency shifts which might result from interactions between reagent molecules, though not sufficient to yield the extremely narrow (1-cm.-l half width) bandwidths characteristic of good matrix isolation. Nonetheless, the resulting bandwidths are sufficiently narrow to permit resolution of vibrational frequencies adjacent to within a few wavenumbers. Because the vibrational bands are well defined and because normal modes are highly sensitive to even small variations in molecular structure, the infrared identity of a single chemical species may be completely characterized by only two o r three fundamentals suitably chosen from its spectrum. It is thus a simple matter to compile a compact table of “selected frequencies” describing the infrared identities of any number of gases. The high specificity of normal vibrations makes accidental degeneracies on the binary and ternary levels upon which an analysis is conducted highly improbable. Furthermore, relative intensity data (independent of experimental configuration) are available for a two-dimensional analysis. Procedure
The actual experimental procedure is simple. Reagent gas is well mixed with a 100-fold mole excess of nitrogen; the resulting solution is deposited in -0.5-mmole pulses onto an alkali halide substrate cooled to 20” K. The crystalline film which results is transparent and highly transmitting throughout the infrared spectral range. Whereas eight aliquots of gas mixture (-40 Mmoles of reagent) are generally deposited when recording the standard spectrum of a reagent gas, as
many as 48 aliquots of a synthetic seven-component mixture have been deposited without any significant loss in signal-noise due t o infrared scattering. While absolute intensities will depend very strongly o n the geometrical properties of the cryostat assembly, frequency reproducibility should be preserved independent of experimental configuration. Thus, standard spectra (frequencies, relative intensities, bandwidths) for gaseous chemical species need be recorded only once t s be permanently cataloged. The proposed method of infrared analysis is amenable to extensive automation. A manifold system equipped with high vacuum solenoid valves, for example, is easily fabricated to provide an automated system for gas expansion, dilution, and pulse deposition. More exciting, however, is the clear application of digital control to analysis of the raw spectral data. Such an approach is encouraged by the high frequency reproducibility of the technique and the well defined spectral bands. A digital computer can compare and match a raw spectrum with a table of “selected frequencies,” even one containing data on hundreds of distinct chemical species, in a matter of seconds. In fact, a digitally controlled analysis might at no further expense be carried out on a two-dimensionsal level-Le., involving relative intensities in addition to frequencies-to ensure against accidental degeneracies. Master File Because infrared spectra are readily digitized, it is optimum to keep a master file of selected frequencies within some centrally located computer and t o perform all analyses via data transmission to this central facility. This is an eminently practical scheme, for it relieves the experimental station of amassing “selected frequency” data, of training personnel in the interpretation of raw spectra, of developing and maintaining computer software t o execute analyses, and of the economic strain in supporting a reasonably sophisticated computer facility. With the degree of automation t o which the sampling procedure admits, analyses of complicated mixtures might be effectively carried out by nonprofessional technical personnel. The telephone provides a method of data transmission which is straightforward and surprisingly economical. I t is inexpensive to record digital spectra on perforated paper tape. Spectra may then be transmitted via Bell System Data-Sets over normal telephone lines at no premium charge. A 3-minute transmission which could involve the transfer of as many as 216,000 bits of information would incur costs a t the same rate as a 3-minute voice conversation. Thus spectral data can be transmitted between any two points in the country a t strictly nominal cost. Transmission of 2000 distinct spectral measurements would require from 2 t o 5 minutes of transmission time, depending on the detailed peripheral equipment; and the required capital investment in suitably flexible peripheral equipment is small. A single computer facility should be capable of handling more than 1500 sets of spectral data per 24-hour period. As raw spectra are received from various experimental stations, they might be queued on a magnetic disk file and as the analyses were completed the results could be voice-telephoned o r teletyped back to the station of origin using the same Data-Set facility employed to transmit the raw spectra. Alternatively, o r in addition, a record of the analysis might be prepared for mailing by auxiliary digital output equipment without human participation. Standard spectra of new chemical species could continually update the master
file without revision of the analysis programs. Thus a single perpetuating master file would be retained, copies of which could be provided t o affiliated centers. Applications The suggested scheme for digitally controlled analyses of spectral data coupled with a practical means of data transmission greatly extends the range of application for the proposed method of analysis. In addition to general use in the research laboratory, for example, where the chief virtue of the method is that nothing need be known about the contents of a gas mixture prier to analysis, there are very real applications in the field of air pollution control, in the general area of airborne atmospheric research, and in the chemical process industry where methods for impurity analysis are invoked as means of quality control. In the field of air pollution control the automated infrared method permits marked standardization in determining the pollutant composition of exhaust gases. For example, inexpensive, evacuated polycarbonate bulbs (100-cc. volume) fitted with some break-seal device might be provided for sampling effluent gases where the pollutant composition of such gases was in question o r where adherence to a pollution code needed to be demonstrated. An unbreakable plastic bulb could be simply c:ipped, after disrupting the vacuum in an effluent stream, and mailed to a local experimental station where a spectrum would be digitally recorded and transmitted via telephone to some central computer facility. The cnmputer would perform the analysis, record the results in its library for future reference or to ensure corrective action: and initiate a certified report on the analysis to the local station. This proposal seems thorough and economically and practically more sound than the construction and support of completely self-contained local analytical stations throughout the country. That there is a need for some kind of analytical network in the area of air pollution control is beyond question. Whether infrared spectroscopy provides the right comproniise between analytical sensitivity and cost is something which should be carefully explored. What is certain is that whatever analytical method is adopted, it must be disposed to very general analysis. The fact that the proposed method requires no knowledge of sample composition prior to analysis suggests its use in interplanetary atmcspheric research. Such an application would permit an orbiting vehicle to define the stratified composition of a planetary atmosphere through a series of sequential samplings: spectral data to be telemetered back to earth for analysis. Infrared gas phase studies, while instrumentally simpler, are difficult to interpret and are not capable of providing the kind of detailed information which repeated samplings of an actual atmosphere would yield. There is also a potential application in the field of exo-biology in the search for extraterrestrial life. In addition t o analyzing natural vapors, a soft-landed package might be employed in the search for organic material via partial thermal reduction of solid inatter to the gaseous state; very low vapor pressures would suffice. The use of solid sampling procedures in lunar research is being considered. Technical details describing experimental procedures and spectrometry have been published (Rochkind, 1967). Literature Cited Rochkind, M. M., Anal. Clienz. 39, 567 (1967). Receiced for reciew Murcli 31, 1967. Accepted M a y 9, 1967.
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