Portable Gas Chromatograph and Electrochemical Detector for Carbon Monoxide J. R. Stetter,* D. R. Rutt, and K. F. Blurton Energetics Science, Inc., 85 Executive Boulevard, Elmsford, N. Y. 10523
To the analyst faced with the detection of trace gases in large and small volumes of gas mixtures, there is always a need for new detectors, especially those which are sensitive and convenient. We wish to communicate our experience in using an electrochemical instrument as a quantitative gas chromatographic detector for the analysis of carbon monoxide in ppm to percent quantities. This technique has the advantage of permitting the design of truly portable gas chromatographic systems. During catalytic studies in dilute gaseous systems, it was necessary to analyze gas mixtures with a composition of 0 4 % HP, 0-3000 ppm CO, 20% 0 2 , and the balance nitrogen. Resolution and analysis of these gaseous mixtures was accomplished by means of the extremely simple chromatographic system illustrated in Figure 1. Samples were syringed into the column via a septum-sealed syringe injection port, and separated on a Supelco 6-ft long, l/a-in. i.d., stainless steel 5A molecular sieve column with Swagelok fittings. The eluant order from this column a t 20 "C was He, 0 2 , Nz, and CO ( I ) . Hydrogen was easily detected in a nitrogen carrier gas, and oxygen and nitrogen in a helium carrier gas by a GowMac katharometer (Model 10-677, a four-filament bridge, and Model 40-002 voltage regulated power supply). However, the CO could not be easily measured quantitatively by this thermal conductivity cell in either helium or nitrogen and, therefore, an alternative detector was required. While flame ionization detectors provide excellent sensitivity for CO and many models are commercially available, methanation of the carbon monoxide prior to analysis is necessary to obtain maximum sensitivity (2). We, therefore, connected a CO analyzer which uses an electrochemical detection principle (3) (Ecolyzer Model No. 2800, Energetics Science, Inc.) in series with the thermal conductivity cell (Figure 1).In this instrument, carbon monoxide is electrochemically oxidized a t a platinum catalyzed electrode, the potential of this electrode being maintained a t a value where neither oxygen reduction nor evolution occurs a t a significant rate. The current generated by the electrochemical reaction is then directly proportional to the CO concentration (3-5). By virtue of operating the sensor in this
potential range, air could be used as the carrier gas in addition to He and Nz. Figure 2 shows three typical chromatographic traces with air as the carrier gas obtained by connecting the analyzer outputs to a Moseley, Autograf M7100B strip chart recorder. The peak was eluted 6-8 min after injection of the 1.0 cm3 sample. Identical traces were observed with helium, oxygen, and nitrogen as the carrier gases. Figure 2 also demonstrates the repeatability of the peak height (approximately 1%)and the rapid instrument response time which is vital for this application. Plotting the peak height of the eluted CO in a 44 ml/min UHP helium carrier gas stream vs. the concentration of the injected carbon monoxide/air samples shows the sensor's response is linear up to about 3000 ppm CO. The gas samples were syringed with 0.1, 0.5, and 1.0 cm3 Hamilton GasTight (accuracy & 1%)syringes from a septum-sealed glass flask which was continuously purged a t 300-500 cm3/min with the CO/air gas mixtures. The concentration of the mixtures was taken as the nominal concentration given by Matheson Gas Products, and the maximum uncertainty in these concentrations was &lo%. The values of the peak heights for the various samples exhibited a small flow rate dependence and were independent of the carrier gas used over the entire range of sample sizes and CO concentrations tested. The peak height was reproducible within 2% with 16 consecutive samples of four CO concentrations in the range 200 to 3000 ppm. A series of syringed sample volumes for the 3000 ppm CO/air gas mixture plotted together with the eluted peak height demonstrates that the sensor's response is linear a t the concentrations of CO eluted from the chromatographic column. Total random noise in this particular detection system was typically f 0 . 1 MV resulting in a detectability [defined as twice the random noise level ( 6 ) ]of 1-2 ppm CO for a l-cm3 gas sample injected into the GC column or 0.1-0.2 ppm for a 10-cm3gas sample (1-2 X lop9 g), The detector response was clearly linear for a l.0-cm3 sample up to 3000 ppm CO, and this was the highest CO concentration tested. Therefore, by the selection of sample aliquots between 0.1 T 30
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CARRIER GAS
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Figure 1. Modified
gas chromatograph
(I) Flow regulator; (2) thermal conductivity detector: (3) sample injection port: (4) 6-ft, 5-A Molecular Sieve column: (5) Ecolyzer: (6) soap bubble flowmeter: (7) variable range recorder
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
Figure 2.
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Chromatographic trace
Carrier gas: zero air at 31 cm3/min; column temperature: 21 OC: recorder attenuation: 0-50 mV full scale, 0.5 in./min; sample: 1.00 cm3 of 200 pprn CO/air mixture
and 10 cm3, gas mixtures between 0.1 and 30 000 ppm (3%) can be conveniently analyzed. Carbon monoxide analysis by nondispersive infrared absorption (NDIR) is another procedure whereby air could be used as the carrier gas but it suffers from limitations such as: 1) its inability to monitor the low CO concentrations (few ppm) met in chromatographic eluants, 2) its relatively slow response time (5, 7 ) making it unsuitable to follow the rapidly changing CO concentrations existing in eluant streams, and 3) the need for ac power, thus making it nonportable. Previous field data (5, 8) have shown that NDIR and this electrochemical method give similar values, and the correlation coefficient of the readings with the two instruments was found to be 0.94 (8). We envision this type of detector not only as a research tool easily augmenting the capabilities of an existing gas chromatograph, but also as a suitable detector in a portable gas chromatographic system. One of the greatest obstacles in the fabrication of a portable chromatographic system is the need to transport carrier gases in cylinders. By using this electrochemical instrument, the carrier gas may be ambient air scrubbed to remove CO or unscrubbed ambient air containing a constant CO concentration in which case the CO would be determined by the differential signal. This permits the chromatograph to be operated in a remote location while handheld [battery operation ( 4 , 5 ) ] . While this discussion refers specifically to the convenient
analysis of carbon monoxide, we can see applications of similar systems to field gas chromatographic detection of other electrochemically active gases for which suitable sensors (detectors) of this type are available such as NO (9), NO1 (9), EtOH (IO), and H2S (11). This technique should be of interest not only in research applications and portable gas chromatographic detection systems, but in all technological problems of detection where only small samples are available for analysis or the aliquot method of sampling gases is preferred.
LITERATURE CITED (1) Supelco. inc., "Chromatography Catalogue", 1974, p 11. (2) L. Dubois and J. L. Monkmann, Anal. Chem., 44, 74 (1972). (3) H. W. Bay, K. F. Blurton, J. M. Sedlak, and A. M. Valentine, Anal. Chem., 46, 1837 (1974). (4) K. F. Blurton and H. W. Bay, Amer. Lab., 6 (7), 50 (1974). (5) H. W. Bay, K. F. Blurton. H. C. Lieb, and H. G. Oswin, Amer. Lab., 4 (7). 57 (1972). (6) C. Harold Hartmann. Anal. Chem., 43 (2), 113A (1971). (7) R. K. Stevens, T. A. Clark, E. C. Decker, and L. F. Ballard, 65th Annual Meetina of the Air Pollution Control Association, Miami Beach, Fla., June 1672. ( 8 ) N. Yamate and A . inoue, J. Environ. Contro/(Japan), 9 (3). (1973). (9) J. M. Sedlak and K. F. Blurton, presented at the Electrochemical Society Meeting, May 1972, Toronto, Canada. (10) H. W. Bay, K. F. Blurton. H. C. Lieb. and H. G. Oswin, Nature (London), 240, 52 (1972). (1 1) J. M. Sedlak and K. F. Blurton, Taianta, accepted for publication
RECEIVEDfor review June 23, 1975. Accepted December 15, 1975.
Sample Preparation for Infrared Emission Spectrometry Gianfranco Fabbri," Pietro Baraldi, and Paolo Frassoldati lstituto di Chimica Fisica, Laboraforio di Chimica Fisica Applicata, Universita di Modena, Italy
In previous research on the ir emission spectra of crystalline inorganic salts carried out in our laboratory ( I , 2 ) ,we noted that the shape and the relative intensity of the bands recorded were markedly influenced both by crystal size and thickness of the emitting layer. While the former is a wellknown factor in ir absorption spectrometry of solids [compare e.g. ( 3 ) ]and is ascribed to insufficiently reduced crystal size, the latter seems to be a peculiar feature of ir emission spectrometry, its mechanism being self-absorption of the emitted radiation by outer and cooler layers of the sample. A study of the mechanism by which self-absorption takes place will be reported in depth elsewhere. As a part of research aimed a t setting up simple techniques for the recording of IR emission spectra with common instruments, we propose here an original method of sample preparation which enables the first factor to be eliminated and the second one to be neglected. Spectra quite similar to those obtained in absorption and, therefore, acceptable a t least for qualitative purposes, can thereby be obtained. The method is applicable to compounds soluble in water or in other sufficiently volatile solvents. A particular method for pulverizing the crystals of the sample, and therefore for reducing the band-broadenings, is based on the fact that the materials generally used in emission spectrometry as supports, such as metals, are both insoluble in water and resistant to thermal shocks. Experiments have shown that a layer of a water-soluble compound can be obtained by instantaneously touching the surface of the solution with one face of the support heated
to a temperature between 200 and 300 "C. The very quick evaporation of the solvent from the film of solution in contact with the support leaves a very adherent layer of crystals that is spread very homogeneously and uniformly, the crystal size, measured by microscope, always being less than cm. The features of the layer (thickness, crystal size) are a function of several parameters such as the support temperature and the solution concentration, and vary, but not widely, with the nature of the salt. However, when working with solutions where the concentration is less than certain values (about one half of the solubility a t 100 "C), after a certain amount of practice the operator will be able to obtain layers giving good-quality spectra, Le., without the well-known features due to crystal size. The operating conditions mentioned above can be said to apply to all inorganic salts; this is not to say that in specific cases (sparingly soluble or decomposable compounds), different conditions should not be adopted. As an example, the emission spectra of sodium oxalate, run between 1800 and 400 cm-' on vibromill-ground powder ( A ) ,suspended in isopropyl alcohol and applied to the support by the technique of Hunt et al. ( 4 ) , and on a sample of the same salt prepared by the method described ( B ) , are shown in Figure 1 in conjunction with the absorption spectrum ( D ) recorded as KBr pellets. All the spectra were recorded with a Perkin-Elmer 457 spectrometer using the techniques described in ( 2 ) . With regard to the second factor, it should be noted that the band-broadenings observed on very intense emission bands and the variation of relative intensity with respect t o ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
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