Chromatographic Analysis of Gas Mixtures Containing Nitrogen, Nitrous Oxide, Nitric Oxide, Carbon Monoxide, and Carbon Dioxide R. NELSON SMITH, JAMES SWINEHART, and DAVID G. LESNlNl Chemistry Department, P ornona College, Claremont, Calif.
b A gas chromatography column using two layers of silica gel separated by iodine pentoxide makes possible the separation and determination of nitrogen, nitrous oxide, nitric oxide, and carbon monoxide or nitrogen, nitric oxide, carbon monoxide, and carbon dioxide in 10 minutes. Simple gas sampling and gas transfer units made from standard equipment are described.
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separation and determination of such simple gases as nitrogen, nitric oxide (NO), carbon monoxide, and carbon dioxide appear straightforward and to be without complication. The classical micromethods using a Blacet-Leighton microgas buret are complicated by the fact that chemical reagents satisfactory for one gas always react to some extent with one or more of the others. Infrared absorption may be used for three of the gases, but the required sample size and insensitivity to nitric oxide and carbon monoxide prevents its use for microdeterminations. Gas chromatography has complications, also. A charcoal adsorption column will completely adsorb carbon dioxide, but it reacts with nitric oxide to produce some nitrogen and carbon-oxygen surface complexes ( 2 ) and does not separate carbon monoxide and nitric oxide. Unreactive coatings on the surface will prevent the reaction of nitric oxide with carbon but will not effect a separation of nitric oxide and carbon monoxide. Silica columns a t any degree of hydration will separate carbon dioxide and will not react with the remaining gases. but the remaining gases are themselves not adequately separated. Molecular sieres completely remove carbon dioxide, but, probably because of partial retention on the surface, the remaining gases break through too insensitively for practical use. Ion exchange resins and sulfuric acid solutions of ferrous sulfate on silica gel also give incomplete separations. An adsorption column which combines both chemical reaction and adsorption, and which is satisfactory for most purposes, is described. Subsequent t o submitting this paper HE
for publication, Szulczewski and Higuchi ( 3 ) described a gas chromatographic separation of these same gases using a dehydrated silica gel column. After the emergence of carbon monoxide, the column temperature is raised from dry ice temperature to room temperature for the elution of nitrous oxide (N,O) and carbon dioxide. The prime advantage of their method is that nitric oxide emerges as a peak, whereas in the method presented, nitric oxide is obtained by difference. The chief disadvantages of the method of Szulczewski and Higuchi are the time required for analysis, 1 hour, and the rather diffuse separation of the gases which have long break times. The present method requires about 10 minutes. CHROMATOGRAPHIC COLUMN
A 10-foot length of l/a-inch copper tubing is half filled R ith silica gel (Code 12, 28-200 mesh, Darison Chemical Co., Baltimore, hId.). This is folloived by about 8 inches of iodine pentoxide powder and about inch of silver metal powder. The remaining length of tube is filled with additional silica gel. The column is vibrated continuously during the filling to ensure good packing. The tube is coiled to the appropriate diameter and connected to the apparatus so that the gas stream passes through the silver ponder on leaving the iodine pentoxide. ilt elevated temperatures, iodine pentoxide n ill completely oxidize carbon monoxide to carbon dioxide (4) and nitric oxide t o nitrogen dioxide (2) with the liberation of iodine. For gas chromatography, it is practical to main-
Figure 1.
A typical chromatogram
tain the column temperature a t 115' C. Here no separation of carbon monoxide, nitric oxide, and nitrogen occurs in the first section of silica gel, but the carbon dioxide lags behind. Nitrogen is unaffected by iodine pentoxide, but carbon monoxide, when converted to carbon dioxide, lags behind and is thus separated. Neither the iodine nor the nitrogen dioxide which are produced a t the iodine pentoxide layer leaves the column; the iodine is probably removed by the copper tube or by the silver metal, and the nitrogen dioxide is probably removed by the silica gel. The amounts of nitrogen, carbon monoxide (as carbon dioxide), and carbon dioxide are determined from peak height, and the nitric oxide is determined by difference. A Gow-Mac thermal conductivity cell, Model 9193 with TE-I1 geometry (Gow-Mac Instrument Co., Madison, S.J.), and a Varian Model G-10 recorder (Varian Associates, Palo Alto, Calif.), .cr-itha full-scale sensitivity of 10 mv., n ere used. Using helium as carrier gas a t a flow rate of 30 cc. per minute, this column gives peak times of 2.8 minutes for nitrogen, 6.7 minutes for carbon monoxide, 9.3 minutes for nitrous oxide, and 10.3 minutes for carbon dioxide. Gas samples containing both nitrous oxide and carbon dioxide cannot be satisfactorily analyzed using a column of this length, but the method works well if either one alone is present with the other gases. 4 typical chromatogram is reproduced in Figure 1. After more than 500 analpcs, the column showed no indication of yielding iodine or nitrogen dioxide in the exit stream or of having insufficient iodine pentoxide for additional analyses. In a series of fairly large samples nhich are predominantly nitric oxide. it is necessary to Imit about 30 minutes b e h e e n analyses. If this is not done. the column passes unconverted nitric oxide a t the peak position for carbon monoxide. The samples analyzed have been of the order of 1 to 2 cc. (at standard temperature and pressure) when collected, but only about a third of this passed through the column. The waiting period between analyses can be avoided by using samples of 1 cc. (STP) or less. VOL. 30, NO. 7, JULY 1958
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The analytical method described has the advantages of rapidity, sensitivity, and giving clean-cut separations, but the nitrogen peak height (or area) is affected by the presence of nitric oxide and carbon monoxide. Because the important variable is composition and not sample size, more extensive calibration is needed than normal. The enhancement of the nitrogen peak height is linear with respect to per cent nitric oxide, but because nitric oxide is not directly determined, its peak height cannot be used to evaluate a suitable correction for the nitrogen content. Up to a t least l5%, carbon monoxide has the same effect as nitric oxide. In one problem, carbon dioxide and nitrogen were of the same order of magnitude, and carbon monoxide was less than 15% of the sample. The calibration was reasonably simple: the nitrogen peak heights were observed for a series of samples containing known amounts of nitric oxide, nitrogen, and carbon dioxide a t known total pressure in the sample bulb. The samples were made by diluting a mixture of nitrogen and carbon dioxide (in about equal amounts) with varying amounts of nitric oxide. The calibration curve was constructed by plotting as ordinate the factor, F, by which the nitrogen peak height must be multiplied to give the known nitrogen pressure and as abscissa the ratio, R, of total sample bulb pressure in millimeters to nitrogen peak height in millivolts. Factor F varied linearly (1.04 to 0.93) for values of R ranging from 1 to 6, corresponding to samples ranging from pure to 167, nitrogen; for R = 12, F = 0.87. SAMPLING TECHNIQUES
The gas sampling and transfer units are simple. Standard vacuum stopcocks (hollow precision-ground, 2-mm.
B
V
b
Figure 2. Gas sampling and transfer apparatus a.
b.
Standard vacuum stopcock with taper joints Altered stopcock
oblique bore) with standard taper joints a ) may be used to transfer samples from the reaction system to the chromatography column. The internal volume of each stopcock and the lead by which each stopcock is evacuated are adjusted to constant volume (to 10.01 cc.) by tedious glass blowing in connection with a simple gas buret. The bulb on the bottom of these stopcocks may be altered appreciably without affecting the bearing surface. A given batch of stopcocks is relatively uniform in volume when purchased. The ready interchangeability of sample tubes is well worth the glass blowing effort. To eliminate dead space and nonrepresentative samples from the system to be sampled, a vacuum stopcock may be altered as shown in Figure 2, b. It is sealed into the gas system a t A and B , so that in a flow system the gas enters a t A and leaves by B. The black section of the stopcock is filled, by
ox each lead (Figure 2,
heating, with Apiezon wax W-100 (James G. Biddle Co., Philadelphia, Pa.) to eliminate dead volume. This stopcock is also sealed to a vacuum system a t V . The sample bulb is connected by its standard taper joint, S’,to standard taper joint, S, and evacuated through V. By turning stopcock X through 180°, a representative gas sample flows through S into the sample bulb, The pressure of the gas sample may be measured by a manometer attached to the gas system. Then X and X’ are turned 90” to close the system and the sample bulb. The gas trapped in leads S and 8’is wasted. In a kinetic study, a series of gas samples may be removed a t recorded time intervals and analyzed at convenience. To introduce the gas sample into the chromatography column in a reproducible manner without introducing foreign gases, the sample bulb is connected to the sampling section of the column. The leads are evacuaFd through V’; Turning stopcock X through 180 permits expansion of the sample into the fixed volume of the sampling section which can be closed off subsequently. The sampie may be swept out and into the column with the helium stream. This method is wasteful of sample in that the sample bulb and leads are left with an appreciable fraction. Variations of this method are possible, depending on the availability of sample and the sensitivity of the sensing and recording units. LITERATURE CITED
(11 Shah. hl. S.. Oza. T. M., J . Chem. Soc. 1931, 32. (2) Smith, R. K , , Lesnini, D., Mooi, J., J . Phys. Chem. 6 0 , 1063 (1956). \-I
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(3) Szulcxewski, D. H., Higuchi, T., ANAL.CHEJI.29, 1541 (1987). (4) Welton, W. M., Drake, N. L., IKD. ENG.CHEIL,AXAI,,ED.1,20 (1929).
RECEIVED for review September 7 , 1957. ;Iccepted March 20, 1958. Work done under Contract K8onr54700 with the Office of Saval Research.
Standardization of Mass Spectra by Means of Total Ion Intensity ARCHIE HOOD’ Houston Research laboratory, Shell Oil Co., Houston, rex.
b A method for standardizing mass spectra of petroleum oils and related pure compounds gives spectra which are standardized with respect to both liquid sample volume and instrumental sensitivity. It is based on the total ion intensity of the spectrum and requires no knowledge of sample size or instrumental sensitivity. Spectral peaks of standardized mass spectra can b e
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ANALYTICAL CHEMISTRY
compared directly; therefore the method has broad application in correlations of mass spectra with molecular structure, leading to development of analytical methods for petroleum oils.
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mass spectrometric methods for analyzing heavy petroleum oils, a major difficulty has
been inability t o determine accurately the amount of sample introduced into the inlet system of the mass spectrometer. When the sample size is not known accurately, one spectrum cannot be compared directly with another. No means is available for measuring
N THE DEVELOPMENT of
Present address, Shell Development Co., Houston, Tex.
