more than 200" C., above which decarboxylation occurs over an extended period of time ( I O , IS). ACKNOWLEDGMENT
The authors are indebted to J. W. Moore, of this station, who supplied the petroleum porphyrin aggregates. LITERATURE CITED
(1) Blumer, Max, -4ri;a~.CnEnf. 28, 1640 (1956).
(2) Brockmann, H., Schodder, H., Ber. 74. 73 (1941). (3) Dunning, H. S . ,Carlton, J. K., ANAL. CHEX28, 1362 (1956).
(4) Dunning, H. N., Moore, J. W., Myers, -4.T., Ind. Eng. Chem. 46, 2000 (1954). (5)Fischer, Hans, Orth, Hans, "Die Chemie des Pyrrols," vol. 11, part I, pp. 579-87, Akad. Verlags, Leipzig, 1937; Edwards Bros., Ino., Ann ,4rbor, Mich., 1943. (6) Groennings, Sigurd, h a L . &En(. 25, 938 (1953). (7) McSwiney, R. R., Sicholas, R.E. H., Prunty, F. T. G., Riochem. J . 46, 147 (1950).
(8) Nicholas, R. E. H., Rimington, C., Ibid., 48, 306 (1951). (9) Nicholas, R. E. H., Rimington, C., Scand. J . Clin. & Lab. Invest. 1, 12 (1949). (10) Treibs, .4.,Angew. Chem. 49, 682 (1936). 1) Treibs. A,. Ann. 509. 103 11934)
RECEITED for review January 5, 1959. Accepted March 25, 1959.
Gas Chromatograph Ionization by Alp ha-Pa rticles for Detection of the Gaseous Components- in the Effluent from a Flow Reactor WENDELL M. GRAVEN Department o f Chemistry, University of Oregon, Eugene, Ore. ,An inexpensive and easily constructed instrument was needed for analyzing mixtures of nitrogen, nitrous oxide, and oxygen which, together with helium, constituted the effluent from a continuous flow reactor. Satisfactory results were obtained with a gas adsorption chromatograph which utilized the ionization current produced by a-particles from an aged radium D source for detecting the presence of gaseous components in the effluent from a column packed with Molecular Sieves. This type of detector is insensitive to fluctuations in temperature or rate of gas flow and is moderately sensitive to all permanent gases except hydrogen. Analytical application can b e extended to other multicomponent mixtures.
analysis of the efRuent gas from a continuous flow reactor was the objectire of the present investigation. TEFLON
APPARATUS
SUPPORT
The design considerations and operational characteristics of an analytical cell n.hich utilizes ionization by aparticles has been reported in detail by Deisler, JlcHenry, and Wilhelni (9). a-Particle Source. A 1-me. sample of aged radium D, in which radioactive equilibrium had been established, n.as obtained from the Canadian Radium and Uranium Corp. The radioactive sample formed a band around the exterior surface of a cylindrical silver foil approuimately inch in diameter. Detector Cell. For the outer electrode a brass cylinder 1 inch high and inch in diameter, to which a wire lead had been attached, was mounted on a Teflon support 7/8 inch in diameter, as shown in Figure 1. Through the center of the Teflon support extended a short length of l/r-inch copper tubing which, m-ith an attached lead, formed the base of the inner electrode.- The silver cylinder which supported the alpha source was inserted in the copper tubing, so that the source was approximately a t the center of the brass electrode. A small Teflon plug inside the cylinder kept the source in the desired position and maintained electrical contact between the foil and the tubing. The entire assembly fitted snugly inside a short length of glass tubing n hich, with attached entrance and exit leads, had an estimated volume of approximately 10 ml. The gas stream passed through the cell from top to bottom as it appears in Figure 1.
LEADS
Figure 1.
Detector cell
270 V
1.5V
G
As-adsorption chromatography has been used for analysis of mixtures of permanent gases in a number of recently reported investigations (5, 6, 9,13,14). It has been shown that each of the active solids-silica gel ( I S , IS), alumina ( 5 ) , charcoal ( 5 ) , and Molecular Sieves (9)- offers certain advantages for specific analytical problems. HOTever, for separations of oxygen and nitrogen Molecular Sieves seem to be superior (9). Programmed heating of the column during elution has been successfully eniployed for separation of a large number of gases on several adsorbents (6, 6). In each of these investigations a thermal conductivity cell served as a detection device. I n addition t o the most commonly
Figure 2.
Measuring circuit
used detection method based on thermal conductivity a number of sensing devices are used in gas chromatography (4, 8, 10). Methods utilizing an ionization gage (12) and an electric discharge tube ( 7 , 1 1 )have been described. A detection method involving ionization of the effluent gas by p-rays ( I , 4 ) appears t o compare favorably ryith that based upon thermal conductivity. I n an attempt to avoid certain disadvantages of high cost and difficult construction, a method of analysis of binary gas mixtures based upon ionization by a-particles (2, 3 ) has been adapted for use in gas chromatography. The application of this device to the
VOL. 3 1 ,
NO. 7, JULY 1 9 5 9
1197
Table I.
Reproducibility of Analytical Determinations
Peak Height, Mm. Mole yo 0.2 0.5 1.0 2.0 5.0 7.5 10.0 15.0
7-
Oxygen 2.0 5.6 13.8 26.5
Nitrogen 7.9 14.8 19.2 '27.0
1.8 5.4 13.6 26.5
8.5 14.9 19.4 26.9
6-
5CI
Measuring Circuit. Three 90-volt dry cells connected in series were used for the cell voltage supply in the final design of the circuit shown in Figure 2. The voltage drop across the 2megohm resistor, produced by the ion current of approximately 10-8 ampere, could be recorded directly on a Leeds & Northrup medium high impedance Speedomax. The recorder used in this investigation had a 10-mv. range, a 2second response time, and a 2-megohm input impedance. By means of the 1.5-volt dry cell, current-limiting resistor, and potentiometer shown in Figure 2 the input to the recorder could be decreased as desired for adjustment of the pen to zero when the cell contained only helium. With the use of a grounded circuit, a shielded cell, and shielded input leads to the recorder the noise was reduced to an insignificant level, as is shown in Figure 4. No objectionable zero drift was observed during &hour periods of
use.
Chromatograph Column. A 10-foot length of l/r-inch copper tubing was packed with 40/80 mesh Type 5A Molecular Sieves obtained from the Linde Co. It was coiled t o form a helix of approximately 5-inch diameter so as t o fit snugly inside a GlasCol heating mantle, which was then packed with borosilicate glass %ool.~' The heating mantle was equipped with a thermocouple which permitted the temperature of the column to be monitored with a millivoltmeter. By controlling the power to the heating mantle with an autotransformer it was possible to raise the temperature of the column rapidly, as well as to operate the chromatograph a t a constant elevated temperature. Before use, the column was kept evacuated at. 350" for 12 hours in order to reactivate the adsorbent. Sampling System. One end of the column was attached t o the detector cell by means of a glass-metal seal, while the other end was connected to a valve which facilitated sampling. Three separate channels were simultaneously controlled by this two-position valve to permit collection of a sample of the reactor effluent and introduction of this sample into the column with only momentary interruptions of gas flow through the reactor and through the column. Thus, in one position of the valve the reactor effluent passed through the sample chamber to the waste exit, while simultaneously a stream of helium passed through the
1 198
ANALYTICAL CHEMISTRY
>
5 -4 -
-> -
w v)
E
E3-
5
Q
v)
W
W
w
a
z 0
a4
2-
Lz:
cn
w n
W
a
a W
n a
J
'1 L 5 10 15 MOLE PER CENT O X Y G E N
Figure 3. Oxygen calibration curve
J F E- D
RESULTS
The relative stopping power of helium for a-particles is much smaller than that of gases other than hydrogen. Therefore, the sensitivity of the detector to all gases, except hydrogen, is adequate for analytical purposes. Assuming oxygen and nitrogen to be representative, it may be seen from Table I that the present apparatus is capable of detecting 0.2 mole % ' without difficulty. Reproducibility of the sampling system permits a high degree of precision in the analyses. The peak heights
B; 6
'
25 20 15 I O 5 E L U T I O N T I M E (MIN.)
Figure 4. Chromatograms component mixture A.
B.
column to the detector. By rotation of the valve handle through a 60" angle the helium stream was diverted through the sample chamber and the sample was swept thereby through the column, while a t the same time the reactor effluent passed directly to the waste exit. A sample chamber with an approximate volume of 20 ml. was constructed from an 8-foot length of l/r-inch copper tubing. Metering Arrangement. A metering valve and rotameter controlled and monitored the helium flow rate. Individually calibrated capillary tube flowmeters were used t o prepare gas samples of known composition.
C
02 Nz
c. co
D. E.
F.
of
six-
CzH, N20 CO2
obtained by repeated removal of samples from a gas stream of known composition often agree to within the limits of measurement, as shown by the data of Table I. By varying the relative amount of helium, binary mixtures having the desired percentage of the second component were made up a t atmospheric pressure and 20-ml. samples were introduced into the column. A helium flow rate of 60 ml. per minute was used to elute the sample. Measurements with oxygen were made with the column a t room temperature; those with nitrogen were made a t 100" C. The results of numerous analyses of nitrogen-oxygen mixtures demonstrated that the peak height of one component was uninfluenced by the absence or presence in varying amounts of the second component. More than a hundred measurements gave evidence that the precision of analysis for oxygen or nitrogen, gases which are eluted from the column a t room temperature, was adequate for kinetic investigations. Although the relation between peak height and mole per cent, which for oxygen is sh0n.n in Figure 3, is not exactly linear, the greater ease and
precision with which peak heights can be measured make their use preferable to use of peak areas for quantitative analytical work. Because the response of the detector is different for each gas, a calibration curve is required in either case. Analyses of mixtures of as many as eight gases n-ere obtained by raising the temperature of the column while the samples were being eluted. The heating rate was controlled so as to obtain adequate separation of the peaks from the individual components without unnecessarily prolonging the total time required for the analysis. Figure 4 shows chromatograms taken directly from a section of strip chart as they were recorded by the instrument. The two tracings correspond to duplicate analyses of a mixture of oxygen, nitrogen, carbon monoxide, ethane, nitrous oxide, and carbon dioxide. The exact composition of the samples was not known. However, it was estimated that they contained approximately 15% of each gas, with the exception of ethane. Initially the coluinn was a t room temperature and heating was begun exactly 3 minutes after introduction of the sample when the first component, oxygen, had been eluted. The temperature of the heating mantle rose uniformly during the elution and had
reached approximately 400” a t the completion of the analysis. The entire analysis requires less than 25 minutes. The reproducibility of the heating program is such that the elution time of each of the six gases varies by less than 2% from run to run (Figure 4). This was accomplished by setting the variable autotransformer which controlled the power input to the heating mantle a t a position such that the desired rate of heating of the colunin was initiated when the switch was thron 11. Although 2 hours had elapsed between the first analysis which gave the lower tracing and the second which gave the upper one, corresponding peak heights differed by no more than 5% in each case, n-ith the exception of carbon dioxide. These variations may be partly the result of the uncertainties involved in preparation of the two gas samples by metering the flow of each of the six gases through the sample chamber. Other gases, such as hydrogen, methane, and nitric oxide, are also separated from oxygen and nitrogen by this type of column 11hen operated a t room temperature. The very small peak which precedes that of oxygen in each of the chromatograms shown in Figure 4 is due to hydrogen which was inadvertently included in trace amounts in each of the samples:.
ACKNOWLEDGMENT
Appreciation is expressed for financial support provided by the Research Corp. and for several samples of Molecular Sieves furnished by Linde Co. LITERATURE CITED
(1) Deal, C. H., Otvos, J. W., Smith,
v. N.,
ZUCCO, P. s., ANAL. CHEM. 28, 1958 (1956). (2) Deisler, P. F., McHenry, K. W., Wilhelm? R. H., Ibid., 27, 1366 (1955). (3) Deisler, P. F., WiIhelm, R. H., Znd. Eng. Chem. 45, 1219 (1953). (4) Uesty, D. H., “Vapour Phase Chromatography,” pp. 127-93, Academic Press, New York, 1957. (5) Greene, S. A., Moberg, M. L., Wilson, E. M., ANAL. CHEM.28, 1369 (1956). (6) Greene, S. A., Pust, H., Ibid., 29,1055 119.571. (7j-Hi
