Determination of Argon in the Presence of Oxygen and Other Atmospheiric Gases by Adsorption Chromatography Kenneth Abel, Laboratory of Technical Development, National Heart Institute, Bethesda, Md.
I
REPORTED that the synthetic zeolite, Molecular Sieve Type 5A (Linde Ccl.) is capable of separating argon from oxygen in adsorption chromatography. Vizard and Wynne (11) reported using a 15-meter column packed with molecular sieve that had been conditioned at 400' C. for 1 hour. Under optimum conditions, using helium as a carrier, up to 1% argon in a mixture containing 20y0 oxygen resulted in complete separation between the oxygen and argon a t 18' C. Lard and Horn (8) reported that a t -72" C. a 6-foot column packed with sieve conditioned for I hour a t 300' C. would separate argon and oxygen but would retain nitrogen. A separate analysis a t room temperature was required to provide the ratio of nitrogen to oxygen. They also stated that the Type 5.4 was the only molecular sieve capable of the separation. Previously, Greene (6) had been unsuccessful in separating oxygen and argon a t - 72" C. using a 10-foot molecLlar sieve column of this same type. I t has been concluded that column conditioning is a critical, and apparently nonreproducible, parameter in successful application of chromatography to this separation. Furthermore, operation a t two temperatures is inconvenient. Krejci, Tesarik, and Janak (7) reported that the conversion of 0 2 to H20on a palladium catslyst is quantitative a t room temperalare when using hydrogen as a carrier. By making two analyses for each sample (one bypassing the catalyst) they could determine argon in the presence of oxygen a t 22" c. We have investigated this catalytic conversion method of analysis in greater detail and have found that with modification it can als:, be used when only single samples ape available for analysis; thereby eliminating the need for two injections. Y O COLUMN 30 INCHES
E
(
EXPERIMENTAL
T HAS BEEN
N
4
. PORT
Figure 1. Series arrangement for the analysis of both argon and oxygen from a single sample in the absence of
co
Materials. Adams Oxide (Pt) catalyst was used as a 50/50 volume dispersion in acid washed Chromosorb W. Linde Molecular Sieve Type 5A (40/80 mesh) was used exclusively in this study after conditioning for 1 or more hours a t 300' C. (Types 44, 5A, lox, 13X, and AW500 all provide adequate separations when conditioned in this manner; Type 3A does not.) Silica gel (Fisher Scientific Co.), SO/lOO mesh, was conditioned a t 200" to 300" C. for 1 or more hours. Electrolytic calcium carbide (Fisher Scientific Co.), 20/30 mesh, was utilized for conversion of HzO to CZH,. The carrier gas was pre-purified hydrogen (Matheson Co.) with water removed using a Molecular Sieve Type 13X drying tube. Apparatus. Borosilicate glass columns (3-mm. i.d.) and reaction tubes were utilized with silicone rubber column seals (Burrell Corp.). Connections between columns, reaction tubes, and detector were with '/Isinch 0.d. stainless steel tubing. The detector was either a single- or dualchamber micro cross-section ionization detector (1) polarized a t +30 volts. A Hewlett-Packard d.c. microvoltammeter was used for signal amplification to a conventional 1-mv. recorder (L and N, Speedomax G). The detector and columns were operated at room temperature without thermostatic control. RESULTS AND DISCUSSION
Adams Oxide (Pt) catalyst was used in preference to palladium because its activation occurs in hydrogen a t room temperature. Krejci et al. used a subtractive method for analysis in which two sample injections are required for analysis. In this method, a short reaction tube is filled with catalyst to convert oxygen to water. This is followed by a molecular sieve column. The first sample injection bypasses the catalyst tube and provides for the analysis of the ratio of the combined oxygen-argon peak to nitrogen (or other peak) in the sample. The second injection, in which the sample passes t,hrough the catalyst, results in the quantitative conversion of 0 2 to HzO which is then retained by the molecular sieve thereby allowing direct determination of argon. Replacing 4 cm. of the catalyst with CaCz results in conversion of O2 to CzHz : 2Hz 2H20
+
0 2
-3 2H20
(1)
+ CaC2 +Ca(OH)2 + CzH2 (2)
Table 1.
Relative Retention Time for Series Arrangement HZcarrier flow rate 35 ml./min. Retention time (min.) Silica Molecular Sample gel sieve He 0.6 1.2 A 0.7 1.9 No 0.7 2 9 ~. eo, 3.8 ads. 0.8 5.2 CH4 0 2 (as e&) 11.5 ads.
dccording to Duswalt and Brandt ( 4 ) and Sundberg and Maresh (IO) the reaction of H 2 0 with CaCz is quantitative when applied to gas chromatographic systems. These investigators reported that the resultant does not adversely retard water from subsequent injections (provided the carbide is changed often) nor does it retain COz. Figure 1 illustrates a series arrangement for analysis of argon and oxygen after conversion of oxygen to acetylene while Figure 2 shows the response of the system to selected atmospheric gases which can be determined from a single sample. The retention times for this system are indicated in Table I. A parallel column arrangement utilizing a single detector surh as employed by Brenner and Cieplinski (3) should also be satisfactory since Hz is not analyzed in this system ( 2 ) . It will be noted from Figure 2 and Table I that carbon monoxide is not shown. We have found when CO is present a t a concentration of about 1% that of Oz (or greater) the conversion of 02 to HzO on the platinum catalyst a t room temperature is completely
b
5
IO TlYL i w YiNUTEP
ia
rn
Figure 2. Chromatogram showing direct analysis for argon and oxygen (as (C2H2) plus selected gases Components: (A) unresolved mixture from silica gel column; (6) helium; (C) argon; (D) nitrogen1 (El carbon dioxide from silica gel column; (F1 methane; ( G ) oxygen (as C2tin) from silica gel column
VOL 36, NO. 4, APRIL 1964
e
953
inhibited. Traces of CO result in partial inhibition. There is no reaction between the CO and Oz. Above 175" C. 0 2 and CO combine on the catalyst to form COz, but the reaction is not stoichiometric; a t various mixtures from approximately 1:99 to 99: 1 of CO to Oz, the products of the reaction include both CO and 0 2 as well as COS. Although we have found no reference specifically regarding the inhibition of the conversion of 0, to H 2 0 by CO when using a platinum catalyst in an excess of Hz, this behavior is consistent with known characteristics of catalytic Carbon reaction retardation (9). monoxide is a poison for platinum a t temperatures below 180' C. (6), and as a result, the series or parallel column (preceeded by the catalyst and CaCJ method of analysis cannot be used when both O2and CO are present. Oxygen and argon can be analyzed in the presence of carbon monoxide when COz is not of interest. It is necessary that CO and oxygen be separated prior to their passage through the catalyst-CaCz chamber. To accomplish this, the arrangement shown in Figure 3 has been utilized. A molecular sieve column is used to retard CO. The argon-oxygen mixture then passes into the catalyst chamber where the oxygen is converted to H20 and subsequently to acetylene in the CaC2
&NJ.
R4
Figure 3. Arrangement for analysis of samples containing CO and 0 2 but not COY
version to HZO, although normally the oxygen-hydrogen mixture will flash back into the carrier if insufficient H2 is present in the catalyst. With the glass catalyst chamber used, combustion of the 0 2 can be seen visually and always occurred within the first millimeter of the catalyst dispersion. Accordingly, very little catalyst need be used and this amount can be discarded when the CaCz is replaced. LITERATURE CITED
chamber. A short length of silica gel then separates the acetylene from the remaining gases under analysis. Since a molecular sieve column is utilized to effect the CO-osygen separation, carbon dioxide cannot be analyzed in this system a t room temperature. The order of elution in this system is helium, argon, nitrogen, methane, carbon monoxide, and oxygen (as C2H2). Assuming that the atmosphere in the laboratory contains the normal 0.94% argon, it was determined from calibration with pure argon, oxygen, and nitrogen that up to 2 ml. of O2 can be injected (at a hydrogen carrier flow of 35 ml. per minute) and conversion of 0 2 to HzO will be complete (within the limits of detection) in the absence of CO. Progressively larger injections sometimes result in incomplete con-
(1) Abel, K., ANAL. CHEM. 36, 954 11964). (2) Abel, K., deschmertzing, H., Ibid., 35, 1754 (1963). (3) Brenner, W., Cieplinski, E., Ann. N . Y . Acad. of Sci. 72, 705 (1959). (4) Duswalt. A. A.. Brandt. W. W.. ANAL. I - -
- - I
( 5 ) Greene,'S. A.,'Ibid.; 31, 480 (1959). (6) Griffith, R. H.,.Marsh, $1. D. F., "Contact Catalysis, p. 207, Oxford University Press, London, 1957. (7) Krejci, M., Tesarik,,,K., Janak, J.,
"Gas Chromatography, H. J. Noebels, R. F. Wall, N. Brenner, ed., Academic Press, New York 1961. (8) Lard, E. W., Horn, R. C., ANAL. CHEM.32, 878 (1960). (9) Psrlin, R. B., Wallenstein, M. D., Zwolinski, B. J., Eyring, H., in "Catalysis," P. H. Emmett, ed., Vol. 11, pp. 272-273. Reinhold. New York. 1955. (10) sundberg, 0. E., Maresh, C . , ANAL. CHEM.32, 274 (1960). (11) Vizard, G. S., Wynne, A., Chem. and Ind. 1959. 196.
An Improved Gas-Tight Micro Cross-Section Ionization Detector Kenneth Abel, Laboratory of Technical Development, National Heart Institute, Bethesda, Md.
Lovelock, Shoemake, and Zlatkis (3) have shown that the sensitivity of the cross-section ionization detector can be increased significantly by decreasing its internal volume. The micro volume detectors reported in the literature to the present time have design features which make them inconvenient to use because they do not allow for direct connection to metal columns, are not gas-tight, or do not provide satisfactory electrical connections. A dual-chamber micro crosssection detector was reported previously (1) for the analysis of gases produced by bacteria which did allow for direct column attachment but did not, have the simplicity, ease of modification, nor as satisfactory electrical connections as the detector described here; furthermore, it did require electrical shielding. The detector described below eliminates the problem of leakage at the detector, provides positive column connections (either solder or detachable metal connectors), provides an excellent, easily-detached electrical connection, is readily constructed of available materials and components, and does not require external shielding. All comECENTLY,
954
ANALYTICAL CHEMISTRY
ponents are of Teflon, silicone rubber, and stainless steel, enabling the detector to be operated a t temperatures up to the tritium foil decomposition temperature of approximately 220' C. (The following sources'for specific components are listed for convenience and do not imply indorsement of these specific products by this laboratory: bulkhead coaxial mounts and coaxial cables, Microdot, Inc., Pasadena, Calif. ; conductive silicone rubber, Union Carbide Corp., New York 17, N.Y.; tritium titanate-coated foils, 100 to 150 mc. per sq. cm., Radiation Research Teflon, Corp., Westbury, N. Y.; Pennsylvania Fluorocarbon Co., Clifton Heights, Pa.) The sample chamber volume of one detector was 12 pl., thereby making the detector suitable for use with capillary columns without the necessity of utilizing a scavenger flow. Even smaller volumes appear to be feasible. The basic design of this detector is shown in Figure 1 in which one side of a dual-chamber version is shown in exploded view. The second chamber is identical. Column connections are brought in through the side with a
connecting hole drilled at right angles (as shown by the dotted lines) into the ionization chamber. A coaxial mount makes electrical connection by means of
-1
imh-
Figure 1. Exploded view of dual chamber micro cross-section detector
