Flame Ionization Detection of Carbon Monoxide for Gas Chromatog raphic Analysis KENNETH PORTER' and D. H. VOLMAN Department o f Chemistry, Universify o f California, Davis, Calif.
b Carbon monoxide may be detected b y a flame ionization detector after catalytic conversion to methane. A high percentage conversion is obtained, mainly independent of catalyst temperature over the operating range. This method is considerably more sensitive than katharometer detection. It i s suggested that carbon dioxide and hydrogen may be detected by use of the same nickel catalyst.
I
was necessary in this laboratory t o determine small quantities of carbon monoxide below the limit of katharometer detection. Bccordingly, a method has been sought and investigated by which carbon monoxide could be rendered detectable in a flame ionization detector with its much greater sensitivity. Carbon monoxide will react with hydrogen in the presence of rein the followduced nickel catalyst ing manner : T
(e)
CO
+ 3H2 S CH, + HZO
According to Groggins (1) this reaction is used to purify hydrogen containing traces of carbon monoxide and under such circumstances a quantitative conversion of carbon monoxide to methane is said to be achieved. All the necessary conditions for conversion -except the catalyst-are present in the normal gas chromatography system employing a flame detector, and investigation has shown that carbon monoxide may be rendered detectable by the above reaction. METHOD
A survey of the literature (2, 3) showed that the preparation of the catalyst is apparently not critical and several diff erent support materials and reducing techniques have been employed. In the present instance 100to 120-mesh Silocel firebrick was soaked in a saturated solution of nickel nitrate and the surplus liquid 11as removed by filtering under vacuum. The firebrick \\as dried overnight at 100" to 110" C. and then heated in air for 5 hours a t a temperature held t o between 400" and 500" C. A quantity of the firebrick Present address, Imperial Chemical Industries Ltd., Fibres Division, Hookstone Road, Rarrogate, England. 748
ANALYTICAL CHEMISTRY
was packed into a &inch length of stainless steel tubing (0.d. inch X id. inch) around which v a s wrapped a heater winding and suitable insulation. This tube was inserted in the flow line immediately prior to the flame-head, Le., just after the flame and carrier gases (hydrogen and helium) were mixed. The current through the heater was adjusted to bring the catalyst temperature to 250' C. and the mixed gas was allowed to flow overnight to reduce the desposited nickel oxide to the metallic state. Injection of carbon monoxide gave a definite response contrasting with the previous behavior of the flame detector in connection with this substance. I n view of this response, it was decided to investigate the possibilities of quantitative analysis of carbon monoxide and the influence of one catalyst parameter (temperature) on the reduction process. The apparatus used was a Loenco Model l5F chromatograph and in the present instance, a fixed helium flow was obtained by passing the gas through a column of alumina a t 100' C.; the f l o ~rate n-as assumed to be unimportant up to the relatively low maximum value which could be employed without extinguishing the flame. Seither carbon monoxide nor methane n s retained on the column. Injections Kere made by a Hamilton 100-pl. syringe against the operating pressure of 40 p.s.i. and the poor reproducibility of this method of sample introduction is reflected in the results below. However, no other method of sampling was conveniently usable. Small samples ( 5 pl. or less) were made by drawing 10 pl. of gas into the syringe followed by 40 pi. of air, followed by expulsion of whatever quantity of the mixed gas was necessary to leave in the syringe a suitable amount of the 207, carbon
Figure 1. Effect of catalyst temperature on carbon monoxide elution Methane shown for comparison. All samples 10 PI. (gas)
RESULTS
The catalyst temperature was first varied over the range 206" to 266" C. While a large amount of tailing was observed a t the lowest temperature (Figure 1) this disappeared almost completely b y 266" C. when the peak width mas almost the same as that of a similar sample of methane Ithose peak width was independent of the catalyst temperature in this quoted range. The areas of five 10-pl. samples were measured over this temperature range and were found to lie in the range 17.4 f 2.6 planimeter units (all results quoted a t the 95% confidence level). This was in agreement with the sensitivity for methane (determined from three 10-111. injections) of 18.9 =t 1.4 units and the sensitivity for carbon monoxide of 18.2 =t 2.9 units a t a fixed temperature of 266" C. This latter value was calculated from a calibration with a range of sample sizes (13 in all) running from 1 to 12 pl.; the best estimate of the peak area for a 10-pl. sample was obtained simply from the appropriate least squares equation. The whole of the carbon monoxide sample appeared to be reduced over this range of sample sizes judging from this latter result, and the modified flame ionization detector n-as adequate for the detection of 2.5 X 10-lo mole quantities of carbon monoxide. The best operating temperature was about 260" C. a t which practically no tailing occurred. The degree
so
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1 i
nionoxide/air mixture for injection to the column.
cc
111001
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of conversion was independent of temperature in the experimental range. CONCLUSION
The catalytic conversion is probably complete and this method offers a way of determining quantities of carbon monoxide at present undetectable b y thermister or hot wire detectors when the catalyst column is coupled to a suitable analytical column (say 5 4 Molecular Sieve). According to lledsforth (3) carbon dioxide may be reduced in a similar
way to carbon monoxide so that this substance could also be analyzed with a flame ionization detector setup. There is also a possibility of detecting hydrogen b y introducing into the carrier gas a small percentage of carbon monoxide or carbon dioxide. I n this case, the catalyst would have to be included in the carrier gas stream only and the sensitivity would be one third of t h a t for methane. ACKNOWLEDGMENT
This work was sponsored by the U. S. Army Research Office (Durham)
and the authors gratefully acknowledge this support. LITERATURE CITED
(1) Groeains, P. H.. “Unit Processes in Orgaic Synthesis,” 5th ed p. 620,
McGraw-Hill, New Tork, 1958.
(2) Hightower, F. W., White, ;2. H., Znd. Eng. Chem. 20, 10 (1928). (3) Medsforth, S., J . Chem. SOC. 123,
1452 (1923).
(4) Sabatier, P., Senderens, J. B., Compt. rend. 134, 514 (1902).
RECEIVED for review Sovember 20, 1961. Accepted April 9, 1962.
Determination of Traces of Water in Hydrocarbons A Calcium Carbide-Gas Liquid Chromatography Method H. S. KNIGHT and F. T. WElSS Shell Development
Co., Emeryville,
Calif.
F A simple gas-liquid chromatographic method has been developed for determining traces of water in hydrocarbons. The sample is passed through a calcium carbide bed where the water reacts to form acetylene, which is then measured by gas-liquid chromatography. The carbide reactor can b e mounted on the sample container, minimizing the length of lines to b e purged. The accuracy has been studied with samples containing as little as 3 p.p.m. of water. The repeatability a t the 20 p.p.m. and 0.3 p.p.m. levels is about 7 and 15% of the value, respectively. The method is applicable to gas or liquid hydrocarbon samples and is readily adaptable to available industrial GLC apparatus.
T
HE determination of traces of water has long been an important and difficult analytical problem. Water is omnipresent and highly polar, and as a result, surfaces that are nominally dry are, in actuality, usually coated rvith a thin film of adsorbed moisture. In passing a relatively dry fluid to be analyzed through tubing to an analytical apparatus, the adsorbed n a t e r will evaporate or dissolve. Many hours of purging may bc neccssary before this water is removed to the equilibrium level and a representative sample waches the apparatus. ,4s the moisture content of the sample varies, the water adsorbed on the surfaces of the connecting lines and fittings will also vary. I t is obvious that all lines leading from the sample container to the apparatus should be
short and that a continuous flow system has a definite advantage over an intermittent one. I n light of this and other factors, the available methods for trace water determination will be discussed. I n one commercially available apparatus, the water reacts with phosphorus pentoxide which is regenerated electrolytically, and the current required is indicated on a meter calibrated in terms of water concentration (3, 7 , I O ) . This apparatus can be operated continuously and is considered useful down to 1 p.p.m. or less. Hoivever, i t cannot be employed directly for liquids, and olefins, particularly diolefins, may polymerize, plugging the cell. Cloud point methods can be developed for nonpolar solvents such as hydrocarbons, but these are usually intermittent and require ?.;tensive calibration since the cloud point depends on other sample componcnts as well as water. Dew point determination of Tvater in gascs is limited to systems containing nothing condensable evcept water. Continuous determination of m-ater in some systems by infrared is feasible (6). The loner limit would depend on the system. l’he Karl Fischer method is useful in the 5 p.p.m. region and with great care can be applied down to 1 or 2 p.p.m, However, a t these low levels the accuracy is poor. Gas-liquid chromatography (GLC) has proved useful as a tool for trace analysis of many types of samples, but direct determination of traces of n-ater by this method is complicated by broad and unsymmetrical water peaks which make area measurement difficult. Kevertheless, some light hydrocarbon systems have been analyzed by the
direct method ( 2 ) . Kyryacos and Boord ( 8 ) avoided the poor n t e r peaks by employing a calcium carbide reactor to convert the water to acetylene. Their work, which is not available in det,ail, was concerned with cool flame oxidation product analysis. Dusn-alt and Brandt (4) and Sundherg and llaresh (9) reported carbon and hydrogen methods in 15-hich the ivater formcld by oxidation of the sample n-as later measured as acetylene by GLC. l h e oxidation was not instantancwus, and their operation was cyclic. hckerinan and Stewart, (1j used t,he calcium carhide reaction technique to determine tracw of n-ater in hydrocarbons. The carbide bed was placed a t the inlet of t’he GLC column, and the carbide reaction \vas intermittent. I n the present paper, a nen- technique vi11 be described in which the carbide reactor is kept scparate from the rest of the apparatus SO that it, can be attached t o the sample container and operated continuously. .I sensitive hydrogen flame ionization drtcctor \vas uscd for most of the work, replacing the less wnsitive thermal conductivity dptcctor uscd in the previous work citcd. APPARATUS
I n the early stages of this study, a thermal conductiTity detector was used to determine n-ater (as acetylene) in benzene. Later a flame ionization detector was employed, mainly for C4 hydrocarbons. The GLC units were of usual construction and \yere built on the premises. The 01-en consisted of a length of aluminum pipe fitted Il-ith heaters, which served as an air bath for VOL. 34, NO. 7, JUNE 1962
749
