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A. C. Simmons, Lea M. Taylor, and Maxwell Nager, Houston Research Laboratory, Shell Oil Ca., Houston 1, Tex.
instances it is desirable t o the bands from gas-liquid :hromatography (GLC) separations as iydrocarbon bands, when identification )f components by trapping and subsequent analysis by spectroscopic meth2ds is of major importance. At other times, the identity of the emerging :ompounds may be known and the :mphasis then is on an accurate quantitative analysis with a minimum of :alibration. Detection of hydrocarbons as C02 ifter combustion over copper osideN MANY
1 detect
Figure 1. tography
using infrared (S), titration (f), and thermal conductivity &-has been demonstrated. Detection as COSprovides uniform response to the amount of carbon present for all compounds. Complete oxidation of CIt o C, hydrocarbons by combustion over hot copper oxide has been prescnted (2, 3, 7). Conversion to Cot in conjunction with thermal conductivity detection provides two distinct advantages in GLC separations of hydrocarbon samplrs. Extensive calibrations due to differences in thermal conductivities of
various hydrocarbons can be eliminated because all known hydrocarbons can be converted quantitatively t o carbon dioxide (and water), and weight percentages of these hydrocarbons can he obtained directly from the area of the resulting CO, peak (after removal of water with Dehydrite). Secondly, there is a net increase in sensitivity, resulting from the larger number of COS molecules seen by the detector, although this is offset somexhat by the lower sensitivity of CO? compared to that of most hydrocarbons.
Portable apparatus far gas-liquid chrama-
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Figure 2.
Schematic flaw diagram of apparatus
Figure 3. sample
Chromatograms of
petroleum alkylate
VOL. 32, NO. 6. MAY 1960
731
A compact, portable COrconversion apparatus has been developed which can be attached t o GLC units. It permits the separated bands t o be detected as hydrocarbon bands which can be trapped for positive identification or more detailed analysis by other methods. Alternatively, the hydrocarbon bands emerging from the GLC unit may be directed into the portable C o r conversion unit, converted t o CO1, and detected with a separate detector. The detector shown in Figure 1 uses a Sargent Model 5-36400 micro combustion furnace. The furnace will heat to the desired operating temperature, 725’ to 825’ C., in 10 minutes; furthermore, it cools rapidly and reduces down time for changing combustion tubes to less than one-half hour. The sensing device is a flow-through thermal conductivity cell (6) housed in a metal box that is insulated with glass wool and attached to the side of the combustion furnace base. This box also contains components of the Wheatstone bridge circuit as well as terminals for connecting the direct current power supply and recorder. The flow diagram (Figure 2) shows that effluent from the GLC column enters the CuO combustion tube through a four-port Circle Seal valve (Circle Seal Products Co., Pasadena, Calif.). This valve and the S/la-inch copper tubing used t o connect i t with the exit of the GLC apparatus are wrapped with heating tape, and both are held a t 20’ t o 25’ C. above the column operating temperature. The combustion tube can be regenerated periodically by rotating the four-port valve 90’ to admit dry air. With the valve in this position effluent from the column vents through the valve and allows trapping of hydrocarbons for additional analysis. Combustion products emerging from the combustion tube pass through a
glass tube filled with Dehydrite to remove water before detection in the thermal conductivity cell. Placement of a n Ascarite-containing absorber between the sample and reference side of the thermal conductivity cell provides some unique features. This provides a Con-free stream of helium for the reference side of the thermal conductivity cell, and gives a continuous monitor on the efficiency of the combustion tube. Incomplete combustion which leaves carbon monoxide or hydrocarbon in the effluent gas shows up as a negative peak on the recorder trace. A positive peak followed by a negative peak on the recorder trace is clear evidence that the CuO should be regenerated (or replaced) or the sample size is too large. An unstable base line indicates that either or both Dehydrite and Ascarite absorbers should be replaced. These absorbers and the one used in the regeneration air line are connected to the apparatus with l/4-inch Swagelock fittings having Teflon ferrules. The combustion tube is filled with about 7 grams of 14- to 50-mesh CuO (wire form), pretreated with aqueous ferric nitrate so that the dried CuO of iron. The comcontains 1 weight bustion furnace is heated to 725’ to 825’ C.; about 0.25 to 0.30 ml. of sample can be injected before it is necessary to regenerate the CuO. The CuO must be replaced after about 10 to 15 regenerations, because it appears t o sinter, causing an increase in pressure drop across the combustion tube. An Ascarite absorber with a total volume of 15 ml. and a Dehydrite tube of 6-mm. glass tubing 15 cm. long are sufficient for many runs. Tests have shown that the portable COrconversion apparatus can be connected t o the end of a GLC unit so that the separated bands can be detected aa both hydrocarbons and Con. I n some cases this procedure can lead to poor
results due to band tailing caused by the design of the GLC unit detector. A flow-through type detection is optimum for this service, although certain d 8 u sion-type detectors have proved satisfactory. Tests made with the popular Gow-Mac Model 9193, a diffusion-type detector, have shown wide variations in band tailing with no obvious difference in geometry. However, any system can be tested easily for tailing by injecting a sample of COZand observing the band in both the GLC detector and the COrconversion unit detector. Typical chromatograms of a petroleum alkylate sample with and without Con conversion are shown in Figure 3. Sensitivity is increased nearly threefold with CO2 conversion. Therefore, sample volumes charged were reduced to 30% of those charged when hydrocarbon detection is used. ACKNOWLEDGMENT
The authors express their appreciation to Shell Oil Co. for permission to publish this work. LITERATURE CITED
(1) Blom, L.,Edelhausen, L., Anal. Chim. Acta 13, 120 (1955); 15, 559 (1956). (2) Brooks, F. R., Lykken, L., Milligan,
W. B.. Nebeker. H. R.. Zahn., V... ANAL. CHEM:21,1005
