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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Determination of Sub-nanogram per Gram Quantities of Light Hydrocarbons (C2-C9) in Rock Samples by Hydrogen Stripping in the Flow System of a Capillary Gas Chromatograph Rainer G. Schaefer,* Bernd Weiner, and Detlev Leythaeuser Kernforschungsanlage Jülich GmbH, Programmgruppe für Erdól und Organische Geochemie, Postfach 1913, D-5170 Jülich Republic of Germany
A method has been developed for measurement of sub-nanogram per gram quantities of light hydrocarbons (molecular range C2 to Cg) in rock samples. The method combines both extraction of hydrocarbons from the rock and subsequent capillary gas chromatography into a single-step procedure carried out in a closed gas-flow system. The stripping gas (hydrogen) acts also as the carrier gas in the capillary column. Splitless sample introduction ensures high sensitivity. The lowest detectable quantity is about 10~11 g butane, representing 0.01 ppb for the maximum weight of 1 g. A backflush
technique removes higher boiling components during the analysis. Total analysis time is about 50 min. The method is applied for geochemical analysis of the light hydrocarbons in rock and fluid samples from oil and gas exploration wells.
Light hydrocarbons (molecular range C6). Finally, Le Tran (8) isolated occluded light hydrocarbons from carbonate rocks by acid digestion of the mineral matrix. Most of these light hydrocarbon extraction procedures have analytical restrictions which seriously limit their application for routine work in petroleum geochemistry, e.g., large sample size requirements of 10-100 g (2, 3, 5), compositional fractionation effects due to partitioning between phases (headspace analysis), restricted over-all sensitivity with respect to sample size (1 ppb per weight (7) or 5 ppt in 100 g of rock (3), respectively), or excessive time requirements (2-h extraction time (7)). Most of these problems are overcome by the new method described in this paper. It represents an innovation to the extent that it combines both removal of light hydrocarbons (molecular range C2 to C9) from the rock sample at ambient temperature and their capillary GC analysis into a single-step 0003-2700/78/0350-1848S01.00/0
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rapid procedure. This is achieved by a flow system designed in such a manner that hydrogen functions both as carrier gas for the GC capillary and as auxiliary gas stripping the light hydrocarbons from the rock sample. In this way the analysis can be carried out on a microscale and the effects of fractionation during extraction and transfer procedures are minimized. The concept of our method stems from the so-called thermovaporization technique of Jonathan et al. (9) designed to cover the molecular range C6 to C15. Their method combines the extraction of small-size rock samples (ca. 200 mg) at elevated temperatures followed by splitless capillary GC analysis in a gas-flow system utilizing helium both as stripping and carrier gas. However, Jonathan et al. take no precautions which might prevent the introduction of pore water into the column, a factor which severely reduces the efficiency of the chromatographic separation. Compounds are identified throughout the chromatogram only by retention times and not by Kovats indices as in the method described below for the C2 to C8 molecular range. Furthermore Jonathan’s procedure does not include a backflush control, and the cold trap is installed in the GC oven, both features not being favorable with respect to the time required and the accuracy of the analysis of C2 to C9 hydrocarbons. EXPERIMENTAL Sample Preparation. Rock samples for light hydrocarbon
analysis should be stored in sealed containers, e.g., gas-tight tin cans. Additionally, storage at deep freeze temperatures to reduce evaporation losses is recommended. Samples containing drilling mud are first washed thoroughly with cold water. Excessive water is then removed with filter paper before sample introduction. Sample preparation consists of crushing the sediment in a steel mortar and sieving followed by introduction of the 0.6- to 2-mm particle fraction into a glass extraction tube. The weight of the rock sample analyzed varies between 0.2 and 1 g according to density and expected light hydrocarbon yield. The glass extraction tube is prepared in the following way (Figure 1). The upstream end of the tube is closed with a plug of glass wool. At the downstream end of the tube, a second plug of glass wool and a small amount of dry CaCl2 (about 4-cm length) prevent the entry of both water and dust particles into the GC capillary during subsequent hydrogen stripping. During sample preparation, extreme caution is essential in order to avoid contamination. To this end all glassware is pretreated with chromic acid while glass wool and CaCl2 are heated to 400 °C for 2 h before storage in glass stoppered bottles. Prior to introducing the rock particles, flushing the glass tube plus CaCl2 with pure oxygen at 500 °C for about 5 min further reduces the hydrocarbon blank values for the whole procedure. Care should be taken that the sample preparation sequence is accomplished in a short and reproducible time. On the other hand, as shown later in a sample aging experiment, evaporation losses of hydrocarbons (except methane) at this stage are minor, compared to those occurring during sample collection and storage. For this reason, in our technique methane is omitted from quantitative analysis. Hydrogen Stripping and Gas Chromatographic Analysis. The analytical apparatus, shown schematically in Figure 2, consists C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Conditions for Hydrogen Stripping and Gas Chromatographic Analysis extraction
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aba
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c
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Figure 1. Preparation of glass tube for hydrogen stripping of light hydrocarbons from rock samples. Dimensions are given In mm. (a) glass wool, (b) CaCI2, (c) rock sample (particle size between 0.6 and 2.0 mm)
stripping gas flow stripping time temperature sample weight sample particle size
phase
detector external standard
of a capillary gas chromatograph which has been substantially modified. Modifications are made to the sample inlet and the carrier gas supply, and include the installation of a backflush system. In detail, the injection port unit has been removed completely. Instead, the front length (ca. 25 cm) of the capillary column, bent into a U-shape, extends outside the GC oven, itself serving as a cold trap when immersed in liquid nitrogen (No. 10 in Figure 2). For the light hydrocarbon extraction step, the glass tube containing the sample (No. 9 in Figure 2) is placed between the cold trap and carrier gas supply. The sample is stripped by passing a measured volume of hydrogen through the tube. Because of its low viscosity, hydrogen is preferred as stripping and carrier gas. However, to reduce the hydrocarbon blank values of the whole procedure to about detection limit, pure-grade hydrogen (99.999%) has to be further purified by passage through a charcoal filled liquid nitrogen cold trap (No. 2 in Figure 2). For the same reason, the flow rate of the hydrogen is controlled by an all-metal precision flow controller (Siemens). Since the gas flow is always held at a known rate, the volume of the stripping gas is deduced directly from the flushing time which in our routine procedure is 10 min. Generally, hydrogen stripping is carried out at room temperature. Since only a fraction of the light hydrocarbons present in the rock is recovered by this method, it is essential that measurements be made under constant conditions with respect to temperature, gas flow, and volume. The development of the backflush technique employed here is based on Deans’ method of column switching (10). For the backflush system the capillary column is cut into two lengths (ca. 1:2 ratio). The carrier gas flow is reversed in the initial length after the last desired compound has passed through; separation continues in the second part. In order to prevent peak-broadening by diffusion in the T-shaped capillary connection (No. 13 in Figure 2), a minimum dead volume design modified after Schomburg et al. (11, 12) is used. A reamed out* 1/i6-inch Swagelok T-union has been used for this purpose. Backflush is started by opening
0.6 to 2
mm
GC analysis
column type
length diameter (i.d.) carrier gas flows through column first part (15-m length) second part (30-m length) reversed flow temperatures column (programmed)
Figure 2. Modified capillary gas chromatographic system for light hydrocarbon analysis of rock samples by hydrogen stripping. (1) hydrogen inlet, (2) hydrogen purification trap, (3) flow controller, (4) solenoid valve, (5) needle valve, (6) pressure regulator, (7, 8) pressure gauges, (9) sample tube; (10) cold trap, (11, 12) capillary column, (13) T-union, (14) flame ionization detector, (15) gas loop, (16) six-port valve, direction of gas flow. (17) inlet for external standard, (18) valve. ->- reversed flow during backflush
hydrogen 10 mL-min"1 10 min ambient (22 0 C)
