Kerogen presumably forms early during sedimentation, locking into its matrix biological debris, which is subsequently reduced. Kerogen, therefore, becomes not only immobilized with respect to migration; but the system is closed to outside contamination subsequent to formation. Kerogen is potentially the most reliable source of biological precursor information. A number of questions come to mind. Which of the two fractions reflects more faithfully the relative abundance of pentacyclic terpanes precursors originally laid down in this sediment? The extractable fraction presumably has been modified by maturation; whereas, the kerogen presumably does not change much with time but could be greatly modified by pyrolysis. But, does pyrolysis modify the relative abundance of surviving biomarkers with respect to those originally present in the kerogen matrix? I t is difficult to answer these questions. Note from Figure 1 that the 5-ring C27 and C29 terpanes, which are of low abundance in the extractable fraction, are, relatively speaking, dominant in the pyrolysis fraction. The first impulse is to suggest that the c27 and C29 terpanes are a product of pyrolysis, i.e., pyrolytic loss of bridgehead or gem-dimethyls from, say, a C30 terpane. There are a t least two reasons why this is probably not the case. First, the negligible concentration of Cas terpanes is not consistent with a statistical ihermodynamic loss of labile C1 or C2 groups from a C30 or higher terpane. Further, the (228 terpane found in the pyrolysis oil is a very specific one, Le., unusually long retention times, also found a t very low levels in the bitumens solvent extract fraction of Green
River shale. No other cz8 terpanes are detected in either fraction. The second reason that comes to mind is that the complete mass spectra of the terpanes found in the pyrolysis oil show no evidence for unusual methyl loss phenomenon; i.e., the bridgehead and gem- dimethyl are apparently in place, a t least as far as the A/B rings are concerned. In other words, the mass spectra look perfectly normal compared to known terpanes. This is also verified by the unusual fact that all terpanes found on the extract fraction are also found in the pyrolysis fraction, apparently without exception. The only conclusion, it seems, is to say that the terpanes which have survived pyrolysis rather than those extracted reflect more faithfully the distribution and identity of terpanoids originally laid down in the sediment. LITERATURE CITED (1) R. H. Dott and M. J. Reynolds, "Source Book of Petroleum Geology", The American Association of Petroleum Geologists, Tulsa, Okla., 1969. (2) B. Tissot, B. Durand, J. Espitolie. and A. Combaz, Am. Assoc. Pet. Geol. Bull., 58, 494-506 (1974). (3) G. Eglinton, Adv. Org. Geochem., 29-48 (1971): H. R. v. Gaertner and ti. Wehner, Ed., Pergamon Press, Oxford, Braunschweig. (4) J. J. Cummins, F. G. Doolittle, and W. E. Robinson, U.S.Bur. Mines, R17924, 1974, 18 pages. (5) E. J. Gallegos, Anal. Chem., 43, 1161 (1971). (6) B. Balogh, D. M. Wilson, P. Christiansen, and A. L. Burlingame, Nature (London), 242, 603 (1973). (7) B. J. Kimble, J. R. Maxwell, R. P. Philp, G. Eglinton, P. Albrect, A. Ensminger, P. Arpino, and G. Ourisson, Geochim. Cosmochim. Acta, 38, 11651181 (1974).
RECEIVEDfor review December 30, 1974. Accepted April 18, 1975.
Radiofrequency Cavity ton Source in Solids Mass Spectrometry D. L. Donohue and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville, VA 22903
A radiofrequency discharge in argon is used to sputter and ionize solid samples for analysis. Metals machined to act as a cavity electrode can be analyzed directly. Samples may also be deposited from solution for subsequent elemental analysis of the residues. The rf discharge allows analysis of both conductors and nonconductors. A glass electrode is used as a cavity electrode for application of multielement solutions. The precision obtainable by electrical scans was about f10% RSD. Lowest detection limit is -10-100 ng.
The high frequency gas discharge has been known for some time. Early work by Wehner ( I ) in 1955 and later by Anderson et al. (2) and Davidse and Maissel ( 3 ) sought to define the electrical properties of such a discharge. Jackson ( 4 ) has reviewed the properties of the high frequency gas discharge, with emphasis on rf sputtering processes. Mass spectral studies of ionic species present in an rf discharge have been described by several workers, including Gilkinson et al. ( 5 ) and Coburn and Kay (6, 7). The latter studies, of particular interest from an analytical viewpoint, involved mass spectral characterization of insulating samples formed in a planar diode sputtering arrangement. 1528
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Recently, a DC gas discharge source has been developed (8-10) which samples ions from a hollow cathode discharge. The DC source in Ref. 10 is almost identical in design to
the rf source described here. The spectra obtained from each are also similar, with certain important exceptions. The work presented here will describe the design, operation, and applications of an rf cavity ion source. EXPERIMENTAL Design. T h e rf cavity ion source (CIS) is shown in Figure 1. Discharge gas enters the mass spectrometer source enclosure through a length of %-inch copper tubing. A flexible stainless steel tubing (Cajon) couples the copper tubing to port A by combination Swagelok and Ultra-torr fittings. The gas flows through the glass tube into the cavity electrode, B. Ions formed in the discharge pass through the small (0.040-inch) hole in the bottom of B, where they are accelerated into the mass spectrometer. A 1-inch diameter stainless steel wire ring, C, acts as the rf counter electrode, which is positioned by an external manipulator (the normal MS-702 electrode controls). Port D is a quartz viewing window which allows optical sampling of the discharge. T h e cavity electrodes are typically Yd-inch long, %-inch o.d., 13/&-inch i.d., and %-inch deep. Metal samples are machined to this size, while glass electrodes have been fabricated from 'i,-inch tubing.
A
nr; ilc
Figure 1. The rf cavity ion source (CIS) (A) gas inlet, (E)cavity electrode, (C) rf counter electrode, (D) quartz optical window, (E) heat sink, (F) liquid N2 cold finger, (G) electrode block, (H) insulators AEI
Provision has been made to cool the electrode by means of a liquid nitrogen cold finger and heat sink as shown in Figure 1. Such cooling was not routinely necessary, however, for the low rf power levels employed in this study. Source Modification. T o accommodate this and other gas discharge sources which are of interest in our laboratories, the standard AEI source compartment was replaced by a larger source, designed and constructed in our departmental machine shop. T h e source is a stainless steel cylinder, 6-inch i.d. and 7 inches deep. A pumping duct (5-inch i d . , 11 inches long) extends directly from the bottom of the source to an NRC type 12935-4 gate valve. An NRC type 316-4 baffle separates the gate valve from a Varian type VHS-4 oil diffusion pump of 1200 l./sec pumping speed. This pump is backed by a high capacity Kinney rotary pump, type 3153 A. By appropriate bypass valves and pipework, this pump is also used to rough out the source. T h e evacuation time is reduced by a factor of ten over the standard AEI system. Five ports are located on the source. Two are for AEI spark source electrode manipulators, P third is located on top of the source for viewing the spark area before the ion exit slit, and two others are available for auxiliary purposes, such as electrical and gas feed through. Three ports on the pumping duct allow connection of the rotary roughing pump, an ionization gauge, and a thermocouple vacuum gauge. Those interested in more detailed plans and dimensions for the source modification may write directly to one of the authors (W.W.H.). T h e new source and pumping system have several advantages. T u r n around time is greatly reduced. Ninety seconds after evacuation begins, the source is ready for spark or discharge analysis. T h e high pumping speed allows the use of flowing gas discharge systems without causing a large rise in analyzer pressure. Even in spark source mass spectrometry, the new system is advantageous in that it responds more rapidly to outgassing and other transient pressure rises. Parameter Measurement. Gas pressure inside the cavity ion source was measured with a thermocouple gauge (NRC type 531) attached to an auxiliary port (not shown in Figure 1) of the glass discharge tube body, about 2 inches from the discharge. Argon was used as the discharge gas for all the experiments in this study. Operation of the discharge at internal pressures of more than 1 Torr was not feasible because of the unacceptably high pressure which resulted in the mass spectrometer analyzer. Pressure in the source chamber outside the CIS was measured with an ionization gauge (AEI insertion type Alpert gauge). A source pressure of 5 X Torr was the upper operating limit. The rf voltage applied to the electrodes was supplied by the spark circuit of the AEI MS-702 spark source mass spectrometer. A pickup coil placed near (-6 inches) the spark generator was used t o monitor the rf by an oscilloscope (Tektronix Model R564B). This permitted the output voltage t o be maximized using the tuning capacitor in the rf tank circuit, and also allowed monitoring of the duty cycle (pulse length, pulses-per-second) of the rf voltage. T h e temperature of the cavity electrode could be monitored with a thermistor (Fenwall type 3ABP1) which passed through a hole drilled in the sample holder and touched the outside of the hollow electrode. The bulk electrode temperature stayed below 100 O C , even without liquid N:! cooling. Acceleration voltage was 10 kV, with typical rf conditions of 30
200
40C TLBE
600
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UT
339
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Figure 2. Effect of argon pressure on ion flux ( % full scale deflection)
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