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Anal. Chem. 1981, 53,938-940
tertiary alcohol, dehydrated to yield mostly l-methylcyclopentene. The LiAIHI apparently serves as a proton source, since similar reaction with the Pt alone yielded unsaturated products. As an example, a-terpineol and isopulegol reacted with Pt to yield only l-methyl-4-(1-methylethyl)benzene whereas they produced the corresponding cyclohexane with Pt/LiAlH4. The acetate esters of several compounds listed in Table I were allowed to react under the same conditions. The products obtained were the same as those generated from the corresponding alcohols, but the reaction was often incomplete, and the ratios of two products were not as reproducible as those obtained with the alcohols. This procedure provides key information for identifying unknown, naturally occurring compounds; this procedure was a key factor in the recent identification of the terpene-like sex attractants of the citrus (ll),Planococcus citri (Risso), and Comstock (12),Pseudococcus comstocki (Kuwana), mealybugs (Table I, 14 and 15,respectively). Mass spectral analysis of the reaction products is a very sensitive and valuable tool in the determination of chain branching, ring
structure, etc. in complex hydrocarbons that comprise the skeleton of such compounds.
LITERATURE CITED Heath, R. R.; McLaughlln, J. R.; Tumlinson, J. H.; Ashley, T. R.; Doolittle, R. E. J. Chem. Ecol. 1979, 5, 941. Beroza, M.; Sarmiento, R. Anal. Chem. 1963, 35, 1353. Beroza, M.; Sarmiento, R. Anal. Chem. 1964, 36, 1744. Blerl, B. A.; Beroza, M.; Ashton, W. T. Microchlm. Acta 1969, 3,837. Kepner, R. E.; Maarse, H. J. Chromatogr. 1972, 66, 229. Adhikary, P. M.; Harkness, R. A. Anal. Chem. 1969, 4 7 , 470. Stanley, G.; Kennett, B. H. J. Chromatogr. 1973, 75, 304. Stanley, G. J. Chromatogr. 1979, 178, 487. Nlgam, I. C. J. Chromatogr. 1966, 24, 188. Tumlinson, J. H.; Hardee, D. D.; Gueldner, R. C.; Thompson, A. C.; Hedln, P. A.; Minyard, J. P. Science 1969, 166, 1010. Bierl-Leonhardt, B. A.; Moreno, D. S.;Schwarz, M.; Fargerlund, J. A.; Pllmmer, J. R. Tetrahedron Left., in press. Bierl-Leonhardt, B. A.; Moreno, D. S.; Schwarz, M.; Forster, H. S.; Pllmmer, J. R.; Devilbliss, E. D. Life Scl. 1960, 27, 399.
RECEIVED for review October 24, 1980. Accepted February 5,1981. Mention of a commercial product in this paper is for information only and does not constitute an endorsement of this product by the USDA.
Potassium Perchromate Standard for Determination of Paramagnetic Spin Concentration, g Values, and Magnetic Moments of Fossil Fuels N. S. Datal" and M. M. Suryan Department of Chemistty, West Virginia University, Morganto wn West Virginia 26506
M. S. Seehra Department of Physics, West Virginia University, Morgantown, West Virginia 26506
We wish to report here that K3Cr08 (potassium perchromate), a Cr(V):3d1paramagnetic compound, can serve as a versatile internal standard for measuring paramagnetic spin concentration and g values of organic free radicals by electron paramagnetic resonance (EPR) spectroscopy and for determining magnetic moments by static magnetic susceptibility techniques. Our search for a new EPR standard resulted from the difficulties we experienced in EPR studies directed at characterizing shales and their pyrolyzed products. Retcofsky and co-workers (I-3), Petrakis and Grandy (4,5), and Singer and Lewis (6, 7) have shown that many fossil fuels can be characterized by quantitative measurements of the concentrations and g values of the free radicals ubiquitously present in most fuels. Also, recent studies by Retcofsky et al. (3)and Kwan and Yen (8)strongly suggest the overall line shape of the free radical EPR signal contains important structural information. An important analytical aid in these studies has been the use of an internal standard, a paramagnetic compound whose EPR signal intensity, line position and line shape could be directly related to the a n a l e ' s concentration, g value, and line shape, respectively. During our EPR studies of shales and related samples, it was found that the signals from these samples overlapped strongly those of the internal standards commonly available, such as DPPH (l,l-dipheny1-2-picrylhydrazyl) (9),nitroxides (9),and pitch (9, IO). Moreover,these EPR standards are known to be unsuitable for accurate concentration studies because of the uncertainty in their purity and, in particular, in their diamagnetic content. These considerations suggested that an inorganic paramagnetic compound with g value close to 2.0 and well-defined structure and purity might prove to be a better internal standard. We shall
argue here that perhaps K3Cr08is such a compound and that it is a more versatile EPR standard than those in current use.
EXPERIMENTAL SECTION Sample Preparation. K3Cr08can be prepared as rose-colored crystals by mixing KOH, K3Cr04,and cold 30% HzOzessentially as described by Riesenfeld in 1905 (11).I h purity can be verified with EPR (I2), wet chemical methods (12),or X-ray powder diffraction patterns (13). K3Cr08,like DPPH, can be stored in a refrigerator for several years. In strongly basic (pH 213) solutions K3Cr08is fairly stable and can be used for g value and magnetic field scan calibrations for several weeks. For accurate spin-concentration measurements, however, K3Cr08solutions should be made fresh and, of course, single crystals would be required for measurements below 273 K. Apparatus. EPR measurements were made at X-band frequencies (-9.4 GHz) with a Varian E-3 spectrometer, equipped with gas-flow type variable-temperature accessory and a Hewlett-Packard frequency counter. Magnetic susceptibility studies were carried out over the 1.2-343 K range with a homemade Faraday balance described recently elsewhere (14). RESULTS AND DISCUSSION Typical EPR spectra of K3Cr08solutions, along with those of the most commonly used standard, DPPH, and of shalederived kerogen are shown in Figure 1. It is seen that the EPR signal from DPPH overlaps and hence distorts the line shape of the kerogen signal. The use of pitch or the nitroxides is beset with the same problem. On the other hand, the K3Cr08 signals cause neither overlap nor distortion. The isotropic g value for K3Cr08 was found to be 1.97120 f 0.000 05 at ambient temperatures (-300 K). In fact variable-temperature measurements on single crystals showed that
0003-2700/81/0353-0938$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
g.1.9953
1
Figure 1. Typical EPR spectra of K3Cr0,, DPPH, and shales under (a) low gain and (b) high gain.
the g value is constant to within ic0.0005 over 77-400 K, Because of this result, derived from five different measurements (15),K3Cr08satisfies an important criterion for a good EPR standard. For g-value measurements, we found it advantageousto use the first two (low-field) b3Crhyperfine components, instead of the central, strong (52Cr)signal. This is because these lines are closer to and yet well resolved from the free radical signal from fuels. The effective g value for the lowest field component is found to be 1.9!35 30 f 0.000 05 an marked in Figure 1. Moreover, the separation of precisely 20.0 G between the two lowest field 63Crlines provides a convenient and simultaneous calibration for mlagnetic field scans. It is noted that none of other standards except nitroxides provides such a convenient field scan standard. However, as mentioned above, nitroxides have the disadvantage that their central line would in general be too close tlo the signal from the fuels to be a desirable standard. It may be argued that the overlap problem with other standards may be overcome by using a dual-sample cavity (9), along with a correction for the difference in the magnetic field strength experienced by the sample and the standard. Besides being expensive, the dual sample cavity cannot be used with some EPR spectrometers such as the Varian E3 and E4 models, the only type available in many laboratories such as ours. Moreover, the signal line width for K9Cr08,0.75 f 0.05 G, is significantly smaller than that of DPPH (-2 G), showing that K3Cr08 has the potential for yielding more accurate results than DPPH, even when the use of a dual sample cavity is feasible. To examine its suitabillity as a standard for spin concentration measurements, we investigatedthe temperature of the magnetic susceptibility of K3Cr08,both by EPR and Faraday balance measurements. For EPR measurements care was taken to ensure that the amount of sample was small enough (5 mg) that the EPR cavity was not loaded. As has been shown elsewhere (16), and also found here. if a much larger sample is used, then the ISPR signal intensity is not strictly proportional to concentration. A good rule of thumb is that the amount of sample should be such that the current through the detector crystal should not be affected when the resonance
939
absorption takes place. It is noted that these considerations apply to DPPH or any other magnetically concentrated material. A plot of the inverse of the EPR signal intensity (taken to be proportional to the area under the absorption curve) against the absolute temperature, T , was found to be linear within the relative-experimentalaccuracy (f5% 1. This temperature dependence followed the Curie-Weiss law, x = C / ( T + e), with the Curie-Weiss temperature 0 = 0 f 3 K. Since such linear behavior of x is an important criterion for a standard for paramagnetic spin concentration, detailed measurements of x were made with a Faraday balance. Again, the plot of x-' vs. T was found to be linear and established the CurieWeiss behavior of x over the temperature range of 1.4-343 K. The Curie-Weiss temperature, 0, was determined to be 0 = 2.7 f 0.1 K. The measured molar susceptibility was consistent with one unpaired spin per K3Cr08molecule and the magnetic moment was in excellent agreement with that found from the EPR g values (17). The fads that K3Cr08does not order magnetically down to 1.4 K (17)with the behavior of x-lfor K3Cr08vs. T is linear make this material a good new standard also for static magnetic susceptibility measurements. In conclusion, the distinguishing features of K3Cr08as an EPR standard are (i) Its paramagnetic (Curie-Weiss) behavior and hence suitability as a spin concentration standard from 1.4 to 400 K, not easily possible with DPPH and pitch since both undergo phase change at low temperatures. (ii) A sharp EPR signal (AHpp= 0.75 f 0.05 G)with g = 1.97120 f 0.00005, together with four weaker hyperfine components 20.0 f 0.1 G apart, providing simultaneousg and magnetic field scan markers for organic free radical studies without use of a dual-sample cavity. (iii) Its ease of obtaining in pure, well-defined structural form, not possible with DPPH or pitch. Pitch is generally a mixture of many structures and commercial DPPH usually contains a significant amount of an aromatic solvent such as benzene, hence additional spectroscopic measurements are needed before they can be used as spin concentration standards. Finally, we note that the usefulness of K3Cr08is not restricuted to fossil fuels only. It can be used in most situations where other standards such as DPPH can be, perhaps with better results. It must be noted that K3Cr08decomposes at 170 "C and may explode at 178 "C (18). In fact, we do not recommend its use above 100 "C.
ACKNOWLEDGMENT We wish to thank Steve Lamey and Mike Paris of Morgantown Energy Research Center for help in the sample characterization and J. M. Millar for growing K3Cr08single crystals.
LITERATURE CITED (1) Retcofsky, H. L.;Stark, J. M.; Friedel, R. A. Anal. Cbem. 1966, 40, 1699-1704. (2) Retcofsky, H. L.; Thompson, G. P.; Hough, M.; Friedel, R. A. ACS Symp. Ser. 1976, No. 71, 142-154. (3) Retcofsky, U. L.;Thompson, G. P.; Raymond, R.; Friedel, R. A. Fuel 1975, 54, 126-128. (4) Petrakis, L.;Grandy, D. W. Anal. Chem. 1976, 50, 303-308. (5) Petrakis, L.; Grandy, D. W. Geocblm. Cosmocbim. Acta 1980, 4 4 , 783-768. (6) Singer, L. S.;Lewis, I. C. Carbon 1964, 2, 115-120. (7) Singer, L. S."Proceedings of the Fifth Conference on Carbon"; Pergamon Press: New York, 1963; p 37. (8) Kwan, C. L.;Yen, T. F. Anal. Cbem. 1979, 1225-1229. (9) P o o h C. P., Jr., "Electron Spin Resonance"; Wiley-Interscience: New York, 1967; Chapter 14. (IO) Vana, N. J.; Unfried, E. J . Magn. Reson. 1972, 6 , 655. (11) Riesenfeid, E. H. Chem. Ber. 1905, 38, 4068-4074. (12) McGarvey, B. R. J. Cbem. Phys. 1962, 37, 2001-2004. (13) Stromberg, R. Acta Cbem. Scand. 1983, 77, 1563-1566.
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Anal. Chem. 1981, 53, 940-942
(14) Seehra, M. S.;Jagadeesh, M. S.Phys. Rev. 1979, 820, 3897-3902. (15) Miiiar, J. M. B.S. dissertation, West Virginia University, Morgantown, WV, 1980. (16) Seehra, M. S. Rev. Sci. Instrum. 1968, 39, 1044-1048. (17) Dalai, N. s.; Millar, M. J.; Jagadeesh, M. s.; Seehra, M. s. J. Chem. Phys. 1981, 74, 1916-1923. (18)Mellor, J. W. “A Comprehensive Treatise on Inorganic and Theoretical
Chemistry”; Longmans, Green and Co.: London-New York, 1931; Vol. XI, p 356.
RECEIVED for review December 22,1980. Accepted February 12, 1981. T K research ~ was suppofied by the E~~~~ ~~~~~~h Center Of West Virginia University, Morgantown, wv.
Pressure Control of a Gas by a Calculator-Operated Mercury Piston Tyler B. Coplen
U,S. Geological Survey, 432 National Center, Reston,
Virginia 22092
Light stable isotope ratio mass spectrometers can measure small differences in isotope ratio by comparing the isotope ratio of a sample with that of a standard ( I ) . For maximum precision, the major ion beam intensity of the sample and the standard should be identical. Adjustment of these intensities is accomplished by adjusting the quantity of gas flowing into the spectrometer through capillary leaks, both of which are connected to their own gas reservoirs. The gas pressure in each reservoir is adjusted by raising or lowering a variablevolume mercury piston. Other devices including servo-operated bellows (2) and sliding pistons with vacuum-tight elastomer O-rings ( 3 ) have also been used to control gas pressure. For conversion of our carbon dioxide double-collecting isotope ratio mass spectrometer from manual operation to unattended multisample operation, a method was devised to adjust the gas volumes automatically. This apparatus (Figure 1) can be used whenever pressure or volume of a gas must be adjusted under automatic control and the presence of mercury vapor can be tolerated. This article discusses control of only a single mercury piston, but two or more can be operated by adding only two control valves for each piston. For elimination of bellows and elastomer O-rings in the spectrometer gas handling system, conventional air-actuated bellows valves were modified so that they controlled satisfactorily variable-volume mercury pistons. The air actuators are controlled by electrically operated solenoid valves, which are controlled by a calculator or computer by means of the electronic circuitry discussed later in this article. To complete the feedback loop for adjustment of the major ion beam intensity, we connected a digital voltmeter between the major ion beam electrometer output and the calculator or computer. This is described in more detail later in this article. The main problems that a computer- or calculator-operated valve system solves are (1) expensive servo-operated bellows that often rupture are eliminated and (2) elastomers that can cause memory effects between samples, especially with COz, can be eliminated. A benefit that may be important in some applications is that the ratio of maximum to minimum volume can be at least 201 in the mercury piston system but is usually limited to about 4:l in the servo-operated bellows system. Therefore, one should be able to analyze smaller samples with the mercury piston system. The vacuum and pneumatic components of the system are shown in Figure 1. The sample gas pressure is adjusted by connecting either a tank of nitrogen or a rotary vacuum pump to the mercury piston by air-operated control valves. Compressed air is delivered to each air actuator by an electrically operated three-way NO or NC solenoid valve (valves 7-10 in Figure 1). For prevention of movement of mercury during a power outage, valves 1,2, and 3 should remain closed. This is accomplished by supplying compressed air to their actuators
through NO solenoid valves. The NC solenoid valve 7 keeps valve 5 closed during a power outage, preventing the sample from being pumped away. The air actuators of valves 3 and 4 are connected in parallel so that both nitrogen and vacuum cannot be connected to the mercury piston at the same time. A gas sample is compressed by opening valves 2 and 3, closing 4 by default. Closing valve 3 (opening 4) and opening valve 2 expand the sample. Valve 1 is used to provide fine pressure adjustment. A single rotary pump is used for the dual functions of operating the mercury piston and pumping the bulk of a carbon dioxide sample from the piston once it has been analyzed. This prevents excessive buildup of carbon dioxide in the liquid nitrogen trap of the mercury diffusion pump connected to the inlet system. When valve 5 in Figure 1 is opened, the sample can be exhausted from the system. Valve 5 is only opened when a pressure sensor connected to the rotary pump indicates a satisfactory vacuum has been attained. Valve 4 remains closed when valve 5 is opened to prevent nitrogen from flooding the inlet system. The selection of valves in Figure 1is critical for satisfactory operation of the system. The valves must be leak-free during thousands of cycles and unaffected by mercury. Nupro stainless steel 4BK bellows valves with air actuators, Kel-F stem tip closure, and Swagelok fittings have provided satisfactory service in our system. Teflon front ferrules are used in all Swagelok stainless steel fittings, and Cajon Ultra-torr fittings provide connection to the glass mercury piston. To minimize the troublesome effects of air leakage past the Kel-F bellows gasket, we oriented the valves with the arrows on the valve body as shown by the arrows in Figure 1. NO valves are selected for the fine- and coarse-adjust valves in Figure 1 because more pressure can be applied to the stem tip than with NC valves. A spring requiring only 200-300 W a to open is employed in the NC valves. However, up to 1 MPa can be applied to the air actuator of the NO valve. A pressure of 480 Pa (70 psig) was found to be satisfactory. The degree of the fine adjustment of the volume is dependent upon (1) the size of the orifice of the fine-adjust valve, (2) the pressure differential across the fine-adjust valve, (3) the volume of the system between the fine-adjust and course-adjust valves and the mercury level in the mercury piston, and (4) the minimum time that the fine-adjust valve can be held open. The orifice of a 4BK-series valve body was decreased to 0.15 mm in the fine-adjust valve (Figure 2). A stainless steel capillary tubing, F in Figure 2, 0.75 and 0.15 mm, outside and inside diameter, respectively, was silver soldered to a stainless steel bushing, E, whose outside diameter is about 0.02 mm larger than the orifice of the valve body. The bushing was cooled in liquid nitrogen and pressed into the valve body, and it expanded on warming for a snug fit. The top of the bushing is beveled (see Figure 2) and positioned
This article not subject to U S . Copyright. Published 1981 by the American Chemical Society
