930
Anal. Chem.
A 1
0
b'p''060c
I
1
I
2
RETENTION
TIME
3
4
(MINI
Figure 3. Net selective N I response chromatogram (Figure 3 = Figure 1 minus Figure 2),350 pm slit.
though the line-to-background ratio is worse). Through replacement of the previous photomultiplier tube (Hammamatsu 7102, in ref 1) with an R456 tube, the nitrogen detection limit for direct gas sampling loop introduction (no chromatographic column) improved from the previous value of 1 pg of N to the present value of 0.25 pg of N. The present detection limit using the GC column is 1 pg of N based on a signal to noise ratio of 2. It should be noted that the R456 gives improved results for the 8216.3-A line of atomic nitrogen when compared to the 7102 tube; however, the R456 tube should not be used for the 8680.3-A line of nitrogen since the response is poor a t this longer wavelength. Precision. The degree of precision achieved for quantitative analysis is 2% RSD. This is presently limited by manual injection reproducibility (since no internal standardization was used in this experiment). Use of an internal standard will no doubt improve the precision in the future. Response Factors. The peak area response (per mole) did not vary (within experimental error) for the compounds Etz" and Et3N. The response (per mole) for undistilled piperidine was initially somewhat lower than that of Etz" or Et3N. However, it was then discovered that the undistilled piperidine had been partially converted to a nonvolatile carbamate by COz adduction. The carbamate did not chromatograph and the undistilled piperidine response was therefore low due to the mass loss as a nonvolatile carbamate. A much better result was obtained after distilling the piperidine so as to remove the nonvolatile carbamate impurity prior to injection.
CONCLUSIONS The selectivity of background-corrected ICP chromatograms (Figure 3), the constant and easily zero-suppressed nitrogen contamination levels, the reasonable precision level, the ex-
1961, 53, 936-938
perimental invariance of response factors, and the fact that torch deposits (reported as being troublesome in microwave-induced plasmas) have not been observed in these studies collectively lead us to conclude that backgroundcorrected inductively coupled plasma emission spectrometers can be analytically useful as selective detectors for the GC determination of compounds containing the element nitrogen. The absence of severe problems associated with torch deposits makes added scavenging gases unnecessary and would allow the simultaneous, selective determination of both nitrogenand oxygen-containing compounds using a multichannel spectrometer with background corrected channels fixed a t the 8216.3-A N I line used in the present work and at the 7771.9-A 0 I line used in the previous papers (21,22).
LITERATURE CITED (1) Northway, S. J.; Brown, R. M.; Fry, R. C. Appl. Spectrosc. 1960, 34, 338-348. (2) McCormack, A. J.; Tong, S. C.; Cook, W. D. Anal. Chem. 1965, 37, 1470-1476. (3) Bache, C. A.; Lisk, D. J. Anal. Chem. 1965, 37, 1477-1480. (4) Bache, C. A.; Lisk. D. J. Anal. Chem. 1966, 38, 783, 784. (5) Bache, C. A.; Lisk, D. J. Anal. Chem. 1966, 38, 1757, 1758. (6) Bache, C. A.; Llsk, D. J. Anal. Chem. 1967, 39, 766-789. (7) Moye, H. A. Anal. Chem. 1967, 39, 1441-1445. (8) Bache, C. A.; Lisk, D. J. J . Gas. Chromatogr. 1966, 6 , 301-304. (9) Dagnall, R. M.; Pratt, S. J.; West, T. S.; Deans, D. R. Talanta 1089, 16, 797-806. (10) Braun, W.; Peterson, N. C.; Base, A. M.; Kurylo, M. J. J . Chromatogr. 1971, 55, 237. (11) Dagnall, R. M.; West, T. S.; Whitehead, P. Anal. Chlm. Acta 1072, 60, 25-35. (12) . . Daanail, R. M.: West, T. S.: Whitehead. P. Anal. Chem. 1072. 44. 2074-2078. (13) McLean, W.R.; Stanton, D. L.; Penketh, G. E. Ana/yst(London) 1073, 98, 432-442. (14) Skogerboe, R. K.; Coleman, G. N. Anal. Chem. 1978, 48, 611A622A. (15) Beenakker, C. I. M. Spectrochim. Acta, Part8 1976, 378,483-486. (16) Beenakker, C. I. M. Spectrochim. Acta, Part8 1077, 328, 173-187. (17) VanDalen; J. P. J.; de Lezenne Coulander. P. A.; de Qalan, L. Anal. Chim. Acta 1977, 9 4 , 1-19. (18) Windsor, D. L.; Denton, M. B. Appl. Spectrosc. 1978, 32, 366-371. (19) Wlndsor, D. L.; Denton, M. B. J . Chromatogr. Scl. 1979, 17, 492-496. (20) Wlndsor, D. L.; Denton, M. B. Anal. Chem. 1979, 51, 1116-1119. (21) Brown, R. M., Jr.; Fry, R. C. Anal. Chem. 1961, 53, 532. (22) Northway, S. J.; Fry, R. C. Appl. Spectrosc. 1080, 34, 332-338.
RECEIVED for review February 27,1980. Resubmitted January 19,1981. Accepted February 3,1981. This paper was supported by the Kansas Agricultural Experiment Station and the Kansas State University Department of Chemistry. This paper was presented in part at the 1980 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ.
Structure Identification of Terpene-Type Alcohols at Microgram Levels B. A. Bierl-Leonhardt * and E. D. DeVilbiss Organic Chemical Synthesis Laboratory, Agricultural Environmental Quality Institute, Agricultural Research, Science and Education Administration, U S . Department of Agriculture, Beltsville, Maryland 20705
Chemical structures of isolated, complex, naturally occurring compounds are not easily elucidated when only a microgram or less of the compound is available. Heath et al. ( I ) showed that IR and NMR spectra can be obtained on 0.2 and 2 pg, respectively, of compound with the application of Fourier transform techniques. On the other hand, mass spectra can be obtained on nanogram quantities. Therefore, we have emphasized the development of microreaction procedures that yield 0-that are more easily identified by gas chromatogra-
phy/mass spectrometry (GC/MS) than is the parent compound. Perhaps the simplest but most important derivative for characterization is the paraffin skeleton. Beroza et al. ( 2 , 3 ) , Bier1 et al. (4)and Kepner and Maarse (5) developed reaction GC procedures that removed functional groups thereby converting the unknown compound into its carbon skeleton which could then be identified by GC retention data. In these procedures, the parent compound is injected into a heated
This article not subject to US. Copyright. Published 1981 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
937
Table I. Alcohols Treat’ed with Pt t LiAlH, no. 1 2 3
4 5 6 7 8 9
10
11
12 13
14
type of alcohol
product
primary primary primary primary primary tertiary secondary secondary
nonane t decane cyclo butane cyclopentane cyclohexane toluene t methylcyclohexane a 1-methylcyclopentene + methylcyclopentane a me thy Icy clopentane 1-methyl-4-(1-methylethy1)cyclohexane
tertiary
1-methyl-4-(1-methylethy1)cyclohexane
secondary
1,1,2,5-tetramethylcyclopentane
primary
2,6-dimethylheptane + 2,6-dimethyloctane t l-methyl-4-(1-methylethy1)benzene a 2,5,5-trimethylheptane + 2,2,5-trimethylhexanea + 2,3,5-trimethylhe~ane~ 2,2-dimethyl-1-( 1-methyle thy1)cyclo butane t 2-ethyl-2-methyl-l-(1-methylethy1)cyclobutane a 2,2-dimethyl-l-(1-methylethy1)cyclobutane( 11 )
compound decanol cyclo butanemethanol cy clopen tanemethanol cyclohexaneme thanol 1-methyl-3-cycllohexene-1-me thanol 1-methylcyclopentanol trans- 2-methylcyclopentanol Bmethyl- 24 1-nnethyletheny1)cyelohexanol (isopulegol) @,a,4-trimethyl~3-cyclohexen-l-methanol
(@-terpineol) endo-l17,7-trimethy1bicyclo[ 2.2. llheptan- 2-01 (borneol) 3,7-dimethyl- 2,6-octadien- 1-01 (nerol and geraniol) 2,2-dimethyl-3-( 2-methylpropen:yl)cyclopropanemethanol (chrysanthemol) (+ )-cis-l-methyl-2-(1-methyletheny1)cyclobutaneethanol (Grandlure I ) (IO)
primary primary
( t )-cis-2,2-dimethyl-3-(1-methyl’ethy1)cyclo-
+ camphanea
primary butanemethanol secondary 15 2,6-dimethyl- 1,5-heptadien-3-01 2,6-dimethylheptane (12 ) tertiary 16 3,7-dimethyl-1,16-octadien-3-01(1 inalool) 2,6-dimethyloctane Minor component. Probable identification; m/e (%): 126 (lo), 111(7), 84 ( 6 0 ) , 69 (loo), 55 (37), 4 1 (50). Probable identification; m/e (%): 142 ( l o ) , 127 (4), 113 (5), 98 (12), 71 (23), 57 (loo), 4 3 (58). -
precolumn of P d catalyst that is swept with the hydrogen carrier gas; rapid reaction saturates double bonds, strips functional groups from the injected compound, and generates the paraffin, which is then swept into the analytical GC column. At a catalyst temperature of 300 “C, compounds having a primary 0- or IV-containing functional group are converted to a paraffin with one less carbon atom than the parent compound since the catalytic reaction removes the terminal carbon to which the heteroatom is linked (4). Adhikary and Harkness (6) described an apparatus for the catalytic reduction of steroids and sterols with Pt; the resulting hydrocarbons from 5- to 10-pg samples !were trapped for spectroscopic analysis. Sitanley (7,8) develloped a technique to carry out reactions in sealed capillaries and then inject the products into the GC/MS system by crushing the capillaries with a device at the injection port. The procedure described here generates the hydrocarbon products but no modification of the GC/MS system is required. In addition, the mass spectrum obtained provides more conclusive structure identification than only the GC retention data. Identification by retention time is a routine procedure only if the reaction product is a straight-chain paraffin; however, a mass spectrum is needled if the original compound is branched and/or contains aliphatic rings. This procedure avoids the problems associated with trapping microquantities of reaction product for subsequent GC/MS analysis. Microgram or submicrogram quantities of the parent compound are allowed to react with Pt catalyst and lithium aluminum hydride (LiA1H4) at 250 “C in a sealed tube; the reaction products are dissolved in solvent ffor injection into the capillary column of the GC-MS system.
EXPERIMENTAL SECTION The 5% F% on neutral alwnina catalyst was prepared according to the procedure of Nigam (5’)and activated at 250 “C in a stream of Hz. LiAlH4(Alfa Division, Ventron Corp., Ilanvers, MA) was mixed in a 1:l ratio (w/w) with the Pt catalyst. The hexane solution (1-10 gL) containing 0.1-20 pg of the compound was injected onto 1 mg of the catalyst powder in a melting point capillary (Kimax No. 34507, 0.9-1.1 mm i.d.) cut to a length of 6 cm; the solvent was evaporated by directing a slow stream of Nz at the sample through a syringe needle. The tube was sealed
near the open end with a torch and then heated in an oven at 250 O C for 30 min. The tube was cooled to room temperature and opened near the top, and a plug of 4-5 ML of solvent was added with a syringe to rinse the walls and leach the catalyst. The choice of solvent (either hexane or hexadecane) is dependent principally on the molecular weight of the expected paraffin products and secondarily on the type of GC column used at the mass spectrometer inlet. With a conventional packed column (such as 1.8m X 3 mm i.d. stainless steel, packed with 10% OV-1 on Supelcoport, SO/lOO mesh), hexane can be used to dissolve products with volatility greater than that of decane; the “tail” of the solvent peak is eluted before the Cll or higher paraffins. Hexadecane is used for earlier eluting products. If a capillary GC column (such as 30 m, SP 2100) is used to provide greater resolution, hexane can be used for C9 and higher paraffins. A Finnigan Model 4000 GC/MS with a Model 6000 data system was used to obtain the data; only electron-impact spectra were recorded. Identification of the product was based on comparison of their mass spectra with those published for reference compounds.
RESULTS AND DISCUSSION Table I shows the alcohols that were allowed to react with Pt/LiAlH4 and the products that were obtained. Little, if any, unreacted alcohol was found. On the basis of area measurementa for given amounts of decanol(l), the yield of paraffins was roughly 30%. Primary alcohols lost the carbon atom adjacent to the OH from all or a significant portion of the sample; in the case of decanol (l), the ratio of nonane to decane varied from 1:l to 1O:l. The cycloalkanemethanols with a saturated C4-C6ring (compounds 2,3,4, and 14) lost -CH20H. The cyclopropane ring of chrysanthemol(12) cleaved to give three acyclic products. The major product from a cyclohexenemethanol ( 5 ) was formed by aromatization and -CHZOH loss. The primary alcohols in which the -CH20H was not attached directly to a ring (1, 11, and 13) gave both the parent hydrocarbon and the paraffin product with one less carbon atom. The monocyclic and acylic secondary alcohols (7,8, and 15) gave only the carbon skeleton, but the bicyclic ring system of borneol (10) was cleaved to yield mostly the substituted cyclopentane. The tertiary alcohols in which OH was not directly attached to a ring (9 and 16) gave the parent carbon skeleton, while 1-methylcyclopentanol (6), a
938
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
