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Anal. Chem. 1981, 53, 1783-1788
Matrix Isolation Fourier Transform Infrared Spectrometric Detection in the Open Tubular Column Gas Chromatography of Polycyclic Aromatic Hydrocarbons D. M. Hembree, A. A. Garrison, R. A. Crocombe, R. A. Yokiey, E. L. Wehry,” and G. Mamantov” Deparfmsnt of Chemistty, University of Tennessee, Knoxville, Tennessee 379 16
A detailed description of a system for gas chromatographic (GC) detection of matrix Isolation Fourier transform infrared (FTIR) spectrometry is presented. Substances eluting from a support-coated open tubular column are deposited dlrectiy on lndivldual faces of a 12-sided movable gold-plated copper dlsk mounted within the cold head of a closed-cycle cryostat. The same gas is used as GC carrier and spectroscopic matrix gas. An optical interface, comprlsing beam condensing and rod optics, permits the modifled cryostat head to be mounted in proximlty to the sample chamber of an FTIR spectrometer. Appiicatlons of the technique to identlficatlon of polycyclic aromatic hydrocarbon constituents of coal-derived materials are demonstrated, and quantitative aspects of the procedure are considered.
matrix isolation FI’IR spectrometry directly to GC. Using this procedure, they have shown that FTIR spectra can be obtained, under true matrix-isolation conditions, of pallar organic molecules which exhibit strong tendencies toward aggregation. The use of matrix isolation IR spectrometry as a high-resolution spectrometric detection technique in the GC analysis of complex mixtures of polycyclic aromatic hydrocarbons (PAHs) and their derivatives would, if successful, enable the identification and determination of isomeric PAHs which are difficult to separate completely and are also difficult to distinguish by mass spectrometry. Accordingly, we have designed and constructed a GC MI FTIR interface which differs in several respects from that described by Reedy et al. (10, 11). Design details of this apparatus, and its application to characterization of mixtures of PAHs, are described herein.
Previous reports from this laboratory (1-7) have described the use of‘matrix isolation (MI) as a microsampling technique in Fourier transform infrared (FTIR) spectrometry. An important analytical advantage of matrix isolation as a sampling procedure for FTIR spectrometry is the production of spectra characterized by relatively small bandwidths and the absence of rotational structure, which facilitate use of the spectra for “fingerprinting” and distinguishing between closely related compounds (including isomers) in multicomponent samples. Other important advantages of matrix isolation include transparence of typical matrix materials in the IR and adherence to Beer’s law by matrix-isolated solutes over extended concentration ranges. Considerable interest has developed in the use of FTIR spectrometry for detection in gas chromatography (GC) (8, 9),the popularity of which is demonstrated by the availability of a t least two commercial instruments which yield vaporphase on-the-fly or stopped-flow spectra. A major drawback of the on-the-fly systems is the limited resolution (ca. 4 cm-’) and number of FTIR scans (ca. 4)possible in the short time the GC effluent traverses the light pipe and is thus in the beam of the spectrometer. Use of stopped-flow systems results in better IR spectra, but the chromatographic resolution may be severely degraded if the gas flow is stopped, and other compounds eluting may be lost if a bypass is used. These experimental limitations all require the use of extremely sensitive, cooled IR detectors to obtain detection limits of about 100 ng per component (8). Matrix isolation is most commonly used as a means of trapping unstable or reactive species in order to study them in a leisurely fashion. For a sample which is liquid or solid at room temperature, matrix isolation involves vaporizing the sample (urually by vacuum sublimation). Accordingly, matrix isolation spectroscopy should be directly applicable to detection in gas chromatography (GC), particularly if the same substance is used both as the GC carrier gas and the matrix material. Indeed, Reedy, Bourne, and Cunningham ( 1 0 , I I ) have recently demonstrated the feasibility of interfacing
GC/Matrix 1sola.tion Interface. A block diagram of the apparatus is shown in Figure 1. The effluent from a glass open tubular GC column (using nitrogen as carrier gas) was passed through a “tee” which divided the stream into two segments. Part of the effluent was directed to a conventional flame-ionization detector (FID) while the remainder of the effluent encountered a “microneedle”valve (MNW/ 100,Scientific Glass Engineering, Austin, TX), which served both as a metering valve for controlling the flow rate of effluent and as a shut off valve necessary for maintaining a vacuum of ca. lo4 torr in the cryostat. With the “microneedle”valve fully open, all of the GC effluent passed into the high vacuum of the cryostat. In the closed position, this vallve directed the effluent to the FID. Intermediate positions could be used to split the stream. The vapor passing through this vallve traversed a heated transfer line to a closed-cycle refrigeration system (Air Products CSW-202, used in conjunction with an Air Products CSW-204compressor). This transfer l i e was fabricated from glass-lined metal tubing, encased in an insulating ceramic material (“Astroceram”,Chemo-ThermicIndustries, Inc., Freeport, NY) in which was embedded a nichrome heater coil which permitted the transfer line to be heated to temperatures exceeding 600 K. The metal tubing was resistively heated at all points from inside the GC to inside the cryostat, to prevent condensation of the effluent at any point within the line. The usual nitrogen flow rate used was 2 std crn3min-’, which yielded a matrix to sample ratio of approximately6001 for a 30-s GC peak containing 10 pg of a typical analyte. Compounds diverted into the cryostat sampling head were condensed on a 12-sidedgold-plated disk constructed of OFHC copper; each “face” on this disk had 4 X 6 mm dimensions. Sampling surfaces were positioned for deposition and for subsequent acquisition of WIR spectra by a stepping motor (Superior Electric S10-SYN M063-FD09, controlled by a Superior Electric SP153A controller) connected to the disk by a rotating vacuum feedthrough (EMB-188-E-N-146,Ferrofluidics Corp., Burlington, MA). A schematic diagram of the cryostat sampling head is shown in Figure 2. A solid Kel-F spacer was used to thermally insulate the sampling disk from two small stainless steel pins connected to the stepping motor drive mechanism. Small steel springs were employed to provide upward pressure on the disk so that thermal contact between it and the cold station was maintained. A dy-
EXPERIMENTAL SECTION
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Figure 2. Diagram of cryostat head design for matrix isolation GC FTIR. The sample is deposited on the 12-sided gobplated oxygen-free high-conductivity copper (OFHC) disk. The dimensions of each face are 4 X 6 mm. For simplicity the ports in the shroud and the fixtures they carry are not shown in this figure. See Figure 3 for details.
namic vacuum of better than lod torr (before cool down) and the nickel-plated radiation shield enabled the cryostat to cool to 17 K at the gold surface, in a time of 70 min. For large nonreactive molecules such as PAHs, this base temperature is sufficient to ensure isolation, but a lower temperature would be desirable for studies of small reactive species such as atoms, ions, or radicals (which are seldom encountered in conventional gas chromatography). The viewing ports indicated in Figure 3 allowed inspection of the matrices, which were usually slightly frosty in appearance, but appeared to be of uniform thickness. Optical Interface, The interface of the cryostat to the FTIR spectrometer is shown schematically in Figure 3. This system (originallyconstructed by Harrick Scientific Corp., Ossining, NY, and subsequently modified by the authors) employed KRS-5 rods to direct the incident light toward, and to convey the reflected light from, a particular disk surface. The spherical mirror was used to provide 6X beam condensationin order to match the beam size at focus of the Digilab FTS-20C/V spectrometer (2.5 cm) to the ends of the KRS-5 rods, which were 3 X 4 mm. In most cases the spectral region of greatest interest was 700-900 cm-l, and the use of the full beam diameter is justified; however, this practice will give rise to slightly diminished resolution above 1000 cm-’. The FTIR spectrometer was equipped with a nichrome wire source and TGS pyroelectric bolometer detector. The TGS detector was moved from its usual position to that shown in Figure 3 in order to increase the overall throughput of the optical system. The detector size of 2 mm X 2 mm sampled a large portion of the end of the KRS-5 rod while minimizing the effect of cryostat vibration arising from the motion of the displacer. To obtain the spectra shown here, we collected between 500 and 1200 scans at 1cm-‘ resolution, requiring 37-90 min. A KBr beam splitter was used and interferograms were subjected to “boxcar” truncation before transformation. The combination of all these elements resulted in a useful spectral range of 4200-500 cm-’. Chromatographic Procedures. All separations were performed by using a Perkin-Elmer Sigma 3 chromatographequipped with a 39-m SE-30 glass SCOT column (Scientific Glass Engineering) and a conventionalflame-ionizationdetector. Nitrogen
INSULATION GC EFFLUENT ELECTRICALLY INSULATED FEED THROUGHS ~
12 SIDED GOLD PLATED OFHC DISK -ELECTRICAL CONNECTION FOR RESISTIVE HEATING
Diagram of the optical interface of the cryostat to the FTIR spectrometer and fittingsin the vacuum shroud. M1 and M2 are planar mirrors; M3 is a spherlcal mirror. Details of the vacuum seals in the shroud are not shown. Figure 3.
(‘‘ultra high purity”, 99.999%, Matheson) was employed as the carrier/matrix gas. Ancillary Separation Techniques. Certain real samples were subjected to cleanup prior to injection into the column. One useful procedure for characterization of relatively volatile constituents consisted of a simultaneous steam distillation/solvent extraction, using a commercial apparatus for this purpose (K-523010,Kontes Glass Co., Vineland, NJ). In typical applications,this apparatus was allowed to function for ca. 100 h, using benzene or toluene as extraction solvent. Following completion of the distillation/extraction, the extraction solvent was evaporated and the resulting residue was dissolved in dichloromethanefor subsequent GC separation. A second preliminary separation procedure was a modification of a column chromatographicmethod described by Schiller and Mathiason (12). Samples of coal-derived materials ranging from 5 to 20 g were dissolved in 50 mL of chloroform, and 60 g of neutral “Brockmannactivity 1”alumina was added to the solution. The resulting slurry was mixed and allowed to “dry” at room temperature for 4 h, and was then added to 4 glass column containing an additional 60 g of alumina. Saturated compounds were first eluted with 300 mL of n-hexane; an “aromatic fraction” was then eluted with 600 mL of toluene. The solvent was then removed from the aromatic fraction, and the residue was dissolved in dichloromethane for subsequent GC separation.
RESULTS GC FTIR Behavior of Pure Compounds. It was desirable initially to ensure that deposits formed from GC effluents were indeed matrix isolated; for this purpose, spectra of pure compounds eluted from the GC column were compared with spectra of the same compounds obtained by the conventional Knudsen effusion method (3). For naphthalene and a wide variety of other PAHs, no discernible differences in band positions or half-widths were noted in comparisons of spectra obtained by the two deposition techniques. In Figure 4,the most intense phenol band a t 1502 cm-’ shows no sign of aggregation, with a 4-pg deposit producing a band having a full width at half-maximum of 1.5 cm-l. Since polar compounds are more susceptible to intermolecular effects than the nonpolar PAHs, this observation was regarded as a sufficient test of isolation for the purpose of PAH analysis. Polar compounds, such as phenol, often exhibit their greatest IR absorption in the 1700-1200 cm-’ region where water vapor absorption interferes. To reduce the amount of
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Matrix isolation IR spectra of components (from chromatogram shown in Figure 5) isolated on three deposition surfaces: (IN) naphthalene, (1,4DHN) 1,4-dlhydronaphthalene,(1,PDHN) 1,2dihydronaphthalene, (2) 2-methylnaphthalene, (1) 1-methylnaphthalene, (2,6DMN)2,6dimethylnaphthalene,(1,3DMN) 1,3dimethylnaphthalene, (2,3DMN) 2,3dimethylnaphthalene,(1,BDMN) 1,5dlmethylnaphthalene, (G) glitch due to 60-Hz Interference. Figure 6.
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water vapor in the optical path, we evacuated the spectrometer except for a small portion of the sample compartment which was purged with nitrogen gas. By use of the spectrum of a face of the disk with no deposit as the reference, the remaining water vapor in the optical path could be eliminated from the spectra. As shown in Figure 4, only a small IR feature due to the matrix isolated water is still visible. Synthetic Mixtures. Preliminary tests were performed by using a synthetic mixture, consisting of naphthalene, 1and 2-methylnaphthalene, 1,3-, 1,5-, 2,3-, and 2,6-dimethyl-
naphthalene, 1,2- and 1,4-dihydronaphthalene,and cis- and trans-decahydronaphthalene(ca. 15 pg of each, diluted to 1 mL with acetone). The chromatogram resulting from an injection of 1p L of the solution is shown in Figure 5. Separate studies indicated that the pairs of compounds naphthalene and 1,4-dihydronaplhthalene and 2,3- and 1,5-dimethylnaphthalene could not be separated using this column under a variety of conditions. Thus only nine peaks are distinguished in Figure 5. The effluent was deposited on three different surfaces such that the first five compounds were deposited on surface 1,the next two on surface 2, and the remaining four on surface 3. The resulting IR spectra are shown in Figure 6, plotted in the 700-900 cm-l region which is most useful faa PAH analysis. cis- rind trans-Decahydronaphthalene, delposited on surface l, exhibit no IR bands in this region (7). Features characteristic of the remaining compounds can be observed in the respective spectra, by comparison with spectr,a of the individual compounds previously obtained in this laboratory (5, 7). In contrast, examination of a matrix formed by deposition of the 11-component mixture using an extern4y cooled Knudsen cell (4)permitted unambiguous identification of only four of the nine compounds which absorb in tho 700-900 cm-' region. "Real"Samples. Figure 7 shows a gas chromatogram of a sample produced by Bimultaneous steam distillation/solvent extraction (see Experimental Section) of a solvent-refined coal ("SRC-1"). The apparatus was allowed to function for 108
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spectrum of the matrix deposit of “region 4” of the chromatogram shown in Figure 8. Comparison of this spectrum with the “library”spectrum indicates pyrene to be a major constituent of this fraction. Flgure 9. I R
Flgure 7. Gas chromatogram of steam distillation/soivent extraction fraction from SRC-1: extraction solvent, benzene: column SE-30, SCOT, temperature programmed from 100 to 260 O C at 8 OC/min; indentified components (1)naphthalene, (2) 2-methylnaphthalene, (3) I-methylnaphthalene,(4) biphenyl, (6) 1,4-dimethylnaphthalene, (7) dibenzofuran, (8) fluorene, (9) phenanthrene, (10) fluoranthene, (1 1) pyrene. No constituents of peak 5 could be identified (see text).
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aromatic fraction from a coalderived crude oil: column SE-30, SCOT, temperature programmed (100-280 O C range, 15 deglmin). The numbers at the top denote regions of the chromatogram deposited on successive surfaces in the cryostat. h using benzene as the extraction solvent. As noted in the caption to Figure 7,lO of the 11numbered chromatographic peaks were assigned to specific PAHs on the basis of comparisons of their spectra with “library” matrix isolation FTIR spectra of authentic samples, and only peak 5 could not be matched. The identifications are consistent with the expectation (13)that the distillate from SRC-1 obtained under these conditions should consist of naphthalene, phenanthrene/anthracene, and pyrene/fluoranthene fractions. A more complex example of a “real” sample is afforded by Figure 8, which is a chromatogram of the “aromatic fraction” (obtained by alumina column chromatography; see Experimental Section) of a coal-derived crude oil. It was obvious that, for such a complex chromatogram, deposition of individual peaks onto individual surfaces of the deposition disk could not be achieved. Therefore, the approach used was to deposit four “fractions” of this effluent into four successive
disk surfaces, as defined at the top of Figure 8. While the IR spectrum of each of these fractions was complex, certain spectral features could be assigned to specific compounds on the basis of comparisons with library spectra of pure compounds. An example of a portion of one such spectrum (that of “fraction 4”) is shown in Figure 9, where it is compared with the GC MI FTIR spectrum of pyrene deposited as pure compound. Despite the complexity of this fraction (and of the resulting IR spectrum), pyrene can be readily recognized as a major constituent of this fraction. Other specific compounds which are indicated by matrix isolation IR spectra of the respective fractions are naphthalene (fraction l),1,4-dimethylnaphthalene,indole, 2-methylindole, and 3-methylindole (fraction 2), and phenanthrene and carbazole (fraction 3). Quantitative Studies. In order to demonstrate the quantitative utility of this apparatus, various volumes of a standard solution of pyrene in acetone were injected onto the column and the absorbance (peak height) of the principal feature of the MI FTIR spectrum of pyrene (846 cm-’) in each of the resulting deposits (formed on a separate deposition surface) was measured. No internal standard was used. The analytical calibration curve shown in Figure 10 was obtained. The detection limit for pyrene (based on a S/N ratio of 2) was observed to be 710 ng, which is larger than that previously observed ( 3 ) for conventional Knudsen effusion sample preparation (200 ng). It is believed that the higher detection
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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Table I. Quantitative Results for Shale Oil Constituents” HPLC extraction,b quantitation by compd LC GC GC/MS phenol o-cresol
2,4,6-trimethylpyridine a
11787
383 330
387 334 912
Results expressed in ppm of the constituent in shale oil.
limit for pyrene (and other PAHs) by GC MI FTIR spectrometry resulted from the relatively low energy throughput (ca. 5%) of the optical interface of the cryostat to the FTIR spectrometer (compared with the use of the transmission mode, wherein 60% throughput was possible). The upper limit of linearity in Beer’s law plots for PAHs (e.g., -8 pg for pyrene) was usually observed to be virtually identical for the GC sampling system and for a conventional Knudsen effusion matrix isolation system (1,3-5). The existence of a negative deviation from Beer’s law (i.e., curvature toward the concentration axis) has been previously observed for PAHs isolated in matrices. In the present case, such deviations can arise from well-characterized limited resolution and instrument line-shape effeds (14).h addition, the matrix-to-sample ratios encountered in this work using larger samples are considerably smaller (ca. 600:l as opposed to ca. 1OOOO:l) than in our previous Knudsen effusion studies, leading to the possibility of intermolecular interactions. Furthermore, although the maximum amount of a compound which can be accommodated by a SCOT glass column is claimed to be on the order of 10 pg (I5),serious tailing can occur at these levels, leading to the possibility that the entire quantity of compound may not be condensed on the cryogenic surface. However, the linear (and observable) range of FTIR absorbance obtained by using this equipment does coincide with the useful working range of typical SCOT columns. An evaluation of the quantitative capabilities of GC MI FTIR for individual constituents of “real” samples was undertaken, with the phenanthrene content of SRC-1 (peak 9 in Figure 7) chosen as the example. Three techniques were compared: (a) GC peak height measurement, using the FID response €or phenanthrene and standard addition; (b) the GC MI FTIR peak height (‘740.5cm-l band) for phenanthrene, also using standard addition; and (c) the GC MI FTIR peak height, using a Beer’s law plot obtained for pure phenanthrene in separate experiments. The results obtained by these three techniques for the phenanthrene content of the particular SRC-1 sample used were, respectively, 0.37, 0.39, and 0.35 mg/g. The agreement between the FID and IR results is extremely encouraging. In the case of more complex samples, the FID response cannot be used quantitatively because base line peak separation is not possible. The agreement between the IR results obtained with and without standard additions is especially noteworthy,because of the relative ease and speed of quantitative TR measurements based on empirical Calibration curves (i.e., Beer’s law plots) compared with the more time-consuming process of successive standard additions. One r e d sample was injected directly into the GC with no prior separation, as a test of the ability to detect compounds known to be prevent in a sample. A 1-pL sample of NBS standard shale oil (16)was analyzed for three previously quantitated polar compounds: phenol, 0-cresol, and 2,4,6trimethylpyridine. A mixture containing 4 r g of each of the knowns WiiS chromatographed to obtain retention times. When the shale oil was chromatographed under the same conditions, the three portions of the oil eluting at the appropriate times were deposited on three of the cold surfaces. Using the FTIR peak heights with Beer’s law plots for the pure components (as discussed above), the results shown in Table
416 350 1214
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GC/MI FTIR
334 322
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988
Data (other than GC/MI FTIR) from ref 16.
I were obtained. Since a 1-pL sample of the oil was injected, these results show detection of approximately 300 ng per component for phenol and o-cresol in the mixture. DISCUSSION There are a number of design similarities, as well as several noteworthy differences, of the interface between GC and MI FTIR designed by Reedy, Bourne, and Cunningham (“RBC”) (IO,11)and that described in this work. Both systems collect the GC effluent in discontinuous “spots” (as distinguished from a continuous “band” (11))on a metallic surface, and obtain the IR spectra by specular reflectance from this metallic substrate. In both Hystems, the deposition surface is moved to a new position by a stepping-motor drive. The system designed by RBC uses helium as the GC carrier gas; because helium cannot be used as a matrix for matrix isolation a t temperatures attainable with closed-cycle cryostats, it is necessary to remove the helium (using a molecular jet separator) prior to entry of the effluent into the cryostat head and to add the matrix gas via a separate line. The present apparatus, using the same gas as both GC carrier and matrix, eliminates the jet separator and the accompanying losses of sample; furthermore, it is possible to deposit the effluent directly without postcolumn dilution with additional matrix gas if open-tubular GC columns are used. The optical arrangements used in the two systems differ significantly, most noticeably in our use of KRS-5 rods. The present arrangement is more compact and somewhat simpller to use, but is optically less efficient, than that described by RBC. The inefficiency of the optics is partially overcome by the ability to coadd scans of the FTIR spectrometer. Substantial improvement could be expected by conversion to a detector having a higher D*in the relevant spectral range and a stronger infrared source (e.g., a globar). With an extended range mercury cadmium telluride detector, an increase in S/N of from 3 to 10 can be expected over the TGS detector currently in use (9). The stepping motfor controller is interfaced to the spectrometer such that data collection for the multiple surfaces is automated. Normally data are collected overnight, with no operator intervention necessary, as the cryostat can continue operation throughout this time. An operator is required only for the GC depositions and interpretation of result,s. In the present system, the GC inlet is placed 150’ from the optical interface, and the disk is rotated after deposition t o obtain the spectrum. Unlike the RBC system, in which the GC inlet and optics are located close to each other, the present system will eventually allow a permanent connection of the GC and cryostat. Some modification will be required to optimize the present GC MI FTIR interface. A “nondestructive” ancillary GC detector should be used instead of an FID or other destructive detector, so that splitter arrangements can be eliminated. .A “universal” detector is needed, upstream of the MI collection system, to provide guidance as to when fresh faces of the deposition substrate should be used. If a photoionization detector (17,18) were used, the entire effluent could be passed through it, increasing both the quantitative reliability and synchronization of the motor-driven deposition substrate with
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Anal. Chem. lQ81, 53, 1788-1792
the detector signal. Second, the optical efficiency of the system must be improved to allow an increase in the number of surfaces on the cold station while still allowing overnight data collection with a reasonable S/N ratio in the spectra. Plans for up to 50 separate deposition surfaces are under consideration in order to analyze the hundreds of compounds found in many environmental samples, and the necessary variations in the optics are currently under evaluation. The present studies augment those of Reedy, Bourne, and Cunningham (10, 11) in demonstrating the Yacticality of matrix isolation spectroscopy in GC detection and point to the utility of the technique in characterization of complex real samples. Of course, there is no fundamental limitation of the technique to IR spectrometry;the use of other forms of matrix isolation spectroscopy (including molecular fluorescence) in chromatographic detection is presently being scrutinized in this laboratory.
ACKNOWLEDGMENT We are grateful to W. E. May (National Bureau of Standards) for the shale oil sample, to W. L. Griest (Oak Ridge National Laboratory) for the coal-derived crude oil sample, and to P. W. Jones (Electric Power Research Institute) for the sample of SRC-1.
LITERATURE CITED Wehry, E. L.; Mamantov, G.Anal. Chem. 1979, 57, 643A. Mamantov, G.;Wehry, E. L.; Kemmerer, R. R.; Hinton, E. R. Anal. Chem. 1977, 49, 86. Tokousbalis, P.; Hinton, E. R., Jr.; Dicklnson, R. B., Jr.; Bilotta, P. V.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1978, 50, 1189, Hembree, D. M.; Hinton, E. R., Jr.; Kemmerer, R. R.; Mamantov, 0.; Wehry, E. L. Appl. Specfrosc. 1979, 33, 477.
(5) Hinton, E. R., Jr.; Mamantov, G.; Wehry, E. L. Anel. Lett. 1979, 12, 1347. (6) Wehry, E. L.; Mamantov, 0.; Hembree, D. M.; Maple, J. R. I n ”Polynuclear Aromatic Hydrocarbons: Chemlstry and Biologlcal Effects”; Bprseth, A., Dennis, A. J., Eds.; Battelle Press: Columbus, OH, 1980; p 1005. (7) Hinton, E. R., Jr. Ph.D. dissertation, University of Tennessee, 1979. (8) Erickson, M. D. Appl. Specfrosc. Rev. 1979, 15, 261. (9) Griffiths, P. R. In “Fourier Transform Infrared Spectroscopy, Appllcatlons to Chemical Systems”; Ferraro, J. R., Basile, L. J., Eds.; Academlc Press: New York, 1978 Vol. 1, p 143. ( I O ) Reedy, G. T.; Bourne, S.; Cunningham, P. T. Anal. Chem. 1979, 57, 1535. (11) Bourne, S.; Reedy, 0. T.; Cunningham, P. T. J. Chromatogr. Scl. 1970, 17, 460. (12) Schiller, J. E.; Mathiason, D. R. Anal. Chem. 1977, 49, 1225. (13) Schiller, J. E. Hydrocarbon Process. 1977, 56 (I), 147. (14) Anderson, R. J.; Griffiths, P. R. Anal. Chem. 1978, 50, 1804. (15) “Chromatography and Mass Spectrometry Products”, Scientific Glass Englneerlng, Inc.: Austin, TX, 1979; p 17. (16) Hertz, H. S.; Brown, J. M.; Chesler, S. N.; Guenther, F. R.; Hllpert, L. R.; May, W. E.; Parrls, R. M.; Wise, S. A. Anal. Chem. 1980, 52, 1650. (17) Drlscoll, J. N. J. Chromatogr. 1977, 134, 49. (18) Hester, N. E.; Meyer, R. A. Environ. Sci. Techno/. 1979, 73, 107.
RECEIVED for review February 27, 1981. Accepted July 22, 1981. This work was supported by Contract RP-1307-1with the Electric Power Research Institute. Purchase of the FTIR spectrometer was assisted by National Science Foundation Research Instrument Grant GP-41711. D.M.H. thanks the Tennessee Eastman Gorp. for a Graduate Fellowship. Portions of this research were described a t the Fourth International Symposium on Polynuclear Aromatic Hydrocarbons, Oct 1979, the Second Chemical Congress of the North American Continent, Las Vegas, NV, Aug 1980, and the 181st National Meeting of the American Chemical Society, Atlanta, GA, March 1981.
Determination of Sulfur and Heavy Metals in Crude Oil and Petroleum Products by Energy-Dispersive X-ray Fluorescence Spectrometry and Fundamental Parameter Approach Leif HmJslet Christensen”’ and Allan Agerbo Department of Chemistty, Aarhus University, OK-8000 Aarhus, Denmark
The combination of energy-disperslve X-ray fluorescence based on the secondary target excltatlon prlnclple and of a quantiflcatlon model based on the fundamental parameter approach provldes a method for the determlnation of sulfur and heavy metals in crude oil and petroleum products. Samples are analyzed dlrectly wlthout any kind of sample preparatlon. For sulfur the analysis is performed under vacuum in an open cell arrangement. Further, a calibration procedure has been developed Independent of both the sample matrlx and instrument settlng. This procedure and the quantification model have been applled to various certlfled reference materlals and the results obtained proved to be accurate to within 2 4 % . The preclslon (lu) of the method Is, however, better than 1.5% as long as the countlng statistics are not the llmltlng factor.
Wavelength-dispersiveX-ray fluorescence spectrometry has often been the method of choice for the determination of Present address: T h e Isotope Division, Rim National LaboraDenmark.
tory, Postbox 49, DK-4000Roskilde,
major, minor, and trace elements in crude oil and petroleum products. In 1967 X-ray fluorescence was approved as an ASTM standard method for the determination of sulfur. The current edition of this method, designated D 2622-77, was approved in 1977, and the 1979 Annual Book of ASTM standards (1) describes the analytical procedure in detail. Gamage and Topham (2) published a paper on a nondispersive on-line analyzer dedicated to the determination of sulfur. The primary excitation beam may be generated either in an X-ray tube or by a radioisotope. Recently, the Institute of Petroleum proposed and approved an X-ray fluorescence method for sulfur (IP 336/78 T) (3)based on a 66Feradioisotope excitation source and a nondispersive analyzer. These standard methods both use a linear calibration procedure and they are prone to the same general problem inherent in X-ray fluorescence, i.e., the problem of matrix effects. For liquid samples the enhancement effect is negligible in most cases and only a matrix absorption correction is necessary. Several papers (4-7) have dealt with different experimental procedures to compensate for matrix effects in the sample. In a very general review by Williams (8) methods based on internal standards, solid internal standards, and incoherently
0003-2700/81/0353-1788$0 1.2510 0 198 1 American Chemical Society
