Energy & Fuels 1993, 797-99
97
Role of Plasma Desorption Mass Spectrometry in the Analysis of Metallogeoporphyrins Karl V. Wood* and Connie C. Bonham Campus-wide Mass Spectrometry Center, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received February 24, 1992. Revised Manuscript Received October 8, 1992
Plasma desorption mass spectrometry (PDMS) has been found to be an alternative mass spectrometric method for analyzing mixtures of metallogeoporphyrins. Carbon number distributions and recognition of both the ET10 and DPEP metallogeoporphyrinanalogs are possible by obtaining PDMS spectra in the reflector mode, in which unit mass resolution can be achieved. Comparisons of the PDMS and isobutane chemical ionization mass spectra of a chromatographic fraction from an Anna shale oil are presented.
Introduction Geoporphyrins are considered important biological marker compounds in characterizing petroleum as well as in petroleum pr~specting.l-~ Considerableeffort has gone into the development of analytical methodologies for identifying the various metallogeoporphyrins. Mass spectrometric analysis offers an advantage over most techniques since it enables molecular weights to be determined, thereby assigning carbon number distributions of various homologous series. However, to obtain this type of information, extensive sample cleanup to isolate the geoporphyrinsfrom the other polar high molecular weight components present in a given oil is often necessary. Furthermore, conventional mass spectrometric analysis of the relatively nonvolatile porphyrin fraction requires considerable heating of the sample which can cause the ion source to rapidly become dirty if sample amounts are not carefully monitored. More novel techniques like fast atom bombardment, tandem mass spectrometry, and electrospray have also been used successfully to analyze metallogeoporphyrins and related tetrapyrrole^.^^ Recently we investigated the potential of plasma desorption mass spectrometry (PDMS) as a means of mass analyzing metallogeoporphyrins.lo Initial indications suggested that many of the components in a complex oil matrix are transparent/less sensitive to PDMS, as an ionization technique, relative to metallogeoporphyrins, thereby reducing the level to which oil samples need to be cleaned up. In addition, PDMS analysis does not necessitate the volatilization of the entire sample, thereby reducing to a (1) Branthaver,J. F.; Filby, R. H. In Metal Complexes in Fossil Fuels; Filby, R. H.,Branthaver,J. F. Eds.;ACSSymposium Series 344: American Chemical Society: Washington, DC, 1987; p 84. (2) Baker, E. W.; Louda, J. W. In Biological Markers in the Sedimentary Environment; Johns,R. B., Ed.; Elaevier: Amsterdam, 1986; p 125. (3) Gallegos,E. J.; Sundararaman, P. Mass Spectrom. Reu. 1986,4,55. (4) Baker, E. W.; Palmer, 5.E. In The Porphyrins; Dolphin, D., Ed.; Academic Prese: New York, 1978; p 485. (5) Beato, B. D.; Yost, R. A,; Van Berkel, G. J.; Filby, R. H.; Quirke, J. M. E. Org. Geochem. 1991,17, 93. (6) Van Berkel, G. J.; Glish, G.L.; McLuckey, S. A.; Tuinman, A. A. Energy Fuels 1990,4, 720. (7) Brodbelt, J. S.; Cooks, R. G.;Wood, K. V. Fuel Sci. Technol. Int. 1986, 4, 683. (8) Keely, B. J.; Maxwell, J. R. Energy Fuels 1990, 4 , 737. (9) Van Berkel, G. J.; McLuckey, S. A.; Glish, G.L. Anal. Chem. 1991, 63, 1098.
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negligible level the problems associated with ion source contamination. In the earlier study,lO PDMS mass resolution was not sufficient for separating the DPEP and ET10 analogs of a given carbon number. In this study, results are presented using a reflector assembly in conjunction with PDMS, which enables the DPEP and ET10 analogs to be mass resolved.
Experimental Section A Bioion 20R (Applied Biosystems AB, Uppsala, Sweden) plasma desorption mass spectrometer was used in this study. The samples were dissolved in a 50/50tetrahydrofuran/acetone solution prior to electrospraying onto a nitrocellulose-coated Mylar target. The samples were inserted into the Bioion sample carousel for mass analysis. The acceleration potential used was 17 OOO V, and for consistency a sampling interval of 1 h was chosen for each analysis, except where noted. The PDMS instrument was upgraded with addition of a reflector assembly since publication of the preliminary report.'O This mode of operation enables better than unit mass resolution data to be obtained in the typical mass range of metallogeoporphyrins, for example,enabling differentiation of the ET10 and DPEP analogs. The vanadium(1V) etioporphyrin-I standard was purchased from Midcentury Chemicals(Posen, IL). The purity was checked using isobutane chemical ionization mass spectrometry and no significant impurities were detected. The polar metallogeoporphyrin fraction of an Anna shale oillowas obtained from M. M. Chou (Illinois State Geological Survey). One drop of this Anna shale oil fraction was diluted with approximately 20 pL of 50/50 chloroformJhexaneand placed on a hexane-prewetted silica Seppak cartridge (Millipore Corp. Milford, MA). The metallogeoporphyrin fraction, wed for PDMS analysis, was eluted with chloroform, the first milliliter of eluate being discarded.
Results and Discussion In the preliminary study,l0the feasibilityof using PDMS for the analysis of metallogeoporphyrins was demonstrated. However, the lack of unit mass resolution was an obvious weakness, because the ET10 and DPEP analogs of a given carbon number could not be resolved. However, this problem has been solved with the addition of a reflector assembly. This can be seen in Figure 1which compares the PDMS spectrum of 100ng of the vanadyletioporphytin (10) Wood, K. V.; Bonham, C. C.; Chou, M. M. Energy Fuels 1990,4, 747.
Q 1993 American Chemical Society
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98 Energy &Fuels, Vol. 7, No. 1, 1993 4
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Figure 2. Comparison of the PDMS spectra of 1and 10 ng of the vanadyletioporphyrin standard electrosprayed on to the sample target. standard in both the normal and reflector modes of operation. In the normal mode, a single Guassian-shaped peak is observed with a centroid around mlz 544. In the reflector mode the resolved mass peaks can easily be distinguished. The two PDMS spectra have similar signal to noise ratios even though the absolute ion count levels are considerably less in the reflector mode (for this reason, collecting reflector mode data can require more analysis
electrosprayed on to the sample target.
time). Notice also that the PDMS spectrum of the standard shows both the M+ and (M + H)+ ions. The presence of fragmentation further complicates the PDMS results. However, as can be seen in Figure 2 (1 ng and 10 ng of the standard vanadyl etioporphyrin on the target), decreasing the amount of metalloporphyrin on the target reduces the relative amount of fragmentation. The molecule ion is optimized when only a monolayer of sample covers the surface, whereas fragment ions form below the surface which is consistent with a increased amount of sample material." In a real sample the amount of any single metallogeoporphyrin component would be expected to be low, therefore minimizing interference in the determination of carbon number distribution or calculation of DPEPIETIO ratios, particularly for monoisotopic metals like vanadium. Multiply isotopic metals like nickel (5*Niand "i) need to be investigated to determine the degree to which the combination of M+ and (M + H)+ion complicates the results. The PDMS spectrum of 1ng of the standard vanadyletioporphyrin (Figure 2) can only be used to give an upper limit €or the detection limit of this standard because in a (11) Nielson, P.F.;Roepstorff, P.Eiomed. Enoiron. Mars Spectrom. 1988, 17, 137.
Energy & Fuels, Vol. 7, No. I, 1993 99
Analysis of Metallogeoporphyrins
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given preset analysis time not all of the deposited material is actually analyzed.12 This is different from a gas (12) Schmitter, J. J. Chromatogr. 1991,557,359.
chromatographic or solids probe mass analysis in which all of the sample becomes available for analysis in a finite period of time. This can be seen in Figure 3 which shows the intensity of the PDMS signal of 1ng of the standard over eight successive 2-h intervals. As can be seen, the intensity of the molecular ion remains strong, being reduced by only 40% from the first to the eighth 2-h interval. This indicates that even after 16 h there is a measurable PDMS signal for the 1 ng of standard. Figure 4 is a comparison of the PDMS spectrum (reflector mode) and isobutane chemical ionization mass spectrum (CIMS) of the chromatographic fraction from an Anna shale oil. Notice the similarity between these two spectra. In both spectra 'the homologous series of vanadyl geoporphyrins, c28-c34, m/z 486-570 (note c31= m/z527 PDMS and m/z 528 CIMS for the DPEP type), can be observed. The spectra are dominated by the (230C32 homologs with the most intense ion in both spectra corresponding to the Cn-DPEP analog (mlz 541 PDMS and mlz 542 CIMS). This figure illustrates that PDMS, in the reflector mode, can provide differentiation of the DPEP and ET10 analogs of a given carbon number in a chromatographic fraction of a real oil sample. The calculated DPEP/ETIO ratios for these two spectra are similar, being 1.6 for PDMS and 2.3 for CIMS. Utilization of PDMS in the analysis of metallogeoporphyrins is still in the preliminary stages; however, this study emphasizes a number of strengths PDMS offers, including the relative sensitivity for the detection of metallogeoporphyrins, determination of carbon number distributions, and elimination of the problems associated with ion source contamination. With the addition of the reflector assembly, the DPEP and ET10 analogs can be separated, thereby enabling DPEP/ETIO ratios to be calculated. Acknowledgment. The authors wish to thank Professor David Freeman (University of Maryland) for his helpful suggestions.
