Liquid chromatography-mass spectrometry of vanadyl porphyrins

Org. Geochem. Vol. 20, No. 7, pp 1099-1104, 1993 Printed m Great Britain

0146-6380/93 $6.00 + 0,00 Pergamon Press Ltd

SHORT NOTE Liquid chromatography-mass spectrometry of vanadyl porphyrins PADMANABHAN SUNDARARAMAN I* and CHRISTINA VESTAL 2 tChevron Oil Field Research Company, P.O. Box 446, La Habra, CA 90631 and 2Vestee Corporation, Houston, TX 77054, U.S.A.

(Received 28 May 1993; returned for revision 21 June; accepted 8 July 1993)

Abstract--A mixture of vanadyl porphyrins was analysed by liquid chromatography-mass spectrometry using a particle beam interface. The resolution of the chromatogram in the LC-MS mode is comparable to that obtained using a u.v.-vis spectrophotometric detector, but the sensitivity was much lower. Vanadyl and nickel octaethylporphyrms were used to compare the performance of the particle beam and the thermospray interfaces. Three times better sensitivity was obtained in the particle beam E1 mode than in the thermospray CI mode.

INTRODUCTION

Discovery of porphyrins in fossil fuel by Treibs (1936), led to the fundamental concept of "Biological Markers". In spite of their early discovery, porphyrins were not widely used in geochemical correlations. This was mainly due to difficulties in analysing them. Unlike steroids and terpenoids, metalloporphyrins are non-volatile and so cannot be analysed by low temperature G C - M S techniques. Advances in analytical techniques such as direct inlet mass spectrometry, high pressure liquid chromatography, high temperature GC, etc. were applied to the analysis of petroporphyrins (Baker, 1966; Baker et al., 1967; Baker and Palmer, 1978; Sundararaman, 1985; Sundararaman et al., 1988; Boreham and Fookes, 1989; Verne-Miser et al., 1991; Barwise et al., 1986). This led to the increasing use of porphyrins in geochemical correlations. For example the DPEP/ ETIO ratio derived from mass spectral and/or high performance liquid chromatographic data (Louda and Baker, 1981; Mackenzie et al., 1980; Barwise and Park, 1983; Barwise, 1987) and the Porphyrin Maturity Parameter (PMP) derived from high performance liquid chromatograms of vanadyl porphyrins are used in the estimation of maturity of source rocks and oils (Sundararaman et al., 1988; Aizenshtat and Sundararaman, 1989; Sundararaman and Raedeke, 1993; Sundararaman and Moldowan, 1993). Successful analysts of petroporphyrins by low temperature G C - M S was achieved by converting the metailoporphyrins into volatile silicon derivatives (Gill et al., 1986). Application of high performance liquid chromatography also led to the isolation *Present address: Chevron Nigeria Ltd, Km 19, Epe Expressway, Lekki Peninsula, Lagos, Nigeria

and structural identification of a number of porphyrins. McFadden et al. (1979) were the first to analyse petroporphyrins by liquid chromatography/mass spectrometry (LC/MS). They used a moving belt interface, where the eluent from the HPLC was dripped onto a moving belt. After evaporation of the solvent, the residue was analysed by direct inlet E1 mode. Since then several workers have analysed petroporphyrins (Eckardt et al., 1991; Van Berkel et al., 1991) using the interfaces developed for LC/MS based on aerosol formation such as therrnospray (Vestal, 1983; Arpino, 1990), particle beam (Willoughby and Browner, 1984; Winkler et al., 1988) and electrospray (Yamashita and Fenn, 1984a, b; Whitehouse et al., 1985). Eckardt et al. (1991) and Van Berkel et al. (1991) have shown that individual metalloporphyrins and polar porphyrins can be analysed by LC-MS. Neither of them have shown that a mixture of vanadyl porphyrins isolated from a fossil fuel can be analysed by L C - M S using either the thermospray or by electrospray ionization techniques. We analysed individual metalloporphyrins and a mixture of vanadyl porphyrins isolated from a Monterey source rock by thermospray and particle beam LC/MS, the results of which are presented in this report. EXPERIMENTAL

Samples

Vanadyl octaethylporphyrin was obtained from Aldrich. The mixture of vanadyl porphyrins was isolated from a Monterey source rock, offshore California. The procedures for isolation and purification of vanadyl porphyrins are described in detail in earlier reports (Sundararaman, 1985; Sundararaman et al., 1988; Sundararaman et al., 1993).

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L C - M S analyses were carried out in thcrmospray positive ion mode as well as particle beam-electron impact modes under the conditions given below: L C pumps. Water 600MS multidelivery system with a Waters 712 WISP Columns• For the analysis of standard vanadyl and nickel octaethylporphyrin Waters Nova-Pack C]a, 3.gram x 15cm, 4 micron particle size column• For the analysis of the vanadyi porphyrin mixture, Shandon Hypersil C~B, 65 cm x 4.6 mm (25 cm + 25 cm + 15 cm in series), 3 micron particle size columns. Flow rate. For the Waters column l ml/min. For the Shandon Column 0.8 ml/min.

Mobile phase. H20/CH3CN/CH3OH 10/45/450/50/50 in 10m in (Vanadium octaethylporphyrin); H:O/CH3CN/CH3OH 40/30/30 (Nickel octaethylporphyrin); H:O/CH3CN/CH3OH 5/47.5/47.5 (vanadylporphyrin mixture). Mass spectrometer. Vestee Model 201 dedicated Thermospray-Electron Impact combined Liquid chromatography-Mass spectrometry system. Data station. A Compaq 386 computer with Teknivent Vector One software. Thermospray conditions. Full scan from m/z 120 to 700 in 3.5 s. Filament-on ionization and positive ion detection. Tj (control temperature)= 130-127°C; T2 Scan at 16.8665

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Fig. 1. Ion chromatogram of (a) vanadyl octaethylporphyrin in the thermospray positive ion mode, (c) vanadyl octaethylporphyrin in the particle beam El mode and (¢) nickel octavthylporphyrin in the particle beam E1 mode. The mass spectra of vanadyl octaethylporphyrin in the positive ion and the EI mode and or" nickel octaethylporphyrin in the El mode are shown in (b), (d) and (t') respectively. The amount of vanadyl octaethylporphyrin injected is the same in the thermospray positive ion (a) and the particle beam EI mode (c). The higher ion count in the particle beam El mode shows greater sensitivity.

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80 90 1O0 60 70 T i m e (rain) Fig. 2. (a) The total ion chromatogram and (b) chromatogram of ions s u m m e d between 400 and 700 mass units of a m]xture of vanadyl porphyrins isolated from a Monterey source rock. (c) The chromatograms of the same mixture obtained with u.v.-vis spectrophotometric detector (l = 4 0 6 n m ) is shown for comparison. The number indicates identical peaks, the structures of which are shown in Fig. 4. 40

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Short Note particle beam electron impact mode is shown in Fig. I. Three times better sensitivity is obtained in the particle beam EI mode [Fig. l(c)] than the thermospray positive ion mode [Fig. l(a)]. The mass spectrum of vanadyl octaethylporphyrin [Fig. l(d)] in the EI mode shows a strong molecular ion (m/z 599) with fragmentation consistent with successive methyl losses via the benzyllic cleavage of ethyl substituents. This mass spectrum also shows a doubly charged ion (m/z 249.5). In the thermospray positive ion mode [Fig. l(b)] there is no fragmentation. The mass spectrum shows only the (M + 1) ion at m/z 600. The EI mode offers limited structural information, i.e. the nature of the substituent groups attached to the periphery of the molecule, compared to the positive ion mode. But even in the EI mode the structural

(Vaporizer temperature)=200-193°C; T3 (source temperature) = 317°C; T4 (Tip heater temperature) = 315°C; 7"5 (vapor temperature)= 282°C; T6 (Lens heater temperature)= 130°C. Particle beam-electron impact conditions. Full scan from m/z 45 to 700 in 4.1 s. T~ (control temperature) = 73°C; T2 (Vaporizer temperature)= 128°C; 7'3 (source temperature) = 310°C; Tc] (spray chamber # 1 ) = 1 2 8 ° C ; Tc2 (spray chamber # 2 ) = 4 2 ° C ; Tram (membrane separator) = 33°C; Tin, (momentum separator) = 118°C; T6 (Lens heater temperature) = 127°C. RESULTS AND DISCUSSION

The L C - M S trace of vanadyl octaethylporphyrin in the thermospray positive mode as well as in the

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Short Note information is limited to the nature of the substituent and not to their exact position around the periphery of the molecule. Complete structural information including the exact arrangements of the substituents around the periphery of the porphyrin nucleus can be obtained using other analytical techniques such as N M R (Callot, 1991). Van Berkel et aL (1991) have demonstrated that limited structural information can be obtained using the electrospray ionization technique. In the electrospray ionization technique the solvent system used in the analysis of vanadyloctaethylporphyrin contains either toluene or methylene chloride, methanol and an acid such as acetic or trifluoroacetic acid (see Table 1 in Van Berkel et al., 1991). This may pose some limitation on the use of this technique for the analysis of geoporphyrins at the present time, since the current HPLC methods (Sundararaman and Boreham, 1993; Barwise et al., 1986; Ocampo, 1985; Boreham and Fookes, 1989; VerneMiser et al., 1991) use different solvent systems. An HPLC method using the solvent system compatible with that used in the electrospray ionization technique has to be developed before this method can be routinely used for the analysis of geoporphyrins. Figures l(e) and l(f) show the LC-MS trace of nickel octaethylporphyrin and mass spectrum in the E1 mode respectively. The mass spectrum shows a molecular ion with fragmentation patterns, characteristic of porphyrins (Baker et al., 1967). A mixture of vanadyl porphyrins obtained from a Monterey source rock was analysed in the particle beam El mode. The total ion chromatogram of the vanadyl porphyrin mixture [Fig. 2(a)] shows several peaks due to non-porphyrin compounds. The nonporphyrin compounds can be eliminated by summing the ions between m / z 400 and 700 mass. Comparison of the resulting chromatogram [Fig. 2(b)] with that obtained using a u.v.-vis spectrophotometric detector (2 = 400 nm) shows that there is only minor loss of resolution due to the parUcle beam interface in the LC-MS mode_ In the LC-MS mode, the resolution can be improved further by plotting the mass chromatograms at various mass units. The mass chromatograms at m / z 499, 501, 513, 525, 527, 541, 553 and 555 and the corresponding mass spectrum of the starred peak are shown in Fig. 3. The mass spectra primarily contain the molecular ion. The fragmentation is confined to the periphery of the molecule. Use of the mass spectrometer as the detector instead of the u.v.-vis spectrophotometer adds another dimension to the chromatograms. Sensitivity of LC-MS is much lower compared to the u.v.-vis spectrometric detector as evidenced by the signal to noise ratio of the LC-MS chromatogram. These results are preliminary and no attempt was made to optimize the conditions. LC-MS offers the advantage of identifying individual peaks in the chromatograms as to their carbon number and skeletal series. Further, tentative identification can be made by coinjectmn of standards and mass spectral comparison.

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1) R I = H, R2 = Et, C30 5) C33DiDPEP

2) R2 = H, RI= El, C30 3) R1 = Me, R2 = Et, C31 4) RI= R2 = Et, C32

Fig. 4. Structures of numbered peaks in F~g. 2. CONCLUSIONS Vanadyl porphyrins can be analysed by LC-MS using a thermospray or a particle beam interface. Three times better sensitivity can be obtained using a particle beam rather than a thermospray interface. However, the production of only quasi-molecular ion (M + 1) ÷ in the thermospray positive ton mode may be advantageous during mixture analysis. Analysis of a mixture of vanadyl porphyrins using LC-MS in the particle beam El mode shows that there is little loss of resolution in coupling an HPLC to MS using the particle beam interface. In the MID mode the MS detector provides an additional dimension to resolution compared to the u.v.-vis spectrophotometric detector. Acknowledgements--We thank Chevron Oil Field Research

Company for permission to publish this work and J. M. Moldowan and J. Dahl for their comments which greatly helped to improve the manuscript. Reviews by J. W. Louda and G. van Berkel helped to improve the manuscript.

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Short Note

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