Comparison of electron ionization and fast atom bombardment-mass spectrometry for the determination of nickel, vanadyl and free-base porphyrins

Org. Geochem. Vol. 14, No. 2, pp. 193-202, 1989 Printed in Great Britain, All rights reserved

0146-6380/89 $3.00+ 0.00 Copyright © 1989 Pergamon Press pie

Comparison of electron ionization and fast atom bombardment-mass spectrometry for the determination of nickel, vanadyl and free-base porphyrins AIDALU JOUBERT CASTRO*, GARY J. VAN BERKELt, FRANK G. DOOLITTLE and ROYSTON H. FILBY Department of Chemistry and Nuclear Radiation Center, Washington State University, Pullman, WA 99164-4630, U.S.A. (Received 15 April 1988, accepted 30 August 1988)

Abstract--Fast Atom Bombardment-Mass Spectrometry (FAB-MS) and Electron Ionization-Mass Spectrometry (EI-MS) at 12 and 70eV, were used to obtain mass spectra of mesoporphyrin IX dimethylester (DME), tetraphenylporphyrin (TPP), octaethylporphyrin (OEP), and the metalloporphyrins, Ni(DME), Ni(TPP), Ni(OEP), VO(TPP), VO(OEP), as well as a VO(II) porphyrin concentrate obtained from the New Albany oil shale bitumen (Mississippian-Devonian). A mixture of dithiothreitol/dithioerythritol ("Magic Bullet") was used as the FAB matrix. Greater fragmentation of free-base and metalloporphyrins was observed in FAB mass spectra compared to the El mass spectra. Adduct ions formed by addition of sulfur and a matrix molecule to the porphyrins were observed. In FAB spectra of the VO(II) complexes, loss of oxygen was noted. The FAB mass spectra of mixtures of VO(II) geoporphyrins are much more complex than corresponding El mass spectra because of the greater fragmentation and the multiplicity of ions (M +, M + H, M + 2H, etc.) observed in the FAB mode. Using the matrices investigated, FAB is less suitable for El for the mass spectrometric analysis of the geoporphyrins. Key words--nickel, vanadyl, free-base porphyrins, FAB-MS, EI-MS

INTRODUCTION

ber range, metal type and DPEP/etio ratio) have been used as source-rock maturation indicators (MackThe discovery by Treibs (1934, 1936) of porphyrins, enzie et al., 1980; Moldowan et al., 1986; Barwise, which are structurally related to chlorophyll (or 1987) and as parameters in oil-oil and oil-source heme), in petroleum, shales, and coal helped to rock correlations (Hajlbrahim et al., 1981; Palmer, confirm the organic origin of petroleum. These geo1983; Branthaver and Filby, 1987). The relative abunporphyrins are found predominantly in crude oils and dances of Ni(II) and VO(II) porphyrins have also mature sediments as Ni(II) and VO(II) complexes of been used as indicators of the physicochemical conditwo major skeletal types: deoxophylioerythroetiotions (Eh, pH, etc.) of depositional environments porphyrin (DPEP; 1) and etioporphyrin (etio; 2), but (Lewan, 1984; Moldowan et al., 1986). lesser amounts of other skeletal types have been Techniques for the analysis of geoporphyrins exidentified (Chicarelli et al., 1987). Copper (II) portracted from sediments or oils must be capable of phyrins have also been identified in immature sedihigh selectivity and/or sensitivity because only small ments (Palmer and Baker, 1978) and small amounts amounts of material are usually available (Quirke, of Fe(III), Mn(III), and Ga(III) porphyrins have 1987). Identification and determination of geobeen identified in coals (Bonnett et al., 1987; Palmer porphyrins may be done using UV-visible (UV-VIS) et aL, 1980). The importance of the geoporphyrins spectroscopy (Smith, 1975a), but little structural inderives, in part, from their use as biomarkers since formation can be obtained, especially in the case of the major geochemical pathways from chlorophylls, the metalloporphyrins. Gas chromatography (GC) and other possible precursors, to the geoporphyrins and GC-MS have had limited application because of have been fairly well established (Baker and Louda, the relative involatility of metalloporphyrins (Mar1983, 1986; Filby and Van Berkel, 1987). For examriott et al., 1982). High performance liquid chrople, metalloporphyrin distributions (e.g. carbon nummatography (HPLC) has been used for porphyrin characterization of free-base porphyrins (Barwise et al., 1986; Chicarelli et al., 1986) and VO(II) por*Present address: Universidad Interamericana de Puerto phyrins (Sundararaman, 1985). Both Nuclear MagRico, Colegio Regional de Ponce, Ponce, PR 00731, netic Resonance (NMR; Ocampo, 1987; Chicarelli et U.S.A. tPresent address: Analytical Chemistry Division, Oak al., 1987) and X-ray crystallography (Ekstrom et al., Ridge National Laboratory, Oak Ridge, TN 37831, 1983) techniques have been used for the deterU.S.A. mination of the absolute structures of single cornOG 14/2--F

193

194

AIDALUJOUBERTCASTROet al.

R--~

~

R

R~

~R

R

RI

R

RI

© 4

I

© OH OH H 5

&.,

H'-~OH

H~-~--H CH2SH 6

&.,

T

pounds, but both methods are limited by the need for relatively large amounts (ca 1 mg) of a pure isomer. Mass spectrometry (MS) is a technique commonly used for the analysis of geoporphyrin mixtures and the availability of several ionization methods (e.g. electron ionization, El; chemical ionization, CI) adds versatility to the technique (Gallegos and Sundararaman, 1985). Depending on the ionization method used and the complexity of the sample, the carbon number range, skeletal type, peripheral substituents, and pyrrole sequence of the porphyrins can be determined. The most popular method of geoporphyrin analysis has been EI-MS since the porphyrins undergo little fragmentation compared to many metal-organic complexes, resulting in high sensitivity and high molecular ion intensities (Smith, 1975b). Also, since most samples are analyzed using a heated solids probe, volatile impurities can be removed by selective ramping of the probe temperature. However, metalloporphyrin mixtures must normally be of high purity when analyzed by El-MS to avoid interactions with non-volatile impurities in the source that may change the volatilization pattern, or possibly demetallate or decompose the complexes. This is particularly true

for the Ni(II) porphyrins. Also, corrections must be applied to the abundances of low carbon-number molecular ions for coincident fragment ions arising from 13-cleavage of alkyl groups on higher molecular weight molecules. This problem can be minimized by operating at very low ionization voltages as suggested by Louda and Baker (1987) (e.g. < 12 eV), but this results in a loss of sensitivity. Fast Atom Bombardment (FAB) is a "soft" ionization technique that has been effectively used for mass spectrometric determination of large involatile and thermally labile molecules (Barber et al., 1982; Fenselau and Cotter, 1987). In the FAB ionization mode, heating of the sample is not required, and for many large molecules, less fragmentation is observed compared to El. Relatively few reports of the analysis of porphyrins by FAB-MS appear in the literature (Barber et al., 1982; Meili and Seibl, 1983; Ekstrom et al., 1983a; Kurlansik et al., 1983; Musselman and Chang, 1985; Gallegos and Sundararaman, 1985; Musselman et al., 1986; Forest et al., 1987). Most of these studies have involved highly functionalized porphyrins such as the chlorophylls and vitamin Bu, which are structurally different from the alkyl geoporphyrins. Application of FAB-MS to the geo-

Comparison of electron ionization and fast atom bombardment porphyrins include the study by Ekstrom et al. (1983b), who presented spectra of a VO(II) porphyrin fraction from the Julia Creek shale (Toolebuc Formation, Australia), the analysis of a VO(II) porphyrin fraction from Meride shale (Venezuela) by Gallegos and Sundararaman (1985), and the identification of a hydroxylated DPEP porphyrin isolated from the Marl Slate (U.K.) by Krane et al. (1984). In these geoporphyrin studies, little information on FAB-MS conditions was given and no attempts were made to relate spectral characteristics to analytical parameters or porphyrin structure. This paper reports a comparison of FAB-MS and El-MS analysis of porphyrins in terms of sensitivity, selectivity, fragmentation, and ease of spectral interpretation. Three porphyrin types were analyzed: (i) a meso-substituted porphyrin, tetraphenylporphyrin (TPP, 3); (ii) an alkyl substituted porphyrin, octaethylporphyrin (OEP; 4); and (iii) a functionalized porphyrin, mesoporphyrin IX dimethylester (DME; 5). The metal complexes, Ni(TPP), Ni(DME), Ni(OEP), VO(TPP), VO(OEP), and a VO(II) porphyrin mixture isolated from the bitumen of the New Albany shale (Mississippian-Devonian, Indiana) were also analyzed. For the FAB-MS analyses, the FAB matrices investigated were sulfuric acid, phosphoric acid, glycerol, glycerol-Triton X and a mixture of dithioerythreitol and dithiothreitol.

195

volatile impurities. The sample was introduced into the EI source and the nominal probe temperature ramped from 250 to 350°C at l°/s, and then from 350 to 460 ° at 10°/s. Data acquisition was continued over the entire volatility range of the porphyrins. Porphyrins in the glass crucible were not exposed to temperatures higher than 350°C because of poor heat transfer between the probe heating coils and the crucible. In the FAB-MS experiments, the porphyrins were dissolved in a small volume of dichloromethane and applied to the FAB probe tip which had been previously coated with the "Magic Bullett" matrix (Gower, 1985). This matrix is composed of the structural isomers, dithioerythritol (6; m.p. 82-84 °) and dithiothreitol (7; m.p. 42--44°; Aldrich Chemical Co.) in a weight ratio of 1 : 5. The sample was bombarded with 8 keV Xe atoms from the FAB gun at a bombarding angle 45 ° to the sample. The FAB spectra were calibrated for mass using glycerol. RESULTS

The Ni(TPP), VO(TPP) (Strem Chemicals, Inc.), OEP, TPP and DME (Aldrich Chemical Co.) were used without further purification. The Ni(DME), Ni(OEP), and VO(OEP) were synthesized from the free-base porphyrins and appropriate metal salts using the procedures described by Buchler (1975). Isolation of the VO(II) porphyrin mixture from the New Albany shale bitumen is described elsewhere (Van Berkel, 1987). The major components of this mixture are VO(II) DPEP and VO(II) etio porphyrins with minor amounts of VO(II) tetrahydrobenzo DPEP porphyrins.

Several FAB matrices were investigated for the analysis of porphyrins. The "Magic Bullet" was found to give reproducible mass spectra of adequate intensity for the porphyrins. Sulfuric acid, phosphoric acid, glycerol and glycerol-Triton X gave low intensity mass spectra, particularly for the metal complexes. In the case of sulfuric and phosphoric acid, low solubility of the metalloporphyrins and partial to complete demetallation of the Ni(II) and VO(II) complexes made these matrices unsuitable. Glycerol and glycerol-Triton X matrices gave low intensity spectra probably as a result of the very low solubility of the metalloporphyrins in these solvents. Musselman et al. (1986) have shown that both dissolution of the porphyrin and the formation of the protonated species in the liquid matrix are necessary for high (M + H) ÷ ion intensities of porphyrins in FAB. These authors showed that OEP gave good FAB spectra in thioglycerol whereas a diporphyrin, insoluble in thioglycerol, could not be effectively ionized.

Mass spectrometry

Free-base porphyrins

EXPERIMENTAL

Porphyrins

All mass spectra were acquired on a VG 7070E Figure 1 compares the E1 (12 and 70 eV) with the double-focusing, high resolution mass spectrometer. FAB mass spectra of free-base DME (Fig. la), TPP The El mass spectra were acquired using a direct (Fig. lb), and OEP (Fig. Ic), respectively. The mass solids probe. The instrument was calibrated using ranges shown cover the molecular ions and the major perfluorokerosene and operated at an accelerating fragment or adduct ions, but are not the same for all potential of 6 keV, a source temperature of 190°C, spectra. For each of the free-base porphyrins anaand at 12 and 70eV ionization energies. The por- lyzed, the odd-electron species, M + ", is predominant phyrin compounds, or mixtures, were dissolved in in the EI mass spectra, whereas the protonated dichloromethane and an aliquot of the solution was molecule, (M + H) ÷, is predominant in the FAB allowed to dry at room temperature in a shallow cup spectrum. For DME, the 12 eV E1 spectrum shows loss of at the end of a 3mm i.d. x 20mm length glass capillary (Strong, 1986). The capillary was placed in 31 amu (---OCH3) from the propionic ester group and the solids probe and heated in the vacuum lock of the the small peaks at m/z 614 and 679 in this spectrum mass spectrometer at 250°C for 5 rain to remove are attributed to memory effects from free base TPP

AIDALU JOUBERT CASTRO et al.

196

,3

.3

±

.~

÷

.3

!

,

÷

w

a

1

~,~..3

< u.

e.,

÷

3

3

+

~,$ .

÷

w~

~.|

Comparison of electron ionization and fast atom bombardment and VO(TPP), respectively, which were run prior to this sample. The 70 eV El mass spectrum also shows fragment ions from the loss of 4EH:COOCH3 (m/z 521) and ~COOCH3 (m/z 535). The (M + 54) + ion is assigned to Fe(DME) formed by metallation of the free-base by iron in the ion source of the mass spectrometer. The FAB mass spectrum shows significantly more fragmentation than the 12 or 70 eV E1 spectra. Losses of 15 mass units (-CH3) via fl-cleavage of the ethyl groups and 31 (--OCH3), 59 (-'CO2CH3) , and 73 (-'CH2CO2CH3) mass units, respectively, from the propionic ester side chain are the major fragmentation modes. An interesting feature in the FAB mass spectrum is the presence of the (MH + 32) + ion believed to result from the abstraction of sulfur from the FAB matrix producing (M + H + S) + or (M + HS) + ions or, less likely, the addition of molecular oxygen (02) to the molecule. The EI mass spectra of TPP show relatively little fragmentation. The peak at m/z 675 is attributed to Zn(TPP), which is believed to be an impurity in the sample since it was also observed in the FAB spectrum. The FAB spectrum again shows more fragmentation that is observed in the El spectra. The m/z 539 peak results from the loss of one phenyl group and, although not shown, losses of two and three phenyl groups were also observed in the FAB spectrum. The (MH + 32) + adduct peak was also observed. The EI mass spectra of OEP show sequential losses of up to 3 methyl groups, resulting from fl-cleavage of the ethyl groups on the OEP macrocycle. In the 70 eV EI spectrum, the molecular ion of Fe(OEP), m/z 588, is observed and was probably formed in the ion source. A similar fragmentation pattern is observed in the FAB mass spectrum, but the degree of fragmentation is greater and the background in the form of clusters around the major ion is much more pronounced. The clusters around the fragment ions in the FAB spectrum may arise in part from fl-cleavage from the molecular ion and both ~- and fl-cleavage from the protonated species. The (MH + 32) + adduct ion is also observed in the FAB spectrum. For all free-base porphyrins, the molecular ion, M ÷', is the most abundant ion in the EI spectra, whereas the protonated molecule, (M + H)+, is the predominant species in the FAB spectra. A higher degree of fragmentation is observed in the FAB spectra than occurs for either 12 or 70 eV, particularly for the alkyl porphyrin, OEP. In the free-base FAB spectra, the degree of fragmentation is lowest for TPP, presumably because fragmentation involves breaking the bond to the meso carbon, whereas in DME and OEP, cleavage along the side chains is possible, and dominant. For all free-base species, fragmentation is greater at 70eV than at 12eV in EI-MS.

197

Ni(OEP) (Fig. 2c). Minimal fragmentation is noted in both El spectra compared to that observed in the FAB mass spectra. In the FAB spectra, fragment ions originating from either the protonated molecule ( M + H ) +, the molecular M +', or the (M-H) + species complicate the mass spectra. Although M +" is the most abundant species in the FAB spectrum of Ni(DME) (Fig. 2a) in contrast to (M + H) ÷ in the free-base porphyrins, the (M + H) + and ( M - H) + ions are also of relatively high intensity, which, when combined with the Ni isotopic cluster pattern, leads to the broad clusters around the molecular and fragment ions. Fragment ion patterns similar to those observed in the FAB mass spectrum of free-base DME are observed, as is the (M + 32) + adduct. The E1 mass spectra of Ni(TPP) (Fig. 2b) show that relatively little fragmentation of the Ni(II) complex occurs compared to that shown in the FAB • spectrum. The peak at rn/z 594 in the FAB spectrum can be assigned to loss of a phenyl group from the (M + H) + ion and the m/z 613 peak is apparently the result of demetallation (i.e. MH - 58) that occurs in the FAB mode, but not to any appreciable extent in El. This demetallation probably occurs in the liquid matrix. The E1 and FAB mass spectra of Ni(OEP) (Fig. 2c) show that more fragmentation occurs in the FAB mode than in El-MS. The FAB spectrum is of considerably greater complexity than the E1 spectra and the FAB spectra of Ni(DME) or Ni(TPP). Beta-cleavage fragments from the ethyl groups is noted for all 8 substituents in 70 eV E1 and FAB. In all mass spectra, free-base OEP was observed [i.e. m/z 534 (El) and m/z 535 (FAB)] and the UV spectrum of this sample indicated free-base OEP from the synthesis of the metal complex as an impurity. In the FAB mode the (M ÷" ion has a higher relative intensity than does the (M + H) ÷ ion. The (MH + 32) ÷ adduct was detected in the FAB mode only. Figure 3 compares the E1 and FAB mass spectra of VO(TPP) (Fig. 3a), VO(OEP) (Fig. 3b), and a VO(II) geoporphyrin mixture isolated from the New Albany shale (Fig. 3c). The El spectra of VO(TPP) indicate that very little fragmentation occurs, even at 70 eV. In all mass spectra, a trace of free-base TPP (m/z 614) is observed, probably as an impurity in the VO(TPP). More fragmentation can be seen in the FAB mode than in El; the peak at m/z 603 results from loss of one phenyl group ( M H - 77). One unusual feature observed in the FAB mass spectrum is the (M - 16) ÷ peak at m/z 663 that can tentatively be assigned to the loss of an oxygen atom from the V~----O2÷ ion. A similar ( M - 16) loss is observed in the FAB mass spectrum of VO(OEP) (Fig. 3b). The loss of oxygen may be associated with reduction of V(IV) to V(VIII) MetalloporPhyrins or possibly to V(II) as suggested by Rochsteiner Figure 2 compares the EI and FAB mass spectra (reported in Gailegos and Sundararaman, 1985). If for Ni(DME) (Fig. 2a), Ni(TPP) (Fig. 2b), and reduction does occur in the FAB ionization mode, it

198

A1DALU JOUBERT CASTRO et al.

4+

+~ -.a

o

4+

.|

a.

r~

~°

>

!

Q

i"

z~

J

.

~

~

Comparison of electron ionization and fast atom bombardment must occur in the FAB liquid matrix because reduction and loss of oxygen does not occur in the E1 mode during electron bombardment. A similar loss of oxygen ( - 1 6 a m u ) is seen in the FAB spectra of TiO(TPP) reported by Forest et al. (1987). An alternative explanation is that fragmentation at the metal-oxygen bond occurs. The E1 spectra of VO(OEP) (Fig. 3b) show much less fragmentation compared to the FAB spectra. Fragmentation via ~-cleavage results in up to eight methyl groups being lost from the molecule. At least nine carbons are lost from the molecule in FAB, indicating that some ~t-cleavage occurs. The molecular ion, M ÷ ', is most abundant in all three spectra. The (MH + 32) + adduct was observed in the FAB spectrum. An interesting feature of the FAB spectrum is the appearance of a peak at m / z 752 (not shown), which is attributed to the addition of one molecule of the matrix to the porphyrin, e.g. (M + 154)+ Similar additions of matrix molecules have been observed in FAB spectra with thioglycerol as the matrix (Kurlansik et al., 1984). The E1 and FAB mass spectra for the VO(II) porphyrin concentrate isolated from the New Albany shale bitumen are shown in Fig. 3c. The carbon number ranges of the VO(II) porphyrins in the El and FAB spectra are similar (C28-C35) and DPEP and etio porphyrins are the major components. The FAB spectrum, however, is clearly more complex. Greater fragmentation is observed in the FAB mode and the clusters around the major fragment and porphyrin homolog ions [from (M + H) +, M ÷', and (M - H) + ions] make the interpretation of mixtures of the DPEP and etio series much more difficult than is the case for El mass spectra. It would appear that the carbon number range extends at least one carbon number lower in the FAB spectrum compared to El-MS, but this cluster of ions may be due to fragment ions arising from fragmentation of higher molecular weight homologs. It is also evident that the DPEP/etio ratio (SID/EIE) of the VO(II) porphyrins in the FAB spectrum is different from that of both El spectra. Table 1 shows some of the possible contributions to the peaks at m/z 513 and m / z 515 that correspond to the C30 VO(II) DPEP and VO(II) etio porphyrins, respectively, in the FAB spectrum. The large number of potential contributions to each peak makes the calculation of individual porphyrin molecular ion intensities very difficult and subject to a number of significant corrections, many of which cannot be determined. DISCUSSION

Although FAB is commonly classified as a "soft ionization" technique, the FAB mass spectra of all the porphyrins analyzed show that greater fragmentation occurs compared to El-MS. An explanation for this may be the multiplicity of protonated and deprotonated ions that are produced in the FAB

199

ionization process which are not produced in EI. Ions such as (M + H)+, the reduced species, (M + 2H) ÷ and ( M + 3 H ) ÷, and ( M - H ) + formed by the matrix-porphyrin interactions may be more susceptible to fragmentation than the M ÷" ion. Thus, protonation and reduction of the molecule appears to open up additional modes of fragmentation besides the normal /~-cleavage from M ÷" that is the major fragmentation mode in El mass spectra. Bombardment of the sample matrix with 8 keV Xe atoms in the FAB method more than likely imparts higher internal energies to the porphyrin molecules than does 12 or 70 eV electron bombardment in E1 mode, resulting in the enhanced fragmentation. In addition, the FAB ionization mechanism appears to cause reduction of vanadium in the vanadyl porphyrins from (IV) to (III) or (II) which results in the loss of the oxygen ligand from the vanadyl ion. This effect was observed for VO(OEP) and VO(TPP), and might occur in the VO(II) geoporphyrin sample. Except for Ni(TPP) and VO(TPP), the major metalloporphyrin ion in the FAB mode is M +" compared to (M + H) ÷ in the free-base porphyrins. This is probably due to the reduced basicity of the metalloporphyrins compared to the free-base species in the FAB matrix, resulting in a lower tendency for protonation. The exceptions, Ni(TPP) and VO(TPP), show the (M + H) + ion as the base peak and this is probably due to increased basicity of these species compared to the metallo alkylporphyrins as a result of the electron releasing character of the phenyl groups on the meso positions. The much greater complexity of the FAB spectrum of the geoporphyrin sample implies that the isolation of VO(II) porphyrins from bitumens, crude oils, and source rocks requires porphyrin concentrates of higher purity for FAB than for EI mass spectrometry. The amount of porphyrin necessary to obtain interpretable FAB spectra was qualitatively determined to be higher than required for El-MS; thus the sensitivity of FAB for porphyrin analysis is less than in El-MS under the conditions of these experiments. The poor sensitivity for the metalloporphyrins may be the result of the low solubility of these compounds in the FAB matrix. Experimentation with other FAB matrices may improve the sensitivity for metalloporphyrins. CONCLUSIONS

From this study it can be concluded that: (i) The "Magic Bullet" matrix gives higher sensitivity for the porphyrins than does sulfuric or phosphoric acids, and allows analysis of metalloporphyrins if available in sufficient quantity and purity. Use of sulfuric and phosphoric acid demetallates Ni(II) and VO(II) porphyrins. (ii) Sensitivity and selectivity in FAB-MS using the "Magic Bullet" matrix appears to be poorer than with El-MS.

AIDALU JOUBERT CASTRO et al.

200

V

~v___~ ~-~ 7~ t~

Q o~

~2

O eL

+-

A

v

. i!

o~

EL

Comparison of electron ionization and fast atom bombardment

201

"Fable 1. Possible ion contributions to C~ VO(II) etio (m/z 515) and C30 VO(II) DPEP (m/z 513) peaks in FAB mass spectrum m/z Assignment Origin of contribution 515

513

C30etio C30D (M + 2H) + C30D(M + 2) + C30D (M + H + 1)+ C3~E (M + H - 15) + C31E (M + 1 - 15) C31D (M + 2 + H - 15)+ C3tE ( M + 2 - 1 6 ) + C32E (M h- H -- 29) + C32E (M + 2 - 30) + C32E (M + 2 H - 30) + C30DPEP C30E ( M - 2H) + C31D (M + H - 15)+ C31D (M + 1 - 15)+ C3~D (M + 2H - 16)+ C32D (M + H - 29) + C32D (M + 1 - 2 9 ) + C32D (M + 2 - 30) +

M +" etio molecular ion di-protonated DPEP species 2 × 13C, 2 × 15N, ~3C-I~N isotopic ion protonated ~3C species -CH3; //-cleavage from (M + H) + -CH3; //-cleavage from ~3C or LSN species -CH3; //-cleavage from 2xl3C (M + H+) ÷ -O from V==0 ion from (M +2H) + or (M+2) + 422H5 (a-cleavage from protonated species) -2CH3; 0-cleavage from 2 × 13C (2 × tSN) -2CH3; //-cleavage from protonated species M +" (normal molecular ion) FAB matrix ion -CH3; //-cleavage from protonated species -CH3; //-cleavage from 13C, ISN species loss of oxygen from VO2+ ion -C2H5; a-cleavage from protonated species ~2H5; ~t-cleavage from 13C, 15N species -2CH3; //-cleavage from 2 x 13C, 15N species

(iii) F r a g m e n t a t i o n o f the p o r p h y r i n s is m o r e extensive in the F A B ionization m o d e c o m p a r e d to El at 12 or 70 eV. This is the case for b o t h free-base a n d metallated porphyrins. (iv) T h e p r o t o n a t e d molecule, (M + H) ÷, is the d o m i n a n t ion observed in the F A B - M S spectra o f the free-base p o r p h y r i n s a n d for V O ( T P P ) a n d Ni(TPP), whereas the molecular ion, M + ", was observed for the o t h e r metalloporphyrins. This is p r o b a b l y a result of the reduced basicity of m e t a l l o p o r p h y r i n s c o m p a r e d to the c o r r e s p o n d i n g free-base species in the F A B matrix. Thus, the ionization of the metallo species occurs p r e d o m i n a n t l y by stripping o f an electron from the molecule, whereas the free-base species are protonated. (v) Reactions o f the p o r p h y r i n s with the m a t r i x occurs p r e d o m i n a n t l y by stripping a n electron f r o m the molecule, whereas the free-base species are A n a d d u c t ion, (M + 154)+, was formed by addition o f a molecule of the matrix (dithioerythritol or dithiothreitol) was observed in the V O ( T P P ) F A B mass spectrum. (vi) In the F A B mode, reduction of V(IV) to V(III) in VO(II) p o r p h y r i n s m a y occur, resulting in loss of the oxygen a t o m from the vanadyl ion. (vii) Multiplicity of ions (M + H + , M + 2 H + , M - H ÷, etc.) a n d greater f r a g m e n t a t i o n in the F A B m o d e m a k e analysis of complex mixtures of geop o r p h y r i n s difficult. (viii) F o r m o s t geochemical purposes, E l - M S is a better technique t h a n F A B - M S for analysis of geop o r p h y r i n mixtures.

Acknowledgements--One of the authors (A. J. C.) would like to acknowledge receipt of a National Research Service Award from the U. S. Public Health Service, N. I. H. (Grant # 5F31-GM09637-04) for partial support of this work.

REFERENCES

Baker E. W., Louda J. W. (1983) Thermal aspects in chlorophyll geochemistry. In Advances in Organic Geo-

chemistry 1981 (Edited by Bjoroy M. et al.), pp. 401-421. Wiley, Chichester. Baker E. W. and Louda J. W. (1986) In Biological Markers in the Sedimentary Record (Edited by Johns R. B.), pp. 125-225. Elsevier, Amsterdam. Barber M., Bordoli R. S., Elliott G. J., Sedgwick D. and Tyler A. N. (1982) Fast atom bombardment mass spectrometry. Anal. Chem. 54, 645A-657A. Barwise A. J. G. (1987) Mechanisms involved in altering deoxophylloerythroetioporphyrin-etioporphyrin ratios in sediments and oils. In Metal Complexes in Fossil Fuels Syrup. Ser. 344 (Edited by Filby R. H. and Branthaver J. F.), pp. 100-109. American Chemical Society, Washington, D.C. Barwise A. J. G., Evershed R. P., Wolff G. A., Eglinton G. and Maxwell J. R. (1986) High-performance liquid chromatographic analysis of free-base porphyrins. I. An improved method. J. Chromatogr. 368, I-9. Bonnett R., Burke P. J. and Czechowski F. (1987) Metalloporphyrins in lignite, coal, and calcite. In Metal Complexes in Fossil Fuels (Edited by Filby R. H. and Branthaver J. F.), pp. 173-185. American Chemical Society, Washington, D.C. Branthaver J. F. and Filby R. H. (1987) Application of metal complexes in petroleum to exploration geochemistry. In Metal Complexes in Fossil Fuels Symp. Set. 344 (Edited by Filby R. H. and Branthaver J. F.), pp. 84-99. American Chemical Society, Washington, D.C. Buchler J. W. (1975) Static coordination chemistry of metalloporphyrins. In Porphyrins and Metalloporphyrins (Edited by Smith K. M.), pp. 157-231. Elsevier, Amsterdam. Chicarelli M. I., Kaur S. and Maxwell J. R. (1987) Sedimentary porphyrins: unexpected structures, occurrence, and possible origins. In Metal Complexes in Fossil Fuels Symp. Ser. 344 (Edited by Filby R. H. and Branthaver J. F.), pp. 40-67. American Chemical Society_, Washington, D.C. Chicarelli M. I., Wolff G. A. and Maxwell J. R, (1986) High-performance liquid chromatographic analysis of free-base porphyrins. II. Structure effects and retention behavior. J. Chromatogr. 368, 11-19. Ekstrom A., Fookes C. J. R., Hambley T., Loeb H. J., Miller S. A. and Taylor J. C. (1983a) Determination of the crystal structure of a petroporphyrin isolated from oil shale. Nature 306, 173-174. Ekstrom A., Loeh H. and Dale L. (1983b) An examination of the petroporphyrins found in oil shale from the Julia Creek deposit of the Toolebuc Formation. ACS Div. Pet. Chem., Preprints 28, 166-176. Fenselau C. and Cotter R. J. (1987) Chemical aspects of fast atom bombardment. Chem. Rev. 87, 501-512.

202

AIDALU JOUBERTCASTROet al.

Filby R. H. and Van Berkel G. J. (1987) Geochemistry of metal complexes in petroleum source rocks and coals: An overview. In Metal Complexes in Fossil Fuels Syrup. Ser. 344 (Edited by Filby R. H. and Branthaver J. F.), pp. 2-39. American Chemical Society, Washington, D.C. Forest E., Ulrich J., Marchon J. C. and Virelizier H. (1987) Comparison of El, FAB, and DCI (electron capture mode) mass spectra of hydrolytically labile dihalogenotitanium (IV) porphyrins. Org. Mass Spectrom. 22, 45-48. Gallegos E. J., Sundararaman P. (1985) Mass spectrometry of geoporphyrins. Mass Spectrom Rev. 4, 55-85. Gower J. L. (1985) Matrix compounds for fast atom bombardment mass spectrometry. Biomed. Mass Spectrom. 12(5), 191-196. Hajlbrahim S. K., Quirke J. M. E. and Eglinton G. (1981) Petroporphyrins V. Structurally-related porphyrin series in bitumens, shales and petroleums--evidence from HPLC and mass spectrometry. Chem. Geol. 32, 173-188. Johnstone R. A. W. and Lewis I. A. S. (1983) Crown ether complexes of metallic cations investigated by fast atom bombardment. Int. J. Mass Spectrom. Ion Phys. 46, 451-454. Krane J., Skjetne T., Telnaes N., Bjoroy M., Schou L. and Solli H. (1984) Isolation and identification of two hydroxy C32 and C30 deoxophylloerythroetioporphyrins. Org. Geochem. 6, 193-201. Kurlansik L., Williams T. J., Strong J. M., Anderson L. W. and Campana J. E. (1984) Desorption ionization mass spectrometry of synthetic porphyrins. Biomed. Mass Spectrom. 11(9), 475-481. Lewan M. D. (1984) Factors controlling the proportionality of vanadium to nickel in crude oils. Geochim. Cosmochim. Acta 48, 2231-2238. Louda J. W. and Baker E. W. (1987) Techniques and applications ofgeoporphrin analysis. 194th ACSNational Meeting, Division of Geochemistry, 30 August-4 September. Abstract. Mackenzie A. S., Quirke J. M. E. and Maxwell J. R. (1980) Molecular parameters of maturation in the Toarcian shales, Paris Basin, France--II. Evolution of metalloporphyrins. In Advances in Organic Geochemistry 1979 (Edited by Maxwell J. R. and Douglas A. G.), pp. 239-248. Pergamon Press, Oxford (1982). Marriott P. J. and Eglinton G. (1982) Capillary gas chromatography and gas chromatography-mass spectrometry of silicon (IV) derivatives of porphyrins with polar substituents. J. Chromatogr. 249, 311-321. Meili J. and Seibl J. (1983) Matrix effects in FAB analysis of cobalamines. 31st Ann. Conf. Mass Spectrometry and Allied Topics, pp. 294-295. Extended abstract. Moldowan J. M., Sundararaman P. and Schoell M. (1986)

Sensitivity of biomarker properties to depositional environment and/or source input in the Lower Toarcian of SW-Germany. Org. Geochem. 10, 915-926. Musselman B. D., Watson J. T. and Chang C. K. (1986) Direct evidence for preformed ions of porphyrins in the solvent matrix for fast atom bombardment mass spectrometry. Org. Mass Spectrom. 21, 215-219. Musselman B. D. and Chang C. (1985) A solvent matrix for fast atom bombardment analyses of porphyrins. 33rd Ann. Conf. Mass Spectrometry and Allied Topics, pp. 899-900. Extended abstract. Ocampo R., Callot H. J. and Albrecht P. (1987) Evidence for porphyrins of bacterial and algal origin in oil shale. In Metal Complexes in Fossil Fuels Syrup. Ser. 344 (Edited by Filby R. H. and Branthaver J. F.), pp. 68-73. American Chemical Society, Washington, D.C. Palmer S. E. (1983) Porphyrin distributions in degraded and nondegraded oils from Colombia. 186th American Chemical Society Meet. Washington, D.C. Abstract. Palmer S. E. and Baker E. W. (1978) Copper porphyrins in deep-sea sediments: a possible indicator of oxidized terrestrial organic matter. Science 201, 49-51. Palmer S. E., Baker E. W., Charney L. S. and Louda J. W. (1985) Tetrapyrrole pigments in United States coals. Geochim. Cosmochim. Acta 46, 1233-1241. Quirke J. M. E. (1987) Techniques for isolation and characterization of the geoporphyrins and chlorins. In Metal Complexes in Fossil Fuels Symp. Ser. 344 (Edited by Filby R. H. and Branthaver J. F.), pp. 308-331. American Chemical Society, Washington, D.C. Smith K. M. (Ed.) (1975a) Porphyrins and Metalloporphyrins, 910 pp., Elsevier, Amsterdam. Smith K. M. (1975b) Mass spectrometry of porphyrins and metalloporphyrins. In Porphyrins and Metalloporphyrins (Edited by Smith K. M.), pp. 381-397, Elsevier, Amsterdam. Strong D. (1986) Vanadium and nickel complexes in the Alberta oil-sands Ph.D. Dissertation, Department of Chemistry, Washington State University, Pullman, Wash. Sundararaman P. (1985) High-performance liquid chromatography of vanadyl porphyrins. Anal. Chem. 57, 2204-2206. Treibs A. (1934) Uber das Vorkommen von ChlorophyllDerivaten in einem Olschiefer aus der oberen Trias. Ann. Chem. 509, 103-114. Treibs A. (1936) Chlorophyll-und H/iminderivate in organischem Mineralstoffen. Angew. Chemie. 49, 682686. Van Berkel G. J. (1987) The role of kerogen in the origin and evolution of nickel and vanadyl geoporphyrins. Ph.D. Dissertation, Department of Chemistry, Washington State University, Pullman, Wash.