Energy & Fuels 1990,4,695-699
695
Systematic Preparative Methods for Petroporphyrin Purification A. L. Johnson**+and D. H. Freeman Department of Chemistry and Biochemistry, University of Maryland, Go1lege Park , Mary land 20 742 Received April 27, 1990. Revised Manuscript Received July 30, 1990 The isolation of porphyrins for geochemical research requires multistage concentrations from 10.1% to >95% purity. In meaningful studies of porphyrin concentration the criteria of enrichment, recovery, purity, and time, need to be considered. The present study was therefore undertaken with the aim of optimizing these parameters, in the systematic concentration of 31 ppm of mixed vanadyl porphyrins (VOP) from 2 kg of powdered New Albany shale. The following preparative scale operations were carried out: extraction, deasphaltation, medium-performance liquid chromatography (MPLC), precipitation, and solid phase bonded sulfonic acid (RS03H) extraction, which are frequently used to precede high-pressure liquid chromatography (HPLC). The overall scheme gave a 9000-fold enrichment, 60% overall recovery, 23% pure VOP, and required approximately 80 h (2 weeks) of time.
Introduction Table I. Review of Petroporphyrin Concentration Procedures The discovery of porphyrins in fossil fuels by Treibs was one of the first indications that crude oil and related step conditions materials are linked to biological sources.1y2 The nickel(I1) sample preparation particle size" and vanadyl(I1) porphyrins, NiP and VOP, respectively, dry millingb extraction extracting solvent bear a close relationship to the natural chlorophylls and techniques survive in sediments due to their thermodynamic ~tability.~ a. Soxhletb They have stimulated interest as biochemical markers for b. ultrasonicationC studying the formation and maturation of oils and shalesa4 impurity removal LC on Sephadexd In order to assess preparative methods in a multistep LC on aluminae concentration procedure, it is necessary to determine the asphaltene precipitation' demetalationg enrichment, improvement in purity, and recovery for each TLC on silica" step. This rationale was used to guide us in the method chromatographic deasphaltation' development. The concentration of the VOP fraction in the New Albany shale starting from 31 ppm to 20% purity " Reference 7. Reference 6. Reference 9. Reference 12. eReference 11. 'Reference 10. #Reference 15. hReference 14. is desired. Reference 22. Progress has not been as rapid as might be desired in porphyrin chemistry: difficulties are associated with their Table XI. Chromatographic Methods Used in isolation and analysis; no general quantitative scheme Petroporphyrin Isolations exists for their concentration and purification; method adsorbent DUrDOse development is by trial and error; and the tendency is that methods be tuned to the nature of the sample, or the study SiOz NiP/VOP separation" A1203 NiP/VOP separationb at hand.5 This suggests that a more systematic approach A1203 isolate VOP groupsc is needed. RSOBH isolate VOP groupsd The methods that have been used in the concentration TLC Si02 isolate VOP groupse of porphyrins from sediments and crude oil include Reference 15. Reference 3. Reference 11. Reference 6. grinding of samples for extra~tion;~~' the use of ultrasoeReference 14. nication to boost the extraction of pulverized rocks8 or the slower, less efficient technique of Soxhlet extraction;6 i.e., as modeled by coal,18 and of porphyrin.lQ Soxhlet porphyrin precipitation in pentane to remove soluble imof metalloporphyrins from asphaltenes is reextraction puritiesQand in cold isooctane to remove insoluble aspha1tenes;'O liquid chr~matography;~*"-'~ thin-layer chro(1)Treibs, A. Angew. Chem. 1936, 49, 682. matograph~;'~ and demeta1ati0n.l~ Table I lists the (2)Treibs, A. Ann. Chem. 1934,510, 42-62. methods that have been used in petroporphyrin concen(3)Barwise, A. J. G.; Whitehead, E. V. Advances in Organic Geotration. Asphaltene interferences with the chromatography chemistry: Douglas, A. G., Maxwell, J. R., Eds.; Peraamon Press: New of geological extracts16 and prior asphaltene removal can York, 19i9 pp i81-192. (4) McKenzie, A. S.; Quirke, J. M. E.; Maxwell, J. R. Aduances in be of critical importance. The removal of shale asphaltene Organic Geochemistry; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon by precipitationlo was reported although similar conditions Press: Oxford, 1979;pp 239-248. have been used for precipitation of porphyrins? Porphyrin (5) Quirke, J. M. E. Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver. J. F.. Eds.: ACS SvmDosium Series 344 American Chemical coprecipitation is more likely during asphaltene precipiSociety: Washington,'DC, 1987;*pp308-322. tation, but such results are not inconsistent with solubil(6) Barwise, A. J. G.; Roberts, I. Advances in Organic Geochemistry; ity" and solubility parameter considerations of asphaltene, Douglas, A. G . , Maxwell, J. R., Eds.; Pergamon Press: New York, 1984; 'Present address: Department of Chemistry, University of South Florida, Tampa, FL 33620.
Vol. 6, pp 167-176. (7)Ferguson, W.S.Am. Assoc. Pet. Geol. Bull. 1962,46, 1613-1620. (8)Freeman, D. H.; OHaver, T. C. Energy Fuels, submitted for p u b lication.
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696 Energy & Fuels, Vol. 4, No. 6, 1990
ported to be very slow.2o Adsorption chromatography is feasible with silica gel but normally requires a high ratio of adsorbent to sample.21 This analytical problem has been solved with silica gel adsorbent that is macroporous,7 using "chromtaographic d e a s p h a l t a t i ~ n " . ~ ~In- ~the ~ present work, the separation of strongly sorbing nonporphyrins from alkylporphyrins in shale extract, as opposed to petroleum,24was accomplished by chromatographic filtration in dichloromethane solvent through macroporous silica gel. The methods that have been used in the separation of nickel and vanadyl porphyrin groups involve chromatography on silica3g6and alumina." The isolation of VOP groups involve the use of alumina," silica functionalized with propanesulfonic acid? and thin layer on ~i1ica.l~ Table I1 summarizes these methods. The shale samples was obtained from the Henryville Bed of New Albany shale (Clark County, Indiana).27
Purification Theory Chemical enrichment involves the preparation of a chemical in a more concentrated form. Emphasis must be placed on removing interferences and impurities, so that their initial mass mo diminishes to m, in the product stream. The mo/ml ratio should be as large as possible. Analyte recovery should be a maximum, referring to the ratio of the mass of the analyte m,* in the product stream to the initial analyte mass mo*, i.e. m,*/mo*
Repeating this a second step
(3) It can be seen by substituion that
Eo,2 = EO,lEl,Z
(4)
and it follows by induction that
EO,^ = EO,1El,z***En-l,n
(5)
This definition is fundamental to process design.23 By taking the mass sum of the substance purified plus the impurities, mT = m* + m, we obtain an alternate form of eq 2.
The fraction of substance after the first unit operation is fl = ml*/mlT and that before is fo = mO*/mOT.This allows us to express the enrichment in terms of the sought substance alone. (7)
It follows that
(1)
The estimation of the amount of purification provided by a multistep procedure is outlined next. The initial enrichment factor E describes the changes in these ratios after an initial unit operation.
(9) Ekstrom, A.; Fookes, C. J. R.; Hambley, T.; Loch, H. J.; Miller, S. A,; Taylor, J. C. Nature 1983,306, 173-174. (10) Blumer, M.; Ommen, G. S. Geochim. Cosmochim. Acta 1964,28, 1147-1 154. (11) Sundararaman, P. Anal. Chem. 1985,57, 2204-2206. (12) Baker, E. W.; Palmer, S. E. The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 1, pp 484-622. (13) Kowanko, N.; Branthaver, J. F.; Sugihara, J. M. Fuel 1978, 24, 769-775. (14) Fookes, C. J. R. J. Chem. Soc., Chem. Commun. 1983,1472-1473. (15) Quirke, J. M. E.; Maxwell, J. R. Tetrahedron 1980,36,3453-3456. (16) Chromatography in Petroleum Analysis; Altgelt, K. H., Gouw, T. H., Eds.; Dekker: New York, 1979; pp 146,196,203,286,293,and 296. (17) Bockrath, B. C.; Schweighardt, F. K. Chemistry of Asphaltenes; ACS Symposium Series 195; American Chemical Society: Washington, DC, 1981; pp 29-39. (18) Weingerg, V. L.; Yen, T.-F. Fuel 1980, 287-289. (19) Freeman, D. H.; Swahn, I. Enerny ~.Fuels, submitted for publication. (20) Strong, D.; Filby, R. H. Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; Americal Chemical Society: Washington, DC, 1987; pp 40-67. (21) Schwager, I.; Yen, T.-F. Fuel 1979,58, 219-227. Angeles, R. M.; Keller, S. K. Prepr.-Am. Chem. (22) Freeman, D. H.; Soc., Diu. Pet. Chem. 1988,33, 231-238. (23) Belter, P. A.; Cussler, E. L.; Hu, W.-S. Bioseparations: Downstream Processing for Biotechnology; Wiley: New York, 1988. (24) Freeman, D. H.; Angeles, R. M.; Freeman, K. H.; Hoering, T. C.; Flynn, J. S.; Lango, T. A.; Homonay-Preyer, C. T. Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; Americal Chemical Society: Washington, DC, 1987; pp 402-422. (25) Giddings, J. C. Anal. Chem. 1967, 39, 1027-1028. (26) Freeman, D. H.; Goldstein, S.; Schmuckler G. Isr. J . Chem. 1969, 7. 741-749. (27) Van Berkel, G. J.; Quirke, J. M. E.; Filby, R. H. Org. Geochem. 1989, 14, 129-144.
because
EO,^ = E0,1E1,2**.En-l,n
(9)
Next, we consider possible losses, on failure of complete recovery. In the first unit operation the corresponding recovery is ROJ = m,*/mo* (10)
For the next unit operation RL2 = m2*/m1* Ro,= ~ R0,1R1,2.**Rn-l,n
(11) (12)
Equations 5 and 12 provide a practical means for expressing the major qualities of a multistep enrichment in terms of the overall enrichment E and the overall recovery R. Chromatographic enrichment will be considered next in terms of an injected whole sample mass mo that is to be separated into a series of n chromatographic fractions. As a working hypothesis, these will be considered to have equal mass mj, so ml = mo/n, and only one of the fractions of mass m* contains the analyte group. The recovered analyte mass by definition is ml* = Rmo*. Substitution into eq 2 gives a chromatographic enrichment, E = nR. This determination of n is known as the chromatographic peak capacity. To a first approximation, according to G i d d i n g ~n, ~ ~ 1 + V I 2 , where N is the number of theoretical plates. To make this a little more conservative, we will use a further approximation, n = N1j2. This enables an estimate of the chromatographic enrichment needed to resolve a particular separation problem. We illustrate in terms of our stated practical goal, namely, the enrichment of VOP from 31 ppm to 20% purity in a series of several preparative steps prior to a final purification by HPLC. To simplify, we assume unit recovery and the enrichment is then calculated as follows
Methods for Petroporphyrin Purification
Energy & Fuels, Vol. 4, No. 6, 1990 697
E = [(0.20)/(0.80)]/[31 X 10*/0.99969] which is approximately lo4. To achieve 10000-fold enrichment in a single chromatographic step, the foregoing approximation that E "= n NJ2 requires N = 108 plates, which is far beyond present practicality. However, the equivalent of los theoretical plates can be achieved in a real and practical sense based upon the multiplicative enrichment effect obtained when nonoverlapping multistep methods are used. This might be obtained, for example, in four similar steps where each nj provides a constant 10-fold enrichment ( N zz 100). The key assumption here is the necessity to use radically different, i.e., nonoverlapping or n o n r e d ~ n d a n tseparation ,~~ mechanisms such as extraction, precipitation, adsorption chromatography, and ion exchange. Proper design will provide for foulants and interferences whose removal must precede HPLC. It is noted that the reasoning employed in this approach can be adapted to other needs or, as followed in this study, by working to maximize EJ and RJ values in order to maximize the overall effect of a multistep enrichment procedure. The order in which the steps are carried out needs to be found empirically, with one important exception: The extraction step as well as the highest sample loads during fractionation should come at the beginning. Experimental Section Materials. Adsorption chromatography was done by slurry packing of macroporous silica gel, 186-A porosity and 37-gm particle size (Impaq samples, PQ Corporation, Valley Forge, PA), into a glass chromatographic column (35 cm x 70 mm i.d.). All solvents were reagent or HPLC grade (J.T. Baker Co., Phillipsburg, NJ). Solvents were evaporated at 25 "C with a Buchi rotary evaporator, evacuated by a high vacuum pump. A diode array UV/vis spectrophotometer (HP8452a), interfaced with an an IBM PCAT, was used to record spectra and to quantitate the porphyrins. The porphyrin concentrations were estimated by use of derivative spectrophotometry of the second order as described elsewhere.8 Procedure. A 1.9-kg sample of New Albany shale was dry ground in a ball mill to a particle size of less than 15 gm as suggested by Ferguson.' The time required for each of three batches was 4-5 days. Three batches of shale were each extracted with dichloromethane for 8 days, with ultrasonication a t 40 "C. A 700-g batch of powdered rock, containing 5.3 g of tar in the dichloromethane extract, was vacuum filtered through 200 g of celite as a filter aid. Next, a chromatographic deasphaltation step (step A)24was carried out on the tar (14 g). In one portion 5.3 g of the tar was dissolved in dichloromethane and applied to 500 g of a macroporous silica column in cyclohexane. The chromatogram was developed with a low air pressure of about 6 psi. Three column volumes of dichloromethane were used to remove a NiP and VOP concentrate. Two column volumes of ethyl acetate were used to remove the acid group containing nickel and vanadyl carboxylic acid porphyrins (NiPCOOH and VOPCOOH). Step B, a group separation step, was next carried out on the NiP and VOP concentrate. The NiP and VOP concentrate, 9.6 g, from above was dissolved in 60 mL of cyclohexane and chromatographed on 400 g of macroporous silica using again a low air pressure of 6 psi. The chromatogram was developed with the use of a concave carrier gradient from hexane to dichloromethane to effect the separation of the saturates and aromatics, Nip, and VOP. T h e saturates and aromatics were eluted in one column volume of hexane, the NiP in two column volumes of 27.5% dichloromethane/hexane, and the VOP in four column volumes of 42% dichloromethane/hexane. A precipitation step was carried out next. In the determination of the order in which this step should be in the purification scheme, precipitation was carried out before and after the MPLC, step B, and the results were compared. Solvents such as hexane and pentane were used to study precipitation before the B step
Grinding
Crude oil
I
1 .Extraction
Dilution
I
2.Deasphaltat ion
I
3.Group separation I
(Sats)
NIP
VOP
NIPCOOH VOPCOOH
F i g u r e 1. General scheme for petroporphyrin concentration. Table 111. Sequence of Steps Used in VOP Isolation step mass, g %VOP % R %R,.II E Eoverdl 1. crushed shale 1900 0.0031 2. extraction/filtration 14 0.43 140 140 3. deasphaltation 11 0.50 90 1.2 170 4. MPLC, step B 0.99 5.1 86 77 9.5 1600 5. precipitation 0.35 12 91 70 2.6 4200 0.11 23 86 60 2.1 9000 6. RSOBH and blends of hexane and methanol after the B step. Basic impurities were removed next, with a sulfonic acid polystyrene/divinylbenzene resin. This adsorbent was made by sulfonating a macroporous polystyrene/divinylbenzeneresin (d, = 30 gm)26obtained from Rohm and Haas and was used as a n acidic solid phase extractant. The VOP concentrate was chromatographed and eluted with etherltert-butyl alcohol/cyclohexane (1:1:98), while polar non-porphyrins were retained.
Results a n d Discussion A general scheme for the concentration of VOP from the New Albany shale was examined (see Figure 1). The order of the steps was determined by optimizing the enrichment, recovery, and time for each step. The scheme begins with the dry milling of the New Albany shale for 4-5 days, to achieve a particle size of 15 pm or belowe7 This particle size was used for complete extraction. Extraction of the ground shale was carried out with the batch time for the extraction varying from 7 to 8 days. Using a shorter time would lead to incomplete extraction, especially of Nip, which extracts more slowly than VOP. The ratio of VOP to NiP a t the end of the extraction was approximately 41. This procedure gave 14 g of tar (0.71% w/w shale) which contained 16 mg (0.11%) of Nip, 60 mg (0.43%) of VOP, and 4.4mg (0.031%) of MPCOOH. This represented a high VOP enrichment of 140 (see Table 111). The second step, the chromatographic deasphaltation step, was carried out with the use of macroporous silica gel. A 1% w/w load ratio of sample to adsorbent was used to effect the separation. This step showed an increase in the VOP purity to 0.50%. The recovery of the VOP was about 90% and the enrichment 1.2 (see Table 111). The time taken was about 6 h. The enrichment is low; however, the aim here was to remove the asphaltic interferences that would otherwise impede the chromatographic separation in subsequent steps. The third step, a group separation step, was carried out on the NiP and VOP concentrate, with macroporous silica gel and a concave carrier gradient from hexane to dichloromethane being used. The purity of the VOP concentrate increased to 5.06%. The time taken was about 6 h and the recovery was 85%. Table I11 summarizes these results. A 9.5-fold enrichment was obtained, and there was a good separation between the NiP and VOP concentrates, as shown in Figures 2-4. This was made possible by the gradient from hexane to dichloromethane, which was de-
Johnson and Freeman
698 Energy & Fuels, Vol. 4, No. 6,1990 NIP
VOP
V W
NIP
20
2oo
I
x
Y
2
10
s
0 1
2
3
4
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4
1
Fraction #
Figure 2. Graph of the step B separation of NiP and VOP groups on macroporous silica, based on % purity of porphyrins over 14
fractions.
NIP
VOP
60
50
L.
E
-
4030
-
3
4
5
6
7 8 9 Fraction #
1011121314
Figure 4. Graph of the step B separation of NiP and VOP groups on macroporous silica, based on % recovery of porphyrins over 14
fractions.
Table IV. Precioitatinu Conditions Used in VOP Isolation precipitating temp, time, sequence solvent "C days E % R index" 2 2.0 14 14 -11 1. before MPLC hexane 2. before MPLC pentane -11 21 3.9 36 8 25 14 1.5 30 3 3. after MPLC methanol 4. after MPLC methanol: 25 2 2.5 91 120 hexane (6:l)
"This is calculated from E(%R)/time,
U
s
2
2010
0
-
L 1
2
3
4
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4
Fraction #
Figure 3. Graph of the step B separation of NiP and VOP groups on macroporous silica, based on mass ( p g ) of porphyrins over 14
fractions.
veloped by an examination of the elution profiles of NiP and VOP standards on silica with increasing concentrations of dichloromethane in hexane.24 From these profiles we saw that going from 20% to 27.5% dichloromethane/ hexane, the selectivity of NiP to VOP increased from about 15 to a maximum a t 27.5%, and fell to about 2 a t 60% dichloromethanelhexane. Using this information, we were able to design a gradient from 20% to 60% for the separation of the NiP and VOP groups from the shale. The amount of coeluting impurities with these two fractions influenced the purity of the products. This step was able to remove most of the lower molecular weight saturates and aromatics, which would interfere with the subsequent steps. The fourth step involved precipitation of the VOP concentrate. Precipitation can be used in two ways: (a) to precipitate asphaltenes'O and (b) to precipitate porp h y r i n ~ .Since ~ the precipitate in a is discarded, there is a risk in discarding coprecipitated porphyrins; the precipitate in b, however, presents no similar risk. Precipitation was therefore carried out at this point. This method was developed as follows: Our results showed that if precipitation was carried out before step B, and hexane used as the precipitating solvent, a t -11 "C after 2 days the recovery was 14%. When pentane was used the operation was slower, with the recovery being 36% after 2 weeks. This may be explained by the presence of solubilizing impurities which dissolve the porphyrins and make their precipitation from these nonpolar solvents difficult. When the precipitation was carried out after step B, and
a polar solvent such as methanol was used, the recovery was 30% after 14 days a t 25 "C. Finally when a mixture of a polar and a nonpolar solvent such as a (6:l) methanollhexane blend was used, a t 25 "C after 2 days the recovery was 91% (see Table IV). This final result was achieved because we were able to dissolve the porphyrins in a polar solvent heated to 50 "C. Methanol was chosen because of its ability to keep the more polar impurities in solution. Thus by dissolving the porphyrins in methanol at 50 "C, and adding hexane to this, we were able to effect their slow precipitation from the solution, with the majority of the more polar impurities remaining in solution. The final purification step was the chromatographic filtration of the VOP precipitate, using an acidic solidphase extractant. In this step the basic impurities were adsorbed by the acidic adsorbent, affording their separation from the VOP. This method was developed in the following way: Our aim was to find a solvent or solvents that would allow the porphyrins to be retained on the column, though not irreversibly, so that the basic impurities could be selectively retained and the porphyrins recovered at >80%. Using cyclohexane as our loading and elution solvent gave an irreversible retention of the porphyrins on the column. We tried other solvents, such as ethyl acetate, methanol, and acetone. However, the retention of the porphyrins was poor and inconsistent. In looking at various solvent blends, the addition of ether to cyclohexane affected the retention and recovery of the porphyrins, but to varying extents. At concentrations less than 1% ether/cyclohexane, the porphyrins were strongly retained. However, their recovery was poor, 540%. At concentrations greater than 2 % ether/cyclohexane the retention was reduced and the recovery was >90%. We thought then that the addition of an appropiate solvent to this mixture, to block the process that caused strong retention of the metalloporphyrins, may lead to their good retention and increased recovery. After trying a number of solvents, such as acetamide, dimethyl sulfoxide, and acetone, the addition of tert-butyl alcohol was found to
Energy & Fuels 1990,4,699-704 Table V. Use of Polystyrene/Divinylbenzene Bonded Sulfonic Acid fraction solvent mass, g VOP, mg %VOP 1 2% ether/C6Hl, 0.13 0.0 0.0 2 1% t-BuOH/ether/C6H12 0.060 8.4 14 3 1% t-BuoH/ether/C6Hl2 0.11 24 23 4 1 % t-BuOH/ether/C6HI2 0.014 1.6 11 5 ether 0.013 1.0 7.0
give the best retention and recovery. This may be explained in terms of the ability of the weakly acidic alcohol (pK, = 19) to hydrogen bond with the sulfonic acid groups, thus reducing the possibility of the demetalation of the metalloporphyrins on the column and aiding their recovery. Finally, an optimum solvent system of etherltert-butyl alcohol/cyclohexane (1:1:98) was developed and used to elute the VOP from the column, with a 86% recovery and an enrichment of 2.1. Table V shows the collection of VOP over five fractions.
Conclusions The development of a preparative method for the purification of a metalloporphyrin from a geological sample
699
was achieved. Enrichment, percent purity, recovery, and time are important factors in the development of useful methods for achieving products of high purity. This five-step purification sequence was used to obtain a 23% pure VOP concentrate, containing 24 mg of VOP. A total purification of 9OOO was achieved for the VOP concentrate. The overall recovery of the VOP in the concentrate was 60%. This sequence is thus quite applicable to the separation of vanadyl porphyrins from a complex mixture of metalloporphyrins in geological materials. The combination of enrichment, recovery, and speed need to be optimized in similar studies of metalloporphyrins, if their high purity is desired. For the preparative HPLC aspect, these factors need to be considered if the desired purity of >95% is to be achieved.
Acknowledgment. A.L.J. thanks the University of Maryland for the award of a postdoctoral associateship (1988-1989). This work was supported by the US. Department of Energy under Grant No. DE-FG05-85ER13308.
Spectrophotometry and Solubility Properties of Nickel and Vanadyl Porphyrin Complexes David H. Freeman* and Irvine D. Swahn Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
Peter Hambright Department of Chemistry, Howard University, Washington, DC 20059 Received April 27, 1990. Revised Manuscript Received August 23, 1990 Spectrophotometric standards of purified synthetic Ni" and VIv complexes with etioporphyrin-I and octaethylporphyrin structures have been evaluated. Extinction coefficients, and their solvent dependence, were measured with a diode array spectrophotometer. Metalloporphyrin absorption bands (ca. 550 nm for the Ni" and ca. 570 nm for the VN0 complexes) obeyed Beer's law with a relative precision of 2% within a 0.3-10 pg/mL concentration range in ethyl acetate and 1-10 pg/mL range in methylene chloride. Nonlinear behavior was found to arise at concentrations above 30 pg/mL in 1,2-dichloroeihane. Precision of the metalloporphyrin extinction coefficient was improved significantly using a baseline correction method. Accuracy depends upon proper baseline definition. A feasibility study was performed to estimate both etio-I metalloporphyrin and octaethylmetalloporphyrin solubility equilibrated at 24 "C. Solubility was found to maximize in chlorinated solvents. The solubility maximum among the solvents tested was found to correspond to a solubility parameter of 9.5 for the Ni or VO porphyrins.
Introduction In this paper, spectrophotometric measurements have been made of absorbance and solubility of nickel and vanadyl complexes of etioporphyrin-I and octaethylporphyrin structures in various solvents. Low concentrations of these porphyrins are commonly found in petroleum, oil shale, and other geological organic meterial.'S2 Their geological occurrence is linked to their biological origin from chlorophyll and related photosynthetic The
physical properties of these biological marker compounds are key to their analysis and will help geologists explore the properties of shales, oils, and bituminous coals. Metalloporphyrins are partly soluble in a number of organic solvents and, once dissolved, absorb both ultraviolet and visible light. With the use of diode array spectroscopy one can obtain the digitized absorbance of a standard solution with high precision. The extinction coefficient is calculated from Beer's law:
(1) Treibs, A. Angew. Chem. 1936,49, 682. (2) Treibs, A. Ann. Chem. 1934, 51 7 103. (3) Siefert, W. K.;Moldowan, J. M.; Jones, R. W. Tenth World Petroleum Congress; Heydon: London, 1979; p 425. (4) Barwise, A. J. G.; Roberts, I. Org. Geochem. 1984, 6 , 167.
(5) Barwise, A. J. G.; Park, P. J. Adu. Org. Ceochem. 1981, 668. (6) Hohl, M. E.; Hajibrahim, S. K.; Eglinton, G. Chem. Ceol. 1982,37, 229. (7) Lewan, M. D.; Maynard, J. B. Geochim. Cosmochim. Acta 1982, 46, 2547.
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0 1990 American Chemical Society
