Energy & Fuels 1991,5, 371-375
371
Characterization of Naphthenic Acids in Petroleum by Fast Atom Bombardment Mass Spectrometry Tseng-PuFan Shell Development Company, Bellaire Research Center, P.O. Box 481, Houston, Texas 77001 Received January 17, 1991. Revised Manuscript Received March 7, 1991
The naphthenic acids in petroleum are considered a class of biological markers. Their potential use in source correlation and as an indicator of biodegradation has been reported in the past. Their presence in waste water at refineries also causes corrosion problems and fish toxicity. Due to their highly complicated nature, detailed characterization of the acids has been difficult. Two newer mass spectrometric methods were recently developed a t Shell to analyze the acid components, namely fluoride ion chemical ionization and negative ion fast atom bombardment mass spectrometry (FABMS). This report demonstrates that FABMS is a simple and efficient approach for analyzing the naphthenic acids, including the heavy, higher molecular weight components. The components are characterized on the basis of group type and carbon number distributions. A comparison of FAB and CI results show that the group type distributions obtained from both methods agree surprisingly well. The geochemical implication of the naphthenic acids is investigated by using a set of well-characterized crude oil samples. It is found that the naphthenic acid distribution can be used as a fingerprint for correlating the sources of oils and source rocks.
Introduction The naphthenic acids represent the carboxylic acids present in the petroleum or crude oils. The attempt to characterize these acidic components dates back to before 1955.' The interest in the naphthenic acids has been twofold: (a) They are a class of biological markers for the studies of geochemical correlations as well as biodegradation mechanisms.2-' (b) Their presence in the refinery streams causes corrosion problems and hazardous wastewaters which leads to fish toxicity. The naphthenic acids are extremely complicated mixtures. They contain predominantly carboxylic acids with a cyclic or polycyclic ba~kbone.~ Many different methods and analytical techniques have been used for analyzing these acids in the past. Seifert and Teeter employed extensive sample preparation such as extraction, separation, and chemical reaction prior to characterization by spectroscopic techniques. Some employed GC/MS with electron impact and positive ion chemical ionization GC/MS provides positive identification of volatile or low molecular weight acids. However, it is not suitable for heavy components with low volatility. Improved methods are continuously in demand to better analyze the acids in both crude oils and refinery streams. Two newer mass spectrometric methods have been recently developed at Shell. One involves fluoride ion negative ion chemical ionization using NF3 as reagent gas, which has been reported earlier.s The other method employs negative ion fast atom bombardment mass spectrometry
c7 roll LC Separation
Chmmooorb T
rCC6 -Neulral
Liquld
+ Basic
cHcl+nt +MBOH 10% FA t Weak
Acid
Asphaltme Preclpllale
MeOH
-
Acld
-FA0 Analy sls
Figure 1. Sample preparation scheme for separating naphthenic acids from crude oil using preparative ion-exchange column chromatography. The acid fraction eluted with acidic methanol is subjected to FAB analysis.
167, 149.
(FABMS) and is described in this report. FABMS is well-known for its ability to analyze polar, nonvolatile, and/or high molecular weight components. Studies in this laboratory found that carboxylic acids, when analyzed in triethanolamine (TEA) matrix, produce relatively simple negative FAB mass spectra containing almost exclusively (M-H)- ions. Their positive ion FAB spectra are much more complicated and consist of various cationized species and fragment ions. The (M-H)- ions were observed in high abundance only in basic matrices, not in neutral matrices such as glycerol. I t is believed that the basic matrix molecule, with relatively higher proton affinity compared to the analyte, induces the formation of RCOOions by removing a labile proton from the acid molecule: resulting in enhanced (M- 1)-signal. This report describes the use of FABMS in analyzing naphthenic acids separated from crude oils. Results from both CI and FAB for several samples are compared. The
1988,60, 1318.
(9) Bush, K. L.; Unger, S. E.; Vincze, A,; Cook, R. G.; Keough, T. J. Am. Chem. SOC. 1982,104, 1507.
(1) Lochte, H. L.; Littmann, E. R. The Petroleum Acids and Bases; Chemical Publishing Co.: New York, 1955. (2) Behar, F. H.; Albrecht, P. Org. Ceochem. 1984,6,597. (3) Cranwell, P. A. Org. Geochem. 1984,6, 115. (4) Brault, M.; Marty, J. C.; Saliot, A. Org. Geochem. 1984, 6, 217. (5) Seifert, W. K.; Teeter, R. M. Anal. Chem. 1970, 42, 750. (6) Schmitter, J. M.; Arpino, P.; Guiochon, G. J . Chromatogr. 1978,
(7) Green, J. B.; Stierwalt, B. K.; Thomson, J. S.; Treese, C. A. A d . Chem. 1986,57,2207. (8) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem.
0887-0624/91/2605-0371$02.50/00 1991 American Chemical Society
Fan et al.
372 Energy & Fuels, Vol. 5, No. 3, 1991
I
R a C O O H \
J
68
55
se IS
a 3s 3a
25 28
IS in S 8
Figure 2. Negative ion FAB spectrum of Kodak Naphthenic Acid Standard. homologues are represented by a general formula geochemical implication based on the distribution of the CnH2n+r02, where n indicates the carbon number and z acid type is demonstrated by using a set of well-understood specifies a homologous series. The z is equal to 0 for oil sample. saturated aliphatic carboxylic acids. It decreases by 2 with Experimental Section increasing hydrogen deficiency from the formation of rings An ion-exchange separation method for the fractionation of or double bonds in the molecule. A number of isomers may petroleum resins has been developed based on the procedure be present in the same homologous z series but cannot be reported by Jewell et al.1° The resin fraction, containing the distinguished on the basis of the observed (M- 1)- ions. naphthenic acids, is first isolated from the crude oil, followed by Homologues in different z series that have the same nomanion-exchange column separation as shown in Figure 1. The inal molecular weights but different formulas, such as components are eluted into four fractions based on acidity by using C14H26O2 ( z = -2) and CI5Hl4O2(z = -161, are not comfour solvents of increasing polarity. Naphthenic acids are found pletely resolved under the operating instrument resolution. only in the acid fraction eluted with acidic methanol. The average Table I lists the major types of naphthenic acids clasrecovery from the column is 95 wt %. sified by their L numbers from 0 to -12. Acids that are All of the FAB analyses were carried out on a VG ZAB-HF mass spectrometer. The instrument was operated in negative ion mode highly aromatic, such as z = -14, -16, -18, etc., are not at 8 kV acceleration voltage. A Xe atom beam at 1 mA ion current included. They are considered minor components of the and 9 kV kinetic energy was used for ionization. The neat acid naphthenic acids since they were not detected in the extract was first dissolved in toluene, approximately 1:5. One previous studies.I5 The presence of other acids such as microliter of the solution was then mixed with triethanolamine diacids, hydroxy acids, carbonyl acids, and olefinic acids (TEA) and loaded on the FAB probe. The scanning rate was 30 is not considered for the same reason. s per decade from 100 to 1200 amu at a resolution of M/LU = Quantitative determination of the components in each 2000. Normally 10-20 scans were acquired and a representative acid class is based on the observed relative ion intensity. spectrum was obtained by averaging 5-10 scans. I t is presented as lists of carbon number distribution or Data Interpretation and Quantification z number distribution. To generate a carbon number list, Since FAB spectra of naphthenic acids consist excluthe masses correspond to a specific carbon number in each sively of ions corresponding to (M - 1)-,the composite z series are selected, and their measured intensities are mass spectrum provides an envelope that follows the used to generate a normalized distribution of components by carbon number in each z series. To generate a list of molecular weight distribution of the mixture. Homologous z number distribution, the intensities of all the components series of components at 14 amu (CHJ difference are clearly in each z series are summed. The resulting list is a relative observed in the spectrum. The identification of the naphthenic acids throughout this report is based on the same distribution in mole percent of each z type within the total z series concept traditionally used in grouping the hysample. Since the absolute response factors of each type drocarbon types in petroleum."-" The naphthenic acid of naphthenic acids are not available, equal molar ionization sensitivities are assumed for all z homologues in all analyses. (10) Jewell, D.M.;Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D.R.Anal. Chem. 1972,44, 1391.
(11) Lumpkin, J. E.; Thomas, B. W.; Elliot, A. Anal. Chem. 1952,24,
1389.
(12) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; Le Tourneau, R. L.; Teeter, R. M. Anal. Chem. 1967,39, 1833. (13) Acxel, T.;Allan, D. E.; Hardin, J. H.; Knipp, E. A. Anal. Chem. 1970.42.~i. - - . -, __, - - -. (14) Scheppele, S. E.; Chung, K. C.; Hwang, C. S. Int. J. Mass Spectrom. Ion Phys. 1983,49, 143.
Results and Discussion Figure 2 shows the negative ion FAB spectrum of the Kodak naphthenic acid standard mixture. The major (16) Seifert, W. K. Carboxylic Acids in Petroleum and Sediments; Springer-Verlag: Vienna, 1975; Chapter 1.
Energy & Fuels, Vol. 5, No. 3, 1991 373
Characterization of Naphthenic Acids in Petroleum
Table I. Major Naphthenic Acid Types' n
D
-2
0
RCooH
-6
-4
R
R(&ow
p
c
a
-8
@- @" -R
R
&-R& -10
-12
w
R
&-
R
Isomers within a given z homologue cannot be differentiated.
h
1-
0.8
0.6
0.4
0.2
12
16
A
w
%$44tZ02
85
2-2
80 75 70 66
I
2-2
28 32 36 40 CARBON NUMBER
+ 2-4
44
48
52
-#- 2-10
Figure 4. Carbon number distribution of three z series, z = -2, z = -6, and z = -10, of the naphthenic acids in a Louisiana oil obtained by FAB.
2-4
I00 05
24
20
NORMALREDRELATIVE DISTRIBUTION% 14
80
2-6
55
M 45 40 35
2-0
30 25 20
2-10
15 10 5 0
1
1171
ana 320
328
330
332
334
1II 336
338
340
Figure 3. FAB spectrum of the acid fraction separated from a Louisiana oil. The distribution of acid types from z = 0 to z = -12 of components containing 22 carbons is shown below the spectrum.
series of carboxylic acid homologues are identified and labeled in the Figure. Only (M- 1)- ions are observed in the spectrum. The observed ion at mlz 148 corresponds to the deprotonated molecular ion of the TEA matrix. A FAB spectrum of the acidic fraction separated from a Louisiana crude oil is shown in Figure 3. All of the even mass peaks found correspond to the 13Cisotope contribution from the molecular ions according to their statistical distributions. The distribution of components by z types within a specific carbon number (C2&is illustrated below. The spectrum shows a distribution of components up to 700 m u with a cutoff point of 2% intensity relative to the
12 14 16 18 20 P
24
I I 50 $2 Y I I40 CARBON NUMBER a C l
u u
4 Y
50
mFAB
Figure 5. Comparison of FAB and CI results for the normalized relative distributions of tricyclic acids in a California crude blend. CI results obtained from ref 8. base (highest) peak. Higher molecular weight naphthenic acids up to 700 amu have been reported in California crude^.'^ The list of carbon number vs 2 group ( z = 0, -2, -4, -6, -8,-10,-12) distribution of the components can be readily obtained from the VG 11-250data system. The carbon number distributions of three groups, z = -2, -6, and -10, are plotted in Figure 4. A shift in the curve maximum toward higher carbon number with decreasing z number
Fan et al.
374 Energy & Fuels, Vol. 5, No. 3, 1991 DRMALIZED DISTRIBUTIONS.MOLE %
35
< I
m
30
1W
CALIFORNIA OIL A
95
1
80
1
:1
25
e5
‘p I1
1I
20
15
10
5
0 -2
0
-6
-4
-8
.lo
-12
MR
2 NUMBER
=
FAB
aCI
Figure 6. Comparison of FAB and CI resulta for the z group distributions of naphthenic acids in a Louisiana oil.
CALIFORNIA OIL 6 w 85 80.
m
mra
75. 95
70.
w.
65
85
II
”* I
80.
75, 70
“I 35
ll MR
Figure 8. FAB spectra of two naphthenic acids from California oils.
1
MONTANA OIL B
40,
35. 30. 25. 20,
i MIZ
Figure 7. FAB spectra of two naphthenic acids from Montana oils.
is clearly observed in the figure. This can be explained by the fact that the minimum number of carbons required for a pentacyclic molecule ( z = -10) is greater than that for a tricyclic (z = -6) or a monocyclic (z = -2) molecule. Comparing FAB results with those from CI analyses, the major difference is that FAB spectra show a wider range of molecular weight distribution and a z maximum at higher mass. Figure 5 compares the normalized carbon
number distribution of the z = -6 group between FAB and CL8 The distribution from FAB shows a carbon number range up to Cm and a maximum at CZ4,compared to CS2 and C16from CI, respectively. However, the relative molepercent distribution of z groups within the entire sample is very similar between CI and FAB. Figure 6 compares the z group distributions (based on CI2-C,) of a Louisiana oil acid fraction obtained from FAB and CI. The agreement between the two methods is excellent considering that these are two completely different techniques. The s u m of the Cm to CWcomponents contributes 20% of the total ion current in FAB. The calculated z group distribution showed no significant different whether it is based on C12-C32 or CI2-C0 This implicates similar z distributions for both the higher and lower molecular weight acids. The geochemical implications of the naphthenic acid distribution, whether for source correlation or as a indicator for biodegradation, are tested with three sets of oil samples. Each of Figures 7,8,and 9 shows the negative FAB spectra of two naphthenic acid extracts obtained from Montana (Ordovician Red River), California (Pismo Basin), and Louisiana (Lake Washington Field) crude oils, respectively. On the basis of results from various geochemical analyses, the two samples from each location are derived from the same source rock. These oils are somewhat different in their geochemical properties. The Montana oils are quite immature, while the California oils are mature and slightly biodegraded. One of the Louisiana oil is normal and mature, while the other is severely biodegraded.
Characterization of Naphthenic Acids in Petroleum
Energy & Fuels, Vol. 5, No. 3, 1991 375
NON-BIODEGRADED LOUISIANA OIL w4
804
IIIIIII E
3 0 W
2 20
5 W
a 10
Louisiana 011
04 0
.2
.4
4
2 NUMBER
MR BIODEGRADED LOUISIANA OIL
.IO
-12 1
L?,
Figure 10. Comparison of carboxylic acid type distribution between oils from Montana, California, and Louisiana.
35 30 25
and -4 series, while the California oils show a more balanced z distribution with slight dominance in the z = -2 type. None of these mature oils contain significant amounts of fatty acids, or the z = 0 series. It has been reported that acid type distributions change during the maturation proces~.'~This indicates that the difference observed between the Montana oils and the other oils is the contribution of both source input and maturation effect, while the difference between the California and Louisiana oils is primarily due to the difference in source. The results also indicate little difference between the biodegraded and the unaltered oil from Louisiana (Figure
20 15
9).
60
.g 55
-ES
-8
M
45
40
10
6 0
M/Z
Figure 9. FAB spectra of two naphthenic acids from Louisiana oils.
The FAB spectra show that the distribution of naphthenic acids, in both carbon number and z type, is very similar for the oils derived from the same wurce. However, it is significantly different between oils from different sources. The differences and similarities in z group distribution between the six oils are illustrated in Figure 10. The naphthenic acid distribution in Montana oils is quite simple. The FAB spectrum is dominated by aliphatic carboxylic acids, mainly cl6 (m/z 255) and C18 (m/z 283) fatty acids. High molecular weight components are not observed. This is understandable since fatty acids derived from living organism are found in recent ~ediments,'~ and the Montana oils are quite immature. The prominent even-mass peaks at 404 and 432 amu are due to the c16 and CI8anions plus a TEA molecule. On the other hand, the naphthenic acid distribution in the mature California and Louisiana oils are much more complicated. All four spectra show wide molecular weight ranges to greater than 700 amu. The Louisiana oils have a dominance of z = -2
Conclusions The results demonstrate that negative ion FAB, using TEA as matrix, provides a simple and efficient mass spectrometric technique for analyzing the naphthenic acids in petroleum. More importantly, FAB analysis detects all of the acidic components, including high molecular weight species. A separation method using ion-exchange chromatography successfully isolated the naphthenic acids from the crude oil. Component information, based on carbon number and acid group type (z series) distributions, is obtained directly from the FAB spectrum. The results clearly show that this group type distribution provides a fingerprint that can be a valuable tool in correlating the sources of crude oils or 89 a maturity indicator. The results also indicate that biodegradation does not appear to affect the characteristics of the naphthenic acid on a macro level. However, the specific carboxylic acid composition may be different. The effect of biodegradation on naphthenic acids awaits further investigation. Acknowledgment. Special thanks are due to C. L. Calkin at Shell Westhollow Research Center in developing the column separation method, and R. I. McNeil at Shell Bellaire Research Center for providing the geological information.
