Anal. Chem. 1996, 68, 46-71
Resolution and Chemical Formula Identification of Aromatic Hydrocarbons and Aromatic Compounds Containing Sulfur, Nitrogen, or Oxygen in Petroleum Distillates and Refinery Streams Shenheng Guan† and Alan G. Marshall*,†
Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310 Stuart E. Scheppele*,‡
Amoco Research Center, Naperville, Illinois 60566 and Research Resources Center (M/C 937), University of Illinois at Chicago, 901 South Wolcott Avenue, Room E102 MSB, Chicago, Illinois 60612-7341
An all-glass heated inlet system has been interfaced to a dual-trap Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The inlet vaporizes a mixture of species of widely different boiling points, and the interface maintains a large (factor of 1010) pressure gradient between the inlet and the mass spectrometer, making possible the analysis of petroleum distillates and refinery streams at very high mass resolution. Ions generated by low-energy electron ionization in the source trap of the spectrometer are transferred to the analyzer trap, where the pressure is at least 2 orders of magnitude lower. Singly-charged ions from a mass window of ∼20 u are isolated by stored-waveform radial ejection, to reduce space charge and increase digital resolution: routine mass resolving power >200 000 (based on magnitude-mode peak full width at half-height) is thereby achieved throughout the full mass window. The mass window may be incremented stepwise to cover the full mass range of several hundred units. The FT-ICR mass spectrum of a gas oil aromatic neutral fraction contained peaks resulting from the resolution of ions having 358 distinct formulas over a mass range of ∼42 u. C3/SH4, 13C/CH, 13CH/N, CH /N, and other mass doublets were 2 baseline-resolved, yielding typical mass measurement inaccuracies of ∼1 ppm. For example, 13C12C17H20S+ and C21H17+, which differ by only 0.0011 u at ∼269 u, were clearly resolved. A 40 000 resolving power low-voltage spectrum of the aromatic neutrals, acquired by use of a Kratos MS-50 double-focusing instrument, was processed with a computer-based deisotoping/formula assignment procedure. The algorithm of the program is outlined and illustrated. Remarkably good agreement exists between the FT-ICR and MS-50 results. However, instrumental rather than indirect resolution of ions clearly enhances analytical accuracy and significantly reduces dataprocessing time. Thus, we have demonstrated that FTICR is the mass analysis of choice for differentiating 46 Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
hydrocarbons from heteroatom-containing compounds in petroleum distillates and refinery streams. In the petroleum industry, knowledge of the composition of aromatic hydrocarbons and aromatic compounds containing one or more of the heteroatoms sulfur, nitrogen, and oxygen in distillates is important in refining of crude oils and in the storage and use of refined products. Sulfur oxides released during combustion of fossil fuels result in air pollution and acid rain. Moreover, sulfur- and nitrogen-containing species and polynuclear aromatics may act as catalyst poisons and must be removed prior to certain oil refinery processes. Finally, sulfur-containing components of crude oil may be hazardous to the environment. High-resolution mass spectrometry is a well-established, widely used technique in methods for determination of compositions of petroleum, synfuel, and refinery stream distillates boiling up to an atmospheric equivalent temperature of 1050 °F (566 °C) according to compound types.1-19 These methods may be classified according to whether composition is determined in †Members of the Department of Chemistry, Florida State University, Tallahassee, FL. ‡ Member of the Department of Chemistry, University of Illinois at Chicago, Chicago, IL. (1) Lumpkin, H. E. Anal. Chem. 1964, 36, 2399-2401. (2) Reid, W. K.; Mead, W. L.; Bowen, K. M. Adv. Mass Spectrom. 1966, 3, 731-745. (3) Sharkey, A. G., Jr.; Shultz, J. L.; Kessler, T.; Friedel, R. A. Proceedings of the 15th ASMS Annual Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Denver, CO, 1967; pp 443-446. (4) Johnson, B. H.; Aczel, T. Anal. Chem. 1967, 39, 682-685. (5) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeToumeau, R. L.; Teeter, R. M. Anal. Chem. 1967, 39, 1833-1838. (6) Kessler, T.; Raymond, R.; Sharkey, A. G., Jr. Fuel 1969, 48, 179-187. (7) Aczel, T. Rev. Anal. Chem. 1971, 1, 226-261. (8) Shultz, J. L.; Kessler, R. A.; Sharkey, A. G., Jr. Fuel 1973, 52, 242-246. (9) Lumpkin, H. E.; Elliott, R. M.; Evans, S.; Hazelby, D.; Wolstenholme, W. A. In Proceedings of the 23rd ASMS Annual. Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Dallas, TX, 1975; pp 235-237. (10) Peters, A. W.; Bendoraitis, J. G. Anal. Chem. 1976, 48, 968-973. (11) Aczel, T.; Williams, R. B.; Pancirov, R. J.; Karchmer, J. H. Chemical Properties of Synthoil Products and Feeds; U.S. Dept. of Energy: Washington, DC, 1977. (12) Dooley, J. E.; Thompson, C. J.; Scheppele, S. E. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 1, pp 467-498.
0003-2700/96/0368-0046$12.00/0
© 1995 American Chemical Society
terms of the amounts of homologs of various hydrocarbon and heteroatom-containing compound types or the total amounts of these types. Resolution (R) and resolving power (RP) are vital considerations for high-resolution mass spectrometric methods that determine composition in terms of homologs of compound types, because the masses and signal magnitudes of the isotopically most abundant molecular ions are used to identify and quantify, respectively, the homologs of the compound types present in a sample. In general, spectroscopic resolution is usually defined as the peak width at a specified percentage of peak maximum height, and resolving power is defined as the ratio of the peak position to that peak width. Alternatively, resolution and resolving power may be based on the closest separation between two peaks of equal magnitude, such that the valley between the two peaks is a specified fraction of the height of either peak. Both definitions obviously depend on the peak shape,20,21 which in turn depends on the data acquisition conditions. Resolution and Resolving Power. In mass spectrometry, the RP for ions of mass-to-charge ratio m/z (in which m is measured in mass units and z is the number of elementary charges per ion) may be defined as
RP(FT-ICR/MS) ) (m/z)/∆(m/z)
(1)
For Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers, ∆(m/z) is usually defined as the full magnitudemode mass spectral peak width at half-maximum peak height for an isolated peak. For double-focusing (DF) mass spectrometers, it is more usual to define ∆(m/z) as the peak separation, (m2/z2) - (m1/z1), corresponding to a 10% valley between two equal-magnitude peaks:
RP(DF/MS) )
(m1/z1) ∆(m/z)
(m1/z1) )
(m2/z2) - (m1/z1)
(10% valley, equal peak heights) (2) However, the valley definition obviously depends not only on peak shape, width, and separation but also on the relative heights of the two peaks (i.e., the relative ion abundances) in question. Therefore, let r be the ratio of the higher-magnitude to the lowermagnitude signals. If the DF mass spectral peak shape may be assumed to be approximately Gaussian, then it can be shown that the RP required to distinguish beams of ions of m1/z1 and m2/z2 takes the form (13) Aczel, T.; Williams, R. B.; Brown, R. A.; Pancirov, R. J. In Analytical Methods for Coal and coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 1, pp 499-540. (14) Scheppele, S. E.; Greenwood, G. J.; Pancirov, R. J.; Ashe, T. R. In Upgrading Coal Liquids; Sullivan, R. F., Ed.; ACS Symposium Series 156; American Chemical Society: Washington, DC, 1981; pp 39-73. (15) Scheppele, S. E.; Benson, P. A.; Greenwood, G. J.; Grindstaff, Q. G.; Aczel, T.; Beier, B. F. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; ACS Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981; pp 53-82. (16) Van Katwijk, K. Int. J. Mass Spectrom. Ion Processes 1981, 39, 287-310. (17) Schmidt, C. E.; Sprecher, R. F.; Batts Anal. Chem. 1987, 59, 2027-2033. (18) Severin, D.; Kollmeier, U.; Jovanovic, J. A. J. Serb. Chem. Soc. 1990, 55, 323-331. (19) Chasey, K. L.; Aczel, T. Energy Fuels 1991, 5, 386-394. (20) Mullen, S. L.; Marshall, A. G. Anal. Chim. Acta 1985, 178, 17-26. (21) Marshall, A. G.; Verdun, F. R. Fourier Transforms in NMR, Optical, and Mass Spectrometry: A User’s Handbook; Elsevier: Amsterdam, 1990; 460 pp.
RP(DF/MS) )
(m1/z1)
( x )
2[(m2/z2) - (m1/z1)]
1+
ln(20r) ln(20)
(10% valley; unequal peaks)
(3)
In eq 3, it is understood that the two peaks are by definition resolved if the valley height is 10% of either peak height if r ) 1, and if the valley height is 10% of the height of the lower-magnitude peak if r > 1.22 (Note that the mass RP defined by full width at half-maximum peak height is about 2.08 times greater than mass RP defined by the 10% valley criterion for two adjacent Gaussian peaks of equal height.) Clearly, the RP required by eq 3 for unequal peak heights is greater than or equal to that required by eq 2 for equal peak heights. The RP required to distinguish, identify, and quantify the ions in a mass spectrum of oil distillates of present interest is a complex function of the origin and nature of the sample and the ionization technique used in the high-resolution mass spectrometer. Although several authors have considered the subject in varying degrees of detail,7,12,23 it is nevertheless appropriate to review the matter briefly here for the reasons presented below. Homolog Type Analysis and Chemical Formula Determination. The general formula for compounds of present interest is CnH2n+ZNaObSc. A compound (chemical) type is designated by the numerical value of Z, followed by the element symbols and coefficients that are greater than zero in parentheses. The symbol H is used for designating hydrocarbons. Thus, compounds are classified according to compound type by specifying their values of Z and a, b, and c. The homlogs of a compound type are specified by the values of n. The various isotopically substituted species making up a given substance are members of the same compound type. Finally, homologs will, in general, consist of more than one isomer. For example, -4(S) designates a homologous series of organosulfur compounds in which the difference between the number of hydrogens in a homolog having n carbons and the number of hydrogens in n CH2 groups is -4. For example, the various isotopically substituted species comprising thiophene, e.g., 12C H 32S, 13C12C H 32S, 12C DH 32S, and 12C H 34S, constitute the 4 4 3 4 4 3 4 4 first members in the -4(S) aromatic series. The second member of this homologous series consists of the isomeric compounds 2-methyl- and 3-methylthiophene. High-resolution mass analysis uses the masses of the isotopically most abundant molecular ions to identify the homologs of the various aromatic hydrocarbon types and aromatic heteroatomcontaining compound types present in distillates. However, molecular ions containing one or more less-abundant isotopes of its constituent elements (e.g., 13C, 34S) contribute significantly to the mass spectra of samples of present interest. In addition, lowvoltage electron ionization can produce nonnegligible quantities of fragment ions. Different combinations of atoms can have nearly the same mass, e.g., 12C3 vs 32SH4, 13C vs CH, CH2 vs 14N, and CH4 vs O. Thus, the presence of these latter ions can complicate the analysis of the isotopically most abundant molecular ions and, hence, complicate the identification and quantitation of the homologs of the various compound types present in a given sample. Resolving Power Needed for Type Analysis. The RP ≈ 1000 obtainable with DF instruments is adequate for field ioniza(22) Scheppele, S. E.; Nutter, G. L., unpublished results. (23) Van Katwijk, K. Int. J. Mass Spectrom. Ion Processes 1981, 39, 273-286.
Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
47
Table 1. Mass Resolving Power (10% Valley Criterion) Required To Distinguish the Isotopic Components of Ions of Nominal Mass, 344, 357, and 511 u, for the Specified Ion Relative Abundances
-Z 36(H)
formula
difference in composition
calcd mass (u)
13C C H 2 25 18
344.147 56 SH4/C3
26(S)
13C C H S 2 22 22
35(H)
13CC H 26 19
34(H)
C27H20
24(S)
C24H24S
26(O2)
13C C H O 2 22 22 2
21(N)
C26H31N
1.10 4.47
21(N)
C37H53N
3.37 8.82 1.0 8.11 357.253 76 4.47
48 Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
88 290 5.0
511.417 80 CH2/N
(24) Mead, W. L. Anal. Chem. 1968, 40, 743-747. (25) Kuras, M.; Ryska, M.; Mostecky, J. Anal. Chem. 1976, 48, 196-198. (26) Scheppele, S. E.; Grindstaff, Q. G.; Grizzle, P. L. ASTM Special Tech. Publ. 1986, 902, 49-80. (27) Lipton, P. A.; Carlson, R. M.; Moldowan, J. M.; Gallegos, E. J.; Rechsteiner, C. E.; Green, M.; Hughes-Davies, T. Proceedings of the 40th ASMS Annual Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Washington, DC, 1992; pp 100-101.
53 173 20.0
357.258 23
tion (FI) mass spectral analysis of saturated hydrocarbon fractions, due to the lack of isobaric homologs of different group types.24-27 A mass RP of 6500-7500 suffices for the FI mass analysis of aromatic distillates ( RP[1+,i + 1]pred
(22)
RP[2+,i + 1]pred > RP[i,2+]pred
(23)
RP[2+,i + 1]pred > RPexp
(24)
M(i) )
S(1)M(F1) + S(2)M(F2) S(i)ADC
(30)
and Inequalities 21 and 24 reveal that ion k+ ) 1+ would not have been resolved from ion i and that ion 2+ would not have been resolved from ion i + 1. Inequality 22 shows that resolution of ion 1+ from ion i requires a greater resolving power than does its resolution from ion i + 1. Inequality 23 shows that resolution of ion 2+ from ion i + 1 requires a greater resolving power than does its resolution from ion i + 1. Given these results, data processing as described in either situation 3 or situation 4 associates ion k+ ) 1+ with ion i and ion k+ ) 2+ with ion i + 1. Situation 6. Ions i and i + 1 were resolved. However, resolution of these ions was unexpected because from eq 3
RP[i,i + 1]pred > RPexp
(25)
Signal magnitudes in the region of the valley between two peaks having voltage values less than the value of the threshold for signal detection is one of several explanations for the occurrence of this phenomenon. As in situation 5, the set L(k+) contains the entries for ions k+ ) 1+ and 2+. However, in this instance, application of eq 3 to the mass and signal magnitudes for ions i, 1+, 2+, and i + 1 results in the following inequalities:
RP[i,1+]pred > RPexp
(26)
RP[1+,i + 1]pred > RPexp
(27)
RP[i,2+]pred > RPexp
(28)
RP[2+,i + 1]pred > RPexp
(29)
Thus, the decision regarding processing mass/signal magnitude pairs in this situation is based on the differences between the LHS of inequalities 26-29. For example, with regard to the present situation, if RP[i,1+] > RP[1+,i + 1], then data processing associates ion i with ion k+ ) 1+. If RP[2+,i + 1] > RP[i,2+], then data processing associates ion k+ ) 2+ with ion i + 1. Alternatively, if RP[1+,i + 1] > RP[i,1+], then data processing associates ions k+ ) 1+ and 2+ with ion i + 1. Conversely, if RP[i,2+] > RP[2+,i + 1], then data processing associates ions k+ ) 1+ and 2+ with ion i. In outlining the formula assignment process, let M(i) designate either M(i) or M(i)corr and S(i)ADC represent either S(i)ADC or S(i)ADC,corr. If a formula is found whose mass M(F) deviates by e|ET| (ET is the error tolerance) from M(i) and if M(i) e M(F) or M(i) g M(F), then the formula, either the experimental or corrected experimental mass, and either the experimental or residual signal magnitude are assigned to this ion. Alternatively, if the formula assignment process finds formulas F1 and F2 having masses M(F1) and M(F2) that meet the error tolerance for formula acceptability criterion and if M(F1) < M(i) < M(F2), then both ions are assumed to be present. The signal magnitude of each of these ions is obtained from
S(i)ADC ) S(1) + S(2)
(31)
in which S(1) and S(2) are the signal magnitudes of the ions having formulas F1 and F2, respectively. The present MS-50 data file was processed according to 14 ppm as the error tolerance for formula acceptability. Interpretation of Table 4. Table 4 lists the results obtained from processing the low-voltage EI high-resolution FT-ICR/MS and MS-50 spectra over the mass range 264 e m/z e 306. The specific Z values, formulas, and formula masses of the ions either observed or deduced to be present in either one or both of the two sets of spectra are listed in columns 2-11. The MS-50 results are shown in columns 12-15 of Table 4. Peak numbers are listed in column 12. The number of ions observed for each peak is shown in column 13. Column 14 reports the distribution of ions between the various ions comprising each peak observed in the MS-50 spectrum. If an MS-50 peak resulted from the detection of ions adhering to either formula I or II, then the values in column 15 are the ppm differences between the experimental mass for the peak and the masses in column 11 for the formulas in columns 3-9. Alternatively, if an MS-50 peak resulted from the detection of ions adhering to both formulas I and II, then the values in column 15 are the ppm differences between the corrected mass for the peak and masses in column 11 for the formulas in columns 3-9. The entries in columns 2-11 are grouped according to the results obtained from the deisotoping/formula assignment analysis of the MS-50 mass/signal magnitude pairs. Compositional differences between ions determined by this analysis to have contributed to the formation of unresolved peaks are listed in column 10. Mass resolving powers corresponding to these compositional differences are shown in parentheses in column 11. In a number of instances, the mass resolution between the highest mass ion associated with peak i and the lowest mass ion associated with the adjacent peak i + 1 exceeded the experimental MS-50 resolving power, i.e., 40 000. The resolving powers required to resolve these ions i and i + 1 were calculated from eq 3 and are listed in square brackets in column 11. These instances relect either situation 5 or 6, depending on whether RP(i,i + 1) e 40 000 or RP(i,i + 1) > 40 000. The FT-ICR/MS results are shown in columns 16 and 17 of Table 4. Peak numbers are listed in column 16. Column 17 lists the ppm differences between the experimental mass for the peak and the masses in column 11 for the formulas in columns 3-9. For example, the experimental masses for peaks 2 and 3 are seen to differ by only -0.8 and 0.2 ppm from the formula masses for 13C12C H 32S+ and 12C H 32S‚+. Column 14 reveals that the 17 15 18 16 deisotoping/formula assigment of the MS-50 peak 2 mass/signal magnitude pair resulted in the same identifications. However, the FT-ICR resolution of ions 2 and 3 signifcantly enhances the efficiency of data processing, because the need to first establish that ion 2 contributed to the formation of MS-50 peak 2 and then to correct both the formula mass and signal magnitude of this peak for contributions from ion 2 before implementing the formula assigment process to identify the presence of ion 3 is eliminated. Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
55
Table 4. Compositional Results for Ions Adhering to the General Formula
13C 12C 1H 14N 16O 32S 34S a b c d e f g
MS-50 ion
-Z
a
b
c
1 2
22(S) 21(S)
1
18 17
14 15
d
e
f
g 1
1 13C/CH
3 4 5 6
20(S) 18(O) 8(S) 8(S)
7
17(H)
2
18 19 17 15
16 20 26 26
1
19
23
1 1 1 1 13CH
3S/C4
C3/SH4 8
7(S)
1
16
27
1 13CH
9
16(H)
20
3S/C4
24 C3/SH4
10 11 12 13
6(S) 21(SO) 22(S) 20(S)
1 1
17 17 17 17
28 13 14 16
1
1 1 1 1 13C/CH
14 15
19(S) 18(O)
16 17
17(O) 7(S)
18
17(H)
1
18 18
17 20
19 17
21 27
18
23
1 1 13C/CH
2
1 1 13C/CH
19
16(H)
1
19
24 C3/SH4
20
6(S)
1
16
28
1 13CH
21
15(H)
20
3S/C4
25 C3/SH4
22 23 24 25 26
5(S) 22(S2) 20(SO) 20(S) 20(S)
2
17 16 17 18 16
29 10 14 16 16
1
1 2 1 1 1 13C/CH
27
19(S)
1
17
17
1 13CH
28
28(H)
21
3S/C4
14 C3/SH4
29 30 31 32
18(S) 16(O) 6(S) 16(H)
2
18 19 17 18
18 22 28 24
1 1 1 C3/SH4
33
6(S)
2
15
28
1 13CH
34
15(H)
1
19
3S/C4
25 C3/SH4
35
5(S)
1
16
29
1 13CH
36
14(H)
20
3S/C4
26 C3/SH4
37 38 39 40
4(S) 21(S2) 19(S) 19(S)
2
17 16 18 16
30 11 17 17
1 2 1 1 13C/CH
41
18(S)
1
17
18
1 13C/CH
42 43 44 45
17(S) 16(O) 5(S) 5(S)
46
14(H)
2
18 18 17 15
19 22 29 29
1
19
26
1
mass (u) (resolnb)
diff in composna
1 1 1 1 13CH
3S/C4
13C/CH
264.077 42 264.092 80 (59 081) 264.097 27 264.151 42 264.171 32 264.182 23 (240 166) 264.183 33 (78 393) 264.186 70 (240 169) 264.187 80 (78 348) 264.191 17 265.068 71 265.080 77 265.100 63 (59 306) 265.105 10 265.154 77 (59 319) 265.159 24 265.179 14 (35 123) 265.186 69 (59 326) [RP(10,11)pred ) 27 557] 265.191 16 (78 691) 265.194 53 (241 525) 265.195 63 (78 693) 265.199 00 266.022 39 266.076 54 266.093 07 266.103 98 (59 531) [RP(14,15)pred ) 36 888] 266.108 45 (241 917) 266.109 55 (78 964) 266.112 92 266.167 07 266.186 97 266.194 51 (78 989) 266.197 88 (241 998) 266.198 98 (78 991) [RP(18,19)pred ) 33 567] 266.202 35 (242 002) 266.203 45 (78 992) 266.206 82 267.030 22 267.100 89 267.111 81 (59 757) 267.116 28 (59 758) 267.120 75 267.174 02 267.194 79 267.205 71 (242 914) 267.206 81 (79 290)
56 Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
peak no.
ions obsd pred
1
6
2
76
3 4
25 4
5
143
5 0
8
72
9 10
11
12 13 14
4.9 19.2
1 2
0.1 -0.8
72
3 4
0.2 0.3
3 1
2.3 2.1 -16.3 28.3
13
24.2
5
-0.3
11
11.4
6
-4.4
51
7.3
7
0.2
-5.5 1.8
8 9
1.6 0.1
2 15
21.8
10
0.9
57 5
4.9 12.7
11 12
0.4 0.2
4 3
-4.2 22.2
13
0.3
1
-6.4
11
25.7
14
-0.1
13
12.9
15
-4.7
33
8.8
16
0.6
75
-3.9
17 18 19
1.5 0.5 0.1
20
-0.6
21
0.2
22 23
0.5 0.7
24
-0.3
9 3
132
5 2 0
3 1 11
15
175
16 17
25 0 8
2.2 10.9
20.4
c 164
18
FTMS peak no. δm (ppm)
11 4
67 6 7
δm (ppm)
3.6 1.1
3 1
3.6
1
-9.1
8
-13.2
14
10.8
13.0 25
19
116
20 21
4 0
48 54
6.7 -6.0 5.8
3 1
41.1
0.5 26 27 28
1.0 0.6 -0.7
22
78
33
24.4
29
