Anal. Chem. 1987, 59, 2027-2033
I
Registry No. PEA (SRU), 24937-05-1; PEA (copolymer), 24938-37-2; PCL (SRU), 25248-42-4; PCL (homopolymer), 24980-41-4; Nylon 6, 25038-54-4; Nylon 6.6, 32131-17-2.
,114
60i I
LITERATURE CITED
1% F
X
5
l 20 j
340 250
2027
3b0
350
-
m I1
Flgure 7. Fast atom bombardment mass spectrum of Nylon 6.
CONCLUSIONS Our results show that FAB mass spectra of the four polymers investigated show peaks due to the low molecular weight compounds already present in the polymer system. Peaks due to selective fragmentation of the polymeric backbone are not detectable in the mass spectra obtained in FAB mode. Recent studies (15-17) on the mechanism of formation of ions in FABMS indicate that the organic species emitted by the liquid matrix are those which do not collide directly with the atom beam,which is in contradiction with the hypothesis of a selective fragmentation of the polymers in the condensed phase with a subsequent desorption and cationization of the fragments originated. The latter process probably occurs in SIMS where a selective fragmentation of nylons has been reported by Bletsos et al. (18).
(1) Barber, M.; Bordoii, R. S.; Sedgwick, R. D.;Tyler, A. N. J . Chem. Soc., Chem. Commun. 1991, 325. (2) Fenseiau, C. J . Mt. Prod. 1984, 47, 215. (3) Mess Spectrometry In the Health and Life Sciences; Proceedings of an International Symposium, San Francisco, California, Sept. 1984; Buriingame, A. L.. Castagnoii, N., Jr., Eds.; Elsevier: Amsterdam, 1985. (4) Foti, S.; Montaudo, G. I n Analysls of Polymer Systems; Bark, L. S . , Allen, N. S.. Eds.; Applied Science: London, 1982, p 103. (5) Schulten, H. R.; Lattimer, R. P. Mass Spectrom. Rev. 1084, 3 , 231. (6) Montaudo, G Puglisi, C. In Developments in fo/ymer Degradation; Grassie, N., Ed.; Applied Science: London, 1987; Voi. 7, p 35. (7) Doerr, M.; Luderwaid, I.; Schulten, H A . Fresenlus’ 2.Anal. Chem. 1084, 378,339. (8) Doerr, M.; Luderwaid, I.; Schuiten, H A . J . Anal. Appl. pvrolysls 1085, 8 , 109. (9) Spanagel, E. W.; Carothers, W. H. J . Am. Chem. SOC. 1935, 57, 929. (10) Garozzo, D.;GiuffrMa, M.; Montaudo, G. Macromolecules 1088, 19, 1643. (11) Garozzo, D.;GiuffrMa, M.; Montaudo, 0. folym. Bull. (Berlin) 1988, 75. . ., 353. - ..
(12) Manoiova, N. E.; Gitsov, I.; Veiichkova, R. S.; Rashkov, I. B. Polym. Bull. (Berlin) 1095, 73,285. (13) Gualta, C. Makromol. Chem. 1984, 185, 459. (14) Mori, S.; Takeuchi, T. J . Chromatogr. 1970, 49, 230. (15) Cooks, R. 0.; Busch, K. L. Int. J . Mass Spectrom. Ion fhys. 1983. 53.111. (16) Wong, S. S.; Roiigen, F. W.; Manz, I.; Przybyiski, M. Blomed. Mess Spectrom. 1985, 72, 43. (17) Desorption Mess Spectrometry; Lyon, P. A., Ed.; ACS Symposium Series; American Chemical Society Washington, DC, 1985. (18) Bietsos, I. V.; Hercules, D. M.; Graifendorf, D.; Benninghoven, A. Anal. Chem. 1985, 57, 2384.
RECEIVED for review October 28,1986. Accepted April 6,1987. Partial Financial Support from the Italian Ministry of Public Education and from Consiglio Nazionale delle Ricerche (Roma) is gratefully acknowledged.
Low-Voltage, High-Resolution Mass Spectrometric Methods for Fuel Analysis: Application to Coal Distillates C. E. Schmidt,* R. F. Sprecher, and B. D. Batts’ US.Department of Energy, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
A low-voltage, high-resolution m a s spectrometric (LVHRMS) method for analyses of complex materiais, such as coaiderived liquids, Is presented. Descriptions of the computations utliired to convert LVHRMS spectra to weight percent data on individual compound types and the associated computer software are reported. Elemental analyses, carbon number distributions, number average molecular weights, and a measwe of aromatktty have been determined for 10 distillate fractions derived from an HCoai llquld by this technique, and the results were compared with those obtained by classical methods. The changes in dlstrlbution of the principal compound classes over the range of distillates can readily be followed. The LVHRMS method descrlbed can be applied to other complex materials containing heteroatoms.
The application of mass spectrometry to the analysis of complex mixtures was developed by the petroleum industry Permanent address: School of Chemistry, Macquarie University, N o r t h Ryde, New South Wales 2113, Australia.
in the 1950s (1-5). All of these techniques involved the use of mass spectra obtained at 50-70 eV ionizing voltages. “Type” analyses for compound classes such as olefins, saturates, naphthenes, and aromatics were determined by matrix calculations. Robinson and Cook (6) extended the group-type analyses to include 21 compound types in petroleum aromatic fractions, accounting for the entire composition of the sample. Robinson (7) later developed a new low-resolution mass spectrometric procedure for determining up to 25 saturated and aromatic types in petroleum fractions having wide ranges of boiling points (200-1100 OF) and composition. The use of low ionizing voltages (10-15 eV) to limit the spectra to molecular ions was introduced by Field and Hastings (8) and further developed by Lumpkin (9). Upon the commercial availability of high-resolution mass spectrometers, Reid et al. (10) and Lumpkin (11) extended the mass spectrometric technique for hydrocarbon analysis to isobaric compound types, such as alkylbenzenes and benzothiophenes, and also naphthenobenzenes and pyrenes, employing a mass-resolving power of 1 part in 10 000. Gallegos et al. (12) published the first multicomponent group-type
This article not subject to U.S. Copyright. Published 1987 by the American Chemical Society
2028
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
analysis utilizing high-resolution mass spectrometry for analyzing high-boiling petroleum stocks without requiring fractionation by silica gel chromatography or its equivalent. Lumpkin (11)and later Johnson and Aczel (13)realized the potential of combining the low-ionizing-voltage technique, which produces simplified mass spectra, with the highmass-resolving power of a high-resolution mass spectrometer and developed a routine low-voltage, high-resolution mass spectrometric (LVHRMS) technique to yield quantitative data for literally hundreds of compounds in complex samples, such as petroleum distillates. Although these workers discuss the LVHRMS technique and its applications in general terms, no detailed description of the computational aspect of the LVHRMS method is available. Sharkey and co-workers (14-16) pioneered the use of high-resolution mass spectrometry for characterization of coal products. White (17) has published a summary of the applications of mass spectrometry to the analyses of coal and coal-derived products, with a review of the historical aspects of the use of the LVHRMS technique. Aczel et al. (18) published a report detailing a comprehensive investigation of coal extracts and liquefaction products by LVHRMS. In this publication, quantitative data were given for nearly 1100 homologues found in coal-derived products, along with bulk chemical information, such as elemental analyses, number average molecular weights, carbon number distributions, and 2 (a measure of aromaticity)-all derived from LVHRMS spectra. Scheppele et al. (19) compared the group-type mass spectrometric analyses obtained from field ionization mass spectrometry (FIMS) and LVHRMS data on oils and asphaltenes from coal liquefaction. In this report, the effect of using sensitivities and approximations to sensitivities on the quantitative results calculated from FIMS were also discussed. The Pittsburgh Energy Technology Center (PETC) has a high-resolution mass spectrometer capable of operating at high resolving power (1 part in 80 000) in the low-ionizing-voltage mode. With the description of the LVHRMS technique available in the literature (11,13)as a guide, the computational tools necessary to obtain the aforementioned chemical information from LVHRMS spectra have been developed. Preliminary results of the application of the LVHRMS technique implemented at PETC to a coal liquid have been reported (20). This paper describes these calculations and computer programs that facilitate the application of the LVHRMS technique to the analysis of complex mixtures. The use of the technique is illustrated in this paper by applying the LVHRMS method to the analysis of distillates from a coal liquefaction product from the H-Coal process. EXPERIMENTAL SECTION
All mass spectra were obtained on a Kratos MS-50 high resolution mass spectrometer interfaced to a Kratos DS-55 data system. The mass spectra represent that portion of the sample volatile at 300 "C and lo4 Torr. The samples were introduced into the ion source of the mass spectrometer via an all-glass heated-inlet system (AGHIS) equipped with a 1-L expansion volume and a molecular leak in the source transfer line. The AGHIS, the transfer line, and the ion source were kept at 300 "C. The mass spectrometer was operated at 1 part in 25 000 dynamic resolving power. Although the Kratos MS-50 is capable of operating at higher resolving powers in the low-electronvolt mode, 25 000 resolution afforded mass separation of the H-Coal distillates (see Results and Discussion) with greater sensitivity. The ionizing voltage was kept at about 11.5 eV to minimize fragment ions and therefore enhance the detection of molecular ions. The ionizing voltage was set before each determination by admitting m-xylene to the mass spectrometer and lowering the ionizing potential until the m/z 91 fragment was 10% of the molecular ion at m/z 106. The instrument was scanned from mass 700 to 60 at a rate of 1 mass decade/1000 s.
Table I. Low-Voltage, High-Resolution Mass Spectrometric Calibration Standards compound
formula
ref formula
pyrrole thiophene fluorobenzene chlorobenzene fluorochlorobenzene dichlorobenzene bromobenzene chloronaphthalene trichlorobenzene chlorobromobenzene bromonaphthalene tetrachlorothiophene chloroiodobenzene iodonaphthalene octafluoronaphthalene
ref Mass 68.0457 84.0034 96.0375 112.0080 129.9986 145.9690 155.9574 162.0236 179.9300 191.9165 205.9731 221.8445 237.9046 253.9592 271.9872 293.9302
3,5-(trifluorodimethyl)-
bromobenzene dibromotetrafluorobenzene
307.8282 333.9840 361.9789 393.6849 455.8218
decafluorobiphenyl decafluorobenzophenone tetrabromobenzene 4,4'-dibromooctafluorobiphenyl
hexabromobenzene pentabromoiodobenzene" a
C6BT6 C6Br51
C679Br3s1Br, 551.5038 C ~ 9 B r ~ 1 B r I595.4939
Impurity in hexabromobenzene.
The spectra were collected and recorded on the DS-55 data system by using software provided by Kratos, Inc. Centroiding arguments, scan parameters, and multiplet separation-a feature that allows mass separation of unresolved multiplets-were implemented by the DS-55 system. In practice, many multiplets requiring greater than 25000 resolution for mass separation were resolved by the multiplet separation procedure. The interface between the mass spectrometer and the computer was equipped with a fast preprocessor microcomputer that enabled on-line centroiding and multiplet detection, and also continuous adjustment of the electronic zero level between the mass spectrometer and the computer. The H-Coal fractions analyzed were 50 OF distillate cuts from a blend of "light" and "heavy" coal oils (1:1.5) from a sample generated at the H-Coal facility in Catlettsburg, Kentucky. Specifically,the original sample was produced from an Illinois No. 6 coal while the Kentucky plant was operating in the synfuel mode. The 50 O F distillate cuts were obtained from Chevron (21) and cover the boiling range from 400-850 O F . In addition, the start-to-400 O F (ST-400 OF) fraction was also analyzed. ANALYTICAL PROCEDURE
Typically, 3-5 mg of sample were placed in the solid inlet vessel of the AGHIS along with an aliquot of the low-voltage calibration standard. The qalibrant used in this study was similar to that used by Lumpkin (11)and is reported in Table I. A heater maintained a t 300 "C was then placed over the solid inlet vessel and vaporized both sample and standard into the AGHIS. Normally, three scans were taken on each sample. At a scan speed of 1 mass decade/1000 s, about 20 min were required to scan from mass 700 to 60. The second scan was used for analysis because it was the most representative sampling of mixtures of compounds having low and high vapor pressures. Experience in this laboratory has shown that the peak shapes were more Gaussian in the second scan, which facilitates mass and intensity assignments. A disk file containing the elemental formulas, measured masses, and intensities of the molecular ions was generated by a modified form of the DS-55 software program ATOM. The ATOM program, as written by Kratos, usually generates several elemental formula choices per measured mass depending on the array of atoms entered by the user for the computer to
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
consider and on the mass deviation permitted for each computer-generated formula match to the measured mass. In this study, elemental formulas for each measured mass were generated by using a combination of one carbon-13 and two oxygen, nitrogen, and sulfur atoms, in addition to carbon and hydrogen atoms. Usually, at least five elemental formulas for each mass were found when using a mass deviation of f 5 mamu between the measured and computed masses. Therefore, the ATOM program was modified to permit the selection of one elemental formula for each molecular ion by using the following additional criteria: even mass peaks require a molecular formula with an even number of hydrogens and nitrogens, and a C/H ratio less than 2; odd mass peaks require a molecular formula with an odd number of hydrogens and nitrogens, and a C/H ratio less than 2. An additional test to distinguish between odd mass molecular ions containing a carbon-13 isotope and a nitrogen-containing moiety was required. An odd mass peak was determined to be a carbon-13 isotope if the difference between the measured and computed mass was less than f l mamu. When an acceptable formula was found, it was written in the disk file along with the measured mass and intensity. The disk file was then edited and corrections were made where appropriate. For example, at m / z 190.082, the molecular ion C15H10would be selected, when, in fact, the correct formula would be Cl2H1,S (see Results and Discussion). After the file was edited, the intensity values were corrected for sensitivity differences a t LVHRMS conditions by using the program CORR, developed a t PETC. (Complete listing available on request.) The sensitivity values used in the program CORR for molecular ions formed at low-ionizing voltage were obtained from the literature (22-24). Computation of the elemental analysis, number average molecular weight, carbon number distributions, average 2,and atomic ratios were accomplished by the program CALC, also developed at PETC (complete listing available on request); CALC uses the elemental formulas, measured masses, and corrected intensities to separate the LVHRMS spectrum into chemical classes (hydrocarbons, oxygenates, etc). The number average molecular weight (M,) can be computed by eq 1,where ni is the number of moles of molecular
weight Mi present in the sample. Since the sensitivites used in the program CORR are gram sensitivities ni = Ii/Mi, where Iiis the corrected intensity of mass Mi.Equation 1 can now be written as
where the term CiIJMi represents the total number of moles of the components determined by LVHRMS. For a particular chemical class, Ii and Mi are the corrected intensity and mass of the ith peak in the chemical class, respectively. Accordingly, the number average Z number for the chemical class was calculated by eq 2, where Zi is the value of 2 for the ith
(a
component in the chemical class with the general formula CnHz,,+Z; 2 is related to the number of aromatic carbons (C,) and rings (R) by the expression 2 = 2 - (C, + 2R). The remaining terms in eq 2 are as in eq 1. The proportion of the total sample contained in the respective chemical class was calculated, along with the carbon number range and the weight percent of the total sample for each Z series in the respective chemical class. The weight percent of the total sample for each carbon number in the respective 2 series was normalized to the largest value in the chemical class and plotted, yielding carbon number profiles for each 2 series (see Figure 1).
2029
Table 11. Resolution Requirements for Mass Doublets Present in Coal Liquids doublet
AM,U
max resolved mass, ua
C-12H CzHB-32S CH4-O 32S-Oz
0.0939 0.0905 0.0364 0.0277
13CH-N
0.0081
2347 2263 910 692 203
C3-32SH4
0.0039
85
Highest mass that could be base-line resolved with a resolving power of 1/25000. After having determined the above for each chemical class detected in the sample, mean analytical data for the total sample were computed. The elemental analyses were calculated, in this case for carbon, by using
%
c = 12.011
CiCiIi / M i (3)
where Ciis the number of carbon atoms in the molecular formula having the mass Mi and corrected intensity Ii for the ith component. The term CiIirepresents, in this case, the total corrected intensity for the sample. Elemental analyses were similarly calculated for hydrogen, oxygen, nitrogen, and sulfur. The number average molecular weight (M,) and average 2 (8for the total sample were determined by eq l a and eq 2, respectively. The atomic ratios H/C, H/O, H/N, and H / S were also computed for the total sample.
RESULTS AND DISCUSSION At a 1part in 25 000 resolving-power dynamic resolution, the maximum masses at which multiplets usually encountered in coal products can be completely separated are given in Table 11. The resolution requirements to mass-separate isobaric compounds, such as acenaphthene (ClzHlo)and undecene (Cl1H2J, benzothiophene (C8H6S)and butylbenzene (C10H14), benzofuran (C8HsO) and indan (C9H10),and dihydroxybenzene (CeH602)and thiophenol (C&$), are much less than the resolution needed to resolve acridine (C13H9N) from the carbon-13 isotope molecular ion from phenanthrene (13CC13Hlo)and to resolve a C3 alkyl substituted dibenzothiophene (C15H14S)from benzo[ghi]fluoranthene (C18H10). Because of the heteroatomic content of coal liquefaction products and the presence of high molecular weight compounds in such liquids, it is virutally impossible to massseparate all the molecular species in materials as complex as coal-derived liquids. However, in many instances, base-line resolution is not required for mass and intensity assignments. Additionally, the multiplet separation feature of the DS-55 data system enables identification of multiplets whose masses are not resolvable at the operating conditions of the mass spectrometer. Therefore, for the H-Coal distillates studied in this investigation, all the molecular ions detected by LVHRMS except the C3-32SH4mass doublet were mass-separated with the aforementioned resolving power. For the C3-32SH4doublet, an extrapolation technique was developed based on the method described by Johnson and Aczel(13) and was used to determine the amount of sulfur heterocyclic compounds. For example, the benzothiophene (C,H2,-loS) homologous series overlaps the 4,5-methylene phenanthrene (C,,Hz,,-m)series at m / z 190. However, the first four members of the benzothiophene (C8H6S1CgHaS, CloHloS, and C11HlzS) homologous series can be measured separately. Aczel (25) noted that the benzothiophenes have the same boiling point distributions as the alkylnaphthalenes. Combining the distribution information given by the alkylnaphthalene series with the unambiguous separation of the first several members
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
Table 111. Elemental Analyses and H/C Ratios Determined by LVHRMS and Ultimate Analysis for the H-Coal Distillatesa % H
% C
70
%N
0
H/C
%S
distillate
MS
ult
MS
ult
MS
ultb
MS
ult
MS
ult
MS
ult
ST-400 O F 400-450 O F 450-500 O F 500-550 O F 550-600 O F 600-650 O F 650-700 O F 700-750 O F 750-800 O F 800-850 O F
83.2 85.1 87.8 88.5 90.2 90.3 89.9 91.3 91.4 91.3
85.1 86.5 88.0 88.3 89.5 89.1 88.8 89.6 88.9 88.7
8.82 9.08 9.32 9.33 8.55 8.16 7.87 7.40 7.25 7.20
10.2 10.0 10.0 10.1 9.3 9.6 8.9 8.1 8.2 7.9
6.49 4.73 2.15 1.51 0.913 0.944 1.16 0.759 0.650 0.652
4.0 2.9 1.6 1.1 0.8 0.6 1.0 1.2 1.7 1.8
1.45 1.09 0.735 0.574 0.291 0.479 0.766 0.542 0.506 0.763
0.7 0.5 0.4 0.4 0.2 0.5 1.0 0.8 0.9 1.3
0.013 0.010 0.047 0.078 0.049 0.122 0.293 0.043 0.150 0.081
0.1 0.1 0.1 0.1 0.1 0.3 0.4 0.2 0.3 0.3
1.26 1.27 1.27 1.25 1.13 1.08 1.04 0.966 0.944 0.940
1.43 1.38 1.35 1.36 1.24 1.28 1.20 1.08 1.10 1.07
Note: Elemental analyses determined classically corrected for water. *Oxygen values determined directly. of the sulfur series permits estimation of the relative concentrations of the succeeding members of the benzothiophene series. In Table I11 are shown the results of the LVHRMS analysis of the H-Coal distillates for elemental analyses and the elemental analysis data obtained by classical elemental analyses methods plus the H/C ratios from the two respective techniques. The most striking difference between elemental analysis data from the two methods is for oxygen in the lower boiling fractions. The LVHRMS method determined 6.5 % oxygen for the ST-400 O F fraction, while 4.0% was measured by the ultimate analysis technique. Similarly for the 400-450 O F distillate, the oxygen value calculated from LVHRMS data was 4.7%, as compared with 2.9% determined classically. In an earlier study (26) of the H-Coal distillates, liquid chromatographic results indicated the presence of saturates, particularly in the lower boiling distillates. Saturate compounds do not form molecular ions a t low ionizing voltages. When the hydrogen and carbon values from saturates are combined with the LVHRMS data, and the elemental analyses renormalized, the percent oxygen decreases from 4.7% for 400-450 O F distillate to 3.3%. Similarly, inclusion of the saturate data in the LVHRMS results decreases the nitrogen value from 1.1%to 0.8% for 400-450 O F distillate, while the percentages of hydrogen and carbon increase slightly. However, the principal reason for the observed differences between the mass spectrometric and ultimate assay values is probably due to fragmentation in the ion source, which, as discussed later, occurs to some extent even at the low ionization voltages used. Overall, the elemental values determined by LVHRMS agree well with the values determined classically and corroborate the general observation that the H/C ratio decreases as the boiling range of the H-Coal distillates increases, indicating an increase in aromaticity and condensation of the components in the distillates. The number average molecular weight (MAand the number average 2 number were computed for the H-Coal distillates from LVHRMS data by using eq la and 2, respectively, and are listed in Table IV. Number average molecular weights determined by vapor-phase osmometry (VPO), and aromaticity values (fa) (27) computed from 'H NMR data are also presented in Table N for each H-Coal fraction. The molecular weights obtained by LVHRMS are slightly less than those determined by VPO, with the difference being fairly constant for all the H-Coal distillates except the 800-850 O F distillate. The LVHRMS molecular weights show the expected smooth increase with increasing boiling point of the distillates. Likewise, the average 2 value becomes more negative as the distillation temperature rises indicating an increase in aromaticity with boiling point, a trend that is also observed in fa determined by NMR. The LVHRMS method of evaluating a complex sample for aromaticity and ring condensation takes into account both heteroatom and hydrocarbon con-
(a
(a
Table IV. Summary of Structural Parameters for the H-Coal Distillates
M" distillate
LVHRMS
VPO
2
fa
ST-400 O F 400-450 O F 450-500 O F 500-550 "F 550-600 O F 600-650 O F 650-700 O F 700-750 O F 750-850 O F 800-850 O F
116 130 142 159 173 191 207 227 238 250
137 144 161 176 196 213 234 250 262 295
-6 -7 -8 -9 -11 -13 -15 -18 -19 -20
0.47 0.49 0.49 0.48 0.56 0.54 0.58 0.65 0.65 0.65
Table V. Summary of the Precision of LVHRMS Method of Analysis for Selected Parameters 600-650
O F
determination
av value
SD
elemental carbon elemental hydrogen elemental nitrogen elemental oxygen elemental sulfur
89.9 8.3 0.5 1.0 0.2
0.21 0.17 0.07 0.07 0.01
M"
rms
192 4.48
700-750 O F av value SD
1.58 0.59
91.0 7.3 0.5 0.9 0.2 226 3.61
0.38 0.16 0.07 0.10 0.07 1.58 0.14
stituents, and therefore is a useful structural parameter in characterizing coal liquids. Three replicate LVHRMS analyses of the 600-650 O F and 700-750 O F H-Coal distillates were performed to estimate the precision of the LVHRMS method (see Table V). The LVHRMS measurements were made at different times each day for three consecutive days on each distillate. Obtaining the replicates in this fashion should indicate bias due to instrument stability and conditioning. Care was taken to charge the same amount of sample (4.0 mg) to the spectrometer for each analysis. Similarly, a constant amount of calibrant (Table I) was added each time. The standard deviations for elemental carbon, hydrogen, oxygen, and number average molecular weight are in agreement with similar data from a petroleum distillate previously reported (25). Included in Table V are the root mean square (rms) deviations of the mass measurements for each replicate. The rms values were estimated from eq 4, where PPMi = 1000 DEVi/M, DEVi is the difference rms = [Ci(PPM?/(N - 1))
(4)
between the measured mass and the precise mass (M) of the ith peak; and N is the total number of peaks. The rms values of less than 5 ppm over the entire mass range are excellent and demonstrate the accuracy of the mass spectrometer in
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
1
2031
!ST.40O0F
i ! i
i
i
14
14
c z W 0 a W
a
c I ? ! W
0 2 4
6
8
IO
12
14 -2
16
18
20 22 2 4 2 6 2 8
Flgure 1. Carbon number distributions as a function of Z number for the hydrocarbons (C, HPn+ z ) from the H-Coal distillates.
determining masses of molecular ions. The low rms numbers also reflect the validity of the multiplet separation feature of the DS-55 data system that enables deconvolution and centroiding of unresolved peaks, and the general observation that a dynamic resolution of 1part in 25 000 for mass determination was sufficient for the analysis of the H-Coal distillates. The rms calculation is a feature of the program CALC and provides a day-to-day measure of instrument stability and accuracy. As mentioned earlier, the program CALC provides carbon number profiles for each chemical class as a function of the structural parameter 2. The carbon number distributions within each 2 series for the hydrocarbons, oxygenates, and nitrogen compounds, respectively, for the H-Coal distillates are plotted in Figures 1-3. The carbon number range within a specific 2 series depends upon the chemical structure, as well as on the degree of substitution of a given homologous series. Consider the carbon number distribution for the naphthalene homologous series (2= -12). The first member, naphthalene, has a carbon number of 10. Thus the carbon number range would start at 10 and increase depending on the degree of substitution. Therefore, the minimum carbon number within a specific 2 series is determined by the chemical structure for aromatic compounds. The carbon number profiles, as a function of 2,reflect the degree of substitution within each distinct chemical structural series.
7
8 6
IO
12
12
5
CARBONNUMBER 8
IO
12
16
14
18 2 0
16
18 18
22
24 26
-Z Figure 2. Carbon number distributions as a function of Z number for the oxygenates (C, H,, +zO)from the H-Coal distillates.
It must be noted, however, that all isomers of a specific 2 series would be included in the carbon number profiles. The carbon number profile for 2 = -22 would include pyrene and fluoranthene components, in addition to the positional isomers for the alkyl-substituted moieties. In Figure 1,which is the carbon number distribution for the hydrocarbons in the HCoal distillates, the carbon number profiles are arranged with
2032
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
Table VI. Structural Distributions (wt % ) of the CnH2,+z Class for the 500-550 OF H-Coal Distillate at Different Ionizing Voltages
z
possible structure(s)
0 olefins/cycloalkanes -2 diolefins/cycloalkenes -4 -6 -8 -10 -12 -14 -16 -18 -20
cyclodiolefins benzenes indans/tetralins
indenes naphthalenes acenaphthenes acenaphthylenes/fluorenes
phenanthrenes cyclopenta[deflphenanthrenes/di-
ionizing voltage, eVn 11.5 11.25 11.0 0.637 0.268 1.27 5.04 3.99 6.24 2.26 2.36 3.41 2.88 2.97 3.24 24.9 25.2 25.4 13.1 13.8 13.5 17.8 20.2 21.2 8.05 8.90 9.34 1.19 0.639 1.02 0.159 0.067 0.030 0.819 0.591 0.429
hydropyrenes -22 pyrenes -24 chrysenes -26 benzo[ghi]fluoranthenes/
1.32 1.03 0.220
1.25 0.965 0.127
0.916 0.923 0.154
cholanthrenes -28 benzopyrenes total
0.032
0.023
0.021
82.2
83.0
I2 W 0
a P W
IS
5?
r"
82.7
=Obtained from ionizing voltage potentiometer.
the lowest boiling distillate, ST-400 O F , on top, and the highest boiling distillate, 800-850 "F, on the bottom. In this fashion, the increase in the aromaticity of the hydrocarbon components with increasing boiling range is evident. The most abundant structure in the hydrocarbon class in the ST-400 O F is CloH,, in the 2 = -8 series, or an indan/tetralin structure. This is in contrast to the distribution in the 800-850 O F distillate, which contains large amounts of three- and four-ring aromatic compounds, such as substituted phenanthrenes and pyrenes. Similar observations can be made for the carbon number distributions for the oxygenates (Figure 2) and the nitrogencontaining compounds (Figure 3). The depiction of the various carbon number distributions within each respective 2 series appears to be a useful method of comparing samples or, in this case, distillates. Previously, Finseth et al. (28) utilized carbon number distributions for the nitrogen-containing compounds as a function of 2 to determine the specificity of a catalyst for removal of nitrogen compounds from a coal liquid. Compounds in the 2 = 0, -2, and -4 series, with carbon numbers less than 10, are unexpected in the higher boiling distillates (see Figure 1). Hydrocarbons with less than 10 carbons in the aforementioned 2 series have low boiling points and should be concentrated in the lower boiling distillates. An explanation could be that even at low ionizing voltages (-11.5 eV), fragmentation is taking place, resulting in the formation of rearrangement products with low appearance potentials. The mass spectrum of 1-dodecene was obtained to determine the extent of fragmentation occurring at 11.5 eV for an olefin. Fragment and/or rearrangement ions at m / z
5
7
9
II
13 15 -2
17
19
21 23
Figure 3. Carbon number distributions as a function of Z number for the nitrogenates (C, H, +=N) from the H-Coal distillates.
78,92, and 106 accounted for 30% of the total ion intensity
of 1-dodecene at 11.5 eV ionizing voltage. In contrast, the molecular ion intensity for 1-dodecene represented less than 1 % of the total ion intensity. The 500-550 "F distillate was analyzed at three different ionizing voltages to assess the extent of fragmentation in the LVHRMS analysis of the H-Coal distillates. Table VI contains the weight percent of each 2 series detected for the CnHPntZclass obtained from LVHRMS data taken at 11.5, 11.25, and 11.0 eV ionizing voltage (measured), and the total weight percent of hydro-
Table VII. Summary of Compound Classes (wt % ) in H-Coal Distillates as Determined by LVHRMS distillate
CH
CHO
ST-400 O F 400-450 O C 450-500 O F 500-550 O F 550-600 O F 600-650 O F 650-700 O F 700-750 O F 750-800 O F 800-850 O F
43.7 53.3 75.1 80.1 86.3 81.5 73.1 80.6 81.1 76.3
45.1 36.7 16.9 12.7 9.93 11.2 12.4 9.29 8.64 9.16
CHO2 0.332 0.694 0.690 0.236 1.63 0.982 0.841 0.437
CHN
CHS
11.1 9.53 6.90 5.89 3.45 6.32 10.8 8.9 8.21 13.5
0.059 0.045 0.225 0.415 0.216 0.725 1.97 0.322 1.18 0.651
CHNO 0.046 0.176 0.179 0.007 0.126 0.273 0.059
CHOS
0.068 0.083
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
carbons found by LVHRMS in each analysis. It seems certain that some low molecular weight fragments are formed at the ionizing voltages used in the LVHRMS method. The weight percent for the 2 = 0, -2, and -4 series decreased from 10.970 collectively at 11.5 eV ionizing voltage to 6.02% at 11.0 eV. However, the total weight percent for the C,,H2,+z class remained fairly constant for each analysis, supporting the probable formation of low molecular weight materials at the expense of the higher molecular weight structures. In fact, the amounts of 2 series -8, -10, -12, and -14 all increase as the ionizing voltage decreases. The largest increases occur at mlz 188 in the 2 = -8 series, which corresponds to a CI4Hm compound, and m / z 156,170, and 184 in the 2 = -12 series, possibly substituted naphthalene structures. In the ionization process, under electron impact conditions, the ejection of an electron from a neutral molecule to produce a molecular ion and bond breaking that results in fragmentation are competing processes. Field and Hastings (8)reported that even under their standard low-voltage operating conditions at 6.9 eV, paraffin-naphthene ionization yielding lower molecular weight molecular ions contributed about 5% to the olefin concentrations. Therefore, since the 500-550 O F H-Coal distillate contains both paraffin and naphthenic material, as determined by liquid chromatography and combined gas chromatography-mass spectrometry, fragmentation to yield lower molecular weight structures is probable. Lumpkin and Aczel(22) noted that at 11.5 eV ionizing voltage, in the case of naphthalene (2 = -12), a small amount of fragmentation occurs. As shown in Table VI, the 2 = -12 series increases from 17.84% a t 11.5 eV to 21.21% at 11.0 eV. It should be mentioned at this point that at 11.5 eV, the mlz 91 fragment of xylene is 10% of the molecular ion at m / z 106. Therefore, sufficient ionization energy is present in fragment xylene. There was a nearly 40% decrease in total ionization going from ionizing voltages of 11.5 to 11.0 eV, accompanied by the loss of some higher molecular weight components with low concentrations. Therefore, even though some fragmentation may be occurring at 11.5 eV, the advantages of analyzing high molecular weight material with low concentrations outweigh the small errors introduced in the distributions of lower molecular weight components. A summary of the compound classes determined by the LVHRMS technique in the H-Coal distillates is presented in Table VII. Seven compound classifications containing homologous series were detected by LVHRMS, with hydrocarbons (CH), oxygenates (CHO), and nitrogen-containing compounds (CHN) being the most abundant. A complete listing of the carbon number distribution for each classification, and the weight percent data for each chemical species detected are contained in ref 29.
2033
The LVHRMS method described in this report is applicable to coal-derived mixtures boiling from 150 to 850 O F . The LVHRMS technique is especially useful for the analysis of small samples and provides a complete chemical characterization; the total analysis is accomplished in about 3 h/sample.
ACKNOWLEDGMENT Joseph Malli and Thomas Link are acknowledged for obtaining the mass spectra.
LITERATURE CITED (1) Brown, R. A. Anal. Chem. 1951, 2 3 , 430-437. (2) Lumpkin, H. E.; Johnson, B. H. Anal. Chem. 1954, 26, 1719-1722. (3) Hastings, S. H.; Johnson, B. H.; Lumpkin, H. E. Anal. Chem. 1956, 28, 1243-1247. (4) Lumpkin, H. E. Anal. Chem. 1956, 28, 1946-1948. (5) Hood, A.; O'Neal, M. J. Advances in Mass Spectrometry; Pergamon: London, England, 1959; pp 175-190. (6) Robinson, C. J.; Cook, G. L. Anal. Chem. 1969, 4 7 , 1548-1554. (7) Robinson, C. J. Anal. Chem. 1971, 4 3 , 1425-1434. (8) Field, F. H.; Hastings, S . H. Anal. Chem. 1958, 28, 1248-1255. (9) Lumpkin, H. E. Anal. Chem. 1958, 3 0 , 321-325. 10) Reid, W. K.; Mead, W. L.; Bower, K. M. Presented at the Institute of Petroleum/ASTM Spectrometry Symposium, Paris, France, Sept. 19AA
(16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
Limpkin, H. E. Anal. Chem. 1964, 36, 2399-2401. Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeTourneau, R. L.; Teeter, R. M. Anal. Chem. 1967, 3 9 , 1833-1837. Johnson, B. H.; Aczei, T. Anal. Chem. 1967, 3 9 , 682-685. Sharkey, A. G., Jr.; Shultz, J. L.; Kessler, T.; Friedei, R. A. Spectrometry of Fuels; Plenum: New York, 1970; pp 1-14. Shuitz, J. L.; Kessler, T.; Friedel, R. A,; Sharkey, A. G., Jr. Fuel 1972, 51. 242-246. - ~. Sharkey, A. G., Jr.; Shultz, J. L.; Schmidt, C. E.; Friedel, R. A. PERC/ RIl75-5, 1975. White, C. M. Handbook of Polycyclic Aromatic Hydrocarbons; Marcel Dekker: New York, 1983; pp 525-615. Aczel, T.; Williams, R. B.; Pancirov, R. J.: Karchmer, J. H. MERC-8007, 1976. Scheppele, S. E.; Benson, P. A.; Greenwood, G. J.; Grindstaff, Q. G.; Aczel, T.; Beier, B. F. Adv. Chem. Ser. 1981, No. 795, 53-82. Klunder, E. 6.; Mima, J. A.; Krastman, D. DOE/PETC/TR-85/4, 1986. Sullivan, R. F. Monthly Report for Feb. 1983, US. DOE Contract No. DE-AC22-76ET10532. Lumpkin, H. E.; Aczel, T. Anal. Chem. 1964, 3 6 , 181-184. Aczei, T.; Lumpkin, H. E. Proceeding of the Nineteenth Annual Conference on Mass Spectrometry and Allied Topics, 1971; Abstract No. S1. Aczel, T. A,, Exxon Research and Engineering Co., personal communication, 1968. Aczel, T. A. Rev. Anal. Chem. 1971, 7(3), 226-261. Perry, M. B.; White, C. M. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 1985, 30(4), 204-214. Brown, J. K.; Ladner, W. R. Fuel 1960, 3 9 , 87-96. Finseth, D. H.; Stiegel, G. J.; Schmidt, C. E.; Sprecher, R. F.; Lett, R. G. Proc. Int. Conf. Coal Sci. 1983, 180-183. Schmidt, C. E.; Perry, M. B.; White, C. M.; Gibbon, G. A.; Retcofsky, H. L. DOE/PETC/TR-87/4, 1986.
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RECEIVED for review November 13,1986. Accepted May 13, 1987. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy.
