L. G. Christophorou, "Atomic and Molecular Radiation Physics", WileyInterscience, 1971, Chapter 6. D. F. Hunt and J. F. Ryan 111, Tetrahedron Lett., 4535 (1971). P. J. Arpino and F. W. McLafferty, "Determination of Organic Structures by Physical Methods", Voi. 6, F. C. Nachod, J. J. Zuckerrnan, and E. W. Randall, Ed., Academic Press, New York, 1975, pp 1-89. H. R. Morris, "New Techniques in Biophysics and Cell Biology", R. H. Pain and B. J. Smith, Ed., Wiley-Interscience, New York, 1973, pp 149-182. J. T. Swansiger, F. E. Dickson, and H. T. Best, Anal. Chem., 46, 730 (1974). A. W. Peters and J. G. Bendoraitis, Anal. Chem., 48, 968 (1976). W. Giger and M. Blurner, Anal. Chem., 46, 1663 (1974). C. J. RobinsonandG. C. Cook, Anal. Chem., 43, 1425 (1971). E. J. Gallegos, J. W. Green, L. P. Lindeman, R. L. Letourneau, and R. M. Teeter, Anal. Chem., 39, 1833 (1 967). T. Aczel, D. E. Allan, J. H. Harding, and E. A. Knipp, Anal. Chem., 42,341 (1970). N. Einolf and B. Munson. lnt. J. Mass Spectrom. /on Phys., 9, 141 (1972). M. McFarland, D. L. Albritton, F. C. Fehsenfeld, E. E. Ferguson. and A. L. Schmeltekopf, J. Chem. Phys., 59, 6529 (1973).
(14) R. C. Dougherty, J. D. Roberts, and F. J. Biros, Anal. Chem., 47, 54 (1975). (15) H. P. Tannenbaurn, J. D. Roberts, and R. C. Dougherty, Anal. Chem., 47,
.-.
A 4 114761 -,. 1",
(16) E. C. Horning, M. G. Horning, D. I. Carroll, I. Dzidic, and R. N. Stillwell, Anal. Chem., 45, 936 (1973). (17) I. Dzidic, D. I. Carroll, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47, 1308 (1975). (18) I. Dizidic, D. I. Carroll, R. N. Stillwell, and E. C. Horning, J. Am. Chem. SOC., 96, 5258 (1974). (19) E. C. Horning, D. I. Carroll, I. Dzidic, K. D..Haegele, M. G. Horning, and R. N. Stillwell, J. Chromatogr., 99, 13 (1974). (20) D. F. Hunt, T. M. Harvey, and J. W. Russell, J. Chem. Soc., Chem. Commun., 151 (1975). (21) D. F. Hunt, C. N. McEwen, and T. M. Harvey, Anal. Chem., 47, 1730 (1975). (22) M. A. Baldwin and F. W. McLafferty, Org. Mass Spectrom., 7, 1353 (1973). (23) D. P. Ridge and J. L. Beaucharnp, J. Am. Chem. SOC., 96, 5595 (1974). (24) A. C. Moffat, E. C. Horning, S. B. Matin, and M. Rowland, J. Chromatogr., 66, 255 (1972). (25) A. C. Moffat and E. C. Horning, Biochim., Biophys. Acta, 222, 248 (1970). (26) D. F. Hunt and G. C. Stafford, Patent Pending. (27) D. F. Hunt and J. F. Ryan Ill, Anal. Chem., 44, 1306 (1972). (28) T. A. Whitney, L. P. Klernan, and F. H. Field, Anal. Chem., 43, 1048 (1971). (29) V. Cermak, J. Chem. Phys., 44, 1318 (1966). (30) D. H. Williams and I. Howe, "Principles of Organic Mass Spectrometry", McGraw-Hill Book Co., New York, 1972, p 24.
RECEIVEDfor review July 16,1976. Accepted September 7, 1976. This research was supported by grants from the National Institutes of Health, USPHS, GM 22039, and the U.S. Army Research Office (DAHC-74-G-0079).
Determination of Field-Ionization Relative Sensitivities for the Analysis of Coal-Derived Liquids and Their Correlation with LowVoltage Electron-Impact Relative Sensitivities S. E. Scheppele," P. L. Grizzle,' G. J. Greenwood, T. D. Marriott, and N. B. Perreira Department of Chemistry, Oklahoma State University, Stillwater, Okla. 74074
Field-ionizatlonand flame-ionization detector sensitivities have been determined for 60 aromatic compounds relative to ethylbenzene. The compounds studied are In general typical of those encountered in coal-derived liquids. The sensitivities are within the limits of experimental precision independent of sample composltion. For predictive purposes, the FI relative cross sections and relative sensitivities are correlated with low-voltage El relative cross sections and relative sensitivlties, respectively, and with -Z. The variations in the FI relative cross sections with change in molecular structure are not as large as those for ionization by low-voltage electrons; however, the trends are similar. For three synthetic blends, quantitative distributions calculated from FI ion abundances assuming unit relative sensitivities are in significantly better agreement with the known distributions that those similarly calculated from the low-voltage El data. Inclusion of sensitivities into both analyses produces excellent agreement with the known distributions.
Field-ionization/mass spectrometry, FI/MS ( I ) , which produces virtually fragment-ion-free mass spectra ( I , 2 ) should be ideally suited to the quantitative analysis of materials related to or derived from fossil fuels. However, publications describing such applications of FI/MS are not numerous. Compositional data have been so obtained for gasoline ( I , 3 ) ,heavy- ( 4 ) and low-boiling ( 5 )petroleum fractions,
Present address, Bartlesville E n e r g y Research Center, E n e r g y Research a n d Development A d m i n i s t r a t i o n , Bartlesville, Okla. 74003.
lubricating oils (6),and nonboiling residues from a crude oil ( 7 ) .To our knowledge, FI/MS has not been employed in either the quantitative or qualitative analysis of materials derived from coal. Such applications are of significance because coal constitutes a potentially important source of alternate liquid fuels (8). The accuracy of the compositional data calculated from FI ion abundances reflects, in part, the availability of sensitivity coefficient data. The dependence of sensitivity coefficients on molecular weight and structure and mixture composition has been investigated for only alkanes, alkenes, cycloalkanes, and low-molecular-weight aromatic hydrocarbons (9, I O ) . However, characterization data for coal-derived materials (11-24) show that the available sensitivity data (5, 6, 9, 1 0 ) are insufficient for routine quantitative analysis of such substances by FI/MS. Consequently, relative FI sensitivity data were determined for a variety of compounds typical of those encountered in coal-derived liquids; relative sensitivities were also determined for flame-ionization detection (FID) gas chromatography (GC).
EXPERIMENTAL Instruments. Field-ionization mass spectra were acquired using a C E C 21-llOB equipped w i t h a m o d i f i e d (25,26) combination FI/EI i o n source ( 2 7 ) .E m i t t e r s are c u t f r o m uncoated stainless-steel razor blades (Personna 74) a n d conditioned in t h e i o n source a t 250 "C a n d in t h e presence of acetone (ca. 2 X 10V T o r r ) . I o n currents are, deFA f o r p e n d i n g u p o n t h e c o n d i t i o n i n g time, t y p i c a l l y 2-12 X acetone pressures o f ca. 1-2 X 10W Torr. An i o n current of ca. 2 X 10+ F A is r o u t i n e l y obtained for a c o n d i t i o n i n g t i m e o f a b o u t 5 min. Emitters were conditioned a n d spectra were obtained w i t h a n emitter (accelerating) potential o f ca. 6.8 kV a n d a counter-electrode potential of -300 t o -500 V. Samples were i n t r o d u c e d i n t o t h e i o n source (at 260 "C) via t h e all-glass i n l e t system ( a t 300 "C).
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2105
Table I. Data for a Typical Mixture Used in Determination of Relative Sensitivities and Cross Sections Ion intensities0 for scan Compound Ethylbenzene 1,3,5-Trimethylbenzene Naphthalene 1-Methylnaphthalene 1,2-Dihydroacenaphthylene 1,6-Dimethylnaphthalene 0
gT, mg
gM,
92.3 103.9 59.8 101.9 78.0 116.0
mg
84.2 93.7 53.4 90.0 68.1
101.2
1
2
3
97.0 118.2 64.5 102.0 72.2 114.0
95.0 104.5 58.5 93.0 74.0 106.8
91.3 99.0 58.0 91.0 68.5 106.0
Arbitrary units.
Gas chromatograms were obtained using a Perkin-Elmer 3920 equipped with a dual FID and a Leeds and Northrup 610 recorder having a 1-s response time. All separations utilized a 5% OV-101 on Gas Chrom Z AW-DMSC (100/120) glass column (12-ft X '/&in.). Compounds. Compounds were obtained from the Bartlesville Energy Research Center, from E. J. Eisenbraun's group at Oklahoma State University,and from commercial sources. Purity of each sample was established by both isothermal FID/GC and FI/MS. Preparation and Analysis of Standard Mixtures. Twenty hydrocarbon mixtures, nine thiophene mixtures, and four mixtures of aromatic nitrogen- or oxygen-containingcompounds were prepared using weighed quantities of the standard samples. Samples of purity 99% of the singly charged ions, a relative mole sensitivity is related to the ionization probability and supply velocity for a compound relative to those for the reference compound, i.e., the relative cross section for field ionization. 2106
where g M + l and g M + 2 are the weights of molecules having one and two heavy isotopic atoms, respectively, and gT is the total weight of the compound taken. Since standard methods (28) were used in calculating the g M + J g M and g M + p / g M ratios, the mathematical formalism will not be presented here but will be provided by the authors with reprints upon request. For the elements of present interest the only isotopes not considered because of negligible natural abundance are I$, :H, and I$N. Calculations were made for a sufficient range of elemental compositions to verify for the compounds presently investigated that the fraction of molecules containing 0 to 2 heavy isotopes is 20.999. Since the uncertainty in any sample weight is ca. 0.14 mg and in the ion intensities is 3-5% and the maximum sample weight is 125 mg, the assumption that the sample is comprised of molecules containing fewer than three heavy isotopes contributes negligible error to the relative cross sections. For a typical mixture used in the determination of relative sensitivities and relative cross sections, Table I lists for each constituent a) the total weight taken and the weight of the lightest isotopic molecules in grams and b) the ion intensities, in arbitrary units, obtained from three spectra of the mixture. For the various hydrocarbons, thiophenes, and aromatic compounds containing oxygen or nitrogen, sensitivities on a gram and mole basis and cross sections relative to ethylbenzene are presented in Tables 11, 111, and IV. A single determination of either the relative sensitivity (gram or mole) or relative cross section for a given compound in a given mixture corresponds to the average value obtained from the three
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table 11. Relative Sensitivities and Cross Sections for Hydrocarbons"
Compound Benzene Toluene Ethylbenzene 1,2-Dimethylbenzene 1,3-Dimethylbenzene 1,4-Dimethylbenzene 2,3-Dihydro-lH-indene I ,3,5-Trimethylbenzene Propylbenzene Naphthalene 1,2,3,4-Tetrahydronaphthalene 2,3-Dihydr0-5-methyl-lH-indene 1-Methylnaphthalene 1,2-Dihydroacenaphthylene 1,6-Dimethylnaphthalene 2,3-Dimethylnaphthalene 1,2,3,4,5,5a-Hexahydroacenaphthylene Fluorene Anthracene Phenanthrene 9,lO-Dihydrophenanthrene 9,lO-Dihydroanthracene 3-Methylfluorene 1,2,3,4-Tetrahydrophenanthrene 1,2,3,4,5,6,7&0ctahydrophenanthrene 4-Methylphenanthrene Pyrene Fluoranthene 2,6-Dimethylanthracene 1,2,3,9,10,10a-Hexahydropyrene 2-Meth ylpyrene
mle ofM+
Z
78 92 106 106 106 106
-6 -6 -6 -6 -6 -6
118
-8
120 120 128 132 132 142 154 156 156
-6 -6
158
166 178 178 180 180 180 182 186 192 202 202
206 208 216
5,6-Dihydro-4H-benz[d,e]anthracene 218 Chrysene 228 2-Ethylpyrene 230
-12 -8 -8
-12 -14 -12 -12 -10
-16 -18 -18 -16 -16 -16 -14 -10 -18 -22 -22 -18 -16 -22 -20 -24 -22
Relative FI Sensitivities" Gram Mole 1.22 f 0.07 1.17 f 0.04 1.00 1.10 i 0.04 1.09 f 0.03 1.11f 0.05 0.97 f 0.01 1.01 f 0.02 0.90 f 0.01 1.04 f 0.04 0.92 f 0.01 0.88 f 0.02 0.95 f 0.03 0.96 f 0.06 0.98 f 0.06 1.05,f 0.02 0.76 f 0.03 0.92 f 0.06 1.06 f 0.09 0.97 f 0.08 0.86 f 0.01 0.80 f 0.04 0.85 f 0.01 0.84 f 0.02 0.82 f 0.08 0.84 f 0.10 0.82 f 0.12 0.89 f 0.08 0.89 f 0.07 0.85 f 0.01 0.81 f 0.04 0.83 f 0.03 0.74 f 0.05 0.78 f 0.01
0.90 f 0.07 1.02 f 0.04 1.00 1.10 f 0.04 1.09 f 0.03 1.11 f 0.05 1.08 f 0.01 1.14 f 0.02 1.02 f 0.01 1.25 f 0.05 1.14f 0.01 1.10 f 0.02 1.27 f 0.03 1.39 f 0.09 1.44 f 0.09 1.55 f 0.03 1.13 f 0.04 1.44 f 0.09 1.78 f 0.15 1.63 f 0.13 1.46 f 0.02 1.36 f 0.06 1.44 f 0.01 1.45 f 0.03 1.43 f 0.14 1.51 f 0.18 1.56 f 0.22 1.69 f 0.16 1.73 f 0.14 1.67 f 0.02 1.65 f 0.07 1.71 f 0.06 1.59 f 0.10 1.68 f 0.03
Cross sections," mole 0.88 f 0.07 1.01 f 0.04 1.00 1.10 f 0.04 1.09 f 0.03 1.11 f 0.05 1.09 f 0.01 1.15 f 0.02 1.03 f 0.02 1.28 f 0.06 1.17 f 0.01 1.13 f 0.03 1.31 f 0.03 1.45 f 0.09 1.51 f 0.09 1.62 f 0.03 1.18 f 0.05 1.53 f 0.10 1.90 f 0.16 1.74 f 0.14 1.56 f 0.02 1.46 f 0.07 1.54 f 0.01 1.55 f 0.04 1.53 f 0.1 5 1.64 f 0.19 1.70 f 0.25 1.85 f 0.18 1.89 f 0.15 1.83 f 0.02 1.82 f 0.08 1.89 f 0.07 1.77 f 0.11 1.88 f 0.04
Relative FID, Sensitivities," gram 1.01 f 0.04 1.02 f 0.06 1.oo
0.90 f 0.04' 1.01 f 0.07 0.96 f 0.01 1.00 f 0.05 0.92 f 0.05 0.95 f 0.08' 1.01 f 0.06 0.94 f 0.06 0.99 f 0.08 0.96 f 0.05 0.95 f 0.05 0.92 f 0.05 1.10 f 0.14 1.04 f 0.05 1.09 f 0.04 0.93 f 0.02' 0.86 f 0.06 0.94 f 0.07 0.90 f 0.02' 0.91 f 0.07 0.99 f 0.16 0.97 f 0.05 1.71 f 0.30' 1.06 f 0.02' 0.96 f 0.01' 1.02 f 0.02' 1.42 f 0.01' 1.04 f 0.05'
Deviations are standard deviations unless otherwise specified. Relative to ethylbenzene. Average of two determinations. Deviations are average deviations.
Table 111. Relative Sensitivities and Cross Sections for Thiophenes"
Compound Thiophene 2-Butylthiophene 2-(2-Methylpropyl)thiophene
mle ofM+
84 140 140 2- (1,l-Dimethylethyl)thiophene 140 2-( 1-Methylbuty1)thiophene 154 2-(2-Ethylbutyl) thiophene 168 2-(4-Methylpentyl)thiophene 168 2-Ethyl-5-(2-methylpropyl)thiophene 168 2-Phenylmethylthiophene 174 2-Ethyl-5-(3-methylbutyl)thiophene 182 2-(2-Ethylhexyl)thiophene 196 2-(3-Methylbutyl)-5-propylthiophene 196 2,5-bis(1,l-Dimethylethyl)thiophene 196 2-Hexyl-5-propylthiophene 210 2-Decylthiophene 224 2-Heptyl-5-propylthiophene 224 2-Undecylthiophene 238 Benzo[b]thiophene 134 6-Methylbenzo[b]thiophene 148 Dibenzothiophene 184
Z
Relative FI Sensitivities gram mole
-6 -6
0.92 f 0.01 0.74 f 0.02 0.76 f 0.02 0.70 f 0.01 0.61 f 0.05 0.63 f 0.03 0.62 f 0.04 0.66 f 0.04 0.70 f 0.01 0.63 f 0.02 0.59 f 0.05 0.61 f 0.02 0.58 f 0.01 0.62 f 0.01 0.48 f 0.04 0.65 f 0.02 0.47 f 0.02 0.86 f 0.03 0.88 f 0.04
+2
0.82 f 0.03
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0.73 f 0.01 0.98 f 0.02 1.00 f 0.02 0.92 f 0.01 0.89 f 0.07 1.00 f 0.05 0.98 f 0.07 1.04 f 0.06 1.15 f 0.01 1.09 f 0.03 1.09 f 0.09 1.13 f 0.04 1.07 f 0.01 1.23 f 0.02 1.02 f 0.08 1.38 % 0.04 1.06 f 0.03 1.09 f 0.04 1.23 f 0.05 1.43 f 0.06
Cross sections,h mole
Reiative FID, sensitivities,b gram
0.74 f 0.01 1.03 f 0.03
0.58 f 0.02 0.66 f 0.03'
1.05 f 0.02
0.76 f 0.01' 0.75 k 0.07 0.62 f 0.09 0.59 f 0.01' 0.68 k 0.09 0.69 f 0.11 0.74 f 0.09 0.81 f 0.02' 0.58 k 0.02' 0.62 f 0.04' 0.84 f 0.10 0.71 f 0.02' 0.71 f 0.10 0.81 f 0.07' 0.72 f 0.01' 0.70 f 0.06 0.65 f 0.14 0.83 f 0.04
0.97 f 0.01 0.95 f 0.07 1.08 & 0.06 1.05 f 0.07 1.12 f 0.06 1.26 f 0.01 1.19 f 0.03 1.20 f 0.09 1.24 f 0.04 1.18 f 0.01 1.37 f 0.03 1.15 f 0.09 1.56 f 0.04 1.21 f 0.04 1.15 f 0.04 1.31 f 0.05 1.57 f 0.07
Deviations are standard deviations unless otherwise specified. Relative to ethylbenzene. Average of two determinations. Deviations are average deviations.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2107
Table IV. Relative Sensitivities and Cross Sections for Aromatic Oxygen- and Nitrogen-Containing Compounds
mle Compound
ofM+
Z
1,3-Benzenediol 1-Naphthalenol Dibenzofuran Diphenyl ether Indole Quinoline Carbazole
110 144 168 170 117 129 167
-2 -10 0 +2 -9 -11 -1
Relative FI Sensitivities" b gram mole 1.32 f 0.02 1.05 f 0.04 0.90 f 0.01 0.91 f 0.04 1.42 f 0.02 1.30 f 0.03 1.07 f 0.06
Cross sections,asb mole
1.37 f 0.02 1.42 f 0.05 1.42 f 0.02 1.45 f 0.07 1.57 f 0.02 1.58 f 0.03 1.68 f 0.10
1.35 f 0.02 1.46 f 0.05 1.49 f 0.02 1.52 f 0.07 1.57 f 0.02 1.61 f 0.03 1.77 f 0,lO
Relative FID, Sensitivities,ath gram 0.56 f 0.01' 1.15 f O , l l < 0.85 f 0.05 0.80 f 0.02' 0.83 f 0.05 0.78 f 0.03
(1 Deviations are standard deviations unless otherwise specified. Relative to ethylbenzene. Average of two determinations. Deviations are average deviations.
Table V. FI- and EI-Relative Cross Sections (RCS)
mle Compound Benzene Toluene Ethylbenzene 1,2-Dimethylbenzene 2,3-Dihydro-lH-indene 1,3,5-Trimethylbenzene Propylbenzene Naphthalene 1,2,3,4-Tetrahydronaphthalene 2,3-Dihydro-5-methyl-lH-indene 1-Methylnaphthalene 1,2-Dihydroacenaphthylene 1,6-Dimethylnaphthalene Fluorene Anthracene Phenanthrene 9,lO-Dihydroanthracene 9,lO-Dihydrophenanthrene 3-Methylfluorene 1,2,3,4-Tetrahydrophenanthrene 1,2,3,4,5,6,7,8-0ctahydrophenanthrene 4-Methylphenanthrene Pyrene 2,6-Dimethylanthracene 1,2,3,9,10,10a-Hexahydropyrene 2-Methylpyrene Chrysene 2-Ethylpyrene Indole Quinoline Carbazole Benzo[b]thiophene 6-Methylbenzo[b]thiophene Dibenzothiophene 1,3-Benzenediol 1-Naphthalenol
of M+
FI-RCS
Set 1
Set 2 n
78 92 106 106 118 120 120 128 132 132 142 154 156 166 178 178 180 180 180 182 186 192 202 206 208 216 228 230 117 129 167 134 148 184 110 144
1.00 1.15 1.14 1.25 1.24 1.31 1.17 1.45 1.33 1.28 1.49 1.65 1.72 1.74 2.16 1.98 1.66 1.77 1.75 1.76 1.74 1.86 1.93 2.15 2.08 2.07 2.01 2.14 1.78 1.83 2.01 1.31 1.49 1.78 1.53 1.66
1.00' 1.87" 2.11' 3.05'8' 2-87' 4.05',' 2.28' 4-52'
1.oo
2.54''aE 6.18 6.59' 6.91'~' 5.57' 8.97"
1.86
EI-RCS Set 3 1.00d 2.05d 2.0gd
2.78 2.72 5.89 2.05 2.38'
6.71d 4.68 ,f 8.2gd
8.09 8.00' J2.95 10.06
9.01d 8.92d 8.0Od3f 7.97dmf
6.75'3' 7.38d,f 6.47d,f 9.56'3' 11.69" 9.70",e
13.11
10.7gd 8.32d,',f
12.18'1' 10.85' 12.42"~~
7.848 3.29a 13.43g 6.00" 6.45''.' 10.56'
6.45 2.9gh 6.61
Set4h
Av
1.00 1.51 1.56 2.60'' 3.17 3.44' 1.62 5.16
1.00 1.82 1.92 2.81 2.92 3.75 1.95 5.57 3.37 2.46 7.12 7.18 8.17 5.57 12.17 9.49 8.00 7.97 6.75 7.38 6.47 9.56 11.86 9.70 8.32 12.18 10.85 12.42 7.84 3.29 13.43 6.23 6.45 10.56 2.99 6.61
6.89' 6.87 9.61' 17.76
See Ref. 29. See Ref. 30. See Ref. 31 and 32. See Ref. 34. e Specific isomeric compgund not reported. f See Ref. 33. i: See Ref.
(I
17. See Ref. 11.
spectra recorded for that mixture. Since the differences in the values so obtained between mixtures were not statistically significant, all determinations of relative sensitivities and relative cross sections were averaged without regard to the standard deviation in each determination. Difficulties were presented by the limited solubilities of anthracene, 2,6-dimethylanthracene, 4-methylphenanthrene, pyrene, 5,6-dihydro-4H-benz[d,e]anthracene, and chrysene. However, the standard deviations in the relative gram and mole sensitivities and relative cross sections for all compounds produce an av2100
erage uncertainty of 0.04 f 0.02,0.06 f 0.05, and 0.06 f 0.05 in these quantities, respectively. Although the differences between the FI-s(rn), and RCS(FI), are generally less than the combined uncertainties in the individual values, the cross sections show a greater variation than do the mole sensitivities with respect to ethylbenzene, as expected. Since the cross section is the quantity of fundamental significance, the effect of molecular structure on ionization by both an electric field and low-voltage electrons is considered in terms of relative cross sections. Conse-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table VI, Correlation Constants" Variables Independent EI-RCS Set 1 EI-RCS Set 2 EI-RCS Set 3 EI-RCS Set 4 EI-RCS Av -2 number -2 number -2 number EI-s ( m )Set 1 EI-s(m) Set 2 EI-s(m) Set 3 EI-s(m) Set 4 EI-s(m)Av -2 number -2 number -2 number
Dependent
Compound type
Slope x 10'
Intercept
FI-RCS FI-RCS FI-RCS FI-RCS FI-RCS FI-RCS FI-RCS FI-RCS FI-s(m) FI-s(m) FI-s (m) FI-s(m) FI-s ( m ) FI-s(m) FI-s(m) FI-s(m)
Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon Thiophene Aromatic nitrogen Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon Hydrocarbon Thiophene Aromatic nitrogen
9.81 f 0.73 8.09 f 0.65 11.01 f 1.61 6.60 f 0.42 9.52 f 0.62 5.80 f 0.77 7.83 f 0.00 3.93 f 0.49 8.82 f 0.75 7.40 f 0.64 9.85 f 1.57 6.12 f 0.38 8.55 f 0.59 4.53 f 0.72 6.50 f 0.10 2.25 f 0.43
0.988 f 0.053 1.033 f 0.047 0.896 f 0.115 1.058 f 0.029 1,000 f 0.047 0.804 f 0.125 0.840 f 0.000 1.415 f 0.059 0.978 f 0.049 1.018 i 0.042 0.907 f 0.103 1.038 f 0.024 0.989 f 0.041 0.869 f 0.118 0.813 f 0.007 1.528 f 0.052
All deviations are standard deviations.
w
G
I
O
O
200
,
,
,
1
.
,
,
,
,
,
,
400 600 800 1000 1200 ELECTRON IMPACT R E L A T I V E CROSS SECTIONS
,
1400
-2 NUMBER
Flgure 1. Correlation of field-ionization relative cross sections with low-voltage electron-impact relative cross sections
Figure 2. Correlation of field-ionization relative cross sections with
Points designated by ( O ) ,(B),and (A)correspond to aromatic hydrocarbons, thiophenes, and aromatic nitrogen-containing compounds, respectively. Line corresponds to best least-squares fit for aromatic hydrocarbons
Points designated by ( O ) ,(B), and (A), and solid, dotted, and dashed leastsquares lines correspond to aromatic hydrocarbons, thiophenes, and aromatic nitrogen-containing compounds, respctively.
quently, Table V presents relative cross sections for ionization by electric fields, RCS(FI), and by low-voltage electrons, RCS(E1). The RCS(E1) values were obtained from low-voltage E1 sensitivities (11, 17, 29-34) as follows. Densities (35, 36) were used to convert low-voltage sensitivities reported as divisions per unit of volume into divisions per milligram. For each compound a g M value was computed using Equation 1 with gT = 1mg. For a given compound, multiplication of the sensitivity in div/mg by the function MW&M yields a mole sensitivity for ionization of the isotopically most abundant molecules, EIS(Mo), which is proportional to their cross section for ionization by electrons possessing energy eV assuming negligible molecular-ion fragmentation. The compound types, the available mass spectral data, and the use of ca. 10-eV electrons in these low-voltage sensitivity determinations indicate the reasonableness of this assumption. If instrumental and experimental factors are assumed to be sensibly constant in the determination of a given set of low-voltage E1 sensitivities, then division of EIS(M0) for a given compound in the set by the EIS(M0) value for the reference compound in the same set provides a first approximation of the RCS(E1) for a given value of the ionizing energy. The data in Table V clearly indicate that variation in molecular structure produces corresponding effects on the cross
section for ionization by either electric fields or low-voltage electrons. As observed for ionization by low-energy electrons (29-32), the cross sections for field ionization in the series benzene, naphthalene, anthracene, phenanthrene, pyrene, and chrysene depend on the size of the aromatic nucleus. It is important to note that the trend in the RCS(F1) and in the RCS(E1) for naphthalene (1.45 and 5.57), anthracene (2.16 and 12.17), phenanthrene (1.98 and 9.49), and pyrene (1.93 and 11.86) appear to correlate with the variation in their ionization potentials (17,30), i.e., 8.12,7.55,8.10,and7.72eV, respectively (37), relative to the ionization potential for benzene (9.24 eV) (37). As seen in Table 11, the RCS(F1) for pyrene could be larger than the value for phenanthrene within the limits of experimental precision. I t should be noted that the variation in both EI- and FI-RCS values does not exhibit a simple dependence on standard expressions of aromaticity for the parent molecule such as delocalization energies calculated by simple Huckel methodology or empirical resonance energies (38). Substitution of an alkyl group for hydrogen in benzene is seen to increase both E1 and FI cross sections. However, the variation in the EI- and FI-RCS values for benzene, toluene, ethylbenzene, and propylbenzene suggests that the substituent effect on the ionization cross section is not strongly de-
-Z
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
* 2109
Table VII. Weight Percents of Components in Test Mixture No. 1 Weight percents and percent deviations by WT
n1
1'I
Compound
Grams GLCasb Si/Sj=
2,3-Dihydro-lH-indene 8.28 1,3,5-Trimethylbenzene 6.28 Naphthalene 4.46 1,2,3,4-Tetrahydronaphtha- 5.46 lene Benzo[b]thiophene 3.21 1-Methylnaphthalene 11.78 6-Methylbenzo[b]thiophene 4.35 1,2-Dihydroacenaphthylene 3.06 1,6-Dimethylnaphthalene 9.43 1,2,3,4,5,5a-Hexahydroace- 8.52 naphthylene Fluorene 3.90 Dibenzofuran 3.54 Phenanthrene 4.05 9,lO-Dihydrophenanthrene 9.11 Dibenzothiophene 2.13 4-Methylphenanthrene 3.47 Pyrene 6.36 2,6-Dimethylanthracene 0.63 C hrysene 1.97
l a l c
llil
Dev, % S i / S j a , l ' Dev, % Si/Sj = l a ' b Dev, %
Si/S,a,h
Dev, o/b
-67.0 -44.4 -30.0 -61.2
8.90 6.35 4.56 5.44
7.5 -0.4
-15.9 -11.5 13.8
3.39 12.03 4.79 3.10 9.31 8.48
5.6 2.1 10.1 1.3 -1.3 -0.5
3.62 3.86 4.16 8.88
-7.2 9.0 2.7 -2.5 3.8 -7.2 -13.2 -17.5 -13.7
6.31 5.02 4.10 4.56
-23.8 -20.1 -8.1 -16.5
7.73 5.83 4.34 5.30
-6.6 -7.2 -2.7 -2.9
2.73 3.49 3.12
I7'l3 3.17 9.17 8.22
2.62 11.25 4.17 3.32 9.94 7.40
-18.4 -4.5 -4.1 8.5 5.4 -13.1
3.18 11.78 4.50 3.16 9.15 8.72
-0.9 0.0 3.4 3.3 -3.0 2.3
2.70 10.42 4.95 3.71 12.02 4.65
27.5 -45.4
3.93 3.53 4.14 8.44 2.31 3.80 5.99 0.37 1.55
4.25 4.06 4.95 10.24 2.42 4.24 7.63 0.97 2.53
9.0
3.91 3.78 4.02 9.29 2.25 3.71 6.50 0.74
0.3 6.8 -0.7 2.0 5.6 6.9 2.2 17.5 7.6
3.51 3.44 6.19 9.37 3.42 5.78 11.05 1.62 4.76
-10.0 -2.8 52.8 2.9 60.6 66.6 73.7 157.1 141.6
8.25 6.20 4.93
2.12
1.1 2.2
9.01
14.7
22.2 12.4 13.6 22.2 20.0 54.0 28.4
2.12
21.2
2.21
3.22 5.52 0.52 1.70
Uncertainty in values is ca. i5%. Average of two determinations. Average of three determinations.
pendent on the nature and size of the alkyl group. Successive replacement of hydrogens in benzene with two and three methyl groups is seen to increase RCS(E1) and also RCS(F1) but not additively. The effect of methyl and ethyl substitution on the cross sections for FI and E1 of the parent aromatic hydrocarbons tends to decrease with increasing "size))of the aromatic nucleus. For example, methyl substitution in benzene, naphthalene, fluorene, and pyrene increases the RCS(F1) and RCS(E1) by ca. 15 and 82,3 and 28,l and 21, and 7 and 3%, respectively. Low-voltage E1 sensitivities for hydroaromatic compounds are lower than those for their aromatic precursors (33).With the exception of 1,2-dihydroacenaphthylene, RCS(E1) values were calculated for the hydroaromatics in Table V by means of equations relating hydroaromatic compound sensitivities to the sensitivity of the parent aromatic compound (33)and the volume sensitivities ( 3 4 )and densities for the latter. For ionization by low-energy electrons and electric fields, it is interesting to note that similar cross sections relative to the parent compound are observed for 172,3,4-tetrahydronaphthalene (0.70 and 0.92), 9,lO-dihydroanthracene (0.89 and 0.77), 9,lO-dihydrophenanthrene (0.89 and 0.89), 1,2,3,4-tetrahydrophenanthrene (0.83 and 0.89), and 1,2,3,4,5,6,7,8-octahydrophenanthrene (0.73 and 0.88). The discrepancy for 1,2,3,9,10,10a-hexahydropyrene relative to pyrene (0.77 for E1 and 1.08 for FI) may reflect the difficulty in determining the RCS(F1) value for the latter due to its relative insolubility. In the structurally related series fluorene, dibenzofuran, dibenzothiophene, and carbazole, replacement of CH2 by 0 and S H is indicated to have little effect on the cross section for FI whereas substitution of NH for CH2 markedly increases the cross section, see Tables 11-IV. The data in Table V for l93-benzenedioland 1-naphthalenol show that substitution of OH for aromatic hydrogen increases both E1 and F I cross sections. Figure 1shows that for the hydrocarbons a reasonable linear correlation exists between the relative cross sections for field ionization and the average relative cross sections for lowvoltage electron-impact-induced ionization. The slope, in2110
tercept, and associated standard deviations for the leastsquares line in Figure 1 are listed in Table VI. Since the four sets of low-voltage sensitivities may reflect somewhat different ionizing energies, each set was used to estimate a set of EIRCS. Consequently, for the hydrocarbons the FI-RCS have also been separately correlated with the individual sets of EI-RCS in Table V. The least-squares slopes, intercepts, and associated standard deviations are given in Table VI. The FIand EI-RCS values for benzene are unity. Thus, the leastsquares parameters in Table VI for each hydrocarbon relative cross section plot should predict unit value for RCS(F1) a t RCS(E1) = 1.The so predicted RCS(F1) values are 1.08,1.11, 1.01,1.12, and 1.10 for RCS(E1) Sets 1through 4 and the average set, respectively. Considering the uncertainties in the experimental FI- and EI-RCS values, these results are consistent with the expected value of 1.The magnitude of the five slopes demonstrates that a given change in hydrocarbon molecular structure has a significantly greater effect on the relative ionization cross section for low-voltage electrons than for an electric field. This result implies, other factors being equal, that omission of sensitivity corrections will more adversely affect the accuracy of quantitative distributions obtained by the low-voltage E1 method than from the technique of field ionization; vide infra. In addition, the observed correlation has definite pragmatic consequences since it allows prediction of an RCS(F1) given an RCS(E1) or vice versa. The data for the heteroatom-containing aromatics in Table V are also plotted in Figure 1. Clearly the limited number of data points presently available precludes drawing conclusions concerning the correlatability of the RCS(F1) and RCS(E1) for sulfur-, nitrogen-, or oxygen-containing aromatics. However, preliminary RCS(E1) data obtained for the thiophenes using the modified CEC 21-102 mass spectrometer a t the Bartlesville Energy Research Center correlate with the RCS(F1) for these compounds. The utility of such correlations clearly warrants determination of additional sensitivity data for these compound classes, especially the ones containing heteroatoms, and a theoretical study of the basis for such correlations. For predictive purposes the relative cross sections for field
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER I976
Table VIII. Weight Percents of Compounds in Test-Mixture No. 2 Weight percents and percent deviations by BT
Ethylbenzene 46.46 Naphthalene 11.67 9.83 Fluorene 5.04 Carbazole Dibenzofuran 13.89 Phenanthre7.03 ne Dibenzothio- 6.08 phene a
FT
46.48 11.62 10.47 6.34 12.80 6.40
38.11 11.64 11.73 6.17 15.74 9.43
-18.0 -0.3 19.3 22.4 13.3 34.0
47.01 11.47 10.03 4.52 13.65 7.13
-1.7 2.0 -10.3 -1.7 1.4
15.68 12.19 12.36 11.54 19.06 14.79
-66.3 4.5 25.7 129.0 37.2 110.4
48.89 11.66 8.33 4.51 14.00 6.51
5.2 -0.1 -15.3 -10.5 0.8 -7.4
5.88
7.17
17.9
6.19
1.8
14.39
136.7
6.10
0.3
1.2
Uncertainty in values is ca. f5%. Average of twddeterminations. c' Average of three determinations.
Table IX. Weight Percents of Components in Test Mixture No. 3
Weight percents and percent deviations by Compound
Grams
8.76 Thiophene 2,3-Dihydro-5-methyl-lH- 11.40 indene 7.28 2-Butylthiophene 8.16 2-( 1-Methylbuty1)thiophene 8.15 2-(4-Methylpenty1)thiophene 6.52 2-Phenylmethylthiophene 1.72 Anthracene 2.30 3-Methylfluorene 6.80 1,2,3,4-Tetrahydrophenanthrene 5.20 1,2,3,4,5,6,7&0ctahydrophenanthrene 2-(2-Ethylhexyl)thiophene 7.45 0.99 Fluoranthene 1,2,3,9,10,10a-Hexahydro- 2.77 pyrene 7.43 2-Hexyl-5-propylthiophene 2.84 5.6-Dihydro-4H-benz[cl,e] anthracene 6.39 2-Decylthiophene 5.86 2-Undecylthiophene a
7.43 9.48
5.55 10.48
-36.6 -8.1
8.60 10.81
-1.8 -5.2
1.85 8.57
-78.9 -24.8
9.17 10.28
4.7 -9.8
7.74 9.37
6.12 6.62
-15.9 -18.9
7.09 8.41
-2.6 3.1
4.22 3.81
-42.0 -53.3
7.65 8.03
-1.6
8.56
7.17
-12.0
8.31
2.0
5.27
-35.3
8.41
3.2
6.30 1.68 1.73 6.32
6.33 2.51 2.99 9.07
-2.9 45.9 30.0 33.4
6.21 1.60 2.35 7.11
-4.8 -7.0
-11.5 162.2 82.2 90.3
6.78 1.46 2.35 7.00
4.0 -15.1
4.6
7.27 4.51 4.19 12.94
5.23
6.11
17.5
4.83
-7.1
6.23
19.8
4.91
-5.6
9.73 0.98 2.34
6.94 1.61 3.81
-6.8 78.9 37.5
7.22
1.08 2.58
-3.1 20.0 -6.9
5.10 2.13 7.31
-31.5 136.7 163.9
7.90 0.98 2.40
6.0 -8.9 -13.4
7.29 2.43
7.98 4.13
7.4 45.4
7.37 2.74
-0.8 -3.5
8.86 8.89
19.2 213.0
7.42 2.81
-0.1
7.60 5.83
6.71 5.85
5.0 -0.2
7.47 6.23
16.9 6.3
4.75 4.06
-25.7 -30.7
6.50 5.93
Uncertainty in values is ca. f5%. Average of two determinations.
ionization of the parent aromatic hydrocarbons, parent thiophenes, and parent aromatic nitrogen-containing compounds were plotted against -2 numbers as shown in Figure 2. T h e slopes, intercepts, and standard deviations of the least-squares lines in Figure 2 are listed in Table VI. For the latter two compound classes, additional data points are required t o verify the indicated correlations. The weight and/or mole percents in a mixture are determined by dividing the ion intensities by the gram and/or mole sensitivities, respectively, and normalizing t o 100. Thus, the relative gram and/or mole sensitivities are the quantities of practical interest. In this regard, reasonable estimates of relative field-ionization mole sensitivities can be calculated from the correlations of RCS(F1) with either RCS(E1) or -2 by Equation 2.
Equations 3 and 4 define the isotopic probability factors for the compound of interest, Ki, and the reference compound, KR.
2.2
5.1
2.2
2.9
-1.1 1.7 1.2
' Average of three determinations.
In Equations 3 and 4, Nk is the number of k atoms in the molecule, M W ( k ) h is the molecular weight for the molecule containing one most abundant heavy isotope of the element k and MW/ is the molecular weight of the molecule containing the most abundant isotope for each of its constituent atoms (for the compounds presently investigated, this represents the lowest naturally occurring molecular weight). In the present study the reference compound is either ethylbenzene or
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2111
benzene. It is important to note that Equation 2 and the correlation of EI- and FI-RCS can be used to predict E1 relative mole sensitivities. The validity of this approach is demonstrated by comparison of the experimental and so calculated s ( m ) ,for FI and EL For example, the values relative to benzene are, respectively, 1.73 and 1.75 and 10.46 and 10.58 for pyrene, 1.52 and 1.53 and 2.97 and 2.98 for 1,3-benzenediol, 1.87 and 1.88 and 12.51 and 12.59 for carbazole, and 1.21 and 1.23 and 5.59 and 5.65 for benzo[b]thiophene. As expected the s ( m ) ,values calculated by Equation 2 are slightly greater than the experimental values due to the neglect of molecules containing more than one heavy isotope. Equation 2 can be appropriately expanded to treat data for compounds containing heteroatoms other than N, 0, and S and more than one heteroatom. A simpler but fundamentally less sound approach is the correlation of s(rn), for FI with the corresponding values for low-voltage EI. Thus, the constants and associated standard deviations obtained from a linear least-squares fit of these data are given in Table VI. Table VI also gives the results obtained from a linear least-squares fit of the s ( m ) ,for field ionization with -2. The relative gram sensitivities for flame-ionization detection (FID) are given in Tables 11,111,and IV. The FID relative sensitivities are seen to be constant for a given class of compounds within the limits of experimental reproducibility. However, the magnitude of these relative sensitivities is dependent upon the particular compound class. These conclusions are consistent with ones previously reported (39, 40). Tables VII, VIII, and IX present results obtained from analysis of three test mixtures. The components of test mixture 1 are typical of those encountered in the hydrocarbon ether fraction obtained from a coal-derived liquid. The field-ionization sensitivities in Tables 11, 111, and IV were determined for mixtures prepared according to compound type except that ethylbenzene and, in most instances, 1,6dimethylnaphthalene were included as reference component(s). Consequently, mixtures 2 and 3 were prepared to test the independence of these sensitivitives on sample composition. For each test mixture the agreement between the weight percents obtained by GC analysis and from the weights of each compound taken (compare the values in columns 2 and 3 in Tables VII, VIII, and IX) confirms the experimental FID sensitivities and the composition of each test mixture. The weight percents of each component in the three test mixtures calculated from the F I and E1 ion intensities assuming unitrelative sensitivities are shown in columns 4 and 8, respectively, of Tables VII, VIII, and IX. For each test mixture, the percent deviations between these values and the weight percents calculated from the weight of each compound are listed in columns 5 and 9, respectively. For each test mixture the largest error is seen to occur for those compounds having the largest and smallest s(g), as expected. The best agreement between experimental and known weight percents is realized for those components whose s(g), most closely approximates the average relative gram sensitivity across each distribution. Since the variation in the s(g), is roughly related to variation in molecular weight and since the compounds in Tables VIIIX are ordered according to increasing molecular weight, the best results are observed for compounds in the middle of each distribution. However, it is significant to note that the error in the compositions incurred by assuming unit-relative sensitivities is, for all three mixtures and except for three compounds in Table VII, at all points across each distribution worse for electron impact than for field ionization. For each mixture, weight percents calculated using the s(g), values for field ionization and low-voltage electrons are shown in columns 6 and 10, respectively, of Tables VII, VIII, and IX. Percent deviations are listed in column 7 for FI data and in
+
2112
column 11for E1 data. Inclusion of the s(g), values is seen to further improve the accuracy of the composition calculated from F I ion abundances for all three test mixtures. These results further substantiate the field-ionization sensitivities in Tables 11-IV and show that under our instrumental operating conditions they are independent of sample composition in terms of types and quantities of the constituent compounds. Inclusion of sensitivities in the conversion of low-voltage E1 ion abundances to weight percents is seen to produce excellent quantitative distributions for the three test mixtures.
CONCLUSIONS For the aromatic hydrocarbons and aromatic heteroatom compounds studied and the mass spectrometer conditions employed, the field-ionization sensitivities do not exhibit a measurable dependence on sample composition. The correlatability of field ionization and low-voltage electron-impact relative cross sections and relative sensitivities has pragmatic consequences. For field ionization of the parent compound in a homologous series, these quantities can be reasonably predicted, given its -2 value. Relative sensitivities and relative cross sections for the homologues can then be estimated from the effect of the substituent on these quantities for structurally similar compounds. If the assumption of unit relative sensitivities is necessary to convert ion intensities to quantitative distributions, the present data clearly suggest, other factors being equal, that more realistic results are obtained from field ionization than from electron-impact mass spectral data. The present investigation points to the need for determination of additional relative sensitivity and relative cross section data especially for higher-molecular-weight aromatic hydrocarbons and for heteroatom-containing aromatic compounds in general, for ionization by both an electric field and low-voltage electrons. Although such FI studies are in progress, the problem is unfortunately dominated by the unavailability of standard compounds.
ACKNOWLEDGMENT We thank J. E. Dooley and E. J. Eisenbraun for providing compounds used in this study and P. W. Woodward and G. P. Sturm for preliminary determinations of low-voltage sensitivity data for the thiophenes. We thank H. E. Lumpkin for stimulating discussions and 0. C. Dermer for comments pertinent to manuscript preparation. LITERATURE CITED H. D. Beckey, "Field Ionization Mass Spectrometry", Pergamon Press, Oxford, England, 1971. H. D. Beckey in "Biochemical Applications of Mass spectrometry", G. R . Walier, Ed., Wiley-interscience, New York, N.Y., 1972,Chapter 30. H. D. Beckey and G. Wagner, Fresenius' Z. Anal. Chem., 197, 58
(1963). W. L. Mead, Anal. Chem., 40,743 (1968). M. Kuras, M. Ryska, and J. Mostecky, Anal. Chem., 48, 196 (1976). D.Severin, H. H. Oeiert, and G. Bergmann, Erdoel Kohle, Erdgas, Petrochem., 25, 514(1972). H. H. Oeiert, D. Severin, and H. J. Windhager, Erdoel Kohle, Erdgas, Petrochem., 26, 397 (1973). Annual Report of the Office of Coal Research, "Clean Energy from Coal-A National Priority", U.S. Government Printing Office, Washington, D.C., pp
67-75,1973. K. G. Hippe and H. D. Beckey, ErdoelKohle, Erdgas, Petrochem., 24,620 '11971\ \,-. ,I.
(IO)M. Ryska. M. Kuras, and J. Mostecky, lnt. J. Mass Spectrom. /on Phys., 16, 257 (1975). (11) A. G. Sharkey, Jr., G. Wood, J. L. Shuitz. I. Wender, and R. A. Friedei, Fuel, 38,315 (1959). (12)A. G. Sharkey, Jr., J. L.Shuitz, and R. A. Friedei, Fuel, 41,359 (1962). (13)A. G. Sharkey, Jr., J. L. Shultz, and R. A . Friedei, "Advances in Coal Spectrometry, Mass Spectrometry", Washington, U S .Department of the Interior, Bureau of Mines, 1963. (14)J. L. Shultz, R. A. Friedei, and A. G. Sharkey, Jr;, "Mass Spectrometric Analysis of Coal-Tar Distillates and Residues", Washington, U . S . Department of the interior, Bureau of Mines, Ri 7000 (1967). (15) T. Kessier, R. Raymond, and A. G. Sharkey, Jr., fuel, 48, 179 (1969). (16)T. Aczel, J. Q. Foster, and J. H. Karchmer, Preprints, Div. FuelChem., Am. Chem. Soc., 13 (l),8 (1969).
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