difficulty in that the matrix containing the rate constants is not a symmetric matrix. It is, however, a “banded” matrixi.e., one in which all elements are zero except those on the main diagonal and the two diagonals adjacent to it. It can be shown that the eigenvalues (but not the eigenfunctions) of this matrix are the same as those of a symmetric matrix with the same main diagonal elements, but off-diagonal elements lii = dkrjkli (20). The eigenvalues are therefore obtained by solving this symmetric matrix and the coefficients
are calculated by solving the five simultaneous linear equations with five unknowns.
for review May 1 9 9 1969. Accepted
22, 1969*
(20) T. Muir, “Treatise on the Theory of Determinants,” Dover Publications, New York, 1960.
Low-Resolution Mass Spectrometric Determination of Aromatic Fractions from Petroleum C . J . Robinson Research and Development Dept., American Oil Co., 2500 New York Avenue, Whiting, ind. 46394
Glenn L. Cook Laramie Petroleum Research Center, Box 3395, University Station, Laramie, W y o . 82070
A mass spectrometric procedure far determining up to 21 compound types in petroleum aromatic fractions is described. The entire composition of any sample is accounted for in terms of 12 hydrocarbon types, 3 thiopheno types, and 6 unidentified groups. Inclusion of the unidentified components avoids the difficulties encountered in earlier methods which described composition in terms of a fixed number of named types. Reasonable results have been obtained for a wide variety of samples.
MASSSPECTROMETRIC methods for determiningcompound types in petroleum have been used routinely for more than 1 5 years. Since the first application to the analysis of gasolines ( I ) , such methods have been extended to the heaviest volatile fractions of petroleum. However, a method to determine compound types in the wide variety of heavy aromatic fractions encountered in the petroleum industry has not been reported. The absence of such a procedure undoubtedly reflects the uncertain knowledge of the composition of petroleum aromatics and also reflects the complexity of the problem of obtaining adequate group type analyses. Hastings et al. (2) determined 12 compound types, including three thiopheno types, by using summations of the most intense (M-I)+peaks for each type. (If a series of molecular ions are designated (M)+the series of ions at each of the next lower masses may be expressed as (M-l>+.) When applied to very light or very heavy fractions, the results are difficult to interpret. Orkin et al. ( 3 ) used monoisotopic peaks to determine seven compound types. Summations over a limited mass range contain both (M)+and (M-l)+ peaks, and equal sensitivities are assumed for each type. No sulfur types are included. This forces a large number of compound types into the seven groups and fails to further resolve the types. Gallegos et al. ( 4 ) developed a high resolution mass (1) R. A. Brown, ANAL.CHEM., 23,430(1951). (2) S. H. Hastings, B. H. Johnson, and H. E. Lumpkin, ibid., 28, 1243 (1956). (3) B. A. Orkin, J . G. Bendoraitis, B. Brown, and R. H. Williams, ASTM Special Tech. Pub. No. 224. Symp. on Composition of Petroleum Oils, 59, ASTM, Philadelphia, Pa. (1958). (4) E. J. Gallegos, J. W. Green, L. P. Lindeman, R. L. LeTourneau, and R. M. Teeter, ANAL.CHEM., 39,1833 (1967). 1548
ANALYTICAL CHEMISTRY
spectral method for application to distillate fractions of petroleum in the 500-950 OF range. The present method gives more detail than the earlier ones. It provides 100 liquid volume per cent accountability for the sample, while avoiding the inclusion of unidentified components with those that have been identified. The calculations are not difficult, but they are lengthy and are practical only with electronic computing. NOMENCLATURE The names given the compound types, Table I, are those believed most likely to occur in greatest abundance in petroleum. Dibenzofurans (5-7) may be present in significant amounts. Because they are indistinguishable from acenaphthenes by this method, both names are included. The compounds in Class It are designated as naphthenephenanthrenes because their occurrence has been reported (8), and enhanced peaks at 217, 231, and 245 in the (M-l)+ series of many samples makes them the preferred choice as the most important constituents. In addition to the named types, the presence of other compound-types should be expected. For example, other naphthenenaphthalenes will probably be present with acenaphthenes, dihydrophenanthrenes with fluorenes, and anthracenes with phenanthrenes. The types of compounds in Classes 112 through VI12 have not been named because there are insufficient background data to decide which of several possible types is the most probable. EXPERIMENTAL Sample Preparation. Concentrates of the aromatics in a petroleum fraction may be prepared by any convenient separation technique. One method that has been found to be useful is that of Snyder (9). ( 5 ) B. H. Johnson and Thomas Aczel, ANAL.CHEM., 39,682 (1967). (6) H. E. Lumpkin, ibid., 36, 2399 (1964).
(7) F. F. Yew and B. J. Mair, ibid., 38,231 (1966). (8) B. J. Mair and J. L. Martin&-Pic6, American Petroleum Institute, Div. of Refining, 27th Midyear Meeting, San Francisco,
Calif., 1962. (9) L. R . Snyder, ANAL.CHEM., 37,713 (1965).
Class Io I10 I110
IVO
vo
VI0 VI10
Nominal type Nominal type Benzenes Naphthenebenzenes Dinaphthenebenzenes Naphthalenes Acenaphthenes, dibenzofurans Fluorenes Phenanthrenes
Table I. Classes and Their Constituents 1st Overlap Class Compound Benzothiophenes 11 111 1111
IV1
v1
VI1 VII,
Pyrenes Chrysenes Dibenzothiophenes Perylenes
Class 12 I12
I112
IV2 VZ
Dibenzanthracenes Naphthobenzothiophenes
VI2 VI12
2nd Overlap Compound Naphthenephenanthrenes Unidentified Unidentified Unidentified Unidentified Unidentified Unidentified
Table 11. Fraction of Total Ionization in (M)+ Plus (M-l)+Series in the Mass Spectra of Aromatic Compounds Used to Construct the Calibration Matrix Carbon No. of number range spectra Fraction of total ionization Nominal compound type: Maximum Maximum 43 Benzenes 6-39 0.668 0.404 14-31“ 19 Benzenes 0.446 0.668 14-32 15 Naphthenebenzenes 0.398 0.615 12-24 Dinaphthenebenzenes 4 0.362 0.580 Naphthalenes 11-25 10 0.444 0.586 2 27 Acenaphthenes 26 1 Fluorenes, dihydrophenanthrenes Phenanthrenes, anthracenes 15-26 8 0.634 0.406 Overlapping compound types: 12-15 Benzothiophenes 10 0.593 0.510 16-24 4 Pyrenes, benzofluorenes 0.514 0.398 4 19-26 Chrysenes, benzanthracenes 0.503 0.347 1 26 Benzpyrenes, perylenes Aromatics fractions in this carbon number range were of particular interest.
Instrumental. Spectra were obtained on a modified (10) CEC 21-103 mass spectrometer connected to a MonsantoNon-Linear Systems Digitizer. (Reference to trade names is made for identification only and does not imply endorsement by the Bureau of Mines.) The digitizer records the spectra on perforated paper tape. The mass spectrometer was run in the focused mode using a 7-mil collector slit. Ionizing voltage was 70 V, ionizing current was 11 PA, and source temperature was 250 “C. Approximately 1 pl of liquid sample was charged t o a 2-liter sample volume maintained at 325 “C. A scan from mass 25 through mass 69 was first made at 3,400 gauss. The field was then switched t o 5,500 gauss and the scan started a t 69 and continued t o the end of the spectrum. Data Preparation. The spectral data from the digitizer were read into an IBM 7044 computer and machine edited to eliminate peaks at fractional mass intervals. Because the mass spectrum of the sample was run a t two magnetic field strengths, the peak heights at mass 25-68 from the low field scan were corrected by multiplying by the height of the 69 peak at high field and dividing by the height of the 69 peak a t low field. The spectrum was then stored as a 1-dimensional array, H(M), where H is the peak height a t mass M . A monoisotopic spectrum was calculated and stored as a second 1-dimensional array, HDI(M). The deisotoping routine used was that suggested by Beynon (11) assuming that only ions of compositions ranging from C,H2,+2 to CnH2n--ll
Average 0.5632 0.5578 0.4997 0.4435 0.5192 0.5075 0.4910 0.5073 0.5376 0.4520 0.3902 0.4525
were present. This is not strictly accurate for aromatics, but the errors introduced are small and have little effect on the results.
RESULTS Class Analysis. The class analysis requires the solution of seven simultaneous linear equations in seven unknowns. Calibration data for the equations were obtained from both published (12) and unpublished spectra (13). Typical values for the sums of (M)+ and (M-l)+ series expressed as fractions of total ionization are given in Table 11. Total ionization is defined as the sum of the polyisotopic peak heights measured from mass 24 to the end of the spectrum. Where data as low as mass 24 were not available, heights for the missing peaks were estimated. Peaks in the (M-l)+ series are all monoisotopic. The use of monoisotopic peaks is essential in subdividing the classes, and it is also advantageous in the class analysis since it reduces interference from the adjacent lower z-number class. An empirical adjustment is made to the heights of the peaks a t masses 175, 176, 189, 190,200, and 213 so that the summations in which they are included more nearly approach the average. The adjustment is made in both calibration and sample spectra by simple interpolation. For example, the “corrected” height a t mass 200 is the average of the heights a t ~~~
(10) H. M. Grubb, C. H. Ehrhardt, R. W. Vander Haar, and W. H. Moeller, ASTM Committee E-14, 7th Annual Conf. on Mass Spectrometry, Los Angeles, Calif., 1959. (11) J. H. Beynon, “Mass Spectrometry and Its Applications to Organic Chemistry,” Elsevier Publishing Co., New York, N. Y., 1960, pp 294.
(12) American Petroleum Institute, Research Project 44, “Selected Mass Spectral Data,” Thermodynamic Research Center, Texas A & M University, College Station, Texas, 1947-1967. (13) ASTM Committee E-14, “File of Uncertified Spectra.” For additional information, contact Dr. A. H. Struck, Perkin-Elmer Corp., Norwak, Conn, VOL. 41, NO. 12, OCTOBER 1969
1549
Z 78
Z Z Z Z Z Z
104 129, 130 128 154 166 178
Benzenes 0.5578 0.0536 0,0036 0. 0028 0.0005
0.0004 O.ooo9
Table 111. Calibration Matrix-Fraction of Total Ionization Nominal type NaphtheneDinaphthenebenzenes benzenes Naphthalenes Acenaphthenes 0.0479 0.0302 0.0097 0.0027 0.4997 0.0524 0.0017 0.0007 0.0613 0.4435 0.0133 O.OOO9 0.0260 0.1142 0.5192 0.0057 0.0016 0.0212 0.0357 0.5075 0.0005 0.0060 0,0063 0.0452 0.0005 0.0034 0,0023 0,0064 92 106 120 . . . . . to end Z 78 = 78 + 91 + 105 + 119 + . , , . . to end (monoisotopic) Z 104 = 104 118 132 146 . . . . . to end + 117 + 131 145 , , . . . to end (monoisotopic) 2 129 = 130 + 144 + 158 172 . . to end + 129 + 143 + 157 + 171 + . . . to end (monoisotopic) Z 128 = 128 142 156 170 . , . . . to end + 141 + 155 + 169 + , , , , , to end (monoisotopic) Z 154 = 154 + 168 + 182 196 . . . . . to end + 167 + 181 195 + , , . . . to end (monoisotopic) Z 166 = 166 + 180 + 194 208 . . . . . to end + 179 + 193 + 207 + , . . . . to end (monoisotopic) Z 178 = 178 + 192 + 206 220 . . . . . to end + 191 + 205 + 219 + . , . . . to end (monoisotopic)
+ +
+
Table IV. Nominal type Benzenes Naphthenebenzenes Dinaphthenebenzenes Naphthalenes Acenaphthenes Fluorenes Phenanthrenes
Z 78
1.8904 -0.1952 f0.0124 -0.0027 -0.0015 -0.0011 -0.0029
+ +
+
+ + + + + + + + +
1550
ANALYTICAL CHEMISTRY
I
Calculation Matrix-Inverse Z 129 -0.1601 -0.0942 -0.2284 2.0479 2.3024 -0.2806 -0.0402 -0.4937 -0.0600 +O, 0081 +0.00ll -0.0154 +O.OOol -0,0088 2 104
186 and 214. If the observed height at 200 is less than the calculated height, the observed height is retained. Similarly, the corrected height a t 213 is the average of heights at 199 and 227. The corrected height a t mass 175 is the height at 161 less one-third the algebraic difference between heights a t 161 and 203; again, if the observed height at 175 is less than the calculated height, the observed height is retained. The same procedure is used at mass 176 using base points a t 162 and 204. Finally, heights a t 189 and 190 are the averages of heights a t 175 and 203 and a t 176 and 204, respectively. The previously calculated or retained values at 175 and 176 are used in these calculations. Tables I11 and IV give details of the peak summations, the original calibration data, and the inverse matrix used in the class analysis. Although two sets of values are shown for benzenes in Table 11, the value based on the higher-molecularweight benzenes was chosen, as the molecular weight range approximates that of the majority of samples of interest to our laboratories. Multiplication of peak-height summations by the inverse matrix yields total-ionization values for each class. Because total ionization is approximately proportional to the liquid volume of sample charged to the mass spectrometer ( I d ) , the results express the relative volumes of the classes, and normalization then yields volume percent of each class. Class Subdivision. Overlap types within each class are resolved using only the monoisotopic (M-l)+ peaks in the respective classes, which are assumed to carry all the information needed. When the heights of the monoisotopic (M-l)+ (14) A. Hood, ANAL.CHEM., 30, 1218 (1958).
+ + + + + + + +
.
Fluorenes 0.0257 0.0075 0 .0025 0.0049 0.0152 0.4910 0.1034
Phenanthrenes 0.0699 0.0342 0.0098 O.OO40 0 .0026 0.0226 0.5073
.
I
.
of Calibration Matrix Z 128 Z 154 -0.0290 -0.0021 +0.0016 -0.0003 -0,0026 -0.0579 1.9406 -0.0195 1,9773 -0.1339 -0.0116 -0.1822 -0.0044 $0.0121
Z 166
-0.0421 +O ,0025 -0.0018 -0.0151 -0.0584 2.0165 -0.4192
Z 178 -0.2345 -0.1069 -0.0266 -0.0020 -0.0057 -0.0904 1.9904
peaks in a series are plotted versus their masses, the curve i s non-linear and, if no overlapping types are present, smoothly approaches zero as the mass increases. This is illustrated in Figure 1 for alkylbenzenes in fractions isolated from a virgin gas oil. However, if the square root of peak height is plotted as a function 1O6/mass2,the plots are essentially linear a t the higher masses, and deviate only a t the lower masses. Figure 2 shows the linearity achieved for alkylbenzenes. A characteristic of distillation fractions is that the highest molecular weight compounds will generally have the fewest rings and double bonds. Hence, within a class the molecular ions near the end of the spectrum will result only from the nominal types, because the first and second overlap compounds will have lower molecular weights. Similarly at the low mass end of the spectrum there will be peaks relatively free of contributions from the first and second overlap types, because the condensed aromatic nuclei d o not fragment to any large extent. Therefore, base lines constructed on the squareroot plots can distinguish between contributions of the nominal and overlapping types. In overlapping regions, from the lowest mass where overlap can occur to the end of the (M-1)+ series, peak heights represented by the base line are attributed to the nominal compound type and values between the base line and the observed heights are attributed to the overlapping types. Data for establishing the base lines are given in Table V. Two points are used to determine a base line for each of the seven (M-I)+series. The high-mass point is the square root of the height a t the highest mass observed in the series. The mass of the low-mass point is empirically fixed for each series
9 I
1000
I
I
1
i
i
0
0
-
I-
I
c? W
I
0 0
0 n.
20 2
500
0 2
0
E
0 O 0
0
aOOOOoOOOo
600
500
400
300
*1
I
I
200
100
M A S S O F (M-1)+ IONS
Figure 1. Intensities of monoisotopic peak heights ES. mass in the mass spectrum of alkylbenzenes from virgin gas oil
at a value below that a t which the first overlap can occur. The peak height a t this fixed mass is multiplied by a constant before taking the square root, so that a straight line through the two points on the square-root plot coincides with the linear region of the curve derived solely from peaks of the nominal type. Points along the base line corresponding to masses in the series represent the square roots of the peak heights attributable to the nominal compound type. These values are stored in the computer in a new “square root” array, SR(M). Each value in the array is then squared so that calculated peak heights for the nominal type are stored. Some peak heights fall off the base line shown in Figure 2. This non-linearity is corrected for by using peak-by-peak correction factors. The masses and correction factors are given in Table VI. To obtain the base line contribution to the nominal types each of the peak heights at the masses shown in Table VI is multiplied by its factor and the result stored in place of the value read from the base line. The total ionization in the (M-l)+series may contain interfering ionization from compounds outside the class, and this
Table V. Data for Establishment of Base Lines Low-Mass Base Point Class Mass Abscissa Ordinate“ I 105 90.71 [0.72 X HDZ(105)]”2 I1 173 34.12 [0.66 X HDZ(173)]1f2 111 185 29.22 [I .OO X HDZ(185)]1/2 IV 183 29.86 r0.25 X HDI(183)]1’2 15.87 [0.64 X HDZ(251)]1’* V 25 I VI 277 13.03 [0.70 X HDZ(277)]1/2 VI1 233 18.42 [0.58 X HDZ(233)]1/2 High-Mass Base Point Let MMAX equal the highest observed mass in a (M-I)+ series. The coordinates for the high mass base point of that series will be: Abscissa = (1000/MMAX)2 Ordinatea = [I.OO X HDI(MMAX)]l/* a Where HDZ(M) is the monoisotopic peak height at mass M.
106/MASS2
I1 447
1
258
1
1
169
200
1 147
1
135
1 124
1
115
1 108
1
103
1
I I
93
98
MASS
Figure 2. Square root of peak height us. mass for (M-l)+ ions in the mass spectrum of alkylbenzenes. -x- - -x- = base points
-
Table VI. Peak by Peak Factors to Correct for Deviation from Base Lines Class Mass Factora Io 147 1.44 110 None None 1110 None None IVO 197 3.10 21 1 2.52 225 2.07 239 1.83 253 1.59 267 1.39 28 1 1.28 295 1.26 309 1.14 323 1.06 265 1.42 vo 279 1.24 293 1.12 307 1.06 291 1.24 VI0 305 1.15 319 1.07 333 1.06 347 1.05 361 1.03 247 1.61 VI10 261 1.50 275 1.44 289 1.37 303 1.28 317 1.24 331 1.21 345 1.10 359 1.09 373 1.07 387 1.05 Heights determined by the base lines are multiplied by these factors to obtain the nominal type contributions. Corrections at other masses are not required.
must be eliminated. Since contributions in the (M-l)+total from the overlapping types are measured from their net heights above the base line, they should be relatively free of outside interference. Therefore, all such interference is assumed to affect only the total for the nominal type. An VOL. 41, NO. 12, OCTOBER 1969
1551
admittedly less-than-rigorous estimate of the interference to the nominal type is made cia the following steps: (1) The total ionization of the class, as determined from the inverse matrix multiplication, multiplied by its diagonal element from the calibration matrix (Table III), gives the sum of the (M)+ and (M-l)+ series free of outside interference. Table VII. (M-1)+ Peaks Required to Determine the First Overlapping Type in Each Class Class 1st Overlap Masses I1 Benzothiophenes 147, 161, 175, 189 111 Pyrenes 215, 229,243, 257 1111 Chrysenes 241, 255, 269, 283 IV1 Di benzothiophenes 197, 211, 225 v1 Perylenes 265,279, 293, 307 VI1 Dibenzanthracenes 291, 305, 319, 333 VI11 Naphthobenzothiophenes 247, 261, 275, 289
Table VIII.
(2) This calculated sum, subtracted from the observed sum of the (M)+and (M-l)+series, yields the total interference in the observed sum. (3) The total interference, multiplied by the ratio of the observed ( M - l ) +sum to the observed (M)+ plus (M-l)+sums, gives the interference in the (M-1)+series. (4) This interference, subtracted from the ( M - l ) +sum for the nominal type, gives the final (M-l)+value for the nominal type. The majority of the ions due to the first overlapping type are found in the four lowest mass peaks that extend above the base line. The peaks required to determine the first overlapping types are listed in Table VII. In petroleum fractions, the first four (M-l)+peaks of a n aromatic type, starting 13 mass units above the parent mass of the unsubstituted compound, generally contain about 7 5z of the total ionization in the series. Therefore, the sum of the first four peaks in an overlapping type, divided by 0.75, should give its contribution to
Comparison of the Compositions of Selected Aromatic Concentrates by the Present Method and by Low Voltage Mass Spectral Analysis Liquid volume Range of carbon numbers C9-c20 ClOrClS C9-Cl2 This method L.V. This method L.V. This method L.V.
Benzenes Naphthenebenzenes Dinaphthenebenzenes Naphthalenes Acenaphthenes, dibenzofurans Fluorenes Phenanthrenes Naphthenephenanthrenes Pyrenes Perylenes Benzothiophenes Class IV2 Total
20.1 8.5 0.6 63.5
16.7 7.3 2.1 59.7
2.5 3.6 0.2 0.9
4.1 3.4 4.2 0.7 1.5 0.2
4.8 4.3 0.4 90.3
3.2 4.6 2.0 86.9
83.5 6.3 10.2
81.4 8.6 0.8 9.2
100.0
100.0
3.2 0.1
0.2
0.1 99.9
100.0
100.0
100.0
Table IX. Analysis of Aromatic Concentrates Liquid volume Virgin gas oil Catalytic gas oil Monoaromatic Diaromatic Diaromatic Triaromatic concentrate concentrate concentrate concentrate
Benzenes Naph thenebenzenes Dinaph thenebenzenes Naphthalenes Acenaphthenes , dibenzofurans Fluorenes Phenanthrenes Naphthenephenanthrenes Pyrenes Chrysenes Perylenes Dibenzanthracenes Benzothiophenes Dibenzothiophenes Naphthobenzothiophenes Class I L Class 111, Class IV2 Class V2 Class VI2 Class VI12 Total
1552
ANALYTICAL CHEMISTRY
32.7 25.5 21.4 3.9
5.6 6.5 8.3 20.7
0.6 0.8 1.5 65.9
3.3 2.6 1.4 0.1 0.3
18.1 14.6 8.3 4.1 3.2 0.7
13.4 5.9 4.2
0.1 0.5 0.1
0.1 1.6 0.1 0.7 0.7 3.5 0.9 0.4 0.3 100.0
0.1 4.2 2.6 0.2 0.7 0.5 0.3 0.5 0.3 0.5 100.0
0.1
2.4 3.9 4.3 8.5 36.4 3.8 19.5 6.1 2.0
5.6 1.8
3.4 8.1 1.7
99.8
100. 1
the sum of ions in the (M-l)+ series. Only the first three peaks are used for dibenzothiophenes because the fourth peak a t mass 239 varies so that its relative height cannot be reliably predicted. The three-peak sum is divided by 0.625 rather than 0.75, because a smaller fraction of the (M-l)+ ionization is included in the summation. If, in each case, the calculated total ionization is less than the sum of heights available for overlapping types, the remainder is assigned to the second overlap. If the quantity is greater than the total available, the latter is assigned to the first overlap and the second overlap is set a t zero. After contributions to the (M-l)+ series are determined for all types in a class, ratios of each contribution to the total are computed. These are then multiplied by the per cent of that class in the sample to obtain concentrations of the individual types in the sample. DISCUSSION
The present method has features that make it suitable for routine use on a wide range of aromatics fractions from petroleum. The class analysis is insensitive to molecular weight or boiling range, because the quantities of all the compound types are determined on the basis of their contributions to the total ionization of the sample. The overlap types, determined by difference, are held within the bounds of their respective classes so that the entire composition is accounted for. Most important, the inclusion of the second overlap groups in the analysis avoids describing the sample in terms of a limited number of compound types that can be specifically named. The method is based on the observation that approximately one-half the total ionization of any aromatic compound occurs in the sum of peak heights in the (M)+ and (M-l)+ series. This is true when, (1) total ionization is defined as the sum of peak heights at all integral masses in the spectrum, (2) the (M)+ summation begins at the mass of the unsubstituted ring compound and includes all higher-mass peaks at increments of 14 mass units to the end of the spectrum, and (3) the (M-1)+ summation begins at the peak 13 mass units above the first peak in the parent series and continues by increments of 14 mass units to the end of the spectrum. There is a decided advantage is first determining the classes as defined here and later subdividing them, as compared to a simultaneous equations procedure that attempts to determine individual components all at once. The latter method can result in large negative values that affect the entire analysis and are difficult to interpret. I n the present method, negative values are usually small. Furthermore, if they occur in the subdivisions, they affect the results in only their own class, and the rest of the analysis is unaffected by them. The present method appears to give results at least as good as might be expected for the usual mass spectrometric type analysis procedures. For low molecular weight samples containing only small quantities of overlapping types, results are in satisfactory agreement with low-voltage results as shown in Table VIII. Qualitatively, the method gives the proper compound types when mono-, di-, and tri- aromatics concentrates, prepared by the adsorption chromatography procedure of Snyder (9), are analyzed, as shown in Table IX. Cross contamination of the fractions is considerably larger than expected. However, detailed examination of the spectra indicates the presence of many of the types found by this method. The indicated presence of monoaromatics in the triaromatics concentrate from a catalytic gas oil is probably erroneous. However, at the usual concentrations of triaro-
Table X. Composition of Aromatic Portion of High Sulfur West Texas Gas Oil before and after Hydrogenation Liquid volume % Original Hydrogenated Benzenes 12.8 24.5 Naphthenebenzenes 11.5 17.4 Dinaphthenebenzenes 12.9 14.5 Naphthalenes 6.8 3.8 Acenaphthenes, dibenzofurans 7.8 5.7 7.8 Fluorenes 6.3 5.3 Phenanthrenes 3.6 Naphthenephenanthrenes 3.8 2.4 5.7 Pyrenes 3.4 Chrysenes 3.5 2.5 2.1 Perylenes 1.5 Dibenzanthracenes 1.1 0.8 Benzothiophenes 7.9 3.3 Di benzothiop henes 5.3 4.7 Naphthobenzothiophenes 2.8 1.2 Second overlap : Class 1112 0.5 0.0 Class IV2 1 .o 2.1 Class V2 0.2 0.5 Class VI2 0.3 0.5 Class VI12 0.6 0.1 Total 99.4 99.1
Table XI. Comparison of Sulfur Contents by Mass Spectrometry and Gas Chromatography Average Weight thiopheno ~ u l f u r ~ * ~ Sample mol wt MS GC Mid-Continent 388 2-ring 0.07 0.14 3-ring 0.11 0.20 4-ring 0.04 0.06 Mid-Continent 253 2-ring 0.09 0.08 3-ring 0.07 0.10 4-ring 0.00 0.00 Mid-East 335 2-ring 0.49 0.67 3-ring 0.53 0.73 4-ring 0.11 0.14 Rodessa 344 2-ring 0.04 0.05 3-ring 0.07 0.09 4-ring 0.02 0.03 High sulfur coker 297 2-ring 0.58 1.07 3-ring 0.48 0.72 &ring 0.15 0.19 a GC results include cyclanothiophenes as well as benzothiophenes. All values are on the original distillate basis.
matics in a whole aromatics fraction, the error so introduced in the monoaromatics value would be trivial. Further qualitative confirmation of the method is shown by the analyses of the aromatics in a high-sulfur West Texas gas oil before and after hydrogenation. The data in Table X show that hydrogenation greatly increases the monoaromatics and decreases the sulfur compounds and polyaromatics. Upon hydrogenation a n increase in concentration of the Class IV components from the second overlap calculation may indicate that trinaphthenebenzenes were produced. Values for per cent sulfur in 2-, 3-, and 4-ring thiophenes based on total gas oil were calculated and compared with values obtained by the gas chromatographic method of Martin VOL. 41, NO. 12, OCTOBER 1969
1553
and Grant (15). In order to convert the mass spectrometrically determined volume per cent sulfur compounds to weight per cent sulfur, weight-per cent analyses of the aromatics fraction were first calculated using estimated densities for the compound types found. Molecular weight values of the original oils before separation of the aromatics were used to complete the calculations. The comparisons are shown in Table XI. Gas chromatography values are generally higher than those found by mass spectrometry. Because the former procedure includes naphthenethiopheno as well as benzothiopheno types, higher values should be expected. The method has some obvious limitations. The highermolecular-weight fractions from petroleum may contain many compound types not considered here. Except for the three sulfur types and dibenzofurans, the heteroaromatic types are not specifically considered. While many of them will be included in the unidentified categories, at least a few types (15) R. L. Martin and J. A. Grant, ANAL.CHEM., 37,649 (1965).
must be calculated with the identified groups. For example, carbazoles contribute to both the acenaphthene and the fluorene values. Analyses of aromatics synthesized from relatively pure starting materials, as, for example, aromatic alkylates may not be satisfactory. Although the class analysis might be successful, irregularities in the (M-l)+ series could result in unreasonable values for the overlap types. Nevertheless the method gives reasonable analytical results for petroleum fractions. It provides more detail than earlier methods, and it can be used routinely when electronic computing of the results is available. ACKNOWLEDGMENT
The authors thank M. E. Fitzgerald, R. P. Page, and Arc0 Chemical Co., F. P. Hochgesang and Mobil Research and Development Corp., H. E. Howard and Union Oil Co., of Calif.; and J. B. Grutka and Universal Oil Products Co. for mass spectra of Wilmington gas-oil aromatics. RECEIVED for review May 20,1969.
Accepted July 31, 1969.
Electrolysis of Organophosphorus Compounds Study of Mechanism of Reduction of Various Diethyl Aroylphosphonates at a Dropping Mercury Electrode K. Darrell Berlin,’ David S. Rulison,* and Paul Arthura Department of Chemistry, Oklahoma State Unioersity, Stillwater, Okla. 74074 Dialkyl aroylphosphonates in acetonitrile were electrolyzed at the dropping mercury electrode (DME) in the presence of benzoic acid to give dialkyl a-hydroxyarylmethylphosphonates as the exclusive product. Coulometric studies revealed an n-value of 2 electrons. In the absence of benzoic acid, benzoins and dialkyl hydrogenphosphonates were the only detectable products by GLC analysis. This situation requires C-P bond cleavage probably after an initial formation of a radical anion. The aroyl radical must couple rapidly at the electrode surface -0 0
0 0
I1 t
ArC-P(OR)2
+e
I t
--* A r $ - P ( O R ) *
4
to give a benzil which is further reduced to a benzoin. A wide variation in n-values was obtained in the electrolysis of dialkyl aroylphosphonates unless benzoic acid was added. A plot of E112 VS. Hammett U-values indicates the reduction is facilitated by electron-withdrawing groups as expected. Half-wave potentials for the dialkyl aroylphosphonates are more negative than for similarly substituted benzils. It is tentatively suggested that a conjugative effect on the carbonyl function by the phosphoryl group may be responsible in part for the decreased ease of reduction in the aroylp hosphonates.
THE POLAROGRAPHIC behavior of organophosphorus compounds at a dropping mercury electrode (DME) has been the subject of relatively few investigations. Reports have been To whom inquiries should be directed.
Du Pont Predoctoral Teaching Fellow, 1967-68; present address, E. I. du Pont de Nemours & Co., Waynesboro, Va. a Deceased. We dedicate this paper to the memory of Dr. P. Arthur. 2
1554
ANALYTICAL CHEMISTRY
published concerning polarographic studies of hydroxyalkylphosphines ( I ) , phosphonium salts (2), and other compounds such as triphenylphosphine (3), 0,O-dimethyl 2,2,2trichloro-1-hydroxyethylphosphonate (4, and various nitrophenyl phosphates (5). A series of papers has been released concerning the polarographic behavior of various metal ions and complex ions in the presence of certain organophosphorus compounds (6-9). A few electrochemical studies which describe the development of polarographic methods for analyses of such compounds as Malathion [S-(l,2-dicarbethoxyethyl) 0,O-dimethyl dithiophosphate] and Parathion (0,O-diethyl 0-p-nitrophenyl thiophosphate) have also been reported (10-13). Santhanam and coworkers (14) examined the behavior of tris-(p-nitrophenyl) phosphate at the DME and ~
~~~~
(1) K. Issleib, H. Matschiner, and M. Hoppe,Z. Anorg. Allg. Chem., 351, 251 (1967). (2) H. Matschiner and K. Issleib, ibid., 354, 60 (1967). (3) S . Wawzonek and J. H. Wagenknecht, “Polarography 1964,” Graham J. Hills, Ed., Macmillan, London, 1966, pp 1035-41. (4) Yu. M. Kargin and K. V. Nikonorov, Izv. Akad. Nauk SSSR, Ser. Khim.,1902 (1966); Chem. Absrr., 66, 4869 (1967). (5) W. M. Gulick, Jr., and D. H. Geske, J. Amer. Chem. SOC.,88, 2928 (1966). (6) H. Sohr, J. Elecrroanal. Chem., 11, 188 (1966). (7) H. Sohr and K. Lohs, ibid., 13, 107 (1967). (8) Ibid., p 114. (9) Ibid., 14, 227 (1967). (10) M. K. Saikina, Uch. Zap. Kazansk. Gos. Univ., Obshch. Sb., 116, 121 (1956); Chem. Abstr., 52, 296 (1958). (11) W. H. Jura, ANAL.CHEM., 27, 525 (1955). (12) C. V. Bowen and F. I. Edwards, Jr., ibid., 22,706 (1950). (13) D. E. Ott and F. A. Gunther, Analyst, 87, 70 (1962). (14) K. S. V. Santhanam, L. 0. Wheeler, and A. J. Bard, J. Amer. Chem. Soc., 89, 3386 (1967).
