Low-Voltage Sensitivities of Aromatic Hydrocarbons H. E. LUMPKIN ancl THOMAS ACZEL Research and Developmenf, Humble Oil & Refining Co., Bayfown, Texas
b A new set of low-voltage calibration data is presented for aromatic hydrocarbons, including compound types from alkylbenzenes to chrysenes. Separate data are provided for alkylbenzenes with different degrees of substitution and for average coefficients for these compound types obtained b y using detlmnined and calculated average isomeric distribution values. Ionization-efficiency curves also have been obtciined for a number of alkylbenzenes in an effort to correlate ion current magnitudes at a given ionizing energy level with molecular structure. Preliminary data indicate that the degree of substitution on the aromatic ring affects the slope o f the ionization-efficiency curves.
T
HE LOW-VOLTAGE METHOD for identification and analysis of hydrogendeficient materials has continued to be one of the most powerful techniques in mass spectrometry since its inception in 1955 (3). A set of calibration data for a number of olefinic and aromatic compound types published in 1958 (6) is widely used in industrial research. Revision and extension of the coefficients obtained originally are justified by the availability of a larger number of pure compounds and increased knowledge about the constitution of petroleum. The rectnt shift in emphasis to chemicals and chemical precursors makes a recalibration on a weight, rather than volume, per cent basis desirable. Concurrently, the scope of low-voltage analysis is being augmented by the advent of instruments featuring resolving powers of the order of 1 in 5000 and above arid thus capable of differentiating among the parent peaks of more than seven compound types. These factors furnished the incentive for obtaining a new and consistent set of calibration coefficients for aromatic hydrocarbons, whic i is presented and discussed here. Correlations between molecular structures and low- voltage ion current intensities have been sought and established for several cltsses of compounds by Crable, Kearns, and Sorris (2). Their data have been corroborated and extended by the presentations of ionization efficiency curvw for a number of alkylbenzenes.
Table I.
Reproducibility of Ethylbenzene Sensitivities
7 . 6 volts
Range for 21 runs Av . Std. dev. Rel. std. dev.
18.2-19.6 18.73 kO.55 12.91
EXPERIMENTAL
Calibration data were obtained on a 180' magnetic deflection (Consolidated Electrodynamics Corp. Model 21-103) mass spectrometer, adapted for lowvoltage work as reported previously (8). The instrument was operated in the focused mode. The level of the magnetic field was about 3800 gauss, and the repeller plates were kept a t about 2.5 volts. Solid materials were introduced through the solids inlet system devised by Lumpkin and Taylor (7). The availability of this system made calibration possible for four- and five-ring condensed aromatics; 62 alkylbenzenes, nine indanes and Tetralins, 22 naphthalenes, five noncondensed 2-ring aromatics, 20 condensed three-, four-, and five-ring aromatics, and nine miscellaneous compounds were used as calibration compounds. These included API standards, whenever available, and materials from various other commercial sources. Purity was a t least 95% in all cases and above 99% for most compounds, as established by mass spectrometric techniques. Significant numbers of alkyl-substituted compounds were available only for benzenes and naphthalenes, but the data obtained in this and previous studies justified the extension of some of the results to other compound types. Data on the reproducibility of the measurements are reported in Table I. These data were obtained for an ethylbenzene standard in the course of one week, during which the ionization curves for 24 alkylbenzenes were determined. The relative standard deviation found was around 3% a t 7.6 volts, 294 a t 15.0 volts, and 1.5% at 70.0 volts (meter readings). The reproducibility of the instrument is around 1% and this accounts approximately, together with errors due to different amounts of sample introduced, for the deviation observed a t 70.0 volts. The poorer reproducibility a t 15.0 and 7.6 volts was
Divisions/pmole 15.0 volts 128.5-135.6 132.53 f2,85 12.15
7 0 . 0 volts 241.6-256.2 247.6 h3.66 3~1.48
caused by additional factors, including the instability of the low-voltage circuitry and the inaccuracy of reading the voltmeter. Considering also the fact that the ionization curve is very steep in this region, causing large changes in the ion current in response t o relatively small changes in the applied electron energy, the standard deviation observed a t lower voltages is considered excellent. All the calibration coefficients reported or plotted in this work were reduced to a standard ratio with respect to a fixed value of ethylbenzene sensitivity. The latter was introduced into the mass spectrometer together with or immediately after each calibration run. The ionization efficiency curves were obtained by determining the values of the ion currents a t intervals of 0.5 volt between 17.5 and 12.5 e.v. and a t intervals of 0.2 volt between 12.5 and 8.9 e.v. Each measurement was repeated several times and a correction factor was introduced to compensate for sample pressure decrease during the experiment. Ionization energies were calculated by comparison with a standard (ethylbenzene). The difference found between the meter reading and the absolute value a t the ionization potential (about 2.5 volts) was added consistently to the values read for other compounds in the whole interval studied. Repeller settings were kept constant. CALIBRATION COEFFICIENTS FOR AROMATIC HYDROCARBONS
The data obtained on all the alkylsubstituted compounds confirmed the dependence of low-voltage sensitivities on the degree of substitution, as pointed out by Crable (2). -4dditional effects due to the position of the substituents in alkylbenzenes and particularly in alkylnaphthalenes were also evident. The decrease in intensities with inVOL. 36, NO. 1, JANUARY 1964
181
__
. ~.
~~-
Ethylbenzene 225 div/rng.
-
600~
500-
F
g 400>
5c 3008 200e7
1000% ~~~-~
0.014
L
013
012
0!1
CIC
ow
008
001
CC,
001
0:1
31:
c::
$01
.
0.012
Table 11. Determination of Average Degree of Substitution in Alkylbenzenes by NMR
syn-
Av. degree
of substitution
thetic
Known
A B
3.06 2.29 1.72
C
182
Detd. 3.03 2.38 1.87
ANALYTICAL CHEMISTRY
Relative error -1.0 +4.8 +8.7
0.006 WEIGHT
0.004
0.002
. J
3
Low-voltage calibration data for alkylbenzenes
creasing molecular weight agreed with that established by Lumpkin (7). Calibrat,ion data were obtained, therefore, by drawing smoothed curves obtained when individual compound sensitivities for each type were plotted against the reciprocal of the molecular weight. The sensitivities were expressed in divisions per milligram. On the instrument used one division corresponds approximately to 2 X ampere. Values for individual compounds or carbon numbers can be easily determined from these curves. The curves for alkylbenzenes are given in Figure 1. Since a sufficient number of alkylsubstituted compounds were available in this case, separate curves could be drawn for the different degrees of substitution. Such a set of curves improves the accuracy of the lowvoltage method greatly if the degree of substitution is known from independent sources. For most problems encountered in petroleum chemistry, this parameter is, however, not known beforehand. One is, therefore, faced with the task of extrapolating or determining an average value for it which can be used for the selection of the appropriate calibration coefficients. Such "average" calibration coefficients can be derived from the dotted line in Figure 1. This curve was obtained in the following manner. An average degree of substitution for
0008 IIMGLECULAR
Figure 2. Low-voltage calibration data for six aromatic compound types
I M O L E C U L A ? WEIGHT
Figure 1.
~
0010
Cs, CS, and Cl0 alkylbenzenes in petroleum products was calculated from data published by Martin, Winters, and Williams (8) on the isomeric distribution of alkylbensenes in 18 crudes. The numbers thus obtained-that is, 1.77, 2.45, and 2.75-were plotted, together with the obvious values of 0.0 for CBand 1.00 for C, alkylbensenes, t o obtain approximate data for the C1l and CIZ compounds. The values derived from this extrapolation were, respectively, 3.07 and 3.22. Both calculated and extrapolated values were then used in the constuction of the calibration curve. If the origin of the sample is known, and composition data are available for the particular crude or a similar one, this approach can be a t once improved and simplified by using specific rather than average values for determining the isomeric distribution. The average degree of substitution can be determined also by nuclear magnetic resonance. This method is capable of furnishing rapid and accurate determination of the per cent of unsubstituted hydrogens on an aromatic ring and, therefore, of the average degree of substitution by measuring the ratio between aromatic proton intensity and the intensities of protons located on carbon atoms a to the aromatic ring (1). Since the latter can be primary, secondary, or tertiary, an appropriate correction has to be introduced to take care of this parameter. Reliability of the NMR method is illustrated by the values reported in Table 11, obtained on synthetics of more than 20 components. It is suggested that the KMR approach be used as a criterion in selecting calibration coefficients whenever justified by accuracy requirements. Calibration data for fractional values of alkyl substitution can be easily derived from Figure 1. Curves comprising a consistent set of calibration data for 13 aromatic com-
pound types are given in Figures 2 and 3. The general shape of the curves for compound types with fewer data points was patterned after those obtained for alkylbenzenes. A11 values reported are on the same basis and can be used directly. It was reassuring to note that the newly obtained data differed only slightly from those published previously (8). IONIZATION EFFICIENCY CURVES
The relationship between ion current intensity at a given electron energy level and the energy necessary to ionize a molecule (ionization potential) has been investigated by several authors (2, 4 ) . It is generally assumed that knowledge of the ionization potential, if it can be determined with sufficient accuracy, will allow the determination of the ion current intensity at a given electron energy level, thus furnishing calibration coefficients for compounds which cannot be examined directly. It has, however, been noted in these laboratories, and also by Crable (2) and Sharkey (9),that isomeric methylnaphthalenes and methylnaphthols give considerably different ion current intensities at the voltages generally used for lowvoltage work. Since the ionization potentials of 1- and 2-methylnaphthalenes are identi-
0 . 0 k
,
0.004 I/MOLECULAR
I
0.002 WEIGHT
1
0
Figure 3. Low-voltage calibration data for seven aromatic compound types
1
,
0
y
a
ion currents(div micromole)
x
=
V-IP,eV
I
1
DEGREE OF METHYL SUBSTITUTION
Figure 4. zenes
Ionization efficiency curves for alkylben-
cal (6),this difference must be correlated with the shape of the ionization efficiency curve, the klowledge of which should also be necesEary for predicting low-voltage calibraticn coefficients. To investigate this phenomenon better, as well as the reasons for the dependence of the ion current intensities on the degrel: of substitution, ionization efficiency curves have been
Table 111.
obtained for 24 alkylbenzenes. Preliminary data indicate that the correlations found for alkylbenzenes are valid also for the alkylnaphthalenes. The following discussion is restricted, however, to the former type. The dependence of the shape of the ionization efficiency curves on degree of substitution is immediately evident from Figure 4. While the curves are
Analytical Expressions for Ionization Efficiency Curves between ca. and ca. 2.0 E.V. above Ionization Potential
Compound Benzene Toluene Ethylbenzene n-Propylbenzene n-Butylbenzene n-Pent ylbenzene Isopropylbenzene o-Xylene m-X y 1ene p-Xylene 1,a-Diethylbenzene 1,2,3-Trimethylbenze11e 1,2,4,5-Tetramethylbcnzene a
Figure 5. Slopes of ionization efficiency curves in alkylbenzenes
Ionization potential, e.v. 9.58 9.20 9.17
Standard error
Quadratic fit 2.08 1.68 2.10 1.45 2.06 3.03 1.09 1.65 0.88
Slopes (using linear fit) 15.9 21.8 21.5 21.5 21.4 22.4 20.3 26 .O 27.6
1.74 2.27 1.55
27.4 31.6 36.9
of approximation
Linear fit 2.78
9.14
9.i3 9 . 12Q
9.13 8.97 9.02 8.88 8.99
8.75 8 .48”
4.94
2.53 1.58 2.60 2.96 1.76 4.02 3.97 4.65 2.91 4.01 5.95
0.6
1.53
25.5
Extrapolated values
Table IV.
Low-Voltage Sensitivities of Some Aromatic Compounds at 10.1 and at
11.5 E. V.” Compound Benzene Naphthalene 1-Methylnaphthalene 2-Methylnaphthalene 1,PDimethylnaphthal ene 2,3,5-Trimethylnaphtlialene 1,3,7-Trimethylnaphtlialene
2-Methylfluorene I-Methylphenanthren3 1-Methylpyrene 4-Methylpyrene Perylene
Parent ion intensities ratios, 11.5/10.1 e.v. 3.48 2.30 2.29
2.25 2.28
2.45
1.89 2.24 2.19 1.93 1.79 1.90
Total fragment ion intensities, yo of parent peaks 10.1 e.v. 11.5 e.v. 1.31 0.39
... ... ...
0.81 0.66
0.52
... ... ... ...
...
... ...
... 0.65 0.48
0.49 0.33
... ... ... ...
Absolute values, determined by comparison with known standard (ethylbenzene).
similar for all the compounds with the same degree of substitution, there is essentially no overlap bet ween the curves given by compounds possessing different numbers of alkyl substituents. The nature of the substituting group has little or no effect, except in the case of highly branched groups such as t e r t butyl which tend to flatten the curve sooner than other compounds. This effect can be ascribed to the competitive processes leading to the formation of fragment ions, which are presumably more significant for these molecules. The portion of the curve which presents the highest interest for the purposes of this investigation is the quasi-linear portion observed slightly above the ionization potential. Linearity of the curves was determined in this interval-that is, between ca. 0.6 and 2.0 volts above the ionization potential-by the least squares method, using first- and second-order expressions. The limits chosen included the energy level at which lowvoltage analyses are usually made. Some standard errors of approximation between experimental and calculated data are reported in Table 111. Although the quadratic fit appeared t o be slightly more accurate, in particular for the polysubstituted compounds, the deviations from the linear fit were still very small, proving the essentially linear character of the curves in the intervals studied. The ionization potentials used were those available in the literature (5) from electron impact studies. When electron impact values were not available, they were estimated from values for lower molecular weight compounds or from values obtained by other methods. While ionization potentials obtained by different methods differ considerably, the average change from a given compound in a homologous series is nearly constant, Agreement of the values thus derived with those obtained in the course of this study was satisfactory. Dependence of the slopes on the degree of alkyl substitution is also VOL. 36, NO. 1, JANUARY 1964
183
Table V.
Ionizing voltage, e.v. 10.1
14.5
17.5 70.0
Partial Spectra of 1,3,5Triethylbenzene Obtained at Various Ionizing Voltages and Currents
Parent peak sensitivity (relative to Ionizing that at 2105 119 current, 10.1 e.v., 133 147 pa. 11 pa.) 161
+ + + +
11
35 45
1 .oo 1.63
1.11
1.06 1.12 ...
11 35 45
3.66 11.45 13.18
81.90 69.11 64.61
35 45
11
4.37 13.93 16.79
156.03 142.90 136.21
11
6.48
407.09
evident from the data reported in Figure 4. Quantitative data supporting this statement are reported in Table 111, column 5, and Figure 5 , The slopes were calculated on the linear portions of the ionization efficiency curves in the same interval selected for the determination of linearity discussed above. It can be concluded from these data that these slopes are nearly identical for compounds possessing the same degree of alkyl substitution, but that they increase linearly as the number of alkyl substituents on the ring increases (Figure 5). Any prediction of low voltage coefficients from theoretical data must thus take into consideration both the ionization potential and the slope of the ionization efficiency curve. If these two parameters can be determined or extrapolated, currents a t a given electron energy level can be satisfactorily predicted. In the course of this work, such predictions were made for nhexyl- and n-heptylbenzenes. The predicted values differed from the experimental values, obtained a t a later time, by about 7% on the average. Considering the overall reliability of lowvoltage measurements, the predicted values are considered acceptable. The constancy of the slope for compounds with the same degree of alkyl substitution, together with the asymptotic behavior of the ionization potential curve for homologous substances, will also result in the construction of theoretical low-voltage calibration curves compatible with experimental data. An application of the work discussed in the preceding paragraph lies in the recognition of the possibility of analytical work at slightly higher voltages than 184
ANALYTICAL CHEMISTRY
Fragment peak sensitivities, % ’ of parent peak sensitivity 106 ...
...
...
120 134 Polyisotopic values ...
... ...
...
...
148
106
120 134 Isotope corrected
... ...
...
...
0.61
...
148
...
...
...
...
...
...
... ...
...
...
0.46 0.24 0.18
0.11 0.15 0.19
1.47
1.30 1.26
0.21 0.18 0.22
...
..*
...
1.35 0.66 0.64
0.36 0.33
6.30 5.49
... ...
... ... ... ... ...
... ...
, . .
...
...
...
5.30
4.59 3.72 3.41
0.62 0.56
0.70
0.42 0.34 0.24
10.08 9.40 9.19
8.87 8.25 7.76
0.09
0.10
0.07 0.07
2.10 1.87 1.95
0.33 0.23
5.10
1.73
20.87
23.74
0.59
0.25
4.46
0.98
...
...
the limit of 10 e.v. adopted generally. At 10 e.v. the ionization process produces primarily aromatic and olefinic parent ions, and less intense ion currents are accepted in order t o maintain a simple spectrum. At least for aromat,ic concentrates, the ion current intensity may be greatly enhanced by increasing the electron voltage slightly without affecting adversely the selectivity towards molecule ions. The data reported in Tables IV and V show that two- to threefold increases can be achieved in this way, accompanied by still minor fragmentation processes. The most abundant fragment peaks are those a t odd mass numbers, and their interferences at parent peaks of homologs of lower molecular weight can be further reduced by isotope correction. Simultaneous increase of ionizing currents leads to further increases in sensitivity, as illustrated by the partial spectra of 1,3,btriethylbenzene (Table V) obtained a t various ionizing voltages and currents. At about 11.5 e.v. and 35 pa. of ionizing current, the parent peak sensitivity for this compound is comparable to that obtained in normal high voltage work at 11 pa., while the fragmentation does not interfere significantly. The parent peak sensitivity values reported in column 3 of Table V indicate that the ion current increases obtained by increasing the ionizing current approach linearity only a t the higher voltage settings. This fact is attributed to the presence of space charges diminishing the collection efficiency. This difficulty might be obviated by operating a t higher repeller voltages. In spite of these deviations from linearity, the reproducibility data obtained thus far on parent peak sensitivities are satis-
...
0.08 0.07
...
factory. iinalyses of synthetics of triethylbenzene in isoparaffins have shown that the former can be determined at a 0.04y0 level Kith the relative error of *lo’%. Another anomaly is the change in pattern caused by variations in ionizing currents. The diminution of the fragment peak sensitivities with respect to that of the parent peak is, however, an obvious advantage ACKNOWLEDGMENT
An expression of appreciation is due G. R. Taylor and J. L. Taylor for obtaining and tabulating the experimental data discussed in this article. The authors also thank K. W. Bartz for obtaining and interpreting the nuclear magnetic resonance spectra. LITERATURE CITED )
Chamberlain, N. F., ANAL.CHEM.31, 1959). rable. G. F., Kearm. G. L.. Norris,
‘ 28, 1248 (1956). (4) Honig, R. E., J. Chem. Phys. 16, 105 (1948). (5) Kiser, R. W., U. S. At. Energy Comm , Rept. TJD-6142 (1960). ( 6 ) LumDkin. H. E., ANAL. CHEM.30, . 321 (1958): (7) Lumpkin, H. E., Taylor, G. R., Zbid., 33, 476 (1961). (8). Martin, R. L.? Winters, J. C., WilLama, J. A., Sixth World Petroleum Congrese, Frankfort/Mahe, June 19-26, 1963. (9) Sharkey, A. G., Jr., et al., ASTM Committee E14, New OrIeane, La., May 1958.
RECEIVEDfor review June 17, 1963. Accepted October 28, 1963. Eleventh Annual Conferenceon Mass Spectrometry, ASTM Committee E14, San Francisco, Calif., May 1963.
