824
Anal. Chem. 1982, 54, 824-825
Table 11. Area Response Factors of CDDs Relative to 1234-TCDD at m / z 32Za
a
component
m/z
1234-TCDD 237 8-TCDD 12378-PCDD HCDD mixture 1234678-H,CDD OCDD
322 322 356 390 426 460
re1 response
( * re1 std dev)
no. of replicates
1.00 k 0.03
5
0.89
i:
5
0.52
f
0.44 0.46 0.32
i i: i:
0.03 0.02 0.02 0.01 0.01
2 4 3 3
100 pg of each component injected, see text for con-
ditions. conditions. By use of analytical reference standards of 1234-TCDD, 2378-TCDD, 12378-PCDD, a mixture of two HCDD isomers, 1234678-H7CDD,and OCDD, GC-MS-SIM area responses were determined for 100-pg injections. The resulting response factors obtained for each CDD congener relative to the response of 1234-TCDD monitored at m , / 2 322 are presented in Table 11. Although these values are expected to differ from those exhibited by the authors' instrument, they do illustrate the possible margin of quantitative error which could be introduced via the assumption of a constant response factor of 1. In view of the described behavior for CDD congener transmiasion through silicone membrane separators, and the relative response factor data we have presented, it is very possible that the authors' CDD determinations (other than TCDDs) are biased low to a significant extent. Hence, the authors' compilation of these data in tabular, bar graph, and ratio formats (authors' Table 11, Figures 2 and 4, and Table
IV, respectively) should not be considered reliable as published. Although the primary purpose of our paper is to call attention to deficiencies in the authors' CDD determinations, certain unsubstantiated assessments involving chlorobenzene data also appear in their publication. These findings should be critically reviewed by an expert in GC-MS if abstracting for comparative purposes i s intended. LITERATURE CITED (1) ACS Committee on Envlronmental Improvement Anal. Chem. 1980, 52, 2242-2249. (2) Elceman, 0. A.; Clement, R. E.; Karasek, F. W. Anal. Chem. 1981, 53, 955-959. (3) Mlllard, B. J. "Quantitative Mass Spectrometry"; Heyden: London, 1979; p 72. (4) Shadoff, L. A.; Hummel, R. A. Blomed. Mass Spectrom. 1978, 5 , 7-13. (5) Langhorst, M. I..; Shadoff, L. A. Anal. Chem. 1980, 52, 2037-2044. (8) Lamparskl, L. L.; Nestrick, T. J. Ana/. Chem. 1980, 52, 2045-2054.
T. J. Nestrick* L. L. Lamparski* W. B. Crummett L. A. Shadoff The Bow Chemical Company Michigan Division Analytical Laboratories Building 574 Midland, Michigan 48640
RECEIVED for review August 3, 1981. Accepted January 11, 1982.
Mass Spectrometric Sensitivity Data for Low Voltage Electron Impact Ionization of Alkylpyrenes Sir: Over the last 30 years, mass spectrometry has been used for qualitative and quantitative analysis of mineral oil fractions. The generation of unfragmented molecule ions by low voltage electron impact has proved to be particularly suitable for this purpose. However, the relative sensitivities of this method to the various molecules present in the complex oil mixtures usually found in the refining industry must first be determined. Lumpkin (I), Crable, Kearns, and Norris (2),Lumpkin and Aczel(3), and Shultz, Sharkey, and Brown (4) quote sensitivity data for the low voltage electron impact method for olefins, various substituted benzenes, aromatics, naphtheno aromatics, and heterocyclic compounds. On the basis of sensitivity values for aromatics and naphtheno aromatics, Severin, Bergmann, and Oelert (5) have derived empirical rules to extrapolate from these values onto unmeasured substances. The majority of these values were obtained from measuring substances without alkyl groups. However, mass spectrometric investigation of high-boiling oil and liquified coal fractions has shown that by far the majority of the components present in the mixture are alkylated ( 5 , 6 ) . Unfortunately, samples of such alkylated components are very difficult to obtain commercially, and thus the development of this analytical method has had to be based up to now on "model components" which represent a very insignificant portion of oil mixtures in commercial practice. As a partial solution to this problem, more relevant to the analysis of liquified coal products, Schiller investigated aromatic systems with up to six methyl groups (7). 0003-2700/82/0354-08!24$01,25/0
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Table I. Calibration Mixture of Alkylated Pyrenes (in n-Hexane) weighed abunmass, portion, dance, component amu mg mol% pyrene
Lme thylpyrene 1-ethylpyrene 1-propylpyrene 1-butyl pyrene 1-pentylpyrene 1-hexylpyrene 1-hexadecvlwrene
202 21 6 230 244 258 27 2 286 426
18.90 11.38 9.50 12.44 9.70 10.06 10.53 8.30
25.33 14.26 11.18 13.80 10.18 9.92 9.97 5.27
Up to now, no systematic work has been done on the influence on sensitivity of the alkyl chain length in larger aromatic ring systems. In addition to this, it has previously not been possible to estimate the changes that must be made to the empirically determined rules with variation in the electron impact energies. For this reason, based on the alkylpyrenes, we have determined the dependency of sensitivity on the alkyl chain length for a number of different ionization energies.
EXPERIMENTAL SECTION Samples. Pyrene and 3-methylpyrene were obtained from the Rtitgers Co., 4620 Castrop-Rauxel, Germany. The higher homologues (Table I) were then synthesized in a two-stage process using Friedel-Crafts acylation with the appropriate acid chloride, followed by reduction of the ketone via a Wolff-Kishner reaction. The impurities arising in this synthesis can be ignored. 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
* 825
Table 11. Relative Molar Sensitivities (=1for Pyrene) Using a Glass Crucible, Standard Deviation f 5% length of alkylchain 8 e V
9eV
lOeV
11eV
1.34 1.32 1.31 1.41 0.42 0.55 0.83
1.19
1.25 1.46 1.44 1.35 1.36 0.78
1.18 1.28
1.09 1.06
1.18
1.01
1.19 1.09 1.17 0.66
0
0
1.06 1.03 0.98 0.64 0.5
1
2 3 4 5 6 16 % o f 215
-
relative molar sensitivities 12eV 1 3 eV 14eV
_ _ . -
s I_ _
l l l l _
_ l l _ l
0.1
1.07 0.95 0.84 0.88 0.85 0.90 0.52 2.5
1.07 0.85 0.76 0.80 0.80 0.82 0.74 10.7
1.10
0.91 0.84 0.93 0.96 1.07 0.82 5.9 I
15eV
16eV
20eV
1.00 0.78 0.68 0.74 0.74 0.77 0.78 14.5
0.93 0.66 0.57 0.66 0.66 0.71 0.79
0.83 0.48 0.42 0.46 0.48 0.53 0.75 31.1
18.1
I _ I _
-
Table 111. Relative Molar Sensitivities (=1 for Pyrene) Using a Porous Sample Carrier, Standard Deviation +5% length of alkylchain 8 e V ~
1
2 3 4 5 6 16 % o f 215
_
l
l
l
l
l
_
_
l
l
_
-
-
l
l
9eV
lOeV
11eV
1.18 1.18 1.19
1.14
1.17
1.12 1.02 0.99
1.03 0.95 0.96 0.97 0.88 0.89
0.05 0.1
'1.19 '1.16 11.17 1.12 0.99 0.95 0.06
1.03 0.97 0.05
0
0
1.15 1.15
relative molar sensitivities 12eV 13eV 14eV
0.05
0.97 0.83 0.82 0.76 0.74 0.70 0.05
0.04
0.89 0.71 0.67 0.64 0.62 0.61 0.06
0.5
2.0
4.0
9.3
I
8 eV
0
1
2
I
4
'
6
16
Length of Alkyl Chain
Figure 1. Relative, molar sensitivities for alkylpyrenes in relation to chain length for different electron Impact energies.
Mass Spectrometry. The calibration mixture (Table I) was introduced to the mass spectrometer (CH4, low resolution) via the batch inlet, using either a glass crucible or a porous sample carrier. The porous sample carrier was prepared by baking out 4 mm sections of pencil lead. The temperature of the inlet section was 220 "C and that of the ion source was 250 "C. The mixture was ionized by electron impact in the energy range from 8 to 20 eV with an electron current held constant at 20 PA. The 13C isotope peaks were added to the isotope-free peaks. Peaks 216 and 217, which become influenced by the isotopic satellite peak 215, were suitably corrected. (Peak 215 originates from the fragmentation of longer alkyl chains.) The relative molar sensitivity was defined as the peak height of the molecule ions plus the isotopic satellites divided by the molar percentage in the calibration mixture, related to pyrene.
RESULTS AND DISCTJSSION Tables I1 and 111 quote the relative (pyrene = 1) molar sensitivities in relationship to the alkyl chain length for various electron impact energies within the range 8-20 eV. In Figure 1,some of these values have been represented graphically. For electron energies below 12 eV, the curves show a maximum, whereas for larger electron energies they show a minimum. This effect can be explained by the superposition of the following factors: (1)As has been demonstrated for benzene (e.g., in ref 5), the probability of ionization increases with increasing alkyl chain length. (2) The degree of fragmentation increases with increasing electron energy. For alkyl groups
0.91 0.90 0.78 0.79 0.74 0.70
15eV
16eV
20eV
0.96 0.69 0.63 0.73 0.66 0.71 0.06
0.92 0.70
0.69 0.46 0.46 0.48 0.46 0.47 0.06
14.0
0.56
0.60 0.57 0.61 0.07 18.2
30.0
with a chain length 2 2 , this usually results in an ion of mass 215, analogue to tropylium. For the mixture investigated, the total percentage of fragmented ions resulting from the decomposition of all larger alkylpyrenes was 0.5% a t ll eV and 30% at 20 eV. (3) A discriminatory effect can take place with the larger alkylpyrenes due to incomplete evaporation. As can be seen by comparing Tables 11and 111,this does not play any role in the case of the glass crucible but must be taken account of when evaporating from the porous sample carrier, In the range between 10 and 14 eV which is usual for low voltage mass spectrometry, the sensitivity is thus seen to be only slightly dependent on the alkyl chain length. Thus it is permissible to assume that the sensitivity values for the homologues present in commercial oil mixtures are the same. However, it can be expected that the degree of alkylation has a strong influence on sensitivity (5, 7). The sensitivities for various basic structures can be classified according to the largest conjugated ring system in the molecule. Related to pyrene, at 10 eV phenanthrene has a relative molar sensitivity of 0.85. Molecules that contain an isolated naphthalene system (e.g., fluoranthene) have a relative sensitivity of 0.69 f 0.08, and those with isolated benzene rings (e.g., 9,lO-dihydrophenanthrene)have a relative sensitivity of 0.43 f 0.11 (5). LITERATURE C I T E D (1) Lurnpkln, H. E. Anal. Chem. 1958,3 0 , 321-325. (2)Crable, G. F.; Kearns, G. L.; Norris, M. S. Anal. Chem. 1960, 32, 13-17. (3) Lumpkln, H. E.; Aczel, T. Anal. Chem. 1964,36, 181-184. (4) Shutz, J. L.; Sharkey, A. G., Jr.; Brown, R. A. Anal. Chem. 1972,4 4 , 1486-1487. (5) Severln, D.; Oelett, H. H.; Bergmann, G. €dol Kohle 1972, 25, 514-521. (6) Swanslnger, J. T.; Dlckson, F. E.;Best, T. E. Anal. Chem. 1974,4 6 , 730-734. (7) Schiller, J. E. Anal. Chem. 1977,4 9 , 1260-1262.
Dieter Severin* Helmut Deymann Otto Glinzer Institut fur Erdolforschung Am Kleinen Felde 30 3000 Hannover, Federal Republic of Germany RECEIVED for review September 9,1981. Accepted December 15, 1981.
