Rearrangement Technique for Analysis of Alkylbenzenes by Mass Spectrometry Unique Heated Inlet System E. W. BOYER, M. C. HAMMING, and H.
T. FORD
Research and Development Department, Continenfal Oil Company, Ponca City, Okla.
b Analysis of detergent alkylate by mass spectrometry is conventionally based upon pattern coefficients obtained by examination of distillation fractions from crude alkylates. A new approach based on quantitative use of rearrangement phenomena and employing sensitivity data independent of volume or pressure measurements is described. This total intensity technique reduces and simplifies computations. Calibration data presented should be directly applicable in other laboratories for molecular distribution analysis on a wide range of monoalkylbenzene systems. A unique high temperature inlet system for sample injection is described.
relative to the C.&, 7 fragment ions (P-1). The rearrangement ions are equal in mass number to the molecular ions of lower homologs (P). Corrections to the polycomponent molecular ion are based on measurements of the parent minus one peak series. Sensitivity data presented are based on total ionization studies of 38 alkylbenzenes of known structure. These compounds were synthesized in this laboratory or obtained from API Research Project 42. Complete spectra for all calibrants employed in this study have been accepted for publication by API Research Project 44 and will be distributed under serial numbers 47-m to 84-m. UPERIMENTAL
A
in the detergent alkylate range are normally accomplished by the use of synthetic cracking patterns. These synthetic patterns are obtained by mass spectrometric examination of narrow fractions separated from crude alkylate by high-efficiency distillation. Analytical techniques using this approach have been described by Brown et al. (4). Modifications of this basic technique have been utilized in many laboratories concerned with the characterization of detergent alkylates by mass spectrometry. The basic concept in calculation by the synthetic cracking pattern method involves using mixed alkylbenzene isomers-of essentially the same molecular weight-to represent the fragmentation patterns of mixed isomers in a sample alkylate. Although the technique is entirely satisfactory for determining molecular distribution of alkylate samples similar in chemical history to that from which the calibration fractions were separated, serious limitations are apparent when attempts are made to apply these patterns to alkylbenzenes of dissimilar structure. The work presented here describes a technique which, to a large extent, overcomes these limitations to conventional analytical methods. Analysis is based upon a sufficient consistency of the rearrangement ions (m/e 162 and above) NALYSES OF ALKYLBENZENES
1168
0
ANALYTICAL CHEMISTRY
All spectra considered in this work were obtained with a CEC-21-103C mass spectrometer equipped with a 40liter getter-ion exhaust vacuum system (3). Samples were admitted to the instrument through a gallium orifice inlet system operated at 350' C. Spectra were recorded on a Consolidated Electrodynamics Corporation Mascot mass spectrum digitizer. Ionizing voltage was 70 e.v.; for most samples, 12 pa. ionizing current was employed. For samples containing components above Cas, 80 pa. were used. The isatron was equipped with a rhenium
,
Air inlet
Dla p hragrn
Bellows
Gaskets Fluorslnt gasket Outtet
Figure 1.
Inlet
Modified air valve
filament, and the source was controlled a t 230' C. The instrument was operated a t high sensitivity and in the focused mode. Scan rate and magnet current were adjusted to give a peak repetition rate of 1 peak per second. Sensitivity for n-hexadecane was 1150 divisions per lambda per microampere. INLET SYSTEM
Several high temperature inlet systems have been described in the literature (2, 4, 6, 9, 17). Efforts to utilize these inlet systems demonstrated a t least one serious shortcoming in each. In an effort to overcome these failures, the inlet system currently used in this laboratory was designed. Unique features of the high temperature inlet system are shown in Figures 1 and 2. Figure 1 shows the air-operated bellowstype valve used on the leak line and on the pump-out line from the expansion volume. Operation of the valve will be apparent from examination of the drawing. The valve, located in the line between the expansion volume and the glass leak, is ceramic clad on all inside parts below the bellows assembly. This reduces significantly the contact of vaporized sample with high-temperature metal surfaces. Figure 2 shows the sample injection device and incorporates several advantages over any orifice system known to the authors. Flow of gallium through orifice I is controlled by proper manipulation of stopcock B. Under normal conditions, stopcock B is turned such that reservoir E is connected to vacuum source D, thereby decreasing the pressure drop across orifice I and preventing the gallium from flowing into reservoir K . A sample is injected by turning stopcock B to vent reservoir E through capillary G. Ball joint H is then removed, and the increased pressure drop causes gallium from reservoir E to flow into reservoir K . The sample is injected during this period in the conventional manner using a standard micropipet. The sample admitted is vaporized in reservoir K and flashed through line F into expansion volume C. Gallium in excess of the amount required to form a seal in the bubble cap ilf flows under the bubble cap, through stopcock A , and into catch flask P. Gallium accumulated in catch flask P is returned to reservoir E by proper manipulation of
the vacuum system thmough the use of stopcocks A , &, and B and through socket joint L. Twenty-five cubic centimeters of gallium are sufficient to operate this system on a routine daily basis. Difficulties normally experienced with orifice injection systems when the various oxides plug orifice I are essentially eliminated with this device. Exposure of gallium SUI face to oxidizing atmosphere a t J is reduced by maintaining this reservoir a t very low pressures. No orifice plug is required in this system, thereby eliminating the problems iiiherent to oside Iluild-up on this plug when it is r1:moved from the hot gallium and exporsed to oxidizing atmosphere. Vacuum source D is obtained by attachment to the mechanical forepump in the cowentional inlet system. This entire system from the heated cover plate to the pump-out valve and including reservoir E tmd reservoir K is maintained a t 350' C. Either pure gallium or gallium-indium-tin eutectic can be used as the sealrtnt in this reservoir assembly. Liquid tin cannot be used, since this would require stopcock A t o be heated. Vapo- barrier seal ilf separates the high temperature zone K from the room temperalure stopcock A. An alternative mode of operation is to have stopcock A in the normally closed position, allowing excess gallium to collect over this stopcock, thereby reducing the pressure drop ac-oss stopcock A. GENERAL THEORY
Xlkylbenzenes cannot form-by simple bond cleavage under electron bombardment-fragment ions equal in mass to molecular ions of lower homologs. Theoretical considerati ins supporting the major premises of this analytical method can be simply stated. Molecular ions or pseudo molecular ions of alkylbenzenes in a mixture spectrum can occur by four processes : by a molecular species actually present in the mixture; by concerted bond cleavage and rearrangement; by contributions from heavy isotopes; and by intermolecular processes. Considering these four possibilities for the formaticm of an alkylbenzene molecular ion, it is possible to eliminate intermolecular processes, since contributions from such processes are negligible a t the preesurcs normally em-
Figure 2. Sample injection device utilizing pressure drop orifice and bubble cap gallium seals
2
ployed in analytical mass spectrometers (1, 6). Effects of contributions from heavy isotopes can be corrected by the use of standard tables of natural abundance such as that of McAdams (10). This leaves only two possibilities for the formation of apparent molecular ions in a mixture spectrum. Since the intensity of the true molecular ions in the mixture is de.iired, it is necessary only to correct this peak for contributions through rearrangement processes. Figure 3 shows the schematic arrangement of beta bond cleavage of the alkylbenzene under electron bombardment. It is important to emphasize that the ion structures shown are symbolic and are not intended to represent the true configuration. Skeletal rearrangements of the phenyl moiety of the molecule may take place. Evidence has been presented by Meyerson and his coworkers (14-16, 19) and others (7) that the C,H,+ ion is the symmetrical seven-
C O N S I S T E N C Y O F T H E 190/189 RATIO AFTER -
MOL.
MOLECULAR STRUCTURES
246
C7 - C - C q
WT PARENT
MOLECULE
PSEJDO
membered tropylium ion. While beta bond cleavage is not the only fragmentation process taking place, the pseudo molecular ions are believed to arise mainly from this type of rearrangement. Mechanism 1 in Figure 3, showing the formation of an ion one mass unit less than a molecular ion, results from the true heterolytic cleavage of the beta bond. Formation of a lower molecular weight parent ion (mechanism 2) demonstrates a typical beta bond cleavage (8, 19) with simultaneous proton shift. These two processes are mutually dependent on the higher molecular species in a manner not yet completely understood. General considerations and mechanisms of rearrangements under electron bombardment have been adequately described by McLaEerty (11, 12). Studies in this laboratory under different pressures have shown that P is not second order dependent on P- 1 by the simple mechanism (P- 1)+ H P. P- 1 is not dependent on P as can be shown by examination of alkylbenzene spectra. If P-1 were dependent on P, the mass spectrum of any alkylbenzene of molecular weight above 162 would show a significant peak at one mass unit below the molecular ion. Low voltage appearance potential studies of these two ions would elucidate the mechanism of their mutual dependence on the originating molecular species. Data presented later will show a remarkable resemblance in the relative abundances of these two ions formed from several different molecular species of monoalkyl substituted benzenes. The basic premise in this work will support the theory that the contribution from any molecular species to a molecular ion of a lower homolog, P, can be calculated from the peak intensity a t P-1. From this consideration and data presented in subsequent tables, it will be shown that the following equation is valid.
BOND CLEAVAGE ~-
'1P-I
RATIOS
0.181
STRUCTURES OF FRAGMENT. IONS
H@
0
/,
\
Figure 3.
P-
0 193
0197
P-I
--
2.
0 E'
H
t R'I
Formation of fragment ions from parent molecule
302
CT-C-CB
/
m / e 190
m/e 189
\,
'\
0 195
Figure 4. Consistency of the 190/189 ratio after beta bond cleavage VOL 35, NO. 9, AUGUST 1963
1169
Di-
Mi=
N ~
i
i =~
Dij
- hi-1 (Di-1)
(1)
j=i
Mi. = Monocomponent peak, M, at m/e i Di = Divisions of peak at m/e i aij = Calibration coefficient, a, at m/e = i, for component j zj = Divisions of base peak, x, of component, j, appearing in mixture spectrum k+l = Correction factor, IC, at m/e i-1 Di-l = Divisions of peak at mje i-1 Intensity factors (I,) are reciprocals of total intensity sensitivities relative to normal hexadecane calculated by the following formula.
where
m/e 226 = Peak height in divisions at m/e 226 for nC1c Z nCls __ = Total ionization of nCle (25 to 228) = Peak height in divisions m/e A at molecular ion for compound A = Total ionization of comZ A pound A (25 t o molecular ion +2)
Table 1. Summary of Data for the Calculation of Alkylbenzenes By the Rearrangement Technique
I t , intensity factor (reciprocal Carbon of total no. of [ M i = Di intensity sensitivity) molecule ki-l (Ll)] 12 0.1629 0.1505 13 0.1795 0.1584 14 0.1944 0.1664 15 0.2055 0.2186 16 0.2058 0.2708 17 0.2100 0.3144 18 0.2191 0.3581 0.3778 0 12262 19 0.2318 20 21 0.2387 22 0.2498 0.2545 23 0.2656 24 0.4869 0.2767 2.5 0.4947 0.2878 26 0.2989 27 0.3100 28 0,3212 29 0.3322 30 0.3434 31 0,3545 32 0.3656 33 0.3767 34 0.3879 35
k, correction factor
~~
36 ..
0.3989
37
0.4101 0,4212 0,4323 0.4434
38 39 40
0,4545 0.4769
41
42
0; 5883 0.5961 0.6039 0.6117 0.6195 0.6273
-1 170
0
ANALYTICAL CHEMISTRY
The products of M , and the corresponding 1, yield approximately the relative partial volume fractions (18) which are subsequently normalized to total alkylbenzene content, independently determined by type analysis, low voltage, or total intensity technique. Relative sensitivities are to a n extent dependent upon the method of reducing the polycomponent peak to a monocomponent peak. The correction factors ( k ) and the intensity factors (I,) from Table I are dependent variable3 and should not be used separately. RESULTS AND DISCUSSION
Figure 4 shows the data obtained by examining fragment ion intensities from beta bond cleavage of four alkylbenzenes of different molecular weights, each having a C7 branch. The two ions formed corresponding to P and P-1 are mje 190 and mje 189, respectively. The ratio P/P-1 is usably consistent. A similar study of 38 alkylbenzene standards provided additional data for calculating the contribution to P as a function of P-1, regardless of the precursor to P-1. A summary of data for the calculation of alkylbenzenes by the rearrangement technique is given in Table I. Correction factors for rearrangement ions vary inversely with the molecular weight (above mass 190) and become quite small at higher masses, while the isotopic contributions become larger with increasing molecular weight. To simplify calculations, these two contributions are combined in the final correction factor, k ; therefore only one mathematical operation is necessary, based on the P - 1 peak, to calculate the contribution from any number of higher homologs to the ion intensity at mass P. Efforts t o evaluate the absolute accuracy of this method face the same dilemma as efforts to evaluate accuracy of the synthetic cracking pattern technique. Because of the complexity of the mixture, a typical detergent alkylate cannot be blended from known compounds. Previous analytical methods were not amenable to component analysis of simple mixtures of single isomers; however the present technique is applicable to most alkylbenzene mixtures above mass 162. To demonstrate this. a blend of seren different alkylbenzenes of known structure was made and analyzed using the figures from Table I. Blend values and analytical results are shown in Table 11. The relative error of 3.4% is not outstanding compared with results expected from component analysis; however, when compared with results obtained from typical detergent alkylate pattern coefficients (Table 11), the accuracy and extended application of the rearrangement technique can be appreciated. Results from analysis of a typical dodecrlbenzene mixture by this tech-
nique, by our synthetic cracking pattern method, and by the only previously published method by Brown e.! al. (4) are shown in Table 111. It should Le emphasized that synthetic pattern data were obtained by calibration on fractions separated from a mixture very similar to the dodecylbenzene analyzed here. The rearrangement method is based entirely upon studies of pure alkylbenzenes, different in structure from those typically present in the dodecyl-type alkylate produced by phenylating polypropylenes. Even so, the analytical accuracy is completely acceptable when compared with results obtained by the other methods. Average molecular weights, calculated from the distribution analysis, compare favorably with those obtained on the Mechrolab Osmometer showing a n average difference of 5 atomic mass units. This difference may result from the fact that the osmometer is measuring the composite molecular weight of the mixture, while the mass spectrometer values represent the average molecular ileight of the alkylbenzenes only; and the sample is known to contain small amounts of lower molecular weight material.
~~
~
Table
II.
Analysis of Synthetic Mixture"
Calcd. bySPY thetic crackRear- ing range- patment tern meth- methCompounds blended Blend od od 1-Phenyloctane 5 . 2 4 . 9 1.9 5-Phenylde cane 17.0 17.6 27.1 3-Phenyldodecane 44.5 43.2 41.7 7-Phenyltetradecane 13.8 13.8 12.2 6-Phenylpentadecane 10.0 10.7 8.4 8-Phenylhexadecane 6 . 0 6.3 4.8 3.5 3 . 5 3 . 9 2-Phenyloctadecane Liquid volume per cent. Q
Table 111.
Analysis of Dodecylbenzene
Carbon no. of alkyl chain 8 9
10 11 12 13 14 15 16
17 hverage molec-
Synthetic RePubcracking arrange- lished pattern ment method method method (4) 1.6
9.6
28.0 47.2 8.7 2.6 1.4 0.6 0.3
0.3 0.8
8.7
27.9 47.8 9.4
1.1
7.6 25.8
49.5
2.6
10.2 2.9
0.8 0.4
0.8
1.3
1.4
0.5
ular weight 242 242 243 Average molecular weight by the Mechrolab Osmometer, 237.
Table IV.
Analysis of Heavy Alkylate and Distillation Cuts
s o . of
Mol.
carbon atoms
302
22 23 ~24 25 26 27 28
wt.
216 _-_
330 344 358 372 386 400 414 428 442 456 470 484 498 512 526 540 554 568 582
29 30 31 32 33 34 35 36 37 38 39 40 41 42
Cut 1
Cut 2
3.1 6.8 16.8 24.9 21.2 13.9 8.5 3.6 1.1
1.2 1.5
. . . . .
. . . . .
. .
...
. . ...
... ... ...
3.0 9.8 17.0 19.2 23.0 17.9 6.4 0.9
... ,..
...
... ... ... ... ... ... ... .*.
Cut3
,..
Cut4 .__
1.5 1.7 4.2 9.5 13.7 24.0 27.4 15.5 2.4
1.9 2.6 6.5 9.9 22.8 31.3 20.9 4.1
...
... ...
... ... ... ...
... ...
*.. .,.
...
...
... ...
... ...
...
...
... ... ...
Cut 5 ...
1.6 1.8 2.9 6.0 16.1 31.5 31.4 6.5 2.3
...
...
... ...
... ... ... ... ...
Per cent of cut 9.1 9.2 9.2 9.1 9.4 Average molecular weight by mass spectrometry Average molecular weight by Mechrolab Osmometer, 396. A more stringent te:k of the applicability of this new method to complex mixture analyses has been made. A heavy alkylate, C22 to (342, was fractionated into nine approximately 10% cuts and a bottoms portion. Each fraction was analyzed, and data obtained are shown in Table X. These data were calculated to original sample composition with results shown under composite analyses. Constant 5% values shown from through C42 are believed to be real in the bottoms fraction but are not considered components of the original sample. Long exposure to elevated temperaturvs during distillation could explain 1 heir formation. Distribution data obtained by the new method are shown in the last column of Table IV. Although absolute agreement is not extremely flattering, when the mass range of the sample and the complexity of such mixtures are considered, these results are entirely acceptable. The average molecular weight, 399, calculated from distribution data, agrees well with the osmometer molecular weight of 396. Agreement between molecular weights obtained by these methods is not offered as proof of accuracy but servls only as circumstantial evidence for reasonable accuracy and consistency of the analytical method. ,4ny component in the mixture, other than alkylbenzenes, which contributes to the P or P-1 peak intensities will interfere with accurate analyses. Generally, such interference can be expected from indanols, condensed tetracyclonaphthenes, certain sulfur-containing compounds, many halogenated compounds, etc. The analytical method is intended for application to detergent alkylate systemswhere typicalaromatic content is
Cut 6
cut 7
Cut 8
... ... ... ...
... ...
...
1.7 3.4 12.8 31.6 39.7 8.9 2.0
...
... ...
... ...
1.1 1.7 6.9 25.9 45.6 13.7 3.8 1.1
...
...
...
... ...
... ... ... ... ...
... ... ... ...
0.9 3.4 17.4 47.6 19.5 7.6 2.9 0.7
... ... ...
...
...
... *.. ... ...
... ... ... ...
9.1
9.3
8.9
...
approximately 99% with a small saturate or cycloparaffin content. The method has been applied successfully to complex systems containing as little as 10% alkylbenzenes in a mixture composed primarily of saturates and olefins. Generally, the method is applicable to any system for which the synthetic cracking pattern method was useful and is far less sensitive to changes in branching of the alkyl substituent. This analytical technique has been in use in this laboratory for the past year. It has been used successfully on detergent alkylate mixtures ranging from Cg through The data presented in Table I should be usable for analyses of similar mixtures from any mixture spectrum obtained on a CEC-103 or comparable mass spectrometer. Recent work, which will be the subject of later papers, has demonstrated a similar applicability to dialkylbenzenes. ACKNOWLEDGMENT
The authors thank Alex Shadan for synthesizing many of the standards used in this work; also we thank Joseph A. Dixon for standards provided from API Research Project 42. Instrumental data were obtained by C. E‘. Maddox, E. E. McKelvey, and W. K. Moore. Mrs. Si7. M. Wright contributed substantially by doing much of the detailed computation and organization of the data presented. LITERATURE CITED
(1) Beynon, J. H., “Mass Spectrometry
and Its Application to Organic Chemistry,” p. 275, Elsevier, Amsterdam, 1960.
Cut 9 Bottoms
...
...
...
... ... ...
... ...
... ... ...
...
1.3 1.5 7.2 37.6 24.5 14.7 9.0 3.1 1.0
...
... ... ...
... ... ...
9.3
3.4 5.2 7.2 9.3 12.4 9.6 8.9 8.2 7.3 6.1 5.6 5.5 5.5 5.7 17.4
Corn osite anafysis
Direct ana1ysiY
0.4 0.9 2.3 4.0 5.5 6.4 10.9 18.4 23.5 8.6 4.4 3.4 2.0 1.6 1.4 1.3 1.1 1.0 1.0 1.0 1.0
1.1 2.6 3.3 5.2 6.6 7.4 12.1 19.5 22.8 7.6 3.5 2.5 1.4 1.1 0.9 0.8 0.6 0.5 0.4
411
...
...
399
(2) Beynon, J. H., Kicholson, G. R., J. Sci. Inst?. 33,376 (1956). (3) Boyer, E. W.,Users’ Clinic, Con-
solidated Electrodynamics Corporation, E-14 Meeting on Mass Spectrometry, Atlantic City, N. J., 1960. (4) Brown, R. A,, Skahan, D. J., Cirillo, V. A., Melpolder, F. XT., ANAL. CHEM. 31, 1531 (1959).
(5) Caldecourt, V. J., ZDid., 27, 1670 (1955). (6) Field, F. H., Franklin, J. L., “Electron
Impact Phenomena,” p. 188, Academic Press, New York, 1957. (7) Foster, N. G., Hirsch, D. E., Kendall, R. F., Eccleston, B. H., Ward, C. C., Am. SOC.Testing Materials E-14 Meeting on Mass Spectrometry, Atlantic City, New Jersey, 1960. (8) Kinney, I. W., Jr., Cook, G. L., ANAL.CHEM.24,1991 (1952). (9) Lumpkin, H. E., Johnson, B. H., Ibid., 26, 1719 (1954). (10) McAdams, R. D., “Isotope Correction Factors for Mass Spectra of Petroleum Fractions,” Esso Research Laboratories, Baton Rouge, La., 1957. (11) McLafferty, F. W., ANAL.CHEM.31,
82 (1959). (12) McLafferty, F. W., “Mass Spectrometry,” in “Determination of Organic
Structures by Physical Methods,” supplementary ed., F. C. Nachod, W. D. Phillips, eds., p. 131, Academic Press, Sew York, 1961. (13) Meyerson, S., Appl. Spectry. 9, 120 (1955).
Meyerson, S., Rylander, P. N.,
Chem. Phys. 27,901 (1957). Meperson, S., Rplander, Phys. Chem. 62,2 (1958).
P. K.,
Meymon, S., Rylander, P. N., Eliel, L.. ZlcCollum. J. D.. J . Am. Chem.
SOC.81,2606 (1959). ’ (17) O’Neal, M. J., Jr., Wier, T. P., Jr., ANAL.CHEM.23,830 (1951). (18) Otvos, J. W., Stevenson, D. P., J . Am. Chem. SOC.78,546 (1956). (19) Rylander, P. N., Illeyerson, S., Grubb, H. M., Ibid.,79,842 (1957).
RECEIVHD for review February 20, 1963. Accepted May 2, 1963. Presented in part a t the E-14 Committee on Mass Spectrometry, A.S.T.M., New Orleans, La., June 1962.
VOL. 35, NO. 9, AUGUST 1963
0
1171
