Chemical Ionization Mass Spectrometry of Pristane Emilio Gelpi and J. Orb
Department of Chemistry, University of Houston, Houston, Texas 77004
THEDETERMINATION of isoprenoid structures by electron impact mass spectrometry is usually made difficult by the fact that the fragmentation processes are so extensive that the intensity of the parent ion peak is greatly reduced o r even nondetectable. This is especially the case for pristane ( I , 2), which being a compound of salient biogeochemical significance (3-6) often requires a more reliable technique for its determination. The application of ion-molecule reactions has led to a new mass spectrometric technique which provides a better identification of the molecular mass by means of the quasiparent ions formed (7-9). The comparison between the spectra of pristane obtained by the conventional electron impact ionization and the c h e n ical ionization mass spectrometry demonstrates the usefulness of the latter approach and is reported here for the first time. The experimental procedure is briefly summdrized as follows: A sample of pristane, obtained from Eastman Organic Chemicals was first analyzed by gas chromatography and found to have a purity of about 98.5%. I t was examined using chemical ionization techniques in the Esso Research and Engineering Chemical Physics mass spectrometer (8). Methane, at a pressure of 1 torr in the ion source, was used as reactant gas, and the technique was that described previously (8). The sample was injected directly into the ionization chamber via a gallium inlet system. As seen in Figure 1A and Table I, the molecule of pristane (CIgH4o)
Table I. Mass Spectra of Pristane (C19HIO) Methane reactant PcH&= 1.0torr. Additive volume: 1.78 pl C19Haa
m/e
Ion
69 70 71 72 73 79 83 84 85 86 87 97 99 100 111 112 113 114 125 127 128 139
CaHs+
Rel. intensity= m/e
Ion
Rel. intensity
141 CioHa+ 67.2 142 7.6 153 C..I I..H ~ 1.96 154 3.8 155 C11H23+ 53.7 156 7.6 169 Ci*Hij+ 44.1 170 7.6 182 Ci3H~s+ 7.6c 183 ci3Ha' 59.Y 184 8.6 197 CirH29' 36.4 198 5.7 211 Ci;H31+ 26.8 212 4.8 225 C16Ha3' 2.8 239 C17H3j' 0.9 253 CisHn40. 3e 254 7.6 267 Ci9H3gC IOOe 268 21.1 269 2.8 4 Excluding ions lower than m/e. 69. Alkenyl ions, C~H~,-I+. c Olefin ions C,H2,+: C8H1at, C13H26+, indicate branching. d e
7 6' 1.9 C;Hii' 78 7 - .. 5.7 4.8 1.9 C6Hii4.8' 1.9 CsHi372.0 5.7 1.9 CiHi,+ 5.7' C;Hi;+ 67.2 5.7 CsH1;6.7' CsHiC+ 11.5' CsHii' 97. gd 9.6 C ~ H I ~ ~3.8' CsH,, 86.4 9.6 CiaHig+ 1.9b
Similarities with electron impact mass spectrometry. Major differences with electron impact mass spectrometry.
I
c l
113j shows by electron impact a spectrum the characteristic features of which are the peaks at mje 183 and mje 113, and a parent ion which is difficult to measure, whenever observed, because of its very low relative intensity. By chemical ionization, this difficulty is overcome since the quasi-parent ion (M-1) becomes the highest peak in the spectrum and shows a relative intensity as high as 10% of the total ionization measured as (1) J. G. Bendoraitis, B. L. Brown, and L. D. Hepner, ANAL.CHEM., 34, 49 (1962). (2) B. Hallgreen and S. Larson, Acta. Chem. Scatid., 2, 17 (1963). (3) W. G. Meinschein, E. S. Barghoorn, and J. W. Schopf, Science, 145, 262 (1964). (4) G. Eglinton, P. M . Scott, T. Belsky, A. L. Burlingame, and M. Calvin, Zbid., p. 264. ( 5 ) J. Orb, D. W. Nooner, A . Zlatkis, S. A. Wikstrom, and E. S. Barghoorn, Zbid., 148,77 (1965). (6) M. Blumer, Zbid., 149, 722 (1965). (7) F. H. Field, and M. S. B. Munson. J . Am. Chem. Soc., 87, 3289 (1 965). (8) M. S. B. Munson and F. H. Field, Zbid., 88,2621 (1966). (9) F. H. Field, M. S. B. Munson, and D. A. Becker, "Ion Molecule Reactions in the Gas Phase," Adcan. Chem. Ser., 58, 167 (1966).
388
ANALYTICAL CHEMISTRY
shown in Figure 1B. In addition, the points of branching are still easily distinguishable. The advantage of chemical ionization mass spectrometry over the electron impact method for paraffins is that less carbon-carbon bond fission occurs and a simpler spectrum is formed. In the chemical ionization process the attack of the methane ions on the pristane molecule involves a random electrophilic attack followed by localized reaction. If the attack is on a C-H bond, the M-1 ion results, whereas if it is on a C-C bond, the different alkyl ions are formed. (See references 8 and 9 for fragmentation mechanisms.) The high M-15 peak in the spectrum (m/e 253) indicates methyl branching in the molecule. Peaks at mje 268 and 269 are isotope peaks corresponding to the ion ClSH39+.The peak at m/e M-29 seems to have a relative intensity higher than expected for branched compounds of this type. Its absence should be expected in pristane spectra because no M-29 ion can be formed by single C-C cleavage. The pristane sample analyzed had a maximum of 1.5% impurities present as determined by gas chromatography, which likely accounts for the intensity value of M-29 peak.
30 25 20
$ W
15
8
IO 5
M- I
B
1
-
IO
0,
W
w
Figure 1. Mass spectra of pristane A : Electron impact. Pristane standard: 2 pl injected in a 180-cm X 3-mm column packed with 1% SE-30 on Gas Chrom-P. Mass spec-
trum taken immediately after elution from the column on a gas chromatograph-Atlas CH-4 mass spectrometer combination B: Chemical ionization. ,Same pristane standard : 1.78 pl directly introduced in the Esso Research and Chemical Physics mass spectrometer. (See ref. 8)
Alkenyl ions (C,H2,-I+) and olefin ions (CnHz,+)are also found on the spectra. 'The alkenyl ions are concentrated on the lower end of the chemical ionization spectra while the olefin ions (which are riot found on the spectra of normal paraffins) appear only at masses corresponding to branching points with the exception of methyl side chains. (No olefin ion is found with the M-15 peak.) The mechanism of formation of these olefin ions is not known at present. Since the contribution of methane to the lower masses has not been substracted here, the intensities (Figure 1B) are reported starting at mje 69, where the contribution of the methane ions begins to bc negligible. No particular effort was made to obtain optimum sensitivity on this first experiment, ;although it could have been increased significantly. Recent experiments (IO) indicate that increases of about three orders of magnitude are possible. On the basis of the diita presented here and other current
work (IO), it is hoped that chemical ionization may become an important analytical tool for the determination of the masses of labile molecules and highly branched structures of natural products. ACKNOWLEDGMENT
We thank F. H. Field and D. W. Nooner for their advice and help.
RECEIVED for review September 12, 1966, Accepted January 9, 1967. Work supported in part by NASA grants NsG-257 and NGR-44-005-020. (10) F. H. Field, Esso Research and Engineering Co., Linden, N. J., private communication, 1967.
VOL. 39, NO. 3, MARCH 1967
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