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Anal. Chem. lW1, 53,34-37
Location of Double Bonds by Chemical Ionization Mass Spectrometry Rosalind Chal and A. G. Harrison' Department of Chemistry, Universiry of Toronto, Toronto, Ontario, Canada M5S 1A 1
The use of vinyl methyl ether in admlxture with N2or Nz/CS2 In locatlng double bonds In olefinic compounds through characteristic fragmentatkn of the four centered complex(es) f m d between the unsaturated ether and the oieflnic compound Is explored. The use of the threecomponent reagent gas system 75% Nz-20% CS2-5% vinyl methyl ether ( W E ) Is shown to give condiderabiy slmpier spectra permitting an easler Identification of the Ions characteristic of double bond kcation as well as g w abundant molecular kns esiaMlshtng the molecular weight. The extent of the cycloaddition-fragmentation sequence is shown to be greater for tennlnai olefins than for internal olefins and to be further retarded by additlonal alkyl substRuth at the double bond. Thus, the method cannot serve to ldentlfy the posltlon of the double bond In Internal olefins substituted at the double bond. The threecomponent system gives chemical ionlzation mass spectra whkh clearly identity the double bond location In unsaturated fatty acid methyl esters.
The electron impact mass spectra of isomeric monoolefins frequently are very similar (I) with the result that it is difficult to locate the position of the double bond with certainty. Similarly, proton transfer chemical ionization mass spectrometry, in most cases, gives mass spectra which do not readily provide information on double bond location (2). Consequently,most electron impact methods (3)and chemical ionization methods (4) for double bond location have utilized prior derivitization. Recently, Jennings et al. (5, 6), in two communications, have reported the use of vinyl methyl ether as a reagent gas in high-pressure mass spectrometry for location of carbon-carbon double bonds. The principle involved is illustrated in Figure 1. Four-centered addition complexes, a and b, are formed by attack of either olefinic molecular ion on the second neutral olefin; these fragment, in part, to characteristic products c and d, the m/z ratios of which serve to characterize the size of R1and & and hence the position of the double bond. In the work reported by Jennings et al. ( 6 6 ) either pure vinyl methyl ether (VME) or C02-VME mixtures were used as chemical ionization reagent gases. In the former system significant yields of high mass ions, such as dimers, proton bound dimers, and higher polymeric ions, complicated the spectra. The spectra are simplified by use of a C02-VME mixture with C02 in excess, although, even in this system, charge transfer from C02+. to VME lowers the yield of the VME molecular ion by formation of fragment ions, while, in mixtures with olefins, charge transfer from C02+. to the olefin produces extensive fragmentation of the olefin which also complicates the spectra obtained. Recent work in this laboratory (7,8)and elsewhere (9,10) has shown that a N2/CS2 mixture (with N2 in excess) is a useful low-energy charge-transfer system giving CS2+. (recombination energy = 10 eV (11))as the major reactant ion. Charge transfer from CS2+.to olefins normally gives a simple 0003-2700/81/0353-0034$01.00/0
spectrum, frequently with the molecular ion, M+., of the olefin as the base peak. In addition, we have observed that the addition of CS2 to an N2/VME mixture simplifies the spectrum and gives a higher yield of the VME molecular ion. Thus, in a 10% VME-90% N2 mixture C3H60+-( W E + . ) is the base peak but peaks for C2H3O+ and C3H70+are observed with intensities 28% and 92% of the base peak with minor peaks at m/z 71,75, and 88. By contrast, in a 5% -20% CS2-75% N2 mixture, C3H60+-is the base peak with C2H30+ and C3H70' being only 6% and lo%, respectively, of the base peak;other peaks observed are S2+(8%)and CS2+.(41%) from the CS2 reagent gas. It appeared that the use of this threecomponent reagent gas system might give simpler CI mass spectra with olefins, thus making easier the identification of the ion signals indicative of double bond location. The p r w n t work explores, first, the advantages of the three-component reagent gas mixture and, second, the general usefulness of the gas mixtures containing VME as reagents for carbon-aubon double bond location in olefins and unsaturated fatty acid methyl esters.
EXPERIMENTAL SECTION Chemical ionization mass spectra were obtained by using a DuPont 21-490 mass spectrometer equipped with a high-pressure source. Spectra were obtained at 70 eV ionizing electron energy and source temperatures of -170 "C with liquid samples being introduced from a heated inlet system held at a temperature of 110 *C and solid samples being introduced by a direct insertion probe. Total source pressures of reagent gases were -0.3 torr. In the two-componentN 2 / W and N2/CS2reagent gas systems, optimum results were obtained with the VME or CS2 reagent gas systems comprising 10% of the total gas pressure. In the threecomponent N2/CS2/VMEsystem variation of the composition showed that 75% N2-20% C+5% VME gave the optimum additive ion signals. All compounds used were commercially available except methyl-d3nonanoate which was prepared by esterification of nonanoic acid with methyl alcohol-d, by standard procedure.
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RESULTS AND DISCUSSION Comparison of N2/VME and N2/CS2/VME as Reagent Gases. The relative merits of the two-component vs. the three-component reagent gas system were explored by using the isomeric n-decenes as test olefins. The recombination energy of N2+.is -15.3 eV (11);as a consequence the energy imparted to the decene molecular ions in the charge exchange is sufficient to result in extensive fragmentation. The Nz+. charge exchange mass spectra show only low-intensity (8-1570 of base peak) M+. ion intensities, the major ion signals observed for all isomers corresponding to C3H5+(m/z 41), C3H7+ (mlz 43), C4H7+(mlz 55), and C a H (mlz , 69), with minor ion signals corresponding to fragment ions containing six, seven, and eight carbons. By contrast, charge exchange from CS2+. in the CS2/N2mixture leads to M+-(Cld-IH,+.;m/z 140) as the base peak with low-intensity fragment ions (
