846 Analyst, September, 1979, Vol. 104, pp. 846-852 Combined Gas Chromatography - Chemical= ionisation Mass Spectrometry of Some Phthalate Esters J. B. Addison Atlantic Regional Laboratory, National Research Couvzsil of Canada, 141 1 Oxford Street, Halifax, N.S., B3H 321, Canada A sample mixture of seven phthalate esters was analysed using combined gas chromatography - chemical-ionisation mass spectrometry (GC - CIMS) . Separation of the various components of the mixture was effected by gas chromatography and subsequent identification by chemical-ionisation mass spectrometry using methane and isobutane as reagent gases. Comparison of electron-impact (EI) spectra and the CI spectra was under- taken in order to illustrate the advantage of CIMS over EIMS in identifying these phthalate esters.CIMS with isobutane gives abundant quasi-molecular ions, [M + HI+, with no adduct ions formed, in contrast to CIMS with methane, and thus provides a means of distinguishing between isomeric phthalate esters. Keywovds : Phthalate estev analysis ; gas chvovnatogvaphy - chemical-ionisation mass spectvometvy No ionisation technique since electron impact (EI) has had as great an acceptance and use in mass spectrometry as chemical ionisation (CI) and the quadrupole mass spectrometer was the first instrument reported that utilises this technique for gas chromatography - mass spectrometry (GC - MS) work.1,2 The use of GC - EIMS for the separation and elucidation of the structure of phthalate esters has been r e p ~ r t e d , ~ and recent work has shown that these compounds are found in the envir~nment.~-~ The use of CIMS is meanwhile being increasingly applied as an aid to structural elucidation by virtue of the prominence of quasi-molecular ions, [M + HI+, and simple fragmentation patterns in the resulting Most of the CI work carried out so far has employed the direct insertion probe technique in identifying phthalate esters.1° This paper presents results obtained by the use of CIMS coupled with GC to obtain representa- tive mass spectra including quasi-molecular ions, [PI + HI+, and other pertinent mass fragments at source temperatures and reagent gas pressures that unequivocally identify the components of typical phthalate ester sample mixtures, and the technique could be used on components of a mixture of phthalate esters separated by gel filtration from lipids or bio- logical and environmental materiakll Experimental All phthalate esters were obtained from Analabs (North Haven, Conn., USA) with a commercial purity of over 98%.Distilled diethyl ether was used to dissolve the esters for GC - CIMS analysis. A mixture of seven phthalate esters (total concentration 100 ng p1-l) was analysed on a Finnigan, Model 4023, system, which includes a Finnigan, Model 4000, quadrupole GC - MS system equipped with dual EI and CI ion sources and a Finnigan Incos data system. Chromatographic separations were achieved using a 1.8-m glass column packed with 3% OV-1 on Chromosorb Q (100-120 mesh) interfaced to the mass spectro- meter via an all-glass jet separator maintained at 280 "C.The reagent gas pressure was regulated at 0.35-0.45 Torr and the injection port temperature was maintained at 220 "C. The temperature programming parameters were as follows : initial temperature, 150 "C for Zmin, then programmed to a final temperature of 240 "C at the rate of 5 "Cmin-l; the final hold time was 5 min and the helium carrier gas flow-rate was 30 ml rnin-l. Methane and isobutane reagent gases were introduced into the mass spectrometer source as make-up gases through a solenoid valve; methane could be used as a carrier gas.ADDISON (a 1 x2 - QI:::::: 847 I I m k Fig. 2. Mass spectra of dimethyl phthalate: (a) EI; (b) CI with methane; and (c) CI with isobutane.848 ADDISON : COMBINED GAS CHROMATOGRAPHY - CHEMICAL- Analyst, VOZ. 104 In the identification of phthalate esters in the environment, the primary interest is to determine the relative molecular mass of the compounds in a sample mixture.Conventional GC - EIMS of these compounds gave weak molecular ions, or no molecular ions with the higher homologues, thus making the specific identification of these phthalate esters from their EI spectra difficult or impossible. Figs. 2-8 clearly show the advantage of GC - CIMS over GC - EIMS for the analysis of these compounds. The former technique provides excellent confirmation of the relative molecular masses. The CI spectra indicate quasi-molecular ions corresponding to the various esters. This result was expected, as the direct insertion probe work by Fales et aZ.10 has shown that at a source temperature of 150 "C and high reagent gas pressures of about 0.8- 1.0 Torr, strong quasi-molecular ions, [M + HI+, were produced with little breakdown of these ions.CI spectra obtained with methane show quasi-molecular ions, [M + HI+, together with adduct ions that sometimes correspond to relative molecular masses of higher homologues, which might therefore interfere with the identification of chromatographically separated components of a sample mixture of phthalates. For example, butyl phthalate produced significant fragment ions at m/e 279 [M + HI+ and 307 [M + 27]+, with the latter adduct corresponding to the quasi-molecular ion of dipentyl phthalate. 50 0 x2 13 a, [M+H] + I . 1 1 . 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 /x2 - (C) 1 [M+H]+ I I I I I I Fig. 3. Mass spectra of diethyl phthalate: (a) EI; (b) CI with methane; and (c) CI with isobutane.CI with isobutane, however, produced far fewer peaks and more intense protonated mole- cular ions without interfering adduct ions, thus making CI spectra obtained with isobutane much easier to interpret than the corresponding CI spectra obtained with methane, and with sufficient information for the identification of these phthalate esters. The conditions under which these CI spectra were collected suggest an advantage in using isobutane as reagent gas in phthalate ester analysis; this observation has also been made in direct insertion probe CI work. Isobutane provides reactant ions of lower energy upon ionisation and therefore exhibits lower exothermic ion - molecule reactions, with the result that fewer fragment ions are produced, which thereby enhances the abundances of ions in the relative molecular mass region concerned.September, 1979 IONISATION MASS SPECTROMETRY OF SOME PHTHALATE ESTERS 50 0 - 849 (4 x2 - [M+Hl+ I ‘ I 1 - I ’ .r - l I I I ‘ # [ I I I I , 1 I - (a 1 - x2 COOC4 Hg COOC4 Hg (6) x2 I . - I. 1 . ,.& , I , ,_ ,\ , . , [M+H] + I I , l mle Fig. 5. Mass spectra of dibutyl phthalate: (a) EI; (b) CI with methane; and (c) CI with isobutane.850 50 . (C) ADDISON : COMBINED GAS CHROMATOGRAPHY - CHEMICAL- Analyst, VoZ. 104 x2 [M+Hl* (6) x2 [ M+H] + I ) . lI . , I ; I . . I .l I . , . , I. . . , 1 ,L . . 0 I I l , ' I . , I 1 1 1 1 40 100 200 mle 300 400 Fig. 7. Mass spectra of di(2-ethylhexyl) phthalate: (a) E I ; (b) CI with methane; and (c) CI with isobutane.September, 1979 85 1 CI spectra obtained with methane and isobutane for dimethyl phthalate and diethyl phthalate show that the most abundant fragments are m/e 163 and 177, corresponding to the loss of OCH, and OC,H,, respectively.The fragment m/e 149 was the most abundant ion in the CI spectra of the other phthalate esters, except for the CI of dioctyl phthalate with isobutane, which gives the quasi-molecular ion [M + HI+ as the most abundant ion. Molecular adducts formed in CI with methane are mainly of the form [M + C,H,]+, [M + C,H,]+ and [M + HI+ as expected when methane is confined to the source chamber of the mass spectrometer at a high pressure.12 Although the adducts formed could interfere with interpretation, as explained above, they could be used as a parity check on particular quasi-molecular ions.IONISATION MASS SPECTROMETRY OF SOME PHTHALATE ESTERS 50 0 40 100 200 mle 300 400 Fig. 8. Mass spectra of dioctyl phthalate: (a) EI; (b) CI with methane; and (c) CI with isobutane. CI of dioctyl phthalate with methane illustrates the case where the spectrum of a com- pound exhibits fragmentation that does not follow the simple scheme of functional group elimination. In contrast to CI with methane, the CI spectrum of dioctyl phthalate with isobutane shows an abundance of quasi-molecular ions and no interference from adducts, therefore making it easy to detect and clarify the presence of ions in the molecular region even with the high GC and manifold temperature requirements.Hence the isomeric dioctyl phthalate and di(2-ethylhexyl) phthalate could be distinguished on the basis of their CI mass spectra obtained with isobutane. Direct insertion probe CI of di(2-ethylhexyl) phthalate at a source temperature of 150 "c and a pressure of about 1 Torr is reportedlo to give strong [M + HI+ ions with both methane and isobutane reagent gases, but no adducts were observed for this GC-CIMS study at 270 "C and reagent gas pressures of 0.35-0.45 Torr. This observation seems to suggest that the formation of adduct ions is temperature dependent at a constant reagent gas pressure. Also, the above low temperature - high reagent pressure probe CI conditioii gave a quasi-molecular ion with 100% relative abundance for di(2-ethylhexyl) phthalate. The GC - CIMS work gave a weak protonated molecular ion and could be due to the high source temperature used for this work.An increase in temperature is known to cause a decrease in CI [M + H]+ ion intensity13; hence, considering that all conditions are satisfactory for chromatographic separations, the temperature requirements for separation dictate the852 ADDISON operating temperatures of the interface and manifold of the mass spectrometer. Although protonation of dicarboxylic acids could be achieved under mild probe CI conditions, the results presented show that protonation could also occur with stable species being formed at high GC - CIMS temperature condtions. Under these conditions, isobutane is highly recommended as the reagent gas for phthalate esters.TABLE I RETENTION TIMES AND RELATIVE ABUNDANCES OF QUASI-MOLECULAR IONS Compound Dimethyl phthalate Diethyl phthalate Diisobutyl phthalate Dibutyl phthalate Dipentvl phthalate .. .. . . . . . . Relative abundance, % r A \ Retention time/ [M + HI+, CI CI . . 0.6 195 3.8 33.2 . . 2.3 223 6.8 37.9 . . 5.0 279 7.8 29.3 . . 6.0 279 8.3 22.2 .. 7.9 307 9.4 18.3 min m/e with methane with isobutane Dii2-ethyihexyl) phthalate , . 11.9 391 6.5 12.9 Dioctyl phthalate . . .. 15.5 391 4.7 100 Table I gives the relative intensities of the quasi-molecular ions, [M + HI+. Other fragment ions are formed from the breakdown of [M + HI+, as reported by workers using the magnetic probe CI technique.13 Elimination of water caused by bifunctional inter- action in protonated dicarboxylic acids1* was not observed in this work; the reason might be that the sample pressures were kept below the normal CI running conditions, and there- fore no protonated dimers were formed.I am grateful to the National Research Council of Canada for financial support and to Mr. Dale Johnson for drawing the figures. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. References Arsenault, G. P., Dolhun, J. J., and Biemann, K., Chem. Commun., 1970, 944. Story, M. S., Paper presented a t the 20th Annual Conference on Mass Spectrometry and Allied Blum, W., “Application Tips,” Finnigan Corp., Sunnyvale, Calif., 1972, p. 42. Blumer, M., Contam. Control, 1965, 4, 13. Chem. Engng News, Sept. lath, 1972, 14. Mayer, F. L., Stalling, D. L., and Johnson, J. L., Nature, Lond., 1972, 238, 411. Field, F. H., and Munson, M. S. B., J . A m . Chem. SOG., 1965, 87, 2289. Field, F. H., and Munson, M. S. B., J . A m . Chem. SOG., 1966, 88, 4337. Field, F. H., J . Am. Chem. Soc., 1969, 91, 2827 and 6334. Fales, H. M., Milne, G. W. A,, and Nicholson, R. S., Analyt. Chem., 1971, 43, 1785. Baker, R. W. R., J . Chromat., 1978, 154, 3. Vander Velde, G., and Ryan, J. F., J . Chromat. Sci., 1975, 13, 322. Week, D. P., and Field, F. H., J . Am. Chem. SOL, 1970, 92, 1603. Grmneberg, T., Org. Mass Spectrom., 1977, 12, 769. Topics, Dallas, Texas, June 4-9th, 1972. Received March 20th, 1979 Accepted April 20th, 1979