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of the dissolved fraction (in labile inorganic and organic monomeric aluminum species) by using the principles outlined by Driscoll (16)has recently become a well-established basis for geochemical and toxicological considerations. The FIAECR/CTA method showed promising properties for development of a speciation method; work along these lines is in progress.
LITERATURE CITED Rayset, 0. Anal. Chim. Acta 1985, 178, 223. Rayset, 0. Anal. Chim. Acta 1986, 185, 75. Marzenko, 2.: Jarosz, M. Analyst (London) 1982, 107, 1431. Barnes, R . B. Chem. Geol. 1975, 15, 177. May, H. M.;Heimke, P. A.; Jackson, M. L. Chem. G e O l . 1979, 2 4 , 259. Ogner, G. Acta Chem. Scand. 1973, 2 7 , 1601.
(7) Sampson, B.; Fleck, A. Analyst (London) 1984, 109, 389.
(8) Wyganowsky, C. Microchem. J . 1981, 2 6 , 45. (9) Wyganowsky,
C.;Motomlzu, S.;
TBei, K. Mikrochim. Acta 1983, 55.
(10) Wyganowsky, C.;Motomizu, S.; T b i , K. Anal. Chim Acta 1982, 140,
313. Wyganowski, C.; Kolezynska, M. Microchem. J . 1982, 2 7 , 37. Dougan, W. K.; Wilson, A. D. Analyst (London) 1974, 9 9 , 413 Seip, H. M.; Myller, L.; Njass, A. Water, Air, SoilPollut. 1984, 2 3 , 81. Rageberg, E. J. S.; Henriksen, A. Vatten 1985, 4 1 , 48. Salbu, 6.: Pappas, A. C.; Steinnes, E. Nord. Hydro/. 1979, 115. Driscoll, C. T. Int. J . Environ. Anal. Chem. 1984, 16, 267. Hue, N. V.: Craddock, G. R.: Adams, F. Soil Sci. SOC.Am. J . 1988, 5 0 , 28. (18) Lingerak, W. A.; Weensveen-Louter, A. M.; Slanina, J. Int. J . Environ. Anal. Chem. 1985, 19, 85.
(1 1) (12) (13) (14)
R~~~~~~~for review J~~~5, 1986. ~ ~ ~ ~ boctober ~ i 7,t t ~ d 1986. Accepted November 20, 1986.
Relationship between Solution Equilibria and Secondary Ion Intensities in Fast Atom Bombardment Mass Spectrometry Michael J. Connolly' and Robert G. Orth*2
Montana State University, Bozeman, Montana 5971 7
The solution equilibrium for the reaction between glycine and copper( I I ) in glycerol was studied, and the results were used to examine the relationship between equilibria in glycerol and the observed secondary ions produced during fast atom bombardment. The solution equilibrium for the reaction between glycine and Cu( I I ) in glycerol is shown to be analogous to the equilibria established in water. The secondary ions only reflect a few of the ions known to exist in solution with a lack of any secondary ions with Cu(I1). The major secondary ion is the protonated glycine ion, which has an intensity that did not reflect the equilibria in solution. The lack of a quantttative relationship is believed to arise from the induced reactions that occur as a result of the fast atom bombardment (FAB). The FAB-induced reactlons include reductbn, dehydration, and protonation. Thus when fast atom bombardment mass spectrometry Is utilized for studying solution equilibrium, it is necessary to be aware of the induced reactions if a direct probe of solution equilibrium is to be undertaken.
In the last 4 years fast atom bombardment mass spectrometry (FAB-MS) has become a very successful technique. It has been used to obtain the molecular weight of and structural information on many nonvolatile and high molecular weight materials (1-4). However, little emphasis has been placed on the evaluation of this technique as a tool for studying the bulk composition of liquids (5). The relationship between the observed ions in the FAB mass spectra and the near bulk and surface layer composition is dependent upon how ions are desorbed from the liquid surface (6). This study examines the relationships between the equilibria that exist Present address: Stauffer Chemical Co., Eastern Research Center Livingstone, Dobbs Ferry, NY 10522. Present address: Environmental Sciences Center, Monsanto Co., 800 N. Lindbergh Blvd., St. Louis, M O 63167.
within a glycerol solution and the observed secondary ions produced during fast atom bombardment. On the time scale of a collision cascade s), particle bombardment of a liquid is identical with that of a solid (7). During this period, the molecules of a typical liquid (with a diffusion coefficient of lo3 cm2 s-l) are essentially stationary and thermal vibrations may be ignored ( 7 ) . One model proposed to account for ionization and desorption from liquids requires the formation of ions in solution followed by desolvation and ion ejection ( 4 ) . The energy for ejection is the energy transferred through the medium due to a collision cascade. Once the ions are formed at the surface, only desolvation remains to be accomplished. This desolvation process has been described by Vestal (8) as the evaporation of preformed ions from splash droplets. Macfarlane (9) described desorption of ions in a FAB-MS experiment by invoking the concept of molecular aggregates. The desorption of the involatile molecule is described as a multiphonon excitation of the bonds holding the analyte ion to the aggregate (9) where the phonons were generated during the primary energy deposition event. Higher secondary ion yields obtained when using liquid matrices have been attributed to the lower surface binding energies of liquids ( 4 , 7 ) . In MacFarlane's model (9) higher secondary ion yields are rationalized in a similar fashion. Macfarlane proposes that the solvated molecular aggregate modifies the strong interactions of other analyte molecules thereby reducing the analyte to aggregate binding energy (9). Therefore, the analyte molecule is more easily dissociated from the aggregate upon excitation. The concept that molecules have to be surface active in order to be observed in FAB-MS was proposed by Barber and co-workers (3). These authors claimed that for maximum sensitivity, sample molecules should form a perfect surface monolayer. However, this does not imply that only molecules in the top layer are desorbed during FAB. In a study designed to separate effects attributable to sputtering efficiency from those attributable to real changes in surface composition,
0003-2700/87/0359-0903$01.50/00 1987 American Chemical Society
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Ligon ( 6 ) found that FAB mass spectra of surfactants represented both the composition of the surface monolayer and the bulk. This implied that the FAB-MS technique was sampling more than just the surface. From this work it was concluded that the observed secondary ion intenqity ratio of two analyte molecules would depend on their bulk coiicentration and their relative surface activity ( 6 ) A relationship between solution concentration and mass spec tral heights has been observed by two different groups (rS, 1 0 ) dotinstone and co-workers (70) found that regardless of the energv transfer mechanism, equilil.riurn or nonequilibriiim, the observed FAR mass spectral peak heights for desorbed crown ether/cation solutions were directly proportional to the concentrations of the qpecies in wlution No secondary ion peaks attributable to glycerol mere observed sirice the crown ether solution was deposited on the glycerol surface This suggests that a rnultilaver solrition existed and that [lie glycerol was acting as a wpport inaterial. 'I'he 1111portant result of this study was the ability t i ) calriilatP crown ether /cation stability constants. C'aprioli (,5) has shown that hy using FAB inass spectrometry i t is pssitde to determine the pf
