The decomposition of 59 different cluster ions (generated by fast atom bombardment) consisting of two different amino acids and a sodium ion was analysed. The only fragment ions of significant abundance could be assigned to sodium ion-bound amino acids. Assuming that the most abundant ion in the fragment ion spectrum corresponds to the amino acid with the highest sodium ion affinity (SZA), the 20 common a-amino acids could be ordered with increasing sodium ion affinity as follows: Gly, Ala, Cys, Val, (Leu, Ile), Ser, Met, Thr, (Phe, Pro), Asp, Tyr, (Glu, Lys), Trp, Asn, Gln, His, Arg. Quantitative determinations were carried out by comparison of the lithium ion affinity (LIA) of Ala with that of dimethylformamide (DMF) in a fragment ion scan of the ion-bound dimer Ala-Li+-DMF. LZA(A1a) was calculated from LZA(Ala) = LIA(DMF) -(l/C)ln [ I(AlaLi+)/I(DMF-Li+),where the constant C was estimated from measurements of proton-bound amine-amino acid clusters. From fragment ion analysis of nine other Li+-bound a-amino acid dimers, the following lithium ion affinities were obtained: Gly 51.0, Ala 52.6, Sar 53.5, a-aminobutyric acid 53.7, glycine methyl ester 54.7 and Val 54.8. SIA(Ala) was estimated to be 75% of the lithium ion affinity and from fragment ion analysis of ten Na+-bound a-amino acid dimers the following sodium ion affinities were obtained: Gly 37.9, Ala 39.4, a-aminobutyric acid 40.3, Val 41.0, glycine methylster 41.0 and Sar 41.2.

A rationalization of the kinetic method for determination of proton affinities (tEPa) has been formulated. When a proton-bound amine dimer with the general structure aminel-amine,-H+ decomposes to either amine,-H+ or amine,-H+ the critical energies for the competing fragmentations can be calculated from a simplified version of the Marcus equation, which is supported by published values of molecular pair proton affinities. Consequently reaction rates of the metastable ions can be calculated from the expression k ( E ) = v [ ( E -E , ) / € ] s -l and ion abundances from the expression J,P(E)F(€)d€, where P ( E ) is the probability of reaction and F ( E ) is the energy distribution function of the metastable ions. It is argued that for metastable ions generated by ionization methods such as CI or FAB, the energy distribution functions will be smooth and that consequently the relative ion abundances from two competing decompositions will not depend on F ( E ) . Model calculations of fragment ion abundances from metastable decomposition of ions with the general structure pentylamine-aminex-H + show a linear relationship between the logarithm to the ratio: /(aminex-H+)/(pentylamine-H+) and the Ep of amine,. This provides a rationalization of the kinetic method that avoids any introduction of a thermodynamic temperature. Determination of the fragment ion abundances from decomposition of metastable protonated clusters with the general structure a-amino acid-aminex-H for 1 7 different a-amino acids gave the following €,,/kcal mol-' values:

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