Analyses of cryogenic samples using ion-induced desorption and multiphoton resonance ionization

In this study, ion-beam-induced desorption with multiphoton resonance ionization detection of desorbed neutral molecules is used to characterize frozen aqueous solutions. This type of matrix is of particular importance since it serves as a model for biological matrices. The time-of-flight mass spectrum, obtained in this way for a millimolar tryptophan/H2O solution, is virtually identical to that for a submonolayer of tryptophan on a silicon wafer. The tryptophan signal from a frozen solution is demonstrated to have a linear dependence on concentration by using 4,4'-biphenyldiol as an internal standard. A detection limit of 2 x 10(-6) M is also demonstrated. Since our ion beam samples one layer of 0.1 cm2 and we assume 10(15) molecules/cm2 of ice, this concentration corresponds to approximately 4 x 10(6) molecules/layer. It is also shown that the signal exhibits an exponential decay with primary ion dose due to the accumulation of primary ion damage in the near-surface molecules.