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 tryptophan on a silicon wafer. The tryptophan signal from a frozen solution is demonstrated to have a linear dependence on concentration by using 4,4'4-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.