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 × 10^-6 M Is also demonstrated. Since our Ion beam samples one layer of 0.1 cm² and we assume 10¹⁵ molecules/cm² of Ice, this concentration corresponds to approximately 4 × 10⁶ 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.