Detection of biomolecules on surfaces using ion-beam-induced desorption and multiphoton resonance ionization.

Multiphoton resonance ionization (MPRI) has been combined with ion-beam-induced desorption to examine a set of thermally labile biological molecules present on surfaces. Specifically, we have examined films of adenine and beta-estradiol, molecules with a rigid skeletal backbone. In both of these cases, molecular ions could be produced efficiently without cooling the neutral molecules into their ground vibrational state. We have also studied other more fragile molecules such as tryptamine, tryptophan, phenylalanine, and serotonin. The base peak in the mass spectra of these molecules is fragment ions formed by losses of the amine side chains. Even with this fragmentation, however, it is possible to achieve sensitivity limits that are many orders of magnitude greater than for secondary ion mass spectrometry, without preparing the samples in special matrices. For serotonin, detection limits of 40 fmol on the surface of a silicon target are achievable. The results also yield a linear relation between the serotonin base fragment ion intensity and the known surface concentration.