Multiphoton resonance ionization has been combined with energetic ion bombardment to examine dopant concentrations ofindium on the surface of silicon. The results yield a linear relation between the indium concentration and the known bulk values and a detection limit of 9 parts per trillion, at a mass resolution exceeding 160. This measurement, which surpasses the limits of any previous surface analysis by a factor of 100, has been made possible with an experimental configuration that optimizes sampling and detection efficiency while reducing background noise to virtually zero. During the analysis, submonolayer quantities of the surface are removed, so that as few as 180 surface atoms may be counted.
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.
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