Application of FT LMMS to inorganic compounds offers the advantage of direct speciation. This means that signals which refer to intact analyte molecules are detected. Cumnt concepts of desorption and ionization in LMMS are mainly based on time-of-llight data and hence on ions formed during the laser pulse. This paper focuses on indications about the desorption and ionization mechanisms for inorganic compounds that can be deduced from the mass spectra in our FT LMMS database. Specifically, the occurrence of continuing ion formation during several hundreds of microseconds is demonstrated. These data are relevant to the possible role of selvedge ionization versus direct ion emission from the solid state.
Laser microprobe mass spectrometry (LMMS) is an analytical tool for local surface analysis, i.e. the characterization of the main components in a microscopic volume.'" A 1-5 p, m laser spot diameter allows LMMS to link micromorphological information directly to the chemical composition. The possibility of obtaining information on both organic and inorganic compounds is a major asset in practical problem solving. Therefore, LMMS is especially useful in applied material research. Current microprobes essentially provide element ratios for the identification of molecules. In contrast, LMMS yields refined qualitative information. Specifically, LMMS generates signals refemng to the intact analytes as well as to specific fragments from which the functionalities and composition of the original molecules can be ded~ced.3,~ This applies to organic as well as inorganic analytes. The term 'direct speciation' denotes identification of inorganic analytes in the same way as organic compounds are structurally characterized. Quantitation in heterogeneous samples is, however, not yet routinely feasible.
Progress in LMMS methodology and applicability depends on complete control over the parameters governing the ion current as a function of the local concentration. Therefore, understanding the desorption and ionization (DI) mechanisms becomes a prerequisite. The first generation of LMMS instruments, equipped with a time-of-flight (TOF) mass spectrometer, permitted an exploration of the analytical capabilities of laser microbeam irradiation but did not provide extensive experimental possibilities for a detailed study of inorganic DI. More specifically, TOF LMMS signals relate almost exclusively to ions generated during the laser pulse, i.e. the so-called 'prompt' ions.' However, from earlier laser desorption (LD) experiments in a &focused mode it could be anticipated that DI in LMMS would continue long after the laser pulse, i.e. the socalled 'post-laser DI' phen~menon.~~ ' Moreover, the TOF LMMS design inherently favours the detection of direct ion emission from the solid state, while it discriminates against the possible contribution of ion-molecule reactions in the selvedge, i.e. the high-density plume just above the sample.' As a result, the concepts of inorganic DI in the literature are inherently biased by TOF LMMS' instrumental features.