Irradiation of an inorganic solid under high-power density conditions leads to spectra that primarily contain signals from elemental ions. This information is comparable to that from other microprobes in the respect that identification of inorganic compounds has to be done by means of element ratios. However, a lower energy regime results in spectra that permit speciation of the analyte in a relatively straightforward way.The procedures for the interpretation of inorganic LMMS data spectra can be based on a substantial amount of reference data, most of which were obtained by TOF instruments. Cluster ion distributions have been recorded for several pure elemental materials, metals, and alloys (140,(168)(169). Studies have been performed on the most common binary salts, such as halides and sulfides (90,(170)(171)(172)(173). The mass spectra of the major oxysalts are available, including nitrates and nitrites, sulphates, sulphites and thiosulphates, phosphates, silicates, and carbonates (90,(172)(173)(174)(175)(176)(177)(178). In addition, salts such as NaBF4 and KPF, were studied (171). Reference spectra have been recorded from an impresive series of oxides (83, 117, 170, 172, 174,(177)(178)(179)(180)(181)(182)(183)(184)(185). The intensity distribution of the cluster-ion spectra from oxides has been found to be similar to the ones in static SIMS, but the empirical model of Plog (186), relating the intensity distribution of cluster ions to the fragment valence, was not entirely applicable to laser-generated ions from oxides (183).In practice, ion signals in threshold spectra can be readily correlated to the original composition of the analyte using a few simple guidelines. Figure 6 gives a few representative examples (83). The identification of binary salts can be based on the low m / z signals in the positive and negative mode and on the detection of a cationized and anionized molecule. Oxides are recognized by the fact that the metal oxide constitutes the building block of clusters in both positive and negative mass spectra, as opposed to oxysalts. Oxysalts produce positive metaloxide-type clusters in the positive mode, but in the negative spectra signals from the anion appear. As shown in Fig. 6, even sodium sulphate and sodium sulphite can be readily distinguished by the rule that the anion with the highest oxygen number refers to the originally present anion. The presence of the cationized molecules in the positive mass spectrum of oxysalts seems to be related to the desorption characteristics of the analyte. Salts requiring more energy to yield a significant ion current do not show this signal. The ultimate test for the speciation capabilities remains the characterization of multicomponent systems. It has been shown that LMMS permits the identification of the individual components from mixtures containing chloride, sulphate, and nitrate salts (187). It is quite clear that the determination of the composition using relative element ratios would be rather difficult in such cases.Note, however, that the local en...
