LASER MICROPROBE FOURIER-TRANSFORM MASS-SPECTROMETER WITH EXTERNAL ION-SOURCE FOR ORGANIC AND INORGANIC MICROANALYSIS

The earlier laser microprobe (LM) mass spectrometer with a time-of-fli ght (TOF) analyser showed that the irradiation of solids by a focused UV laser represents an interesting means of performing local analysis. However, the subsequent measurement of the generated ions is done und er low-resolution conditions. The characterization of heterogeneous sa mples and the occurrence of poorly understood ion formation mechanisms motivated the development of an instrument with superior mass spectro metric performance. Specifically, Fourier transform (FT) mass spectrom etry potentially provides the required high mass resolution and accura te mass determination in combination with adequate sensitivity. In thi s work an FT-LM mass spectrometer with an external source was develope d. Analysis is performed in the reflection mode with a 5-mum spot from a Q-switched Nd: YAG laser. Transfer of ions from the external source into the cell is performed by static electrical fields. The resulting mass separation imposes limitations on the panoramic spectrum registr ation in the case of pulsed laser microbeam ionization of solids, prod ucing ion bunches over not more than a few hundred microseconds. This paper addresses instrument design and performance with respect to sens itivity, spatial resolution and accuracy of m/z determination and the problem of panoramic registration. Attention is focused on the compari son of results from FT- and TOF-LM mass spectrometry in relation to th e determination of the speciation of inorganic compounds and diagnosti c analysis of organic molecules.