Pulsed extraction in LAMMS

Abstract

In laser microprobe mass spectrometry (LAMMS), the ablated ions are normally extracted using a 3-kV potential difference. This constant extraction bias continuously extracts ions as they are formed in the plasma and reduces the chance of any thermal equilibration. In contrast, in pulsed extraction a delay between ion formation and extraction should allow for partial thermal equilibration. This would reduce the energy spread of the ions entering the spectrometer and would allow more of the ablated ions to be detected, both effects giving a spectrum more representative of the original sample composition. In this paper, the energy distributions of ions from single element samples are shown experimentally to be reduced at defined delays. This arises from some extent of thermal equilibration (as predicted) and also bunching of the ions at the detector due to the transient extraction profile which causes velocity compaction. An additional result is that heavier elements require longer delays before extraction for the ions to equilibrate, because of their larger initial energy spreads. This mass-dependence of the time to reach equilibrium limits improvement in quantification of LAMMS by pulsed extraction without further modification. A LAMMS instrument designed to extract the ions initially at a low potential (to allow for equilibration) and then to apply a pulsed voltage to each bunch of ions of different mass would improve quantification.