THE MANCHESTER-ISOLAB-54 ION MICROPROBE

The Isolab 54 ion microprobe installed at the Department of Earth Scie nces at the University of Manchester has been designed to measure isot opic abundances of elements with high precision from ions formed by se condary ionisation (SIMS), laser resonance ionisation (RIMS) or therma l ionisation (TIMS). This instrument has been extensively modified sin ce its original installation to improve its precision and capabilities , and these improvements are detailed, as are applications for which t he instrument has been used. The principal modifications were the inst allation and testing of a new type of ion detector (conversion dynode system, CDS), which has an ion-to-pulse conversion efficiency that has been constant to better than 2 parts per thousand over periods of man y months, a method of integrating secondary ion currents to eliminate the effects of irreproducible instrumental isotopic fractionation, and a method of rapid electrostatic peak centring which can centre high m ass resolution peaks on a detector to a precision of 2 ppm in mass in 1.2 s. Particular attention has been paid to in situ SIMS analysis of oxygen isotopes from insulating materials, where a spatial resolution of 10-20 mu m with a precision of 1-2 parts per thousand has been achi eved for both O-17/O-16 and 18O/O-16. Analytical techniques for the me asurement of carbon, nitrogen and osmium isotope ratios by SIMS are al so described, together with the development of a technique for the mea surement of strontium isotope ratios using laser resonance ionisation of sputtered neutral atoms.