The ASU IMS 3f secondary ion microanalyzer has been converted to a multiple ion detection mass spectrometer with double focusing for three neighbouring masses. We present preliminary experimental tests of the modified instrument. For that purpose we chose sulphur isotope ratio measurements in three pyrite samples using a small triple Faraday cup assembly as a multicollector. We achieved high mass resolution for off-axis masses and an accuracy of S-32/S-34 isotopic ratio of similar to 0.2%. (c) 2006 Published by Elsevier B.V.