In this paper we describe results from a new design of plasma ion mass-spectrometer (PIMS) which can resolve the ion energy parallel to the total magnetic field. The experimental ratios of the collector signals for different charge states are fitted to the results from a three-dimensional Monte Carlo code. This procedure determines the impurity ion temperature and transmission function for the instrument. Transmission corrected spectra are then used to calculate impurity concentrations, charge state distributions and Z eff in the boundary.