A home made PTFE micro-column loaded with polyaniline (50mg), prepared freshly by a chemical method was used for the on-line separation of arsenite [As(III)] and arsenate [As(V)] followed by determination at ultra trace levels in natural waters by flow injection-hydride generation-inductively coupled plasma mass spectrometry (FI-HG-ICPMS). The species were determined using time resolved mode of data acquisition, by monitoring ⁷⁵As. The volume of sample injected was 100 µl. Both the species eluted within 3min. The effects of variation in sample pH, eluent concentration and the hydride generation conditions were investigated. The calibration in the range of 0.5-50 µg L^-1 was found to be linear with a regression coefficient, R² ³ 0.997. The detection limits (3 sigma) 
were calculated to be 0.05 and 0.09 mu g L^-1 for As(III) and As(V) respectively and the precision (%RSD) at 1 µg L^-1 level was found to be 2.0% for As(III) and 2.5% As(V). The method validation was carried out by analyzing two 
BCR groundwater certified reference materials, BCR609 and BCR610, certified for total arsenic. The developed speciation method has been applied to groundwater samples collected from West Bengal, India, where there have been 
many instances of arsenic contamination.