PERFORMANCE OF AN ION-TRAP MASS-SPECTROMETER MODIFIED TO ACCEPT A DIRECT INSERTION MEMBRANE PROBE IN ANALYSIS OF LOW-LEVEL POLLUTANTS IN WATER

Modifications to a Finnigan ITS40 ion trap mass spectrometer are descr ibed which allow its use with a direct insertion probe. Details are gi ven of the fabrication of a membrane probe for such an instrument. The membrane probe, which includes facilities for heating the fluid, empl oys a tubular membrane which is located just outside the electrode str ucture of the ion trap. Direct analysis of organic compounds in aqueou s solution is demonstrated using a silicone membrane, with compounds s uch as benzene, chlorobenzene and dichloroethene being studied below t he 1 ppb level. The effects of operating parameters including probe te mperature, ion trap temperature, solution flow rate, mass spectrometer scan speed, and instrument tune procedures are explored in detail. Op timum performance characteristics are identified and trace level detec tion of eight organic compounds in the parts per trillion range is dem onstrated. In seven of the eight cases studied, detection limits are b elow the EPA practical limit of quantitation levels. It is shown that the most sensitive mode of operation is when steady state passage of t he analyte across the membrane is achieved, however, the time required for this is long in the case of some samples, and a dynamic flow inje ction analysis procedure is then favored. Use of the modified inlet sy stem for solid sample introduction via a standard solids probe is also demonstrated.