EVALUATION OF FLOW-INJECTION TECHNIQUES FOR MICROWAVE PLASMA TORCH ATOMIC EMISSION-SPECTROMETRY

Flow injection (FI) sample introduction was coupled with a recently de veloped atomic emission source, the microwave plasma torch (MPT), oper ating at 200 W with Ar as the plasma gas. The use of FI in conjunction with an ultrasonic nebulizer was compared with direct nebulization; p arameters taken into consideration include carrier solution flow-rate, aerosol carrier gas flow-rate, sample volume, dispersion coefficients and the effect of two easily ionized concomitant elements. Detection limits and precision of the FI technique were assessed for four sample -loop volumes: 75, 200, 250 and 500 mul. Flow injection was found to o ffer several advantages over continuous sample introduction: rapid sam ple throughput and a reduction of memory effects without a loss in sen sitivity or precision. Furthermore, by appropriate choice of sample di spersion, a significant reduction of the Na and K interferences can be achieved without a substantial deterioration in sensitivity. An FI sy stem incorporating a microcolumn of bonded silica with octadecyl funct ional groups was used to determine Cu in synthetic sea water. The samp le preconcentration time was one minute at a sample solution flow-rate of 1.0 ml min-1; pure ethanol was used as the eluent. The detection c apability in this preconcentration mode was approximately 25 times bet ter than that offered by a comparable continuous nebulization system. The detection limit for Cu was 0.16 ng ml-1, the relative standard dev iation at 500 ng ml-1 was about 3.2%, and recovery rates were in the 9 4-97% range.