Optimization of instrumental parameters for flow injection analysis-thermospray tandem mass spectrometry

Summary

Optimization of signal-to-noise ratios (S/N) by optimizing electron multiplier (EM) voltage and resolution of the first and second mass analyser in a thermospray (TSP), tandem mass spectrometer system is studied. Using flow injection analysis (FIA) of samples containing eight chlorophenoxy carboxylic acid herbicides and bentazone, and a FIA system, the signal and background (noise) intensity i.e. S/N are studied with selected reaction monitoring (SRM) at different EM voltages. An EM voltage of 2500 V improves the S/N ratios 5.5–13 fold compared to the usual 1700 V. Applying additional resolution voltages of +3 to +4 V to the first and second mass analyser decreases resolution in each mass analyser, but there is no overall loss in selectivity, while the S/N ratios further increase 3–4 fold. The selectivity of measurements was studied using 2,4,5-T ((2,4,5-trichlorophenoxy)acetic acid) and triclopyr, which differ only one mass unit in the selection of the parent and daughter ion mass in the applied SRM method. Resolution could be decreased to 36–54% valley definition, while still ensuring the selectivity. With the herbicides studied, screening of surface water samples spiked at the 1 μg l⁻¹ level, corresponding to 25 pg component s⁻¹ into the MS, is easily achieved under optimum conditions without analyte concentration. Some sample clean-up is recommended, however, because ionization efficiencies tend to diminish with some of the raw sample studied.