Using human insulin (MW 5808 Da) as a model compound, the possible strategies towards optimization of sensitivity and selectivity of measurement by electrospray ionization with a standard triple quadrupole mass spectrometer were investigated. For measurement in selected ion-monitoring (SIM) mode, these strategies involved systematic variation of instrumental parameters and spray pH. In this investigation four different operating modes were used corresponding to positive/negative ionization modes with acidic/basic sprays and pH reversed (hereafter termed ‘wrong-way-round’ operation); the cone voltage was optimized for each mode of operation. When collision-activated dissociation (CAD) is employed, two additional operation modes are possible: namely, low collision energies (10–35 eV, CAD-l) for the generation of sequence-specific fragments and high collision energies (>80 eV, CAD-h) for the generation of nonspecific fragments. Overall, this results in twelve different modes of operation.

Loop-injection of aqueous insulin standards were run for each of the twelve operating modes and measurements made for five different charge states (n = 2–6) observable with our instrument that has an upper mass limit of m/z 4000. The signal/noise (S/N) ratio was optimized for each charge state, resulting in 60 measurements. The best S/N ratios (20 000) were achieved under positive SIM conditions with charge state 6 (m/z 969) and under ‘wrong-way-round’ negative SIM conditions with charge state 3 (m/z 1935). Lower S/N ratios were observed under positive CAD-h conditions with charge state 5 (m/z 1163, S/N 15 000) and positive CAD-l conditions with charge state 6 (m/z 969, S/N 10 000). All other operating modes gave maximum S/N ratios of 4000.

For measurement of insulin standards, the results obtained show SIM to give the best S/N ratio. However, for samples in complex matrices, our general experience suggests CAD to be the preferable operating mode. Consequently, for the development of a quantitative method for proteins in general, it might be advocated that all of the twelve operating modes and all relevant charge states be investigated to find the optimum S/N ratio. Copyright © 2001 John Wiley & Sons, Ltd.