Optimization of electrospray interface and quadrupole ion trap mass spectrometer parameters in pesticide liquid chromatography/electrospray ionization mass spectrometry analysis.

Optimization of both the ionization process and ion transportation in the mass spectrometer is of crucial importance in order to achieve high sensitivity and low detection limits and acceptable accuracy in liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) analysis. In this paper four optimization procedures of electrospray interface and quadrupole ion-trap mass spectrometer parameters (ESI-MS) (nebulizer gas and drying gas flow rate, end plate voltage, capillary voltage, skimmer voltage, octopoles direct current and radio frequency, trap drive and lens voltages) were studied on three pesticides--thiabendazole, aldicarb and imazalil. The results demonstrate that the methodology of optimization strongly influences the effectiveness of finding true optima of the operating parameters. Both eluent flow rate and composition during optimization have to mimic the situation during real analysis as closely as possible in order to achieve parameters giving the highest sensitivity. Therefore, post-column addition of analyte to the mobile phase identical in composition to the one in which analyte elutes during real analysis combined with software-based optimization was found to be the most effective and fastest method for achieving intensity maxima. The parameters most strongly affecting ion formation and transportation, hence sensitivity, were capillary voltage, direct current of the first octopole, trap drive and the second lens for all pesticides under study. In addition to sensitivity and detection limit matrix effect was considered in the optimization process. It was found that the matrix effect can be reduced but not eliminated by adjusting the ESI and MS parameters. The optimal parameters from the point of view of the matrix effect can only be found with factorial design. Parameters giving higher sensitivity tended to be more affected by matrix effect causing higher ionization suppression by co-eluting compounds.