High-performance liquid chromatography–tandem mass spectrometry with post-column pH modification: Independent pH optimization for chromatographic separation and electrospray ionization

Rationale: In recent environmental research, multi-methods using high-performance liquid chromatography–mass spectrometry (HPLC/MS/MS) have become more and more important for the analysis of organic micropollutants in environmental matrices. As the targeted compounds usually have different physicochemical properties, the optimization of these methods is challenging. Methods: The pH in the electrospray of the mass spectrometer ion source was modified independently of the one used for the HPLC separation, using a post-column HPLC flow to make the spray acidic or alkaline. The method development was carried out manually in a systematic multistep way. Results: The method used for the analysis of organic biocides (e.g., terbutryn, propiconazole, and isothiazolinones) was optimized. It was shown that the use of a neutral as well as an acidified gradient could not be optimized for all target compounds, whereas a neutral gradient with post-column acidification was optimum for all target compounds. Conclusions: Acidic or alkaline post-column infusion allows pH optimization for ionization conditions, independent of the pH optimization for chromatographic separation. The introduction of an additional modifier might also allow the minimization of matrix effects or directed formation of sodium adducts, without affecting the chromatographic separation.