Positive + negative is not equal to zero: Use of negative ionisation as analyte identity confirmation tool in LC-ESI-MS analysis.

In this study the use of negative electrospray ionisation mode as a confirmation tool for identifying derivatized amino acids using LC-ESI-MS, was evaluated. The derivatization reagent was based on azobenzene N-hydroxysuccinimide carbamate. The results indicate that even though negative ionisation mode produced less intense peaks, the ratio of peak area of quantifier ion (obtained in positive mode) to the qualifier (or identifier) ion measured in negative mode meets the requirements established by two prominent validation guidelines: SANTE/11312/2021 and 2002/657/EC. Therefore, the use of product ions obtained via negative transitions as qualifier ions, while using product ions from positive transitions as quantifier ions is a fruitful approach that widens the choice of transitions to choose from for obtaining suitable qualifier ions. This methodology was applied to the LC-ESI-MS/MS determination of amino acids in different beverages (tomato juice, watermelon juice, kvass).