Generally, ionisation in positive ion mode gave better results compared to the negative ion mode. To reach highest sensitivity, optimal MS–MS parameters proved to be quite different for the four analytes. Since MS sensitivity was found to be lowest for the AH and SC derivatives, parameter sets were optimised in view of these compounds, accepting a decrease in sensitivity of 20–30% for AOZ and AMOZ derivatives. Substance identification and quantification was performed by multiple reaction monitoring (MRM) selecting one parent ion and two product ions for each analyte, which is in accordance with the EU guidelines for unambiguous positive identification and quantification of analytes [19].