APLI is a soft ionization method. Therefore, the mass spectra generally show the radical cation signal as the base peak representing the derivatisation product after reaction of the label with the analyte. There are only minor signals which are caused by impurities and very few small fragments (e. g. m/z 191). The wavelength envelopes of REMPI spectra of the derivatised analytes are only marginally different from the spectrum of the ionisation label itself. The comparison of the spectrum of anthracene with spectra of FL20101 or labelled analytes, respectively, suggests that the presence of a C-atom next to the ring system shifts the spectrum to the red, whereas further distant groups do not significantly impact on the absorption features. It thus follows that the ionization yield for labelled analytes is virtually identical. Naturally, the derivatisation yield can be different for different analytes, but instead of expensive stable-isotope labelled standards, one or more homologous compounds serving as internal standards can be added to the sample in known amounts. Since derivatisation yields are essentially identical in a homologous compound series, this simplified approach guarantees quantitative results at significantly reduced cost.

APLI-(TOF)MS of ethanol (a), cholesterol (b), dodecyl amine (c) and oleic acid (d) after derivatisation with FL20101 or FL20102 with direct syringe injection

However, in a number of matrices there may be a problem with this derivatisation concept. For example, fatty acids present in a fatty alcohol matrix cannot be derivatised with FL20102 since the fatty alcohols compete with the ionisation label. For such applications FL20103 can be used as alternative ionisation label. In this case, a reaction with the interfering fatty alcohols is not possible and the acids are labelled in high yields.