This study used 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) as a derivatization reagent to establish an LC-MS/MS analytical method for the quantitative detection of a class of amine drugs, mesalazine(5-ASA) and two thiol drugs, methimazole (MMI) and captopril (CAP).

Plasma samples were processed using derivatization and protein precipitation methods. Optimal derivatization conditions (derivatization reagent concentration, reaction temperature, and derivatization time) were investigated for different drugs. The analytical columns used were Thermo AccucoreTM C₁₈ (2.4 μm, 30 mm×4.6 mm), Agilent Poroshell 120 EC-C₁₈ (4 μm, 50 mm×4.6 mm) as analytical columns, with 0.1% formic acid-acetonitrile, 0.1% formic acid-methanol, and 5 mM ammonium acetate-methanol: acetonitrile (V∶V=50∶50) as the gradient mobile phase, with flow rates of 0.5 mL·min^-1 and 0.6 mL·min^-1, respectively, using an ESI source for positive/negative ion detection in multiple reaction monitoring (MRM) mode.

5-ASA, MMI, and CAP all undergo nucleophilic substitution reactions with NBD-Cl, reacting rapidly and completely under suitable derivatization conditions. The established MMI analytical method has a linear range of 1.00-500 ng·mL^-1, with good linearity (r=0.998 9). The linear ranges of the CAP and 5-ASA analytical methods are both 2.00-1 000 ng·mL^-1, with good linear relationships (CAP: r=0.999 3, 5-ASA: r=0.999 6). The intra- and inter-batch precision (CV) and accuracy (RE) were both less than 15%, making the methods suitable for the analysis of actual samples of the corresponding drug formulations and successfully applied to the pharmacokinetic studies of MMI.

The established NBD-Cl derivatization LC-MS/MS analytical methods for the three drugs exhibit advantages such as high sensitivity, strong specificity, and low background noise, and successfully address the issues of low mass spectrometry response, poor spectral retention, and the tendency of CAP to oxidize. The methods require minimal plasma sample volume and have short analysis times, providing a feasible solution for the analysis of biological samples of the three drugs. They also demonstrate the applicability of NBD-Cl derivatization technology in the analysis of amine- and thiol-containing drugs.