Total microcystins analysis in water using laser diode thermal desorption-atmospheric pressure chemical ionization-tandem mass spectrometry

A new approach for the analysis of the cyanobacterial microcystins (MCs) in environmental water matrices has been developed. It offers a cost efficient alternative method for the fast quantification of total MCs using mass spectrometry. This approach permits the quantification of total MCs concentrations without requiring any derivatization or the use of a suite of MCs standards. The oxidation product 2-methyl-3-methoxy-4-phenylbutyric acid (MMPB) was formed through a Lemieux oxidation and represented the total concentration of free and bound MCs in water samples. MMPB was analyzed using laser diode thermal desorption-atmospheric pressure chemical ionization coupled to tandem mass spectrometry (LDTD-APCI-MS/MS). LDTD is a robust and reliable sample introduction method with ultra-fast analysis time (<15 s sample⁻¹). Several oxidation and LDTD parameters were optimized to improve recoveries and signal intensity. MCs oxidation recovery yield was 103%, showing a complete reaction. Internal calibration with standard addition was achieved with the use of 4-phenylbutyric acid (4-PB) as internal standard and showed good linearity (R² > 0.999). Limits of detection and quantification were 0.2 and 0.9 μg L⁻¹, respectively. These values are comparable with the WHO (World Health Organization) guideline of 1 μg L⁻¹ for total microcystin-LR congener in drinking water. Accuracy and interday/intraday variation coefficients were below 15%. Matrix effect was determined with a recovery of 91%, showing no significant signal suppression. This work demonstrates the use of the LDTD-APCI-MS/MS interface for the screening, detection and quantification of total MCs in complex environmental matrices.