Electrolyte-Induced Ionization Suppression and Microcystin Toxins: Ammonium Formate Suppresses Sodium Replacement Ions and Enhances Protiated and Ammoniated Ions for Improved Specificity in Quantitative LC-MS-MS

This study examines the effects of electrolytes on microcystin (MC) electrospray ionization (ESI) mass spectrometry and quantitative LC-MS-MS. Sodium replacement ions (SRI) are prominent in MC ESI spectra in protic solvents such as HPLC grade methanol. In a methanol-water-0.006% acetic acid (v/v) gradient, envelopes with up to 11 SRI were apparent in both the +1 and +2 charge states with structures [M + Na-x - (x-1)H](+) and [M + Na-x - (x-2)H](+2). The m/z 135 product ion, [empty set-CH2-CH=O-CH3](+), widely used in tandem LC-MS-MS determination of MC, is a low collision energy fragment of many doubly charged MC precursor ions (e.g., [M+Na+H](+2), [M+Na+NH4](+2), M+Na+H+CH3OH](+2), [M+2H](+2)). These phenomena impair congener-specific LGMS-MS detection of MC and degrade quantitative accuracy and precision. Pulse addition experiments established that ammonium formate (AF) strongly suppresses SRI in both +1 and +2 charge states and enhances MH+ and MNH4+ adducts in neutral MC. Addition of the buffer either post-column or by incorporation in the mobile phase increases specificity for all of the MC which were determined as the MNH4+ > MH+ and MH+ > [MH - 134](+) transitions for neutral MC (MCLA, MCLF, MCLW) and [M+2H](+2) > 135(+) and [M+2H](+2) > [M+2H - 135](+) transitions for arginine-containing MC (MCLR, MCYR, MCRR). These findings shed light on mechanisms of electrolyte-induced ionization suppression, and demonstrate beneficial use of a buffer electrolyte for improved specificity and analytical ruggedness in quantitative LC-MS-MS.