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, [Ø-CH(2)-CH=O-CH(3)](+), 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+NH(4)](+2), M+Na+H+CH(3)OH](+2), [M+2H](+2)). These phenomena impair congener-specific LC-MS-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 MNH(4)(+) 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 MNH(4)(+) > 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.
California Department of Public Health, Sanitation & Radiation Laboratory, 850 Marina Bay Parkway, Richmond, California 94804, USA. william.draper@cdph.ca.gov
William M
WM
Draper
Dadong
D
Xu
S Kusum
SK
Perera
D000641:Ammonia / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D005561:Formates / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D052998:Microcystins / Q000032:analysis*; D012964:Sodium / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
D005561:Formates; D007477:Ions; D052998:Microcystins; C030544:formic acid; D000641:Ammonia; D012964:Sodium