Improved tuning and calibration in liquid chromatography/thermospray mass spectrometry using acetic acid cluster ions

Tuning and calibration of a thermospray ion source can cause rapid contamination of the source, and can have the disadvantage of introducing substances or solvents that are not used in the subsequent analytical work on the mass spectrometer. The use of aqueous 0.05 M ammonium acetate alone to produce acetic acid clusters for calibration in the mass range up to m/z 800 is reported. A low temperature of the vaporizer and the source block is used in order to maintain the clusters formed. The low temperature makes possible a very good signal-to-noise ratio, since the only ions formed are clusters of the general formula (CH₃COOH)_m(NH₃)_n(H₂O)_pNH₄]⁺. Owing to the use of the ammonium acetate mobile phase, no interference from other sources was observed, e.g. clusters of solvents with ammonium acetate or impurities.