Improvements for absolute quantitation using electrochemical mass spectrometry

We recently reported a new quantitation method using mass spectrometry (MS) in combination with electrochemistry (EC, J. Am. Soc. Mass Spectrom., 2019, 30, 685). The strength of this method is that no reference standard or isotope-labeled compound is required for absolute quantitation. The method relies on electrochemical oxidation of an electrochemically active target compound to determine the amount of the oxidized compound using Faraday's Law. On the other hand, the oxidation reaction yield can be determined based on the MS signal change following electrolysis. Therefore, the absolute amount of the analyte can be calculated. Our experiment is carried out using a coupled liquid chromatography/electrochemistry/mass spectrometry (LC/EC/MS) apparatus. In this study, the method is further optimized. First, quantifying the compounds in a mixture is possible after the chromatographic separation. Gradient elution is used for separation and each compound can be quantified using the electrochemical mass spectrometry method. Second, for compounds that are already purified, LC column is not necessary and can be removed (i.e., flow-through analysis), thus shortening the analysis time for each injected sample from 10 min to 2 min. With using an LC auto-sampler, multiple samples can be injected sequentially. All the quantitation errors shown in this study are within 5%, indicating a good accuracy of our method.