Qualitative and quantitative analysis of quaternary ammonium alkaloids from Rhizoma Corydalis by matrix-assisted laser desorption/ionization Fourier transform mass spectrometry coupled with a selective precipitation reaction using Reinecke salt

A major problem hampering the use of MALDI-MS for quantitative measurements is the inhomogeneous distribution of analytes and matrices in sample preparations. In this study, an aerospray method was utilized for sample preparation method to improve sample homogeneity across stainless steel targets for quantitative analysis of quaternary ammonium alkaloids (QAAs). A selective precipitation reaction with Reinecke salt known to selectively trap QAAs was used to facilitate the separation and purification of QAAs from the complex crude plant extracts. Palmatine and berberine as the representative QAAs in commercial Rhizoma Corydalis were successfully quantified by introducing an internal standard with similar molecular properties as analytes. The LODs were found to be 0.07 fmol, for palmatine, and 0.24 fmol, respectively, for berberine. The content of QAAs of three commercial Rhizoma Corydalis was between 0.201 and 0.245% for palmatine, and 0.049-0.057% for berberine. Furthermore, MS/MS experiments based on the accurate-mass measurements were carried out by infrared multiphoton dissociation (IRMPD) for QAAs and the corresponding tertiary alkaloids, which offered additional selectivity for this quantitative analysis method. In the fragmentation of precursor ions from QAAs, only cleavage of substituted groups attached to the A- or D-ring was observed, while cleavage between B- and C-ring from tertiary alkaloids had occurred. This study offers a perspective into the utility of MALDI-FTMS as an alternate quantitative tool for QAAs, especially in complex plant extracts.