Thin-layer chromatography/desorption electrospray ionization mass spectrometry: Investigation of goldenseal alkaloids

Desorption electrospray ionization mass spectrometry was investigated as a means to qualitatively identify and to quantify analytes directly from developed normal-phase thin-layer chromatography plates. The atmospheric sampling capillary of a commercial ion trap mass spectrometer was extended to permit sampling and ionization of analytes in bands separated on intact TLC plates (up to 10 cm x 10 cm). A surface positioning software package and the appropriate hardware enabled computer-controlled surface scanning along the length of development lanes or at fixed R-f value across the plates versus the stationary desorption electrospray emitter. Goldenseal (Hydrastis canadensis) and related alkaloids and commercial dietary supplements were used as standards and samples. Alkaloid standards and samples were spotted and separated on aluminum- or glass-backed plates using established literature methods. The mass spectral signal levels as a function of desorption spray solvent were investigated with acetonitrile proving superior to methanol. The detection levels (similar to 5 ng each or 14-28 pmol) in mass spectral full-scan mode were determined statistically from the calibration curves (2.5-100 pmol) for the standards berberine, palmatine, and hydrastinine spotted as a mixture and separated on the plates. Qualitative screening of the major alkaloids present in six different over-the-counter goldenseal dietary supplements was accomplished by obtaining full-scan mass spectra during surface scans along the development lane in the direction of increasing R-f value. In one sample, alkaloids were detected that strongly suggested the presence of at least one additional herb undeclared on the product label. These same data indicated the misidentification of one of the alkaloids in the TLC literature. Quantities of the alkaloids present in two of the samples determined using the mass spectral data were in reasonable agreement with the label values, indicating the quantitative ability of the method. The advantage of mass spectral measurements in identifying and quantifying materials within overlapping bands and in providing positive identification for even minor species in a mixture was also demonstrated.