Analysis of glycoalkaloids from potato shoots and tomatoes by four-sector tandem mass spectrometry with scanning-array detection: comparison of positive ion and negative ion methods.

A wide range of glycoalkaloids from potato shoots and tomatoes, including trisaccharide-containing glycoalkaloids (alpha-chaconine, alpha-solanine, and alpha-solasonine), tetrasaccharide-containing glycoalkaloids (alpha-tomatine and demissine), and disaccharide-containing glycoalkaloids (beta 1-chaconine, beta 2-chaconine, and beta-solamargine), have been studied by both positive and negative ion liquid secondary ion and four-sector tandem mass spectrometry with scanning-array detection. In positive ion mode, collisionally induced dissociation tandem mass spectra of the [M + H]+ ions induce three major fragmentation processes, Z cleavage, Y cleavage, and 1,5X cleavage, which are structurally informative. Signals resulting from Z0, Y0, and 1,5X0 cleavages provide information on the nature of various aglycone moieties in all glycoalkaloids. Linkages and positions of the sugars in trisaccharide- and tetrasaccharide-containing glycoalkaloids are indicated by the presence or absence of the ions corresponding to Z alpha/beta and Y alpha/beta cleavages and intensity differences of the peaks due to 1,5X alpha and 1,5X beta cleavages, respectively. In negative ion mode, collisionally induced dissociation tandem mass spectra of the [M - H]- ions induce Y cleavage as the major fragmentation process. The location of the terminal sugars in branched trisaccharide and tetrasaccharide glycoalkaloids is indicated by the difference in intensity of the ions due to Y alpha cleavage and Y beta cleavage. Isomeric structures cannot, however, be differentiated unambiguously; complete structural assignment is only possible by NMR of purified components. Both positive and negative ion tandem mass spectrometry are considered to be suitable for the characterisation of glycoalkaloids in mixtures. The positive ion method has the advantage of (i) a lower detection limit than in conventional mass spectrometry; (ii) numerous and intense fragment ions which are structurally informative; and (iii) the capability of analyzing minor components in crude extracts. Comparable analysis by other analytical means would not have provided the amount of structural information on the components in the glycoalkaloid mixtures.