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The analysis of cardiac glycosides by the desorption/ionization (D/I) mass spectrometric technique potassium ion ionization of desorbed species (K(+)IDS) is presented. K(+)IDS mass spectra of digitonin, digoxin, digoxigenin, digitoxin and ouabain are discussed to demonstrate the capabilities of this D/I method. The K(+)IDS analysis consists of two steps: thermal desorption of neutral molecules representative of the analyte, followed by gas-phase additon of K(+) ions to these species. Structural and molecular weight information of the cardiac glycosides is obtained wit the K(+)IDS technique. The most intense peak in the K(+)IDS mass spectrum of an analyte, M, is frequently the (M)K(+) ion. Interpretation of the K(+)IDS mass spectra is simple, since one thermal degradation mechanism dominates. This mechanism is a 1,2-elimination process. A variation of the original K(+)IDS technique, performed by changing the ionizing metal from K(+) to Na(+) (i.e. Na(+)IDS), is presented for the analysis of digoxin. The Na(+)IDS mass spectrum of digoxin contains more structural information obtainable with this D/I technique by varying the cation that is thermionically generated. K(+)IDS analyses can be performed rapidly, no sample derivatization is necessary, no matrix is required and little instrument modification is necessary.