Mass spectrometric analysis of cardiac glycosides by the desorption/ionization technique potassium ion ionization of desorbed species

Abstract

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 addition of K+ ions to these species. Structural and molecular weight information of the cardiac glycosides is obtained with 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 than the K+IDS mass spectrum of that compound. This may lead to a means of controlling the types of 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.

Ionization processes in matrix‐assisted laser desorption/ionization mass spectrometry: Matrix‐dependent formation of [M + H]+ vs [M + Na]+ ions of small peptides and some mechanistic comments

“…While proton affinities can be very large, with amides having proton affinities higher than 200 kcal mol-', alkali metal ion affinities are an order of magnitude smaller, frequently in the 20-30 kcal mol-' range for polar molecules. Thus, the small alkali metal ion affinities make process (15) substantially endothermic and, if both processes (14) and (15) could be photoinduced, one would certainly expect them to have considerably different thresholds. This evaluation assumes that intact NaCl molecules exist in the solid MALDI target.…”

“…While this mechanism parallels mechanism 1 for protonation, and the amount of salt required for the formation of sodium ion adducts is of the same order of magnitude as the molar amount of matrix used, there are problems with this mechanism, largely thermochemica1 in nature. Consider the processes (14) and (15) These two processes contribute to the overall reaction enthalpy. First, the mH or mNa bond must be heterolytically cleaved.…”

Mechanistic considerations of the protonation and fragmentation of highly functionalized molecules in fast atom bombardment: High resolution mass spectrometry and tandem mass spectrometry analysis of the ions formed by fast atom bombardment of digoxin and related cardiac glycosides

“…This creates a difficult situation because the identities of the frag- ments are necessary for determining the fragmentation mechanism(s) and likewise the fragmentation mechanism is required to know the exact structure of the fragments that we wish to describe in the ionic assignments. Whereas mechanistic possibilities are being evaluated throughout this article, the nomenclature used to identify different fragments is the Light/Kasselj'Allison scheme, which has previously been described [8]. The Light /Kassel/Allison scheme is designed as a precise shorthand for discussing both ionic and neutral variants of the types of molecules addressed in this article.…”

“…Energy is imparted into the molecular ion, which fragments as it would if energy were added to the corresponding neutral molecule. In this context, we have reported the K+IDS mass spectrometric analysis of cardiac glycosides [8]. K+lDS, K+ ionization of desorbed species, is a technique that combines rapid thermal processes, thermal degradation, and vaporization, with gas phase K+ attachment.…”