Field desorption mass spectrometry of carboxylate salts: characteristic spectra and effects of added complexing agents

Field desorption mass spectra are reported for a series of sodium carboxylates. Above 20 mA the major ions in the spectra belong to the series [xNa+ + (x − 1)RCOO−]+ with values of x normally extending throughout the measurable mass range. Desorption characteristics and spectral composition have been shown to be strongly modified by the addition of fructose and other polyhydroxy compounds. Thus, the sodium acetate spectrum is more intense and contains higher aggregates when 0.1–0.2 mol of fructose is added. With further additions the ionized free acid, AcOH+•, becomes the base peak in a low intensity spectrum recorded at 17–18 mA. The main ion from fructose is the sodium complex which does not exceed 20% of the base peak. Added fructose also has a beneficial effect on sodium 12-hydroxystearate but the improvement is less dramatic and there are some complications in the interpretation. A sample of technical lithium stearate contains the usual complexes up to [Li4(RCOO)3]+ and the presence of important amounts of palmitate gives rise to all possible combinations of the two carboxylate anions in these complexes. Smaller amounts of Li2Na, Li3Na, and Li3K complexed with the appropriate number of anions give evidence of alkali metal impurity.