Characterisation of fulvic acids and glycyrrhizic acid by time-of-flight secondary ion mass spectrometry

Fulvic acids of different origin, spray deposited on polished silicon after dissolution in high-purity water without any additives, were analysed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) in combination with oblique 24...36 keV SF5+ ion bombardment. The observed, highly reproducible mass spectra cover more than five orders of magnitude in dynamic range, without background subtraction. Apart from lines due to atomic ions and low-mass ion fragments, the mass spectra exhibit broad maxima between m/z 200...350, mainly due to a beat-like superposition of lines at every single mass number, up to at least m/z 400. In the negative ion spectra the beats have a spacing of m/z 14, corresponding to a CH2-unit. The high-mass tails of the spectra extend well beyond m/z 5000, with similar slopes in the positive and the negative ion mass spectra. The negative spectra appear to be less affected by fragmentation products than the positive spectra. Fulvic acids (FAs) of different origin show distinctly different spectra, with mean masses ranging between m/z approximate to 450 and 580 (for a low-mass cut-off m/z 150). To further verify the ability of TOF-SIMS to detect molecules and clusters with masses significantly above the maxima of the FA spectra, samples of glycyrrhizic acid (GA, as GA ammonium salt with molecular weight 840) were also analysed. Parent ions as well as multimers (GA)(n) were observed as positive and negative ions, up to n = 4 (m/z 3320). The results are compared with spectra recently obtained by other mass spectrometric techniques.