Evidence for the contribution of humic substances to conditioning films from natural waters

The presence of humic substances in conditioning films deposited on solid surfaces from natural waters was investigated using electron impact (EI), chemical ionization (CI) and secondary ion time-of-flight mass spectrometry (TOFSIMS). EI and CI spectra of a freshwater sample from a pond in Centennial Park, Sydney, Australia, showed a high degree of similarity with spectra of humic acids purchased from Fluka and Sigma as well as with reference humic acid and fulvic acid from the International Humic Substances Society, suggesting that most of the organic matter in the pond water was of humic origin. All the complex electron impact mass spectra feature series of high-intensity ions separated by 14 Da or 18 Da, which can be attributed to CH(2) and OH(2) respectively. Thermal desorption profiles of all samples generated by EI and CI were qualitatively similar. The secondary desorption peaks were less well-defined in CI compared to EI. Positive ion thermal desorption profiles displayed a two-step ionisation, with a sharp and well-defined initial desorption peak at t～50s, followed by a broader desorption peak with a maximum intensity at t ～ 100 s post-heating. The Centennial Park natural organic matter (NOM) differed from the other humic fractions in having two additional broad desorption peaks between the two described previously, and a less-defined initial peak. Infrared spectroscopy showed that proteinaceous matter in the lake water was insignificant in comparison with functional groups indicative of humic substances. TOFSIMS characterization showed almost identical spectra for Aldrich humic acid and Centennial Park NOM in the high mass region of 2000 Da to 3000 Da. Each spectrum contains approximately 25 groups of ion peaks, separated by 74 Da from group to group. Each group is composed of 6 or 7 individual peaks. The spectral features are consistent with a macromolecular structure of humic acid where aromatic rings are joined to the macrostructure via aliphatic spacer molecules.