Effects of sample properties and mass spectroscopic parameters on electrospray ionization mass spectra of size-fractions from a soil humic acid |
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Authors: | A Piccolo M Spiteller A Nebbioso |
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Institution: | 1. Dipartimento di Scienze del Suolo, della Pianta, dell’Ambiente, delle Produzioni Animali, Università di Napoli Federico II, Via Università 100, 80055, Portici, Italy 2. Institute of Environmental Research (INFU), University of Dortmund, Otto-Hahn-Strasse 6, 44227, Dortmund, Germany
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Abstract: | Size-fractions from a soil humic acid were separated by preparative size-exclusion chromatography (SEC), desalted, and concentrated
by ultrafiltration and vacuum centrifugation without being subjected to any freeze-drying process. After having assessed the
lack of formation of any multiple-charged ions by high-resolution Fourier transform ion cyclotron resonance electrospray ionization
(ESI) mass spectrometry (MS), the size-fractions were used by direct infusion to compare the molecular ion distribution by
both atmospheric pressure chemical ionization (APCI)- and ESI-MS in negative mode. The weight- (Mw) and number-averaged (Mn) molecular weight obtained by ESI-MS were invariably larger than by APCI-MS for all size-fractions, thereby indicating that
ESI is more efficient than APCI to evaluate the molecular mass distribution of humic samples. No substantial difference was
observed when concentration and pH of unfreeze-dried humic size-fractions were varied. The negative mode was applied to assess
the effect of cone voltage from −20 to −60 V on ESI of the humic size-fractions further separated through an on-line SEC column.
The resulting mass spectra and Mw and Mn values suggested that the variation of cone voltage in ESI-MS affects the ionization potential of associated humic molecules
more in solution rather than their fragmentation. These findings agree with previous observations which indicated a limitation
of ESI in providing consistent mass detection for a complex mixture of heterogeneous humic molecules, especially when they
are aggregated by a freeze-drying process. |
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