Ionization and Fragmentation of Humic Substances in Electrospray Ionization Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry

Abstract

Electrospray ionization (ESI) was combined with ultra-high-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FTICR MS) to characterize complex humic and fulvic acid mixtures. Lower than expected molecular weight distributions previously observed for humics when analyzed by ESI-MS have fueled speculation about a bias in favor of low molecular weight. Multiply charged ions, ionization suppression, and sample fragmentation have all been suggested as sources of this low molecular weight bias. In this work, resolution of the individual components of humic mixtures within a 1 mass-to-charge unit window was accomplished by FTICR MS at 9.4 T. At mass resolving powers between 60,000 (high mass) and 120,000 (low mass), it was possible to determine that virtually all ions present in spectra of Suwannee River fulvic and humic acid are singly charged, thus eliminating inadequate accounting for multiply charged ions as a primary source of any low molecular weight bias. The high-resolution mass spectra also revealed the presence of molecular families containing ions that differ from each other in degree of saturation, functional group substitution (primarily CH vs N and CH4 vs O), and number of CH2 groups. Ionization suppression and ion fragmentation were addressed for humic and fulvic acid mixtures and well-characterized poly(ethylene glycol) (PEG) mixtures with average molecular weights of 8000 and 10,000. Although these high molecular weight PEG mixtures fragment extensively under traditional positive-ion mode ESI conditions, similar fragmentation could not be confirmed for humic and fulvic acid mixtures.

Comprehensive characterization of natural organic matter by MALDI- and ESI-Fourier transform ion cyclotron resonance mass spectrometry

“…_ 1 ) T D $ F I G ] On the other hand, because in most cases the majority of peaks at even m/z are assigned to the 13 C isotope of peaks in high resolution mass spectra of NOM, only peaks at odd m/z were considered for CHO-molecular formulas assignment in the previous studies [5,19]. However, interestingly, during processing mass spectra in MALDI negative mode in our study, a significant portion of peaks at even m/z can also be assigned as CHOmolecules except for the 13 C isotope peaks.…”

“…As a result, the extent of molecular information about NOM is largely restricted. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), due to its ultrahigh resolution and mass accuracy, can provide molecular elemental information about NOM without chromatographic separation [5,6], and has been used for accurate characterization of NOM [7][8][9]. Electrospray ionization (ESI) is the most commonly used ionization method for FT-ICR-MS characterization of NOM because it is one of the softest ionization methods and can be easily coupled with FT-ICR-MS [10,11].…”

Electrospray and photoionization mass spectrometry for the characterization of organic matter in natural waters: a qualitative assessment

“…Kendrick mass defect analysis-Kendrick mass defect (KMD) analysis identifies homologous series of molecules by identifying ions that are separated by integer multiples of a given functional group (Kendrick 1963;Hsu et al 1992;Stenson et al 2002). For example, Equation 1 has the effect of converting to a mass scale based on CH 2 rather than 12 C (12.00000 Da).…”

Comparative study on humic substances isolated in thermal groundwaters from deep aquifers below 700 m

“…For example, Aiken et al (1989) determined using various methods the average molecular weight of SRFA as 800 Da. This discrepancy between m/z values was justified by the existing charge suppression and competition that undergoes under the applied electrospray ionization conditions (Stenson et al, 2002). The lower m/z values for HS may suggest decomposition.…”