Complementary Elucidation of the Molecular Characteristics of Groundwater Dissolved Organic Matter Using Ultrahigh-Resolution Mass Spectrometry Coupled with Negative- and Positive-Ion Electrospray Ionization

Abstract

The formula assignment of the Fourier transform ion cyclotron resonance mass spectrometry coupled with positive-ion electrospray ionization [ESI(+)-FT-ICR MS] is challenging because of the extensive occurrence of adducts. However, there is a paucity of automated formula assignment methods for ESI(+)-FT-ICR MS spectra. The novel automated formula assignment algorithm for ESI(+)-FT-ICR MS spectra developed herein has been applied to elucidate the composition of dissolved organic matter (DOM) in groundwater during air-induced ferrous [Fe(II)] oxidation. The ESI(+)-FT-ICR MS spectra of groundwater DOM were profoundly impacted by [M + Na]+ adducts and, to a lesser extent, [M + K]+ adducts. Oxygen-poor and N-containing compounds were frequently detected when the FT-ICR MS was operated in the ESI(+) mode, while the components with higher carbon oxidation states were preferentially ionized in the negative-ion electrospray ionization [ESI(−)] mode. Values for the difference between double-bond equivalents and the number of oxygen atoms from −13 to 13 are proposed for the formula assignment of the ESI(+)-FT-ICR MS spectra of aquatic DOM. Furthermore, for the first time, the Fe(II)-mediated formation of highly toxic organic iodine species was reported in groundwater rich in Fe(II), iodide, and DOM. The results of this study not only shed light on the further algorithm development for comprehensive characterization of DOM by ESI(−)-FT-ICR MS and ESI(+)-FT-ICR MS but also highlight the importance of appropriate treatment of specific groundwater prior to use.

Spectral Quality Assessment Strategy Based on the 13C-Isotopic Pattern for the Fourier Transform Ion Cyclotron Resonance Mass Spectra of Dissolved Organic Matter

“…The 13 C-isotopic signal is a critical feature in organic mass spectrometry and has been employed to estimate the carbon number and assign the optimum chemical formula for monoisotopic peaks. 4,12,16 In this study, the 13 C-isotopic pattern has been implemented to indicate the extent of the space-charge effect for the FT-ICR MS spectra. Subsequently, the universal 13 C-isotopic pattern-based strategy has been developed to optimize the data quality of FT-ICR MS by inspecting the mass error and 13 C-isotopic peak intensity deviation.…”

“…Dissolved organic matter (DOM) is a complex mixture of organic compounds that play a vital role in the global carbon cycle. DOM transports trace metals and pollutants, and it affects the quality of drinking and effluent water during chlorine disinfection. − The molecular characterization of DOM is challenging due to its highly complex nature until the advent of ultrahigh-resolution mass spectrometry, including Orbitrap mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Particularly, FT-ICR MS is capable of revealing tens of thousands of peaks in DOM samples by providing the best achievable mass accuracy over a broad dynamic mass-to-charge ( m / z ) range and has been the first option for the molecular characterization of DOM at the molecular level. − …”

“…Sample preparation and FT-ICR MS measurements are critical to comprehensively characterize the molecular composition of DOM from various matrixes. In contrast to numerous studies focused on the sample preparation methods, ,,, ionization methods, , and postdata analysis, − few attempts have been conducted to investigate the effects of instrument parameters on the data quality of FT-ICR MS spectra. The FT-ICR MS spectra data quality is vital to the mass accuracy and peak intensity, which profoundly affect the subsequent formula assignment for DOM samples.…”

FT-ICR MS Spectral Improvement of Dissolved Organic Matter by the Absorption Mode: A Comparison of the Electrospray Ionization in Positive-Ion and Negative-Ion Modes

“…The absorption mode will facilitate the FT-ICR MS-based complementary analysis of DOM because multiple ionization sources and ionization charge modes favor the complementary analysis of organic mixtures. 26,40,41 Furthermore, FT-ICR MS peaks of the deuterium (D)labeled DOM sample in the hydroxyl and/or carboxylic functional groups are frequently overlapped and require higher mass resolution to identify D-labeled peaks satisfactorily. 31,42,43 For example, as illustrated in Figures S31 and S32, the overlapped ESI( −)-FT-ICR MS peak series in the magnitude mode caused by mass doublets of 13 C 1 H 1 versus C 1 D 1 (Δmass = 2.92 mDa, ΔDBE = 0) and D 2 versus H 4 (Δmass = 3.10 mDa, ΔDBE = 1) at nominal masses of 551 and 552 can be entirely separated by the absorption mode.…”

“…The molecular parameters [such as modified aromaticity index (AI mod ), the nominal oxidation state of carbon (NOSC), and DBE], chemical classes, and elemental composition were calculated using the equations and criteria described elsewhere. 26 For the statistical analysis, a one-way analysis of variance (ANOVA) was employed to statistically test the significant difference at p < 0.05 for variables that passed the normality test. Otherwise, the nonparametric Kruskal−Wallis one-way ANOVA was used.…”