Fact or artifact: the representativeness of ESI-MS for complex natural organic mixtures

Abstract

Because mass spectrometers provide their own dispersion and resolution of analytes, electrospray ionization mass spectrometry (ESI-MS) has become a workhorse for the characterization of complex mixtures from aerosols to crude oil. Unfortunately, ESI mass spectra commonly contain multimers, adducts and fragments. For the characterization of complex mixtures of unknown initial composition, this presents a significant concern. Mixed-multimer formation could potentially lead to results that bare no resemblance to the original mixture. Conversely, ESI-MS has continually reflected subtle differences between natural organic matter mixtures that are in agreement with prediction or theory. Knowing the real limitations of the technique is therefore critical to avoiding both over-interpretation and unwarranted skepticism. Here, data were collected on four mass spectrometers under a battery of conditions. Results indicate that formation of unrepresentative ions cannot entirely be ruled out, but non-covalent multimers do not appear to make a major contribution to typical natural organic matter spectra based on collision-induced dissociation results. Multimers also appear notably reduced when a cooling gas is present in the accumulation region of the mass spectrometer. For less complex mixtures, the choice of spray solvent can make a difference, but generally spectrum cleanliness (i.e. representativeness) comes at the price of increased selectivity.

Complexity of dissolved organic matter in the molecular size dimension: insights from coupled size exclusion chromatography electrospray ionisation mass spectrometry

“…Generally, each m/z in the range 150-800 was detected over a broad retention time range from 9-12 minutes, and all assignments were singly charged, as is typical for DOM. 49 High resolution mass spectrometry allows accurate formula assignment of the material eluting from HPSEC. The resolution and mass accuracy of the Orbitrap instrument dictates the mass range over which formulas can be assigned (up to about m/z 800).…”

An international laboratory comparison of dissolved organic matter composition by high resolution mass spectrometry: Are we getting the same answer?

“…In the vast majority of recent DOM research, HRMS has been coupled to electrospray ionization (ESI), as this 'soft' ionization technique allows intact molecular ions to enter the mass spectrometer without fragmentation (Fenn et al 1989;Henry et al 1989;Novotny et al 2014). With ESI-HRMS, signal response is not necessarily linearly related to the concentration of analyte (Kujawinski et al 2002), or the linear response range may be very narrow.…”

Comparing the Spectroscopic and Molecular Characteristics of Different Dissolved Organic Matter Fractions Isolated by Hydrophobic and Anionic Exchange Resins Using Fluorescence Spectroscopy and FT-ICR-MS

“…One of the prevailing problems in FT-ICR MS analysis is the formation of adducts and/or multimers during ionization [22,45]. Although adduct formation is a more common phenomenon in the positive versus the negative ionization mode, it can also occur in the negative mode, especially when samples contain significant amounts of inorganic salts and oxyanions [46]. These adducts may interfere with ionization in the electrospray process, resulting in a low-ion intensity of the analyte and contributing to low molecular detection [45,47].…”