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The molecular level characterization of complex mixtures remains an analytical challenge. We have shown that the integration of complementary, high resolution, gas-phase separations allow for chemical formula level isomeric content description. In the current work, we revisited the current challenges associated with the analysis of dissolved organic matter using high resolution trapped ion mobility separation (TIMS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In particular, we evaluate the separation capabilities provided by TIMS-MS compared to MS alone, the use of ICR complementary data acquisition (DAQ) systems and transient processing strategies, ICR cell geometries (e.g., Infinity cell vs harmonized cell), magnetic field strengths (7 T vs 9.4 T vs 21 T) for the case of a Harney River DOM sample. Results showed that the external high-performance DAQ enables direct representation of mass spectra in the absorption mode FT (aFT), doubling the MS resolution compared to the default magnitude mode FT (mFT). Changes between half- vs full- apodization results in greater MS signal/noise vs superior MS resolving power (RP); in the case of DOM analysis, a 45% increase in assigned formulas is observed when employing the DAQ half (Kaiser-type) apodization window and aFT when compared to the default instrument mFT. Results showed the advantages of reprocessing 2D-TIMS-FT-ICR MS data with higher RP and magnetic field chemical formulae generated list acquired (e.g., 21 T led to a 24% increase in isomers reported) or the implementation of alternative strategies. 

In the present work, the advantages of ESI-TIMS-FT-ICR MS to address the isomeric content of dissolved organic matter are studied. While the MS spectra allowed the observation of a high number of peaks ( e.g. , PAN-L: 5004 and PAN-S: 4660), over 4× features were observed in the IMS-MS domain ( e.g. , PAN-L: 22 015 and PAN-S: 20 954). Assuming a total general formula of C x H y N 0–3 O 0–19 S 0–1 , 3066 and 2830 chemical assignments were made in a single infusion experiment for PAN-L and PAN-S, respectively. Most of the identified chemical compounds (∼80%) corresponded to highly conjugated oxygen compounds (O 1 –O 20 ). ESI-TIMS-FT-ICR MS provided a lower estimate of the number of structural and conformational isomers ( e.g. , an average of 6–10 isomers per chemical formula were observed). Moreover, ESI-q-FT-ICR MS/MS at the level of nominal mass ( i.e. , 1 Da isolation) allowed for further estimation of the number of isomers based on unique fragmentation patterns and core fragments; the later suggested that multiple structural isomers could have very closely related CCS. These studies demonstrate the need for ultrahigh resolution TIMS mobility scan functions ( e.g. , R = 200–500) in addition to tandem MS/MS isolation strategies.