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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.