Response to Comment on “Laser Desorption/Ionization Coupled to FTICR Mass Spectrometry for Studies of Natural Organic Matter”

We have recently illustrated that the LDI and MALDI spectra of the Suwannee River fulvic acid (SRFA) and Suwannee River natural organic matter (NOM) contain complementary signals to spectra obtained by negative electrospray (Blackburn, J. W. et al., Anal. Chem. 2017, 89 (8), 4382–4386). In that work we presented an ESI‐(‐) FTICR MS spectrum of SRFA that showed bimodal distribution of peaks (200‐400 m/z and 400‐800 m/z). He et al. Anal. Chem., 2018, 90 (9), pp 5965–5967, DOI: 10.1021/acs.analchem.7b05023 commented on this unusual appearance and suggested that the higher m/z values are due to dimerization. In our response we demonstrate that (i) a monomodal spectrum, acquired using different experimental parameters, produced 86% identical molecular formulae assignment to that obtained for the bimodal distribution spectrum; (ii) the heteroatom class distribution was practically identical for both spectra; (iii) taking several different experimental approaches, we did not find any evidence of oligomerisation in our spectra (iv) we showed that different experimental parameters enhance peak intensities in different m/z regions of the FTICR MS
spectra and produce spectra of SRFA containing peaks outside of the narrow 200‐700 m/z window (iv) we showed that it is beneficial to explore these different experimental settings to obtain a fuller coverage of the molecular space of NOM and that one should always treat the peak intensities with caution. This analysis allowed us to reaffirm the conclusions of our original paper: ESI and LDI are complementary techniques and both should be used for a more complete characterisation of complex NOM.