The task of identifying dissolved organic matter (DOM) molecular components presents a significant challenge because they are polyfunctional and heterogeneous (containing C, H, N, O and S), and vary greatly in molecular weight and concentration.  FT-ICR MS at high magnetic field (>7 T) is unrivaled for achieving the resolution and accuracy required to determine DOM molecular composition over a wide mass range (200 < m/z < 1000).  With this technique, it is possible to characterize DOM samples from different sources by determining the molecular constituents (see composition assignments below).