A further processing step was applied to ensure complete removal of any residual column material which might be adding to the total sample mass. The mixture of material was dissolved in 500 mL of a 0.1% solution of 25% ammonium hydroxide in LCMS grade water. This mixture was washed with 500 mL of CHCl3 three times. This led to three separate liquid components: an aqueous phase, an organic phase, and an emulsion layer. The aqueous and organic phases were concentrated under reduced pressure using a rotary evaporator, then freeze dried. The emulsion layer (~ 100 mL) was mixed with 500 mL of a 0.1% solution of 25 % NH4OH in LCMS grade water and stirred for 30 minutes to break down the majority of the emulsion. The mixture was then washed with 500 mL of CHCl3 another three times. Again, this led to an aqueous phase, an organic phase, and an emulsion layer. These liquid components were concentrated under reduced pressure, before being freeze dried. Five separate dried components were obtained: two aqueous, two organic, and one from the emulsion. H NMR and LC-MS analysis of these five fractions showed that the material presumed to be from the column had been transferred to the organic phases, while the vast majority of the DOM had remained in the aqueous phases. While both the column material and the DOM were observed in the emulsion fraction, this accounted for just ~ 2% of the total dried mass and was discarded, along with the organic portions. The two aqueous portions were re-dissolved in 50 mL of a 0.1% solution of 25% NH4OH in LCMS grade water, combined, dried under reduced pressure, and freeze dried to produce a dried mass of 1058 mg. This was re-dissolved in 100 mL of 10 % MeOH in MilliQ water with 0.1 % NH3, aliquoted for distribution, and dried down a final time using a Speedvac (1 mg and 10 mg aliquots) or freeze drier (larger aliquots). A 1 mg sub-sample was re-diluted in 41 mL MilliQ water, giving a DOM concentration of 24.7 mg L -1 . DOC concentration in this sample was 10.1 mg L -1 , equivalent to 41 % DOM, which is in line with expectations and, along with detailed characterisation data (see later), suggests that removal of column material was successful.