Rios 5

The 100 ml of original freshwater samples were filtered using cellulose filters (Millipore) with a pore size of 0.45 μm and freeze dried. Before the analysis 1 mg of each dry DOM sample was completely dissolved in 2 ml of 30 mM phosphate buffer, pH 6.5 (PB). The 0.3 ml of each sample solution was diluted ten times using 30 mM PB and absorbance at 270 nm was measured for diluted solutions with C=50 mgL -1 . Using this data the A270 for the concentrated DOM solutions (C=500 mg L -1 ) was calculated. Than all concentrated DOM solutions were diluted using 30 mM PB to A270 = 5 a.u. As a result, the stock DOM solutions had identical A270=5 a.u. and different concentration (C SRDOM =362 mgL -1 , C L1DOM =371 mgL -1 , C L2DOM =423 mgL -1 ). Before further spectroscopic and chromatographic analyses stock DOM solutions were stored in sealed glass bottles at +4°C. Phosphate buffer (pH 6.5) was prepared in water at concentration of 30 mM by mixing three volumes of 30 mM Na 2 HPO 4 solution (pH 9.8) and seven volumes of 30 mM NaH 2 PO 4 solution (pH 4.5). Water was purified using a reverse osmosis RIOS 5 and Synergy (Millipore) device (resistivity 18 M cm, DOC < 0.1 mg L -1 ). All reagents were of the highest grade available.