Determination of hydrophobicity of NOM by RP-HPLC, and the effect of pH and ionic strength

A reverse phase high performance liquid chromatography method for the determination of the hydrophobicity of natural organic matter (NOM) was developed by modification of an existing method based on elution with acetic acid/ acetonitrile. The modified method separates the NOM into two well-defined groups according to their retention on a C18 column. The recovery was 101 +/- 11% for samples containing between 4 and 40 mg/L dissolved organic carbon (DOC). The method was tested for effects of ionic strength up to 0.8 eq. /L. Testing the method on eleven well-described NOM samples at pH 4.7 and 7.0 revealed the strong pH dependency of the hydrophobicity. The effect of pH on the hydrophobicity could be predicted from the proton binding properties of the NOM. Application of the method to NOM samples fractionated by HPSEC demonstrated considerable variations in hydrophobicity among the fractions, particularly at low pH.