Determination of low-molecular-weight organic acid concentrations in seawater and pore-water samples via HPLC

A method is described for the preparation of 2-nitrophenylhydrazide derivatives of low-molecular-weight organic acids in aqueous samples and their subsequent analysis via HPLC with absorbance detection. The method has been optimized for derivatization of marine sediment pore-water samples and is suitable for use with fresh or saline waters with variable carbonate alkalinity. The HPLC analysis utilizes a reversed-phase column and ion-pairing solvents. Baseline resolution of lactate, acetate, formate, propionate, n- and iso-butyrate, and n- and iso-valerate is demonstrated. The chemical behavior of the derivatives allows their preconcentration on a concentrator column in the sample loop. The concentration sensitivity of the method is thereby extended into the sub-micromolar range and is currently limited only by the blank. The derivatives are colored and the absorption of derivatized samples at 550 nm can be used as the basis of a colorimetric assay for total carboxylic acids. Examples of the use of this method for quantifying C-1 to C-5 organic acids and total carboxylic acids in marine sediment pore waters are given.