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 C1 to C5 organic acids and total carboxylic acids in marine sediment pore waters are given.