Influence of filtration, preservation and storing on the analysis of low molecular weight organic acids in natural waters

The suitability of three filter materials (polycarbonate, cellulose acetate and glass fibre) for trace organic acids analysis (acetic, butyric, β-hydroxy-butyric, formic, lactic, maleic, methyl-succinic, malonic, oxalic, succinic and valeric) was tested by determining leached components and sorption/desorption properties. Analysis of the organic acids was made with capillary zone electrophoresis (CZE). Polycarbonate membranes were easily cleaned by leaching with Milli-Q water and showed limited adsorption of the acids. Cellulose acetate filters required a more elaborate washing before use and had a higher retention capacity. Glassfibre filters are not recommended due to severe contamination of organic compounds and release of colloids. Preservation of natural water samples (eutrophic, dystrophic) by addition of chloroform, sterile filtration and freezing (−20°C) were quantitatively evaluated, after spiking with the organic acids (500 μg/l each) and storing for 11 weeks. Freezing was the only treatment where the added acids were quantitatively recovered after 4 weeks. Sterile filtration in combination with storing of the samples in the dark at 4°C can be suitable for short time preservation (less than 30 days). However, these conditions resulted in a high variation of recovered amounts among the replicates. Storing of untreated samples at room temperature or in a refrigerator, as well as addition of chloroform should be avoided.