Comparison of sample storage protocols for the determination of nutrients in natural waters

There have been several reports on storage protocols for the determination of nutrients in natural waters but each one has been limited to a particular sample matrix and they have reached different, matrix specific conclusions. The aim of this study was therefore to systematically apply the various recommended storage protocols to a range of natural water matrices. Samples from four contrasting sites in the UK, collected in late winter (February, 1999), were filtered and stored under different conditions (-80 degrees C, -20 degrees C, 4 degrees C and at 4 degrees C and -20 degrees C with 0.1% (v/v) chloroform) for up to 247 days prior to analysis. The sites were the River Frome in Dorset (a chalk stream catchment) and three sites from the Tamar Estuary (draining a non-chalk catchment) with salinities of 0.5 per thousand, 10 per thousand and 34 per thousand, Samples and controls were analysed for total oxidised nitrogen (TON) and filterable reactive phosphorus (FRP) using a segmented flow analyser with spectrophotometric detection. To investigate possible seasonal effects (particularly changes in biological and chemical matrix composition). a second sampling campaign was undertaken in early autumn (October, 1999). The results showed that the optimum storage conditions for the determination of TON and FRP were highly matrix dependent. with significant differences in FRP stability between the Frome and Tamar catchments (due to different calcium concentrations) and between samples of different salinities (due to different bacterial populations and/or dissolved organic matter). General guidelines for sample handling and storage are listed and matrix specific recommendations presented for samples rich in calcium and dissolved organic matter.