Depend-ing on the particle concentration, 500-2000 mL of the sample was filtered through 0.2 ím cellulose nitrate filters (Sartorius). The dry net weight of the particles on the filter was determined. Next,4mLofa210-3 M phosphate solution was added to the filter. After 15 min, the solution was removed by a syringe and filtered through a 0.45 ím filter. The net weight of the remaining phosphate solution was measured. The subsequent treatment was the same as for dissolved EDTA. The method gives an overall preconcentration factor off )f1f2, where f1is the volume of water filtered per 4 mL andf2is the volume of phosphate solution after desorption. This factor is in the range between 200 and 1500 for surface waters with particle concentrations between 0.5 and 10 mg/L.