Optimization and quality control of suspended particulate matter concentration measurement using turbidity measurements

The dry weight concentration of suspended particulate material, SPM (units: mg L −1 ), is measured by passing a known volume of seawater through a preweighed filter and reweighing the filter after drying. This is apparently a simple procedure, but accuracy and precision of SPM measurements vary widely depending on the measurement protocol and experience and skills of the person filtering. We show that measurements of turbidity, T (units: FNU), which are low cost, simple, and fast, can be used to optimally set the filtration volume, to detect problems with the mixing of the sample during subsampling, and to quality control SPM. A relationship between T and ‘optimal filtration volume’, V opt , is established where V opt is the volume at which enough matter is retained by the filter for precise measurement, but not so much that the filter clogs. This relationship is based on an assessment of procedural uncertainties in the SPM measurement protocol, including salt retention, filter preparation, weighing, and handling, and on a value for minimum relative precision for replicates. The effect of filtration volume on the precision of SPM measurement is investigated by filtering volumes of seawater ranging between one fifth and twice V opt . It is shown that filtrations at V opt maximize precision and cost effectiveness of SPM. Finally, the 90% prediction bounds of the T versus SPM regression allow the quality control of SPM determinations. In conclusion it is recommended that existing SPM gravimetric measurements be refined to include measurement of turbidity to improve their precision and quality control.