Appendix 2
During preliminary tests to measure suspended sediment concentrations with the WTS-Tr, we suspected that the internal top frit supplied with the filter holder by the manufacturer (Figure 5) was removing (pre-filtering) some fraction of the natural assemblage of the suspended matter sample that was being suction filtered onto the Nucleopore filter. This was evaluated in the laboratory by vacuum filtering aliquots from freshly collected seawater through in-line filter holders with and without the internal top frit. The samples were filtered at approximately the same vacuum as pressure created by the WTS-Tr instrument (10-15 inches of mercury). Test results showed that the average mean concentration of suspended matter collected with a top frit in place was 44% lower than the mean value with the frit removed (Table 1). This indicates significant trapping of material by the top frit. The frit has large openings (1mm x 2mm) but can trap filamentous organic matter on its gridwork. Based on these test results, the top frits have been removed from all filter holders during laboratory and field-testing and actual long-term field deployments.
In addition, these results were compared to replicate samples filtered through a standard glass Millipore chimney type filter holder that has no tubing or other potential constriction to the flow. The suspended matter concentrations determined with the in-line filter holder without top frit were an average of 14% lower than measured using the Millipore holder. A probable explanation for the lower values is that some adsorption of particles by the tubing and filter housing takes place. Samples of surface coastal waters containing filamentous or gelatinous particles of biological origin are expected to experience greater losses flowing through tubing and connectors than resuspended bottom sediment that is likely to have a greater fraction of lithogenic particles
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