The filtration bottle valve is opened and 100-200 ml is run through the transfer tube and valve assembly to rinse the sampling bottle. The bottle and cap are rinsed 3 times in this fashion. During the rinsing and filling operation, the filtration bottle cap valve is used to control and direct the sample flow. After filling, the filtration bottles are inverted and placed into the filtration rack. Following filtration, clean forceps are used to transfer each filter to a combusted 16 x 100 mm glass test tube which is then covered with a 3.5 cm square piece of combusted foil.
Any water remaining in the carboy is measured to calculate the volume filtered.
This information and any other appropriate data are entered on the data sheet. Blank Determination Standards are corrected for reagent blanks while samples are corrected for field filter blanks. The mean absorbance from three field filter blanks, stored and processed in the same manner as samples, are used to correct individual sample absorbances for filter, reagent and systematic procedural contamination. Samples are combusted in 16 x 100 mm test tubes at 450oC for 4.5 hours in a muffle furnace. The samples are then allowed to cool and are immersed in 10 ml of 0.5 M HCl.