Samples for orthophosphate, nitrate/nitrite and ammonium are collected together with a peristaltic pump and tubing and in-line filtered (through a 40 micron polycarbonate filter) into new, 20 ml HDPE plastic containers with conical caps. The samples are stored frozen until analysis, which should occur within 6 months of freezing. The samples are analyzed for orthophosphate (nitrate/nitrite, and ammonium simultaneously) by automated colorimetric spectrophotometry, using a segmented flow autoanalyzer. Orthophosphate is determined by forming a phosphoantimonylmolybdenum complex and the absorption is monitored at 880 nm.
The detection limit for orthophosphate is approximately 3 ppb and the analytical range for the method extends to 400 ppb.
Method Log: LTER began running orthophosphate during the summer of 2007 following the method described above. Orthophosphate is determined by an Astoria-Pacific Astoria II segmented flow autoanalyzer.
field & laboratory