Table B.3 Precision and accuracy (expressed as %) of phosphate, silicate, nitrate+nitrite, 
ammonium, and nitrite Data. 
Measure 
P0 4 ' 
Si(OH) 4 
NO 3 +NO 2 " 
nh 4 + 
N0 2 ‘ 
Precision, % 
97.6 
(n=265) 
98.3 
(n=258) 
98.2 
(n=266) 
98.0 
(n=265) 
94.6 
(n=7) 
Accuracy, % 
98.2 
(n=82) 
98.2 
(n=253) 
96.5 
(n=195) 
96.8 
(n=140) 
97.9 
(n=4) 
Total Phosphorous and Total Nitrogen 
Total nitrogen and total phosphorous analysis was performed by the Willamette Research 
Station (WRS), an EPA (WED) research facility in Corvallis Oregon. A Lachat Quikchenv 
8000 Two-Channel FIA was used for analysis. WRS adheres to strict EPA QA/QC procedures 
and the following procedures are detailed in the standard operating procedure document WRS 
34A.3 (2005). 
A second source check standard (SSCS) (NIST traceable) was included in each 
automated analyses run. Instrument calibration and stability was validated after every tenth 
samples. A blank (Reagent water) was run after each SSCS to ensure negligible carryover. 
Calibration verification was monitored throughout the run by checking SSCS recovery. If 
measurement exceeded ±10% of the theoretical value, the instrument was recalibrated. A control 
check standard (QCCS) is a bulk sample digested at least once each digest batch set to show 
inter-run consistency. The bulk sample is collected as needed from a local stream or river and 
prepared in the same manner as samples. An analytical duplicate was run as a separate analysis 
(digestion and analysis) no less than once every 10 samples. Inter-run consistency and column 
performance was monitored with the QCCS (bulk sample) that is analyzed once each analytical 
run. 
Three of each of the TN and TP quality control check standards (nicotinic acid and 
ascorbic acid) were digested with each batch to verify TN and TP recovery. Three reagent water 
blanks were digested to determine the nitrogen and orthophosphate blank present in the mixed 
persulfate digestion reagent. Digested standards and blanks were used to monitor digestion 
efficiency and background contribution of the persulfate. A method detection limit is the 
minimum concentration of an analyte that can be measured and reported with 99% confidence. 
138 
