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Office of Ground Water and Drinking Water

February 18, 2016

MEMORANDUM

SUBJECT: Basis for Expedited Approval of Thermo Fisher Scientific Orthophosphate Discrete 

Analyzer Method for Drinking Water

FROM: William A. Adams, Chemist,

SRMD/TSC

TO: The Record

Thermo Fisher Scientific submitted an orthophosphate discrete analyzer method as an alternate test 

procedure to Standard Method (SM) 4500-P E and Standard Method (SM) 4500-P F. SM 4500-P E and 

SM 4500-P F are approved in the national primary drinking water regulations at 40 CFR 141.23(k)(1)

for the determination of orthophosphate by ascorbic acid reduction followed by colorimetric 

detection.

In the Thermo Fisher discrete analyzer orthophosphate method, orthophosphate is reacted with 

ammonium molybdate and antimony potassium tartrate in an acidic medium to form an antimonyphospho-molybdate complex. The complex is then reduced by ascorbic acid to form an intensely 

blue-colored complex that can be measured spectrophotometrically (880 nm). Only orthophosphate 

forms the blue color in the analysis. The Thermo Fisher orthophosphate method uses an automated

discrete analyzer, which decreases cost through minimizing the use of chemical reagents and 

generation of waste products, along with a decreased possibility of human error compared to the 

approved manual SM 4500-P E method. In turn, the analysis of drinking water samples becomes 

easier without risking the quality of results.

A validation study was conducted to compare the performance of the Thermo Fisher 

orthophosphate method with SM 4500-P E. An initial demonstration of capability was performed 

using different matrixes (e.g. tap water from surface and ground water sources) at various 

orthophosphate concentrations. Reagent blank data to evaluate contamination, precision and 

recovery data, and detection limit data were; in addition, a method minimum reporting level (MRL)

was confirmed. The complete data summary is provided in the Thermo Fisher Scientific ATP

Validation Report. Briefly, the results are as follows:

 The detection limit calculated as MDL for the Thermo Fisher Scientific method was 

determined to be 0.00036 mg P/L compared to 0.0032 mg P/L for the reference SM 4500-P

E. MRLs were confirmed at 0.01 mg P/L for the Thermo Fisher Scientific method and 0.015

mg P/L for the reference SM 4500-P E.

 The recovery for initial precision and recovery (IPR) samples ranged from 102% to 103% 

with 0.1 to 1.0 %RSD for the Thermo Fisher Scientific method and 101% to 102% with 0.4 

%RSD for the reference SM 4500-P E. The recovery for precision and accuracy laboratory 

fortified blank (LFB) samples ranged from 102% to 103% with 0.2 to 1.1 %RSD for the 
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Thermo Fisher Scientific method and 92.0% to 104% with 0.29 to 0.96 %RSD for the 

reference SM 4500-P E.

 Mid-level continuing calibration check (CCC) recovery results ranged from 100% to 105% 

with 1.3 to 1.9 %RSD for the Thermo Fisher Scientific method and 97.5% to 104% with 1.5 

%RSD for the reference SM 4500-P E.

 The average matrix spike recovery results ranged from 92.0% to 97.9% with 0.1 to 

0.7 %RSD for the Thermo Fisher Scientific method and 94.4% to 101% with 0.24 to 3.0 

%RSD for the reference SM 4500-P E. 

Based on the results of the validation study, the Thermo Fisher Scientific discrete analyzer method was 

determined to be equally as effective as the reference SM 4500-P E for the determination of

orthophosphate in drinking water.