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APPLICATION NOTE:

Drinking Water Orthophosphate for Thermo 

Scientific Gallery discrete analyzer

Name of the method: Drinking Water Orthophosphate

Reference: Standard Methods (SM) 4500-P E. Phosphorous /Ascorbic Acid Method. 

EPA approved method under 40 C.F.R. § 141.23 (National Primary 

Drinking Water Regulations, NPDWR, based on SDWA, Safe Drinking 

Water Act).1

Intended use: This paper presents Drinking Water Orthophosphate method for Thermo 

Scientific GalleryTM discrete analyzer. The method was designed to 

follow the standard method SM 4500-P E. as closely as possible. 

Revision number: 5

Revision date (mm/dd/yyyy): 02/18/2016

1. Scope and Application 

1.1 This automated method covers the determination of orthophosphate (CAS: 14265-44-2) in 

drinking water with the Thermo ScientificTM GalleryTM discrete analyzer. 

1.2 The method is based on reaction that is specific for the orthophosphate ion.

1.3 The applicable range for this method is from 0.0125 – 0.5 mg PO4-P/L. An extension of the 

range to 5 mg PO4-P/L is achieved when 1:10 automated dilution is configured. The 

automated dilution feature must be confirmed with acceptable analysis of quality control 

samples by the user.

2. Summary of Method

2.1 Ammonium molybdate and antimony potassium tartrate react in an acidic medium with 

dilute solutions of phosphorous to form an antimony-phospho-molybdate complex. This 

complex is reduced to an intensely blue-colored complex by ascorbic acid. The color is 

proportional to the orthophosphate concentration.

2.2 Only orthophosphate forms a blue color in the test. Polyphosphates (and some organic 

phosphate compounds) may be converted to the orthophosphate form by sulfuric acid 

hydrolysis. Organic phosphate compounds may be converted to the orthophosphate form by 

persulfate digestion. These conversion methods are not covered in this Application Note.
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3. Definitions

3.1 Units and symbols from the international metric system (SI) are used. Definitions, acronyms, 

and abbreviations are explained as they occur for the first time.

4. Interferences

4.1 No interference is caused by copper, iron or silicate at concentrations many times greater 

than their reported concentration in seawater. However, high iron concentrations can cause 

precipitation of, and subsequent loss, of phosphorous.

4.2 Arsenates react with the molybdate reagent to produce a blue color similar to that formed 

with orthophosphate. Thus, it should be considered when present in concentrations higher 

than orthophosphate. Concentrations as low as 0.1 mg as/L can interfere with the 

orthophosphate determination.

4.3 Hexavalent chromium and nitrite-ion can interfere with the orthophosphate determination 

yielding in approximately 3 % lower results at concentrations of 1 mg/L and 10 % to 15 % 

lower results at 10 mg/L concentrations.

4.4 Sample turbidity is not to be removed by filtration prior to analysis for orthophosphate, 

unless dissolved reactive phosphorous is studied. Samples for total phosphorous may be 

filtered only after digestion. Sample color that absorbs in the photometric range used for 

analysis will also interfere. The sample blank is measured automatically in the application 

for sample color and turbidity correction.

5. Safety 

5.1 The toxicity and carcinogenicity of each reagent used in this method have not been fully 

established. Each chemical should be regarded as a potential health hazard and exposure 

should be as low as reasonably achievable.

5.2 The following chemicals have the potential to be toxic or hazardous. Consult MSDS 

(Material Safety Data Sheet) for details.

5.2.1 Sulfuric acid (contained in Phosphate R1) (CAS# 7664-93-9)

5.2.2 Ammonium molybdate tetrahydrate (contained in Phosphate R1) (CAS# 12027-67-7)

5.2.3 Antimony potassium tartrate (contained in Phosphate R1) (CAS# 28300-74-5)

5.2.4 Ascorbic acid (contained in Phosphate R2) (CAS# 50-81-7)

5.2.5 Potassium phosphate monobasic (CAS# 7778-77-0)
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6. Equipment and Supplies 

6.1 Balance: Analytical, capable of accurately weighing to the nearest 0.0001 g. 

6.2 Water purification system for producing suitable water for autoanalyzers. Refer to instrument 

user manual.

6.3 Thermo Scientific Gallery, automated discrete photometric system.

6.4 Filter, wavelength 880 nm.

6.5 DECACELLTM Cuvettes for Thermo Scientific Gallery. DECACELL cuvettes must always 

be used with Thermo Scientific Gallery. Cuvettes are for single use only.

6.6 Washing solution 4.5 % hypochlorite solution, 4 × 20 mL, is used for daily instrument 

cleansing.

7. Reagents and Standards

Reagents for Gallery: THERMO SCIENTIFIC ORDERING NUMBERS

984366 Phosphate R1, 4 × 20 mL 

984367 Phosphate R1, 20 × 20 mL

984368 Phosphate R2, 4 × 20 mL

Standards for Gallery: THERMO SCIENTIFIC ORDERING NUMBER

984729 Phosphate (as P) Std, 1000 ppm, 500 mL (calibration)

984726 Phosphate (as PO4) Std, 1000 ppm, 500 mL (QCS)

7.1 Preparation of reagents needed in this method is described under. Also ready to use reagents 

are available for this method.

7.2 Reagent water — Distilled or deionized water, free of the analyte of interest. Water stored in 

bottles should be substituted by fresh water after one week.

7.3 Phosphate R1 — Prepared by adding 15 mL of ammonium molybdate solution (20 g 

(NH4)6Mo7O24∙4H2O /500 mL water) to 50 mL of sulfuric acid solution (70 mL conc. H2SO4

/500 mL water) followed by 5 mL addition of antimony potassium tartrate solution (1.5 g 

K(SbO)C4H4O6∙1⁄2H2O /500 mL water) to this mixture. 

7.4 Phosphate R2 — Ascorbic acid solution containing 1.76 g ascorbic acid in 100 mL water. 

This solution is stable for 5 days when refrigerated between 2 -8 °C. 

7.5 Phosphate (as P) Std, 1000 ppm — Phosphate standard 1000 mg/L as P is for calibrating the 

Gallery instrument.

7.6 Phosphate calibration solutions — Prepare an appropriate series of standards by diluting 

suitable volumes of Phosphate standard (stock) with distilled water, or use automated 

Gallery calibration dilution feature.

7.7 Quality Control Solution (QCS) — A second source standard from an external source, e.g.

Phosphate (as PO4) Std, 1000 ppm (326.2 ppm as P). Dilute to appropriate concentration 

with distilled water. Do not use Phosphate calibration solutions as QCS-samples.
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7.8 Laboratory Fortified Blank (LFB) — Phosphate standard (stock) 1000 mg/L or self made

standard solution. Dissolve 0.4394 g of predried (105 °C for one hour) potassium phosphate

monobasic (KH2PO4) in distilled water and dilute to 1000 mL. 1.0 mL of this solution 

contains 0.1 mg P. Dilute to appropriate concentration with distilled water. Do not use 

Phosphate calibration solutions as LFB-samples.

8. Sample Collection, Preservation, and Storage

8.1 Sample containers may be of plastic material or of Pyrex glass. All bottles must be 

thoroughly cleaned and rinsed with distilled water. Volume collected should be sufficient to 

insure a representative sample, allow for replicate analysis (if required), and minimize waste 

disposal. Do not store samples containing low concentrations of phosphorous in plastic 

bottles because orthophosphates may be adsorbed to the walls of plastic bottles.

8.2 Samples should be analyzed as soon as possible, after collection, within 48 hours at 

maximum. Samples must be cooled and maintained at 4 °C.

9. Quality Control

9.1 Each laboratory using this method is required to operate a formal quality control (QC) 

program. The minimum requirements of this program consist of an initial demonstration of 

laboratory capability and the periodic analysis of laboratory reagent blanks, continuing 

calibration check standards, fortified blanks and fortified samples as duplicates as a 

continuing check on performance. The laboratory is required to maintain performance 

records that define the quality of the data that are generated.

9.2 The initial demonstration of capability (IDC) is used to characterize instrument performance 

in an individual laboratory by determination of calibration curve and analysis of quality 

control samples (QCS) and laboratory performance by determination of method detection 

limit (MDL), minimum reporting level (MRL) and the initial precision and recovery (IPR) 

test.

9.2.1 Calibration Curve — Calibration curve must be determined initially and reanalyzed at 

least every six months or whenever a significant change in instrument response is 

observed or expected. The calibration curve must use a minimum of five standards and 

a blank to ensure that the resulting curve is fitted correctly. The Gallery analyzer fits the 

calibration first as a linear curve. The user can then fit the calibration to 2nd degree 

curve, if the fitting is better thereby. The correlation coefficient of the calibration curve 

should be equal to or greater than 0.995.

9.2.2 Quality Control Sample (QCS) — When beginning the use of this method, whenever 

new standard materials are used, on a quarterly basis or as required to meet data-quality 

needs, verify the calibration standards and acceptable instrument performance with the 

analyses of a secondary standard solution (QCS) from an external source. If the 

determined concentrations are not within ± 15 % or ± 20 % when the concentration is ≤ 

2x MRL of the stated values, performance of the determinative step of the method is 
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unacceptable. The source of the problem must be identified and corrected before 

proceeding with the initial determination of MDL or continuing with on-going analyses.

9.2.3 Method Detection Limit (MDL) — An MDL should be established using reagent water 

(blank) fortified at a concentration of one to five times the estimated instrument 

detection limit (MDLest). The estimate is calculated as three times the standard 

deviation of replicate instrumental measurements of the analyte in reagent water. 

MDLest = 3 × (S0)

where S0 = standard deviation of the replicate analyses (n=10) of reagent 

water.

Prepare a laboratory fortified blank sample (LFB) at a concentration which is at least 

equal to or in the same concentration range as the estimated method detection limit 

(recommended 1 – 5 x MDLest). Perform all calculations defined in the method and 

report the concentration values in the appropriate units. Each individual result must be 

within 70 – 130 % of the theoretical value. Calculate the MDL as follows:

MDL = (t) × (S)

where t = Student's t-value for a 99 % confidence level and a standard 

deviation estimate with n-1 degrees of freedom [t = 3.14 for 7

replicates]

S = standard deviation of the replicate analyses

MDL should be determined every six months, when a new operator begins work, or 

whenever there is a significant change in the background or instrument response.

9.2.4 Minimum Reporting Level (MRL) Confirmation — The minimum concentration that 

can be reported by a laboratory as a quantified value for the method analyte in a sample 

following analysis. The MRL must be at or above the level of the lowest fortified 

calibrator, where it must meet the criteria set for MRL confirmation. It would also have 

to be considered that criteria for Laboratory Reagent Blank (LRB) must be met (LRB ≤

1/3 x MRL).

Fortify and analyze seven replicate LFBs at the proposed MRL concentration.

Calculate the mean (Mean) and standard deviation (s) for these replicates. Determine 

the Half Range for the Prediction Interval of Results (HRPIR) using the equation:

HRPIR = 3.963s

Where s = standard deviation 

3.963 = constant value for seven replicates

Calculate the upper and lower limits for the Prediction Interval of Results (PIR = Mean

+HRPIR) from the results and confirm that the results meet the criteria. Accepted results 

confirm the MRL to be validated.
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Version 5 Page 6 of 20 100 



Fortified Concentration

Mean HR

PIRUpper Limit PIR 100 



Fortified Concentration

Mean HR

PIR Lower Limit PIR

Criteria: The Upper PIR Limit must be ≤ 150 % recovery.

The Lower PIR Limit must be ≥ 50 % recovery

9.2.5 Initial Precision and Recovery (IPR) — For initial precision and recovery test the 

laboratory should analyze four replicate volumes of reagent water spiked with the 

analyte of interest (LFB). Calculate accuracy as percent recovery and precision as 

relative standard deviation (% RSD) as shown under. 

 



 

n

i n

X X

X

RSD

1

2

1

1

100 ( )

%

where

X

= mean of replicate measurements

X1

= measured value of the replicate

n = number of replicates

Recovery % 100





s

Cs C

where Cs = spiked sample concentration

C = sample background concentration

s = concentration equivalent of analyte added to sample

Criteria for % Recovery: ±15% (or 85 - 115 %), when c is > 2 x MRL and 

±20% (or 80 - 120 %), when c is ≤ 2 x MRL 

Criteria for % RSD: ±15% 

9.3 Assessing laboratory performance with ongoing QC includes the use of Laboratory Reagent 

Blank (LRB), Laboratory Fortified Blank (LFB), Continuing Calibration Check (CCC)

9.3.1 Laboratory Reagent Blank (LRB) — The laboratory should analyze at least one LRB 

with each batch of samples. Data produced are used to assess contamination from the 

laboratory environment.

Criteria for LFB: Must be below 1/3 of MRL 

9.3.2 Laboratory Fortified Blank (LFB) — The laboratory should analyze at least one LFB 

with each batch of samples. Calculate accuracy as percent recovery as shown in 9.2.5. If 
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the recovery of any analyte falls outside the required control limits, the source of the 

problem should be identified and resolved before continuing analyses. LFB analyses 

data is used to assess laboratory performance against the required control limits.

Criteria for % recovery: ±15% (or 85 - 115 %), when c is > 2 x MRL and 

±20% (or 80 - 120 %), when c is ≤ 2 x MRL 

When enough data from LFB samples analyzed as part of method ongoing QC is 

gathered the laboratory can assign its own control limits from the standard deviation of 

the results.

9.3.3 Continuing Calibration Check (CCC) — For all determinations the laboratory should 

analyze the CCC (a mid-range check standard) and a calibration blank immediately 

following calibration, after every tenth sample and at the beginning and end of the 

sample run. Analysis of the CCC solution and calibration blank immediately following 

calibration must verify that the instrument is within ± 10 % of calibration. Subsequent 

analyses of the CCC solution must verify the calibration is still within ± 10 %. This 

procedure is done automatically by the analyzer. Note! Application has manual result 

acceptance instead of automatic acceptance to ensure CCC protocol is followed.

If the calibration cannot be verified within the limits specified in the application, the 

analyzer gives a message of outlier result. In such case reanalyze the CCC solution by 

triggering the Ongoing QC procedure. If the second analysis of the CCC solution 

confirms calibration to be outside the limits, sample analysis must be discontinued, the 

cause determined and/or in the case of drift the instrument recalibrated. All samples 

following the last acceptable CCC solution must be reanalyzed, even if the resumed 

CCC is acceptable.

The analysis data of the calibration blank and CCC solution must be kept on file with 

the sample analyses data.

Criteria: CCC % recovery ±10% (or 90 -110 %)

9.3.4 Quality Control Samples (QCS) — Standard solution samples (9.2.2), that must reach 

laboratory acceptance criteria are analyzed with each calibration, whenever new 

standard materials are used, on a quarterly basis or as required to meet data-quality 

needs in addition to the IDC measurements.

Criteria: ±15% (or 85 - 115 %), when c is > 2 x MRL and 

±20% (or 80 - 120 %), when c is ≤ 2 x MRL 

9.4 Laboratory Fortified Matrix Sample

9.4.1 Laboratory Fortified Matrix Sample (LFM) (also termed Matrix Spike, MS and Matrix 

Spike Duplicate MSD) — The laboratory must add a known amount of analyte to a 

minimum of 10 % of the routine samples. The analyte concentration must be high 

enough to be detected above the original sample and should not be less than four times 
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the MDL. The added analyte concentration should be the same as that used in the 

laboratory fortified blank in section 9.3.2.

Calculate the percent recovery for each analyte, corrected for concentrations measured 

in the unfortified sample, and compare these values to the designated LFM recovery 

range 85-115 %. If the recovery of any analyte falls outside the designated LFM 

recovery range and the laboratory performance for that analyte is shown to be in 

control, the recovery problem encountered with the LFM is judged to be either matrix or 

solution related, not system related. The Gallery analyzer has a standard addition feature 

for difficult sample matrixes.

100 ( )LFM Recovery %, 

 



s

C f C

R

s

where Cs = spiked sample concentration

f = spike dilution correction

C = sample background concentration

s = concentration equivalent of analyte added to sample

Note! If the added spike volume is less than 1% of the total LFM sample volume, the 

factor f can be excluded.

Criteria: LFM % Recovery: ±15%

9.4.2 Laboratory Fortified Matrix Sample (LFM) Precision — The precision of the LFM 

determinations is also assessed by measuring LFM samples in three replicates followed 

by calculation of relative standard deviation (% RSD) of the results as shown in 9.2.5. 

In routine laboratory performance evaluation LFM duplicate samples (LFMD, also 

termed Matrix Spike Duplicate or MSD) are used. Precision is then calculated as 

follows:

1⁄2 ( )

% 100

LFM LFMD

LFM LFMD RSD

 



 

where LFM = analyte concentration measured in LFM sample

LFMD = analyte concentration measured in LFM duplicate

Criteria: % RSD ±20%

10. Calibration and Standardization

10.1 Dilute Phosphate standard (stock) (Thermo Scientific) with distilled water to get a suitable 

phosphate standard for calibration. Use the automated Gallery calibration dilution feature for 

a calibration curve. Alternatively, prepare a series of at least five standards, covering the 

desired range, and a blank, for a calibration curve.

10.2 Process standards and blanks as described in Section 11.0 Procedure.
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10.3 The Gallery analyzer plots automatically instrument response against standard concentration. 

The user must accept this calibration curve before the analyzer starts to measure blanks and 

samples. The calibration correlation coefficient shall be equal to or greater than 0.995.

10.4 After the calibration has been established, it must be verified by the analysis of a suitable 

control sample (QCS). If measurements exceed ± 15 % of the established LFB value, the 

analysis should be terminated and the instrument recalibrated. Periodic reanalysis of the QCS

is recommended quarterly at minimum. Ongoing QC is done automatically by the analyzer 

and it includes analyzing LRB, CCC and LFB samples with each batch of ten samples. In 

addition LFM samples are to be done in duplicates with every batch of ten samples. 

11. Procedure

11.1 Preparation before analysis — Add all required reagents, samples, other consumables and 

requests for tests following the instrument instructions according to the manufacturer.

11.2 Gallery Drinking Water Orthophosphate test flow — To 120 μL of sample or standard or 

blank 14 μL of Phosphate R1(Thermo Scientific) is added and solution is mixed. After blank 

measurement, 6 μL of Phosphate R2 (Thermo Scientific) is added and solution is mixed. 

After 9 minutes incubation, the absorbance is measured at 880 nm. The application is shown

in Appendix A.

12. Data Analysis and Calculation

12.1 The Gallery analyzer plots automatically instrument response against standard concentration 

making a calibration curve. The instrument computes sample concentration by comparing 

sample response with the standard curve.

12.2 Results are reported in mg P/L.

13. Method Performance

13.1 According to a validation study of the Gallery Drinking Water Orthophosphate method, 

single laboratory result for MDL was 0.00036 mg/L as P and MRL was confirmed to be 

0.0125 mg/L as P. Initial precision was analyzed to be 0.1-1.0 % RSD and accuracy 102-103 

% recovery at concentrations 0.1 - 0.4 mg/L as P. 

13.2 Precision for LFB samples was 0.2 – 1.1 % RSD and for LFM samples 0.1 – 0.7 % RSD 

corresponding to sample concentrations 0.1 – 0.4 mg/L as P.

13.3 Accuracy for LFB samples was 102 – 103 % recovery and for LFM samples 92 – 98 % 

recovery corresponding to sample concentrations 0.1 – 0.4 mg/L as P.

13.4 CCC standard recovery was 100 – 105 % and LRB was always below MDL.

13.5 Method limits for LFB and LFM samples are 15 % RSD and 85 - 115 % Recovery and for 

CCC 90 - 110 % Recovery. LRB must be below MDL.
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14. Pollution Prevention

14.1 The instrument uses small amounts of reagents, which reduces the quantity of wastes 

significantly compared to manual methods or flow analyzers. The small packing size

facilitates the use of reagents during their shelf lives and thus reduces disposal cost of unused 

materials. 

15. Waste Management

15.1 Excess reagents, samples and method process wastes should be characterized and disposed 

of in an acceptable manner.

15.2 The containers for cuvette and liquid waste must be emptied and rinsed with water at the end 

of the day.

16. References

16.1 4500-P E. PHOSPHOROUS/Ascorbic acid method. Standard Methods for the Examination 

of Water and Wastewater, 22nd Edition, American Public Health Association (APHA), 

2012.

16.2 Code of Federal Regulations 40 § 141.23. Inorganic chemical sampling and analytical 

requirements.

16.3 Code of Federal Regulations 40 § 136, Appendix B — Definition and Procedure for the 

Determination of the Method Detection Limit – Revision 1.11. 

16.4 4020 Quality Assurance/Quality Control. Standard Methods for the Examination of Water 

and Wastewater, 22nd Edition, American Public Health Association (APHA), 2012.

16.5 1020-B.4 Method Detection Level Determination and Application. Standard Methods for the 

Examination of Water and Wastewater, 22nd Edition, American Public Health Association 

(APHA), 2012.

17. Tables, diagrams, flowcharts and validation data

17.1 Test parameters and test flow for Gallery Drinking Water Orthophosphate method are 

presented in Appendix A.
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17.2 Method performance data 

17.2.1 Calibration and QCS-samples

Calibration curve was prepared between 0 - 0.5 mg P/L for DW o-PO4P application. 

Calibration was done with automatic dilution from working standard solution of 1.5 mg

P/l and laboratory reagent water. Calibration was verified with QCS-samples.

Figure 1. Calibration curve for DW o-PO4P application.

Table 1. Results for Phosphate QCS-samples after calibration.

Orthophosphate as phosphorous

QCS-sample Duplicate % RSD % Recovery

PO4P 0.1mg/l 0.13 % 96 %

PO4P 0.4mg/l 0.19 % 103 %

17.2.2 Method Detection Limit (MDL)

MDL was established using reagent water (blank) fortified at a concentration

approximately five times the estimated instrument detection limit (MDLest). The 

estimate was calculated as three times the standard deviation of replicate instrumental 

measurements of the analyte in reagent water. 

MDLest = 3 × (S0)

where S0 = standard deviation of the replicate analyses (n=10) of reagent 

water.
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Table 2. Values for determining MDLest for DW o-PO4P application.

Sample Results (mg/L)

LRB m in. 0.001800 

LRB m ax. 0.002122

average = 0.00188

n = 10

std. deviation (SD) = 0.00013

MDLest calculated = 0.000402

MDLest assigned= 0.0022

NOTE! The MDLest had to be assigned higher than the calculated MDLest, because the 

highest individual LRB result of this test was as high as 0.002122 mg/l as P, and criteria 

for LRB sample in this test is, that it should never be above the MDL est.

A laboratory fortified blank sample (LFB) at a concentration which was approximately

five times the MDLest, was analyzed in seven replicates. Each individual result had to 

be within 70 – 130 % of the theoretical value. MDL was calculated as follows:

MDL = (t) × (S)

where t = Student's t-value for a 99 % confidence level and a standard 

deviation estimate with n-1 degrees of freedom [t = 3.14 for 7 

replicates]

S = standard deviation of the replicate analyses

Table 3. Values for determining MDL for DW o-PO4P application. 

Sample Result (mg/L) % Recovery

LFB 0.01 mg/l PO4P 0.00793 79 %

LFB 0.01 mg/l PO4P 0.00791 79 %

LFB 0.01 mg/l PO4P 0.00795 80 %

LFB 0.01 mg/l PO4P 0.00768 77 %

LFB 0.01 mg/l PO4P 0.00796 80 %

LFB 0.01 mg/l PO4P 0.00803 80 %

LFB 0.01 mg/l PO4P 0.00797 80 %

average = 0.007919 79 %

n = 7

std. deviation (SD) = 0.000114

% RSD = 4.5 %

MDL = 0.00036
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17.2.3 Minimum Reporting Level (MRL)

Seven replicate LFBs were analyzed at the proposed MRL concentration.

The mean (Mean) and standard deviation (s) for these replicates were calculated. The

Half Range for the Prediction Interval of Results (HRPIR) was determined using the 

equation:

HRPIR = 3.963s

Where s = standard deviation 

3.963 = constant value for seven replicates

The upper and lower limits for the Prediction Interval of Results (PIR = Mean +HRPIR)

were calculated from the results. 

100 

Fortified Concentration

Mean HR

PIRUpper Limit PIR 100 



Fortified Concentration

Mean HR

PIR Lower Limit PIR

Criteria: The Upper PIR Limit must be ≤ 150 % recovery.

The Lower PIR Limit must be ≥ 50 % recovery

Table 4. Results for MRL confirmation for DW o-PO4P application.

Sample Result (mg/L) % Recovery

LFB 0.01 mg/l PO4P 0.010088 101 %

LFB 0.01 mg/l PO4P 0.009663 97 %

LFB 0.01 mg/l PO4P 0.009568 96 %

LFB 0.01 mg/l PO4P 0.009378 94 %

LFB 0.01 mg/l PO4P 0.009891 99 %

LFB 0.01 mg/l PO4P 0.009616 96 %

LFB 0.01 mg/l PO4P 0.009605 96 %

average = 0.00969 97 %

n = 7

std. deviation (s) = 0.000232

HRPIR = 0.000921

Upper PIR Limit = 106.1 %

Lower PIR Limit = 87.7 %

% RSD= 2.4 %

Calculated limits met the criteria and confirmed the MRL to be validated.
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17.2.4 Initial Precision and Recovery (IPR)

Four replicate volumes of reagent water spiked with the analyte of interest (LFB) were 

analyzed at three different concentrations. % recovery and % RSD were calculated. 

Results are shown in table 7.

Criteria for % recovery: 85 % - 115 %, when c is > 2 x MRL and 

80 % - 120 %, when c is ≤ 2 x MRL 

Criteria for % RSD: ±15% 

Table 5. Results for initial precision and recovery (IPR) for DW o-PO4P application.

Sample Result (mg/L) % Recovery

LFB 0.1 mg/l 0.1027 103 %

LFB 0.1 mg/l 0.1028 103 %

LFB 0.1 mg/l 0.1027 103 %

LFB 0.1 mg/l 0.1028 103 %

average 0.1027 103 %

std.deviation 0.000074

% RSD 0.1 %

Acceptable YES YES

LFB 0.3 mg/l PO4P 0.3056 102 %

LFB 0.3 mg/l PO4P 0.3020 101 %

LFB 0.3 mg/l PO4P 0.3096 103 %

LFB 0.3 mg/l PO4P 0.3051 102 %

average 0.3056 102 %

std.deviation 0.0031

% RSD 1.0 %

Acceptable YES YES

LFB 0.4 mg/l 0.4074 102 %

LFB 0.4 mg/l 0.4083 102 %

LFB 0.4 mg/l 0.4088 102 %

LFB 0.4 mg/l 0.4083 102 %

average 0.4082 102 %

std.deviation 0.00059

% RSD 0.1 %

Acceptable YES YES
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17.2.5 Precision and accuracy from LFB

Precision (%RSD) and accuracy (% Recovery) were determined using LFB samples at 

0.1, 0.3 and 0.4 mg/L concentrations. Results are shown in table 8.

Criteria for % recovery: 85 - 115 %, when c is > 2 x MRL and 

80 - 120 %, when c is ≤ 2 x MRL 

Criteria for % RSD: ±15% 

Table 6. Precision and accuracy data for DW o-PO4P application.

Sample: 

Concentration:

LFB 0.1 mg/l PO4P

0.1 mg P/L

LFB 0.3 mg/l PO4P

0.3 mg P/L

LFB 0.4 mg/l PO4P

0.4 mg P/L

average = 0.10257 0.30541 0.40891

n = 10 10 10

std. dev. (SD) = 0.000209 0.003242 0.000767

% RSD = 0.2 % 1.1 % 0.2 %

Avg. % Recovery = 103 % 102 % 102 %

Acceptable YES YES YES

17.2.6 Precision and accuracy from LFM samples

Laboratory Fortified Matrix (LFM) samples were prepared by spiking three different tap 

water samples with Phosphate standard solution to gain spike concentrations of 0.1 and 

0.4 mg/L as P for each sample. The analyte concentrations were high enough to be 

detected above the original sample and were not less than four times the MDL. The 

added analyte concentrations were chosen to be the same as those used for LFB. 

Precision was estimated by analyzing four replicates and calculating the %RSD. 

Accuracy was estimated from LFM and corresponding unspiked samples by analyzing 

them and calculating the % Recovery using the next equation. Results for LFM 

precision and accuracy are shown in table 9.

100 ( )LFMRecovery %, 

 



s

C f C

R

s

where Cs = spiked sample concentration

f = spike dilution correction

C = sample background concentration

s = concentration equivalent of analyte added to sample

Note! Because the added spike volume was less than 1% of the total LFM sample 

volume (0.4 % and 0.12 % of total LFM sample volume), the factor f could be excluded.

Criteria for LFM % recovery: 85 - 115 %, when c is > 2 x MRL and 

80 - 120 %, when c is ≤ 2 x MRL 

Criteria for LFM % RSD: ±15% 
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Table 7. Laboratory Fortified Matrix (LFM) sample results for DW o-PO4 application.

Sample Result 

avg. n = 3

(mg/L)

Spike. 

mg/L

% 

Recovery

% RSD. 

n =3

Acceptable

TW-02 0.0021

TW-02 +0.1 mg/l PO4P 0.0946 0.10 92 % 0.7 % YES

TW-02 +0.4 mg/l PO4P 0.3906 0.40 97 % 0.1 % YES

TW-03 0.0026

TW-03 +0.1 mg/l PO4P 0.0944 0.10 92 % 0.3 % YES

TW-03 +0.4 mg/l PO4P 0.3943 0.40 98 % 0.2 % YES

TW-04 0.0028

TW-04 +0.1mg/l PO4P 0.0989 0.10 96 % 0.7 % YES

TW-04 +0.4mg/l PO4P 0.3754 0.40 93 % 0.3 % YES

Average = 95 % 0.4 % YES

17.2.7 Continues Calibration Check (CCC)

Continues Calibration Check (CCC) solution was analyzed in intervals of every 10th

sample, at the beginning and end of run. CCC sample concentration was from the mid 

calibration area – 0.25 mg/L. CCC sample result summary is in table 10.

Criteria for CCC % recovery: 90 – 110 %

Table 8. PO4P CCC result summary. Sample: PO4P CCC, concentration 0.25 mg/L.

Analyzer 1 Analyzer 2

Average: 0.254 0.256

% Recovery 102 % 102 %

n = 20 6

std. deviation (SD) = 0.0033 0.0048

% RSD= 1.29 % 1.87 %

Minimum result: 0.249 0.252

Maximum result: 0.261 0.261

Acceptable YES YES
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17.3 The validation study results showed the Drinking Water Orthophosphate method for Thermo 

Scientific Gallery discrete analyzer is equally effective in meeting the QC acceptance criteria 

given in the reference method. 

Table 9. Performance of the Drinking Water Orthophosphate method for Thermo Scientific Gallery

discrete analyzer compared to reference method acceptance criteria.

QC acceptance criteria for SM 4500-P E Performance of DW o-PO4P for 

Gallery analyzer

Minimum detectable concentration: 10 μg 

P/l

Recovery IPR: NA (laboratory specific)

% Recovery CCC: 90 – 110 %

Recovery LFB: Mean ± 3 x SD (n ≥ 20)

Recovery LFM: NA (laboratory specific)

% RSD IPR: NA (laboratory specific)

% RSD LFB: NA (laboratory specific)

% RSD LFM: NA (laboratory specific)

Method Blank: ≤ 1⁄2 x MRL

MDL calculated: 0.00036 mg P/L

MRL: 0.0125 mg P/L

% Recovery IPR: 102 – 103% 

% Recovery CCC: 100 – 105%

% Recovery LFB: 102 – 103%

% Recovery LFM: 92 – 98 %

% RSD IPR: 0.1 – 1.0 %

% RSD LFB: 0.2 – 1.1 %

% RSD LFM: 0.1 – 0.7 %

Method Blank (LRB): ≤ 1/3 x MRL
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APPENDIX A.

Test flow for DW o-PO4P application in the Gallery analyzer.
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