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PTA wishes to gratefully acknowledge the technical assistance provided for this program by Ms R Ryan, Global Proficiency go to Global Proficiency Ltd (New Zealand) and Global Proficiency Pty Ltd (Australia) for the supply and distribution of the samples.
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Waters Proficiency Testing
Total
PTA wishes to gratefully acknowledge the technical assistance provided for this program by Ms R Ryan, Global Proficiency go to Global Proficiency Ltd (New Zealand) and Global Proficiency Pty Ltd (Australia) for the supply and distribution of the samples.
PO Box 7507 SILVERWATER NSW 2128,
Report No.
Waters Proficiency Testing
Total Recoverable Oil and Grease
PTA wishes to gratefully acknowledge the technical assistance provided for this program by Ms R Ryan, Global Proficiency go to Global Proficiency Ltd (New Zealand) and Global Proficiency Pty Ltd (Australia) for the supply and distribution of the samples.
PTA wishes to gratefully acknowledge the technical assistance provided for this program by Ms R Ryan, Global Proficiency go to Global Proficiency Ltd (New Zealand) and Global Proficiency Pty Ltd (Australia) for the supply and distribution of the samples.
PTA wishes to gratefully acknowledge the technical assistance provided for this program by Ms R Ryan, Global Proficiency Ltdgo to Global Proficiency Ltd (New Zealand) and Global Proficiency Pty Ltd (Australia) for the supply and distribution of the samples.
This report summarises the results of a proficiency testing program on the determination of total recoverable oil and grease in waters. This is round 150 in a planned series of programs involving the analysis of chemical and physical parameters of waters. The exercise was conducted in November 2012 by Proficiency Testing Australia (PTA). The main aim of the program was to assess laboratories’ abilities to competently perform the prescribed analyses. The Program Coordinator was Ms D Mihaila and the Technical Advisor was Ms R Ryan from Global Proficiency Ltd, New Zealand. This report was authorised by Mrs F Watton, PTA Quality – Business Development Manager.
2. Program Features and Design 2.1 Each laboratory was randomly allocated a unique code number for the program to
ensure confidentiality of results. Reference to each laboratory in this report is by code number only. Please note that a number of laboratories reported more than one set of results and, therefore, their code numbers (with letter) could appear several times in the same data set.
2.2 Laboratories were provided with the "Instructions to Participants" and "Results Sheet"
(see Appendix C). Laboratories were requested to perform the tests according to their routine methods.
2.3 Participants were provided with two glass vials (labelled PTA 1 and PTA 2) containing
solutions of total recoverable oil and grease. 2.4 A total of 56 laboratories received samples, comprising:
- 39 Australian participants; and
- 17 overseas participants, including:
- Brunei Darussalam (1), Indonesia (2), Malaysia (6), New Zealand (2), Papua New Guinea (3), Singapore (1), Thailand (1), Vietnam (1).
Of these 56 laboratories, 8 were unable to submit results by the due date. 2.5 Results (as reported by participants) with corresponding summary statistics (i.e.
number of results, median, uncertainty of the median, normalised interquartile range, robust coefficient of variation, minimum, maximum and range) are presented in Appendix A (for each sample and for each of the analyses performed).
2.6 A robust statistical approach, using z-scores, was utilised to assess laboratories’
testing performance (see Section 3). Robust z-score and z-score charts relevant to each test are presented in Appendix A.
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The document entitled Guide to Proficiency Testing Australia, 2012 (reference [1]) defines the statistical terms and details the statistical procedures referred to in this report.
2.7 A tabulated listing of laboratories (by code number) identified as having outlier results
can be found on page 10. 2.8 Prior to sample distribution, a number of randomly selected samples were analysed
for homogeneity. Based on the results of this testing (see Appendix B) it was considered that the samples utilised for this program were homogenous. As such, any results later identified as outliers could not be attributed to any notable sample variability.
3. Statistical Format
For each test, where appropriate, the following information is given:
- a table of results and calculated z-scores;
- a list of summary statistics; and
- ordered z-score charts.
3.1 Outlier Results and Z-scores In order to assess laboratories’ testing performance, a robust statistical approach,
using z-scores, was utilised. Z-scores give a measure of how far a result is from the consensus value (i.e. the median), and give a "score" to each result relative to the other results in the group.
A z-score close to zero indicates that the result agrees well with those from other
laboratories, whereas, a z-score with an absolute value greater than or equal to 3.0 is considered to be an outlier and is marked by the symbol “§”.
Each determination was examined for outliers with all methods pooled. The table on
page 10 summarises the outlier results detected. 3.2 Results Tables and Summary Statistics Each of these tables contains the results returned by each laboratory, including the
code number for the method used, and the robust z-score calculated for each result. Results have been entered exactly as reported by participants. That is, laboratories
which did not report results to the precision (i.e. number of significant figures) requested on the Results Sheet have not been rounded to the requested precision before being included in the statistical analysis.
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A list of summary statistics appears at the bottom of each of the tables of results and consists of:
- the number of results for that test/sample (No. of Results);
- the median of these results, i.e. the middle value (Median);
- the uncertainty of the median;
- the normalised interquartile range of the results (Normalised IQR);
- the robust coefficient of variation, expressed as a percentage (Robust CV) - i.e. 100 x Normalised IQR / Median;
- the minimum and maximum laboratory results; and
- the range (Maximum - Minimum).
The median is a measure of the centre of the data. The normalised IQR is a measure of the spread of the results. It is calculated by multiplying the interquartile range (IQR) by a correction factor which converts the IQR to an estimate of the standard deviation. The IQR is the difference between the upper and lower quartiles (i.e. the values above and below which a quarter of the results lie, respectively). For normally distributed data, the uncertainty of the median is approximated by:
n
normIQR×2
π
Please see reference [1] for further details on these robust summary statistics.
3.3 Ordered Z-Score Charts On these charts each laboratory's robust z-score is shown, in order of magnitude, and
is marked with its code number. From these charts, each laboratory can readily compare its performance relative to the other laboratories.
These charts contain solid lines at +3.0 and -3.0, so that outliers are clearly
identifiable as those laboratories whose "bar" extends beyond these "cut-off" lines. The y-axis of these charts has been limited, so very large z-scores appear to extend beyond the chart boundary.
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4. PTA and Technical Advisor’s Comments 4.1. Metrological Traceability and Measurement Uncertainty of Assigned Values Consensus values (median) derived from participants’ results are used in this program. These values are not metrologically traceable to an external reference.
Sample preparation was undertaken according to Global Proficiency Ltd’s Standard Operating Procedures to ensure samples were fit-for-purpose, homogenous and stable. Weight checks were undertaken on all samples to ensure that the variability on doping concentrations for all samples was less than 2.0%.
Sample PTA 1 was prepared from high purity mineral oil which has a low viscosity in distilled water. Sample PTA 2 was prepared from a mixture of sunflower oil and anhydrous milk fat in distilled water. Samples were prepared as concentrates (~30 mL) equivalent to 1 litre of wastewater, packaged in Teflon capped glass vials and preserved with acid to pH 2.
Solutions were stable and homogeneous, and medians obtained from this proficiency round were in good agreement with the expected levels (dope Concentration), as shown in Table 1.
As the assigned value for this program is the median of the results submitted by the participants, the uncertainty of the median has been calculated and is presented in Table 1.
Analyte Sample Dope
Concentration (mg/L)
Median (mg/L)
Uncertainty of the median
(mg/L)
Total Recoverable Oil and Grease
PTA 1 21.7 +/- 0.2 20.80 0.64
PTA 2 92.6 +/- 0.3 85.10 2.18
Table 1. Comparison of manufacturing concentrations (Dope) and proficiency medians. The values of the calculated uncertainty of the median are also presented. Overall, the performance of participants in this round was good, with robust CVs of less than 20%, consistent with the results of the last two rounds. The median concentrations indicated that on average 95% of total recoverable oil and grease was recovered for sample PTA 1 and 92% for sample PTA 2.
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4.2. Analysis of Round 150 Results The following table compares the Total Recoverable Oil and Grease medians and robust CVs from this round to those obtained in previous PTA rounds.
Round Sample Median (mg/L) Robust CV (%) No. of Res ults
This study PTA 1 20.80 17.1 48
PTA 2 85.10 14.1 48
Report 735 PTA 1 40.75 17.2 53
PTA 2 52.70 12.9 51
Report 678 PTA 1 21.20 32.2 58
PTA 2 24.70 15.9 58
Table 2. Comparison of current round variability and proficiency medians of Total Recoverable Oil and Grease testing with the results of the previous two rounds. Bias / Accuracy
Total Recoverable Oil and Grease testing was successfully carried out, with satisfactory results (|z-score| ≤ 2.0) ranging between 13.7 – 27.9 mg/L for sample PTA 1 and 61.0 – 109.2 mg/L for sample PTA 2.
Out of 48 participants, two questionable results (2.0 < |z-score| < 3.0) were obtained for sample PTA 1 (laboratories 306 and 651) and three questionable results were obtained for sample PTA 2 (laboratories 150, 188 and 403).
Six outlier results (|z-score| ≥ 3.0) were obtained for sample PTA 1 requiring follow-up action by laboratories 159, 188, 255, 335, 596 and 608. Four of the outlying results are approximately twice the doped concentration and could be due to calculation errors. The remaining two outlying results are lower than the doped concentration. They are most likely due to analyte loss during sample extraction.
Two outlier results were obtained for sample PTA 2, requiring follow-up action by laboratories 159 and 596. These two outliers are lower than the doped concentration and are also most likely due to analyte loss during sample extraction.
The Total Recoverable Oil and Grease dataset formed a normal distribution with no significant bias attributable to any one method (Figures 1 and 2). The main method used in this round was APHA 5520B - Liquid-Liquid, Partition-Gravimetric method, which was used by almost half of the participants, followed by the APHA 5520D - Soxhlet extraction method, which was used by approximately 20% of participants.
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Figure 1. Spread of results for Total Recoverable Oil and Grease proficiency testing of Sample PTA 1, with a median of 20.80 mg/L.
Figure 2. Spread of results for Total Recoverable Oil and Grease proficiency testing of Sample PTA 2, with a median of 85.10 mg/L.
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Fre
quen
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Results (mg/L)
Total Recoverable Oil and Grease - Sample PTA 1
APHA Part 5520B
APHA Part 5520C
APHA Part 5520D
ASTM D 7066-04
US EPA 1664A
Modified US EPA1664A method
Other
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0 15 30 45 60 75 90 105 120 135 150 165 180 195
Fre
quen
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Results (mg/L)
Total Recoverable Oil and Grease - Sample PTA 2
APHA Part 5520B
APHA Part 5520C
APHA Part 5520D
ASTM D 7066-04
US EPA 1664A
Modified US EPA1664A method
Other
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Measurement Uncertainty (MU)
The results submitted for Total Recoverable Oil and Greaseparticipants4 belowthe expected uncertainty for these samples based on the reproducibility
Out of 48 participantsaccurately reflect the difference between the median and the participantsproficiency sample.
Laboratories452, 472, 512, 523, 568,calculations, as To keep it in perspective, confidence in the medians is 20.8and 85.1
Figure 3. compared to median
Figure 4.compared to median
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surement Uncertainty (MU)
results submitted for Total Recoverable Oil and Greaseparticipants compared to the median 4 below. In figures 5 and 6, the expected uncertainty for these samples based on the reproducibility (95 %CI).
ut of 48 participantsurately reflect the difference between the median and the participants
proficiency sample.
Laboratories 112, 142, 150, 159, 237, 241, 250, 255, 285, 306, 335, 368, 372, 403, 409, 436, 452, 472, 512, 523, 568,calculations, as at least To keep it in perspective, confidence in the medians is 20.8and 85.10 ± 2.18 mg/L for sample
Figure 3. Total Recoverable Oil and Greasecompared to median
Figure 4. Total Recoverable Oil and Greasecompared to median
surement Uncertainty (MU)
results submitted for Total Recoverable Oil and Greasecompared to the median
In figures 5 and 6, the the expected uncertainty for these samples based on the
(95 %CI).
ut of 48 participants, 34 (71%) submitted MU informatiourately reflect the difference between the median and the participants
of their results was further from the median than their To keep it in perspective, confidence in the medians is 20.8
PTA 2.
Total Recoverable Oil and Grease results concentrations.
Total Recoverable Oil and Grease results for sample PTA 2 including concentrations.
results submitted for Total Recoverable Oil and Greaseconcentration
s displayed by the methods used,the expected uncertainty for these samples based on the 95% confidence interval for overall
(71%) submitted MU information. Severalurately reflect the difference between the median and the participants
112, 142, 150, 159, 237, 241, 250, 255, 285, 306, 335, 368, 372, 403, 409, 436, 569, 596, 608, 634, 651 and 656 may wish to re
further from the median than their To keep it in perspective, confidence in the medians is 20.80 ± 0.6
results for sample
results for sample PTA 2 including
results submitted for Total Recoverable Oil and Grease including the are displayed
he methods used, giving an indication of 95% confidence interval for overall
everal of the stated urately reflect the difference between the median and the participants
112, 142, 150, 159, 237, 241, 250, 255, 285, 306, 335, 368, 372, 403, 409, 436, may wish to re
further from the median than their ± 0.64 mg/L for sample PTA 1
for sample PTA 1 including
results for sample PTA 2 including
including the MU stated by displayed in Figures
giving an indication of 95% confidence interval for overall
of the stated MUsurately reflect the difference between the median and the participants’ results for each
112, 142, 150, 159, 237, 241, 250, 255, 285, 306, 335, 368, 372, 403, 409, 436, may wish to re-examine their MU
further from the median than their stated mg/L for sample PTA 1
PTA 1 including
results for sample PTA 2 including
stated by in Figures 3 and
giving an indication of 95% confidence interval for overall
MUs did not for each
112, 142, 150, 159, 237, 241, 250, 255, 285, 306, 335, 368, 372, 403, 409, 436, examine their MU
stated MU. mg/L for sample PTA 1
PTA 1 including MU
results for sample PTA 2 including MU
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Figure 5. MU for Total Recoverable Oil and Grease proficiency testing of Sample PTA 1, as reported by participants, compared with 95% confidence interval for overall reproducibility (----) (± 6.9 mg/L) in this round.
Figure 6. MU for Total Recoverable Oil and Grease proficiency testing of Sample PTA 2, as reported by participants, compared with 95% confidence interval for overall reproducibility (----) (± 23.6 mg/L) in this round.
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quen
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Measurement Uncertainty (mg/L)
Total Recoverable Oil and Grease ± MU - Sample PTA 1
Total Recoverable Oil and Grease ± MU - Sample PTA 2
APHA Part 5520B
APHA Part 5520C
APHA Part 5520D
ASTM D 7066-04
US EPA 1664A
Modified US EPA1664A method
Other
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Using the t-value (outliers removed, 95% confidence interval), overall results indicated that the estimate of reproducibility (~2SD) for Total Recoverable Oil and Grease analysis was 20.8 ± 6.9 mg/L for sample PTA 1 and 85.1 ± 23.6 mg/L for sample PTA 2.
Using the t-value, results submitted by laboratories using method APHA 5520B (n=24) indicated a method reproducibility of ± 5.5 mg/L for sample PTA 1 and of ± 23.6 mg/L for sample PTA 2.
4.3. Analysis of Results by Method Groups Further analysis of results by method groups is undertaken to provide specific information on individual method performance. The APHA Liquid-Liquid, Partition-Gravimetric method (APHA Part 5520B - method code 1) was most frequently employed for Total Recoverable Oil and Grease analysis in this round. Out of 48 participants, 22 laboratories indicated the use of this method for analysis of sample PTA 1 and 24 laboratories for sample PTA 2. Table 3 below presents the median, uncertainty of the median and robust CV for Total Recoverable Oil and Grease results obtained by method 1.
Sample Method code Participants
Median ± Uncertainty of the Median
(mg/L)
Robust CV (%)
PTA 1 1 22 19.95 ± 0.79 14.8
PTA 2 1 24 82.70 ± 2.92 13.8
Table 3. Variability and proficiency median of Total Recoverable Oil and Grease results obtained by method 1. The median concentrations determined for results submitted using method APHA 5520B indicated that on average 92% of total recoverable oil and grease was recovered for sample PTA 1 and 89% for sample PTA 2. These results compare well with the average recovery published for method APHA 5520B (Standard Methods 22nd edition) of 93%. In order for methods to be grouped for analysis, PTA requires more than ten sets of results from the same method group. For methods other than those presented above, there were less than ten results submitted, therefore reliable conclusions cannot be drawn from analysing them separately and these results have not been analysed by grouped methods.
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5. Outlier Results
Laboratories reporting outlier results are listed in the following table:
Lab Code
Total Recoverable Oil and Grease
PTA 1 PTA 2
159 § §
188 §
255 §
335 §
596 § §
608 §
1 A “§” indicates the occurrence of a z-score outlier result (i.e. those results for which
|z-score|≥3.0). 6. References
[1] Guide to Proficiency Testing Australia, 2012 (This document can be found on the PTA website, www.pta.asn.au).
[2] APHA “Standard Methods for the Examination of Water and Wastewater” 22nd Edition.
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APPENDIX A
Results and Data Analysis Total Recoverable Oil and Grease Sample PTA 1 ................................................................ A1
Total Recoverable Oil and Grease Sample PTA 2 ................................................................ A4
1 Where reported, results are shown with their corresponding
measurement uncertainty (MU). 2 "§"s denote outliers (i.e. those results for which |z-score| ≥ 3.0).
Robust z-scores are calculated as: z = (A - median) ÷ normalised IQR, where A is the participant laboratory's result.
3 Please refer to Appendix C (page C3) for method code descriptions.
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Total Recoverable Oil and Grease - PTA 2 - Robust Z -Scores
Robust Z-Scores
A6
Total R
ecoverable Oil and G
rease - PT
A 2
Ordered R
obust Z-S
core Charts
596
159
188 15
0 403
405
240
523
548a
548b 656
242
436
409
452
626
639
241 507
181
142
136
224
285
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-3
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-1
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z-sc
ore
lab code
-5
-4
-3
-2
-1
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z-sc
ore
lab code
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APPENDIX B
Sample Homogeneity and Stability
B1
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Homogeneity and Stability Testing
Samples for this program were obtained from Global Proficiency Ltd, New Zealand. All samples underwent quality control weight checks to ensure a consistent mass of oil and grease was delivered to each vial. Sample PTA 1 contained 21.7 ± 0.2 mg/L (3x Standard deviations = 99% CI) of mineral oil, and sample PTA 2 contained 92.6 ± 0.3 mg/L (99% CI) of a mixture of vegetable oil and anhydrous milk fat. Random selections of ten samples were chosen from sample PTA 1. Seven were stored chilled and three were subjected to 35ºC for 3 days for an accelerated ageing stability trial. Samples were then analysed by Hill Laboratories for Oil and Grease, using method APHA 5520D 21st ed. 2005. All stability samples showed no increased variability when compared to chilled samples. Based on the assumption that the rate of ageing doubles with every 7ºC elevated temperature, these samples were assumed stable for more than 96 days (3days*2(35/7)) from the date of manufacture (1 November 2012).
Table H-1. Extraction (Test) and QC Weight Check Results of Oil and Grease Samples.
From statistical analyses based on the results of this testing and rigorous quality control, it was considered that all samples were sufficiently homogenous and stable, so that any results later identified as outliers should not be attributed to any notable sample variability.
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APPENDIX C
Documentation Instructions to Participants ................................................................................................... C1
4. Safety i) Samples are for laboratory use only. ii) Participants should have sufficient experience and training to take the necessary
precautions when handling the samples and reagent chemicals and during disposal.
iii) Use of safety glasses, gloves, and fume hoods, where appropriate during the
determinations, is recommended. 5. Reporting
i) Report results using one decimal place, e.g 10.1 mg/L.
ii) Report results in milligrams/litre (mg/L) - i.e., the total mg extracted/sample treated as 1L for reporting.
iii) Do not correct results for recovery.
iv) In addition to reporting the results, record the method of analysis using the
attached codes.
v) Laboratories are also requested to calculate and report an estimate of measurement uncertainty (MU) for each reported measurement result. All estimates of MU must be given as a 95% confidence interval (coverage factor k ≈ 2) and reported in mg/L.
6. Testing should commence as soon as possible after receiving the samples and results
reported NO LATER THAN 14 DECEMBER 2012 to:
Delfina Mihaila Proficiency Testing Australia PO Box 7507 SILVERWATER NSW 2128 AUSTRALIA Phone: +612 9736 8397 Fax: +612 9743 6664 Email: [email protected]
7. For this program your laboratory has been allocated the code number shown on the
attached Results Sheet. All reference to your laboratory in reports associated with the program will be through this code number, thus ensuring the confidentiality of your results.
8. As a guide, ranges for the samples can be expected to be (in mg/L):