Analytische Qualitätssicherung Baden-Württemberg

Transcription

1 Analytische Qualitätssicherung Baden-Württemberg Proficiency Test 7/16 - Ions in waste water - ammonium-nitrogen, nitrate-nitrogen, nitrite-nitrogen, total phosphorous, total cyanide, cyanide (week acid dissociable), chromium (VI) provided by AQS Baden-Württemberg at Institute for Sanitary Engineering, Water Quality and Solid Waste Management, University of Stuttgart Bandtäle 2, Stuttgart-Büsnau, Germany on behalf of the Ministry of the Environment, Climate Protection and the Energy Sector Baden-Württemberg Stuttgart, in February 2017

2 Responsibilities: Scientific director AQS: Dr.-Ing. Dipl.-Chem. Michael Koch PT manager: Dr.-Ing. Frank Baumeister AQS Baden-Württemberg at Institute of Sanitary Engineering, Water Quality and Solid Waste Management at University of Stuttgart Bandtäle Stuttgart-Büsnau Germany Tel.: +49 (0)711 / Fax: +49 (0)711 / info@aqsbw.de

3 PT 7/16 LIST OF CONTENTS 1. GENERAL PT DESIGN SAMPLE PREPARATION SAMPLE DISTRIBUTION ANALYTICAL METHODS SUBMISSION OF RESULTS EVALUATION AND ASSESSMENT PROCEDURE EVALUATION EXPLANATION OF APPENDIX A EXPLANATION OF APPENDIX B EXPLANATION OF APPENDIX C MEASUREMENT UNCERTAINTY TRACEABLE REFERENCE VALUES INTERNET Appendix A Ammonium-nitrogen A-1 Nitrate-nitrogen A-11 Nitrite-nitrogen A-21 Total phosphorous A-31 Cyanide (week acid dissociable) A-41 Total cyanide A-51 Chromium (VI) A-61 Appendix B Appendix C Ammonium-nitrogen C-1 Nitrate- nitrogen C-28 Nitrite- nitrogen C-56 Total phosphorous C-85 Cyanide (week acid dissociable) C-112 Total cyanide C-139 Chromium (VI) C-166

4 PT 7/16 page 1 1. General This PT was provided by AQS Baden-Württemberg in the framework of the nationwide conducted waste water PT scheme in Germany. This PT scheme is based on the requirements of the German Working Group on water issues of the Federal States and the Federal Government (LAWA), which fixed the way of doing PTs in the regulated environmental sector in its AQS-leaflet A-3 for the analysis of water, waste water and sludge. These PTs are conducted together with several PT providers nationwide in Germany in a harmonised way. The PT was executed and evaluated according to the requirements of DIN A45 and ISO/TS PT design Each participant received the following samples: 3 samples for the determination of ammonium-nitrogen, nitrate-nitrogen in 1000-ml-glass bottles with screw cap. Preservation by cooling. 3 samples for the determination of nitrite-nitrogen in 100-ml-glass bottles with screw cap. Preservation by cooling. 3 samples for the determination of total phosphorous in 500-ml-plastic bottles with screw cap. Preservation by acidification with HNO 3 (ph ca. 2,1) and cooling. 3 samples for the determination of cyanide (weak acid dissociable) and total cyanide in 500-ml-glass bottles with screw cap. Preservation by alkalinising with NaOH (ph ca. 11,4) and cooling. 3 samples for the determination of chromium (VI) in 250-ml-plastic bottles with screw cap. Preservation by alkalinising with NaOH (ph ca. 9) and cooling. 9 different concentration levels/batches were produced. The concentration levels were randomly allocated to the participants. It was ensured that each participant received one concentration level from the lower concentration range (level 1-3), one level from the middle concentration range (level 3-6) and one level from the higher concentration range (level 7-9). 3. Sample preparation The samples for the determination of ammonium-nitrogen, nitrate-nitrogen, nitritenitrogen, total phosphorous, total cyanide and cyanide (week acid dissociable) were based on a real waste water (effluent of the microsieve of the waste water treatment plant Stuttgart-Büsnau). The waste water was kept one day in an intermediate bulk container for sedimentation. For the preparation of the samples, the waste water was filtered by using 5 µm and 1 µm filter cartridges to eliminate particles. To reduce germs, the waste water was irradiated with ultraviolet light and was pasteurised at 80 C in a stainless steel tank overnight. During pasteurisation the waste water was aerated with a mixture of nitrogen and carbon dioxide to prevent degassing of carbon dioxide followed by precipitation of calcium carbonate. For the cyanide parameters the waste water was additionally treated to eliminate carbon dioxide after pasteurisation. The special treatment was necessary to prevent chalk deposition in the batches. Therefore the waste water was acidified with sulphuric acid (ph < 4) and were aerated with nitrogen to remove all carbonic acid spe-

5 PT 7/16 page 2 cies (decarbonisation). Afterwards the ph of the matrix was adjusted to a value of 11,4 directly before producing the batches. The samples for the determination of chromium (VI) were based on drinking water as matrix. The drinking water was filtered with 5 µm and 1 µm filter cartridges and irradiated with ultraviolet light before use. For the preparation of the samples, the matrices were spiked with stock solutions. The concentrations covered waste water relevant ranges. The samples were cooled directly after preparation. The samples were dispatched with freezer packs added to the packages. 4. Sample distribution The samples were distributed on 07 November 2016 with a post express service (TNT).

6 PT 7/16 page 3 5. Analytical methods The participants were restricted to use analytical methods according to the requirements of the Technical Module Water from the German Working group on water issues of the Federal States and the Federal Government (LAWA) from 13 November parameter NH 4 -N NO 3 -N NO 2 -N P total method DIN EN ISO : (E23) DIN EN ISO : (E23) DIN E 5 : DIN ISO : (D49) DIN EN ISO : (D20) DIN EN ISO : (D20) DIN EN ISO : (D28) DIN D 9 : DIN D 9-2 / 9-3 : DIN ISO : DIN EN : (D10) DIN EN ISO : (D20) DIN EN ISO : (D20) DIN EN ISO : (D28) DIN EN ISO : (D49) DIN EN ISO 6878 : (D11) DIN EN ISO : (E22) DIN EN ISO : (E22) DIN EN ISO : (E29) DIN EN 1189: , Ziffer 6 (D11) DIN EN 1189: , Ziffer 7 (D11) DIN EN ISO : (D45) DIN EN ISO : (D46) CN (wad) DIN D13-2 : DIN EN ISO : (D6) DIN EN ISO : (D2) DIN EN ISO : (D3) CN total DIN D13-1 : DIN EN ISO : (D6) DIN EN ISO : (D2) DIN EN ISO : (D3) Cr (VI) DIN D 24 : DIN EN ISO : (D22), section 6 (dissolved chromate) DIN EN ISO : (D41)

7 PT 7/16 page 4 Following limits of quantification were required: parameter limit of quantification [mg/l] ammonium-nitrogen 1,0 nitrate-nitrogen 2,0 nitrite-nitrogen 0,05 total phosphorous 0,25 total cyanide 0,1 cyanide (wad) 0,05 chromium (VI) 0,05 The samples had to be analysed in duplicate over the complete method (sample preparation and measurement). The participants were asked to submit the results as average values in mg/l with three significant digits. 6. Submission of results The deadline for the submission of results was on 25 November Evaluation and assessment procedure The values were assessed according to the tolerance limits determined in the assessment of 43. LÜRV. In 43. LÜRV the assigned value x pt was calculated as consensus mean using the Hampel estimator. The standard deviation for proficiency assessment σ pt was taken from the Q-method for the calculation of z U -scores according to DIN A45 (chapter 10.4) or ISO/TS respectively. The standard deviation for proficiency assessment σ pt was taken from the variance function. σ pt was limited as follows: parameter limits for σ pt in % lower limit upper limit NH 4 -N, NO 3 -N, NO 2 -N, total phosphorous 5 10 cyanide (wad) cyanide (total) Chromium (VI) 5 15 A z-score for a result x was calculated for each measurement result derived from the assigned value x pt and the standard deviation for proficiency assessment σ pt : z = x σ x pt pt The z-score was modified to a z U -score with a correction factor for proficiency assessment (as described in the above mentioned standards). The tolerance limits were defined as Iz U I=2.0.

8 PT 7/16 page 5 According to ISO (2015) the single results were assessed as follows: z u < z u < 3.0 z u 3.0 successful (s) questionable (q) unsatisfactory (u) There was no overall assessment of the proficiency test round, but the single parameters were assessed. A parameter was assessed as successful, if more than half of the values were correctly determined (2 out of 3 values are within the tolerance limits). Not successful were: 1) Values which were not determined (if the other samples of this parameters were analysed), 2) Values, which were indicated with lower than limit of quantification, 3) Values, which were subcontracted, 4) Values, which were submitted after the deadline of submission of results. 8. Evaluation Number of participants: 141 Number of reported values: 2667 Number of accepted values: 2379 (89,2%) 1 laboratory reported no results In the following figure the successful and not successful laboratories for each parameter are illustrated.

9 PT 7/16 page 6 9. Explanation of Appendix A Appendix A contains for each parameter - parameter tables - a figure of participants means versus the spiked amount for the determination of the recovery rate and the matrix content - a figure of the relative standard deviations versus the concentrations - a figure of the tolerance limits in the PT versus the concentrations - the frequency of application of analytical methods - the method specific evaluation - a comparison of mean and reference values for each concentration level - a comparison of the relative standard deviations of the different methods - the statistical characteristics of the method specific evaluation - a tabular comparison of the means with the reference values and their uncertainties Parameter tables In these tables the following values for each concentration level are listed: assigned value expanded uncertainty of the assigned value in %, calculated according to ISO using the formula rel. standard deviation U = number of values standard deviation, calculated using robust statistical method standard deviation for proficiency assessment for the calculation of z U -scores rel. standard deviation for proficiency assessment tolerance limits above and below permitted deviations above and below in % number of values in this level number of not satisfactory values below and above the assigned value and the percentage of these values in total. Determination of recovery rate In the diagrams of the assigned values versus the spiked amount of analyte a linear regression line was calculated using a generalized least square regression which takes into account the uncertainties of the values in both directions. From these values the recovery rate for each parameter was determined (see diagrams). The slope of the line indicates the average recovery rate. The diagrams also contain the expanded uncertainty (k=2) of the mass values and the assigned values. Relative standard deviations and tolerance limits The diagrams for the relative standard deviation vs. the assigned value show the concentration dependency of the standard deviation and the tolerance limits in percent. The relative standard deviations calculated from participants data are the stars connected by an interrupted line, the rel. standard deviation taken from the variance function (and sometimes limited by the upper or lower limit) are given by squares, connected by a continuous line.

10 PT 7/16 page 7 Method specific evaluation For each parameter the methods used by the participants are shown in a diagram. In a second diagram for each method with a frequency of more than 5 %, values are sorted in 5 categories: too low results with z U -score < -2 low results with 2 z U -score < 1 correct results with 1 z U -score +1 high results with +1 < z U -score +2 too high results with z U -score > +2 Comparison of means and reference values for each concentration level Finally the mean value calculated from all results (used as assigned value) is compared with mean values calculated for all methods separately (in this case using the Hampel estimator described in ISO/TS 20612). Mean values were calculated only, if more than 7 results were within a z-score-range of ± 2. The means are reported with their expanded uncertainty calculated according to ISO Explanation of Appendix B Participants were asked to report expanded uncertainties of their results on a voluntary basis. In this diagram for each parameter the reported uncertainties for all concentration levels with the reproducibility standard deviation (horizontal line) are displayed. Values which deviate from the reproducibility standard deviation with a factor more than 2 are usually not realistic. 11. Explanation of Appendix C In the last part of the report, for all concentration levels the results of all participants are illustrated. Confidentiality of participants is ensured by using lab codes. The lab codes were sent to participants with the certificates. In detail Appendix C contains: - a table with all data - figures with o all reported results o all z U -scores o all reported expanded uncertainties o all ζ scores Table with all data The assigned value with the expanded uncertainty and the tolerance limits for the concentration level is illustrated in the table. For each participant the following data are given: lab code reported result measurement uncertainty of the value (if reported) ζ-score for this value, calculated with the following formula ζ = x x pt 2 2 ulab + ux pt, with x x pt = difference from the measured value and the assigned value u lab = standard uncertainty of the value, reported by the participant

11 PT 7/16 page 8 u pt x = standard uncertainty of the assigned value z U -score for proficiency assessment assessment of the value according to its z-score Meaning of ζ-scores: The assessment of ζ-scores is similar to that of z-scores. If the data are normally distributed and the uncertainties are well estimated, ζ-scores will lie between -2 and +2 with a probability of around 95 %. ζ-scores are mainly influenced by the measurement uncertainties reported by the laboratory. Therefore ζ-scores are usually not appropriate for the assessment of the reported results, unless the reported measurement uncertainty is checked for fitnessfor-purpose. Therefore we do not use the ζ-scores for the assessment of the laboratories. Nevertheless ζ-scores are appropriate to check the plausibility of the reported measurement uncertainty: If the z-score of a result is within the tolerance limit and the ζ-score is outside, then the measurement uncertainty is underestimated. If the z-score is outside the tolerance limits and the absolute value of the ζ-score is lower than two, then the requirements of the proficiency test were stronger compared with the reported measurement uncertainty. Diagrams of uncertainty data In the first figure for all lab codes the measurement uncertainty (together with the reproducibility standard deviation) is illustrated. The second figure shows the associated ζ-scores. 12. Measurement uncertainty 59 (42,4%) out of 139 laboratories with valid values reported measurement uncertainties. In total 1070 (41,3%) out of 2589 valid values were given with the measurement uncertainty. The following table displays the number of values with measurement uncertainty against the accreditation status. Accreditation status of the values Number of values Number of values with measurement uncertainty accredited (43,7%) not accredited (45,4%) not specified (27,9%) We would like to put emphasis on the fact that reporting of measurement uncertainties in our PT scheme is absolutely voluntary. The only objective is to help all participants to reasonably handle measurement uncertainties and their estimation. The diagrams show that the spread of reported uncertainties is vast, from unrealistic low values up to very high. A plausibility check using reproducibility standard deviations of the PT round could be helpful here. If measurement uncertainties are underestimated values assessed as satisfactory in the PT ( z U 2), will have a large ζ-score. ζ > 2 means that the own requirements (defined in terms of estimated uncertainty) are not fulfilled. 156 (16%) of the 978 values reported with uncertainties and having a z U -score z U 2.0 had a ζ-score > 2.0. This means that the requirements of the PT scheme have been fulfilled, but not the own requirements, the uncertainty is underestimated.

12 PT 7/16 page Traceable reference values Traceability of analytical results to national and international references is of increasing importance in all laboratories. This is not easy to realise for chemical analyses and often can only be done by analysing certified reference materials. But availability of these reference materials in the water sector is very limited. Therefore we try to provide reference values for the proficiency test samples, traceable to national and international references. Since our PT samples without exception are spiked, real water samples, reference values can be calculated from the sum of matrix content and spiked amount of analyte. For both summands traceable values and their uncertainty have to be determined. It is assumed that no unrecognised systematic deviations occurred during sample preparation and dispatch and that all uncertainty components were discovered. Determination of the spiked amount and its uncertainty All spiking of samples was controlled gravimetrically. Conversion to concentration was done by measuring the density of the resulting samples using a pycnometer. This procedure allows the preparation of a complete uncertainty budget. The first step is the specification of the measurand with a formula. This shows the links between the result and all influence quantities for the parameter. c lot = ( m m ) ( m m ) P s _ ss, b s _ ss, t ss _ lot, b ss _ lot, t ( m m ) m m ) K ss, b ss, t ( lot, b lot, t F ρ lot with: c lot m s_ss,b m s_ss,t m ss_lot,b m ss_lot,t ρ lot m ss,b m ss,t m lot,b m lot,t P F K concentration of the analyte in the lot resulting from the spike in g/l mass of the substance added for preparation of the stock solution in g (gross weight of the vessel) mass of the substance added for preparation of the stock solution in g (tare weight of the vessel) mass of the stock solution into the lot in g (gross weight of the vessel) mass of the stock solution into the lot in g (tare weight of the vessel) density of the lot in g/l total mass of the stock solution in g (gross weight of the vessel) total mass of the stock solution in g (tare weight of the vessel) total mass of the lot in g (gross weight of the vessel) total mass of the lot in g (tare weight of the vessel) purity of the chemical conversion factor buoyancy correction Based on this formula the uncertainty budget can be prepared and all components can be quantified. The following figure shows the distribution of the contributions for the parameter ammonium-nitrogen. The main contribution results from the purity of ammonium-sulphate. According to the certificate of analysis, the purity was 99,8% and we estimated the uncertainty estimated to be 0,1% as rectangular distribution.

13 PT 7/16 page 10 All used balances are proofed annually by an accredited calibration laboratory and for each balance a calibration certificate is provided. The uncertainties of the weighings are taken from these certificates. The determination of the density was also made using weighings (of the pycnometer). Temperature measurement was made with a calibrated thermometer. With all these uncertainty components the combined uncertainty, as described in the EURACHEM/CITAC-Guide Quantifying Uncertainty in Analytical Measurement, was calculated using the sensitivity coefficients determined by partial derivation of the formula to the respective influence quantities. So traceability was assured by using calibrated balances and thermometers. Determination of the matrix content Because the same matrix was used for preparation of all samples, the matrix content could be calculated from the mean values of the participants and the spiked amounts in a standard-addition-like way 1,2. The uncertainties of the spiked amounts were known from the uncertainty budgets. The expanded uncertainties of the mean values of participants result were calculated according to ISO (Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparisons) as u mean = 2 1, 25 s R n 1 Rienitz, O., Schiel, D., Güttler, B., Koch, M., Borchers, U.: A convenient and economic approach to achieve SI-traceable reference values to be used in drinking-water interlaboratory comparisons. Accred Qual Assur (2007) 12: Koch, M., Baumeister, F.: Traceable reference values for routine drinking water proficiency testing: first experiences. Accred Qual Assur (2008) 13:

14 PT 7/16 page 11 with: s R reproducibility standard deviation n number of data for this level 2 coverage factor for the expanded uncertainty 1,25 correction factor (according to ISO to be used for robust methods) The content of the matrix can be derived from a linear regression of means vs. spiked amounts. Since uncertainties of all data points were available for x- as well as y-direction a generalised least square regression was used as described in DIN EN The computer program LIN_LEAST (from BAM) was used for this purpose. With this method a value for matrix and its uncertainty are obtained. Because of statistical variation of the input values the calculated matrix content might result in a negative value. In those cases the matrix content is set to zero. The lower end of the uncertainty range of the matrix content also might be negative. Therefore the expanded uncertainty of the matrix content was set to the matrix content itself in this case. The matrix content is not directly traceable to national or international references, but it does not considerably compromise the traceability of the final content due to its comparably low contribution. 14. Internet The report is available on the following webpage:

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