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Consiglio Nazionale delle Ricerche Istituto per lo Studio degli Ecosistemi Ve r b a n i a P a l l a n z a R E P O R T CNR-ISE, 07.10 (E) Quality Control IC Proficiency Test QC-IC 2010-2 A. Marchetto, G. A. Tartari, P. Giacomotti E. Zabiello & F. Abballe 2010 National Research Council of Italy Institute of Ecosystem Study Dionex S.p.A. – Milano Quality Control IC Proficiency Test QC-IC 2010-2 Aldo Marchetto, Gabriele A. Tartari, Paola Giacomotti CNR – Institute of Ecosystem Study (ISE), Largo Tonolli 50, 28922 Verbania Pallanza, Italy e-mail corresponding author: [email protected] web page: http://www.idrolab.ise.cnr.it Eugenio Zabiello, Franco Abballe Dionex S.p.A. via XXV Aprile, 6, 20097 San Donato Milanese (Milano) web page: http://www.dionex.com 24 November 2010 TABLE OF CONTENTS 1. INTRODUCTION 1 1.1. Aims 1 1.2. Description 1 1.3. Participants 1 2. PROFICIENCY EVALUATION 2 2.1. Assigned values 2 2.2. Web site for result upload 3 3. SAMPLES 5 3.1. Preparation and shipping 5 3.2. Homogeneity 6 3.3. Stability 7 3.4. Expected values 8 4. RESULTS 9 4.1. Data analysis 9 4.2. Results 9 5. DISCUSSION 24 6. CONCLUSIONS 25 7. REFERENCES 26 1. INTRODUCTION 1.1. Aims This Quality Control IC Proficiency Test is organized by Dionex (Italy) S.p.A. supported by the Institute of Ecosystem Study (ISE) of the National Research Council of Italy. The aim of proficiency tests is to give to the participants a tool to evaluate the quality of their results in order to improve them. This program follows international ISO e ILAC rules. The proficiency of each laboratory is evaluated by means of a z-score calculated using an assigned standard deviation (SDa) for each analyte and sample. SDas were obtained using relative standard deviation reported in official analytical methods or in previous exercises. 1.2. Description This test follows ISO/IEC 43-1:1997 e 43-2:1997, ILAC G13:2007 and ISO/IEC 17043:2010 rules. A coordination team was formed, including: Aldo Marchetto CNR-ISE Verbania Gabriele Tartari CNR-ISE Verbania Paola Giacomotti CNR-ISE Verbania Eugenio Zabiello Dionex S.p.A. Milano Franco Abballe Dionex S.p.A. Milano Leader After registration, each laboratory received a code and a password to upload their results into the website. To maintain each laboratory anonymous, its code is known exclusively by the laboratory itself and the coordination team. In the report, laboratories are numbered, without any reference to their codes. Two 280-mL polyethylene bottles containing natural water samples (C1 and C2) were sent to each participating lab. A single determination of chloride, nitrate, sulphate, sodium, potassium, magnesium and calcium was requested, without replication. Expected values were obtained by the robust average (x*) of the results, following point 5.6 of ISO rule 13528:2005. Laboratory proficiency was evaluated using a z-score calculated using an assigned standard deviation (SDa) for each analyze and sample. SDas were obtained using relative standard deviation reported in official analytical methods or in previous exercises, following ISO rules 57255 (point 6.2) and ISO 13528. Registration was open from June 2010 to September 27, 2010. Samples were sent on October 1st and the deadline for result upload was set to November 8th, 2010. 1.3. Participants In February, 2010, a first proficiency test opened to Italian laboratories was organized. This second test was opened to laboratories of different countries, namely 37 in Italy, 15 in the Netherlands, 2 in Belgium and one each in Australia, Malaysia, Nigeria, Qatar, Russia, Switzerland and the Arab Emirates. 50 laboratories uploaded the results. -1- 2. PROFICIENCY EVALUATION 2.1. Assigned values Laboratory proficiency was evaluated using z-scores on the basis of the robust average of the results and of assigned standard deviation. Assigned standard deviation were estimated on the basis of method reproducibility reported in official analytical methods APAT IRSA-CNR 4020 (Italy) and ISO 10304-1 (2007) for anions and APAT IRSA-CNR 3030 for cations, and on the basis of other proficiency tests, such as Italian UNICHIM proficiency tests for drinking water and wastewater and the previous QC-IC 2010-1 test. Chloride 25 50 40 CVR % CVR % 20 15 10 30 20 10 5 0 0 0 100 200 0 300 20 40 60 80 mg/L mg/L Sulphate 15 CVR % Nitrate ISO 10304-1 10 IRSA 4020 UNICHIM 5 QC-IC 2010-1 0 0 100 mg/L 200 300 Coefficients of variation of the reproducibility for anions, reported in analytical methods (ISO 10304-1 and IRSA 4020) and in previous exercises (UNICHIM 2005-2009 and QCIC 2010-1). -2- IRSA 3030 Sodium 15 50 Potassium UNICHIM 40 10 CVR % CVR % QC-IC 2010-1 5 30 20 10 0 0 25 50 75 0 100 mg/L 15 0 60 Magnesium 2 mg/L 4 mg/L 100 6 Calcium 50 10 CVR % CVR % 40 5 30 20 10 0 0 10 20 30 40 0 50 0 mg/L 50 150 Coefficients of variation of the reproducibility for cations, reported in analytical methods (ISO 10304-1 and IRSA 4020) and in previous exercises (UNICHIM 2005-2009 and QCIC 2010-1). Assigned relative standard deviations (RSDa) are reported in the following table: Anions Chloride Nitrate Sulphate Expected value mg/L ~ 1.8 – 2.2 RSDa assigned Cations RSDa assigned Sodium Expected value mg/L ~3–4 15 ~3–4 10 Potassium ~ 1.5 – 2.5 10 ~ 50 - 65 8 Magnesium ~ 7 - 12 8 Calcium ~ 40 - 50 8 8 2.2. Web site for result upload In this proficiency test, a website has been used to upload the results. During data upload, each lab can compare its results with provisional expected values. This comparison is possible after final confirmation of results by the lab and when a minimum of 10 results have been uploaded for the analyte. Eight labs did not use the website and sent their results to the coordination team by e-mail. -3- In the following pictures, statistical plots and tables obtained by the laboratory of the Institute of Ecosystem Study for sulphate are shown as an example. In the plots, the point referring to this laboratory is in red. Web page showing uploaded results and statistical tables Web page with distribution plots of the results for the two samples -4- Web page with the Youden plot 3. SAMPLES 3.1. Preparation and shipping Samples were prepared mixing different drinking waters in 100-L containers two weeks before sample shipping. Samples were sent in 250-ml polyethylene bottles, washed with deionized water, left full of deionized water for one day, and then washed twice with the samples. No preservative was added and the samples were not refrigerated during shipping. Samples were sent on October the 1st (to Italian labs) and September 28th (to the other labs) and in most cases arrived to the participating laboratories before October 7th. However some labs received the samples later, up to October 20th. -5- 3.2. Homogeneity Sample homogeneity was evaluated following ISO e ILAC rules for proficiency tests, and in particular Thompson et al. (2006) and Tholen et al. (2007). Homogeneity tests were performed at the laboratory of the Institute of Ecosystem Study in Verbania. For each sample (C1 and C2), twelve bottles were analyzed twice in a single batch but in random sequence. The presence of outliers was detected using Cochran test, with a critical value of 0.54, for p<0.05 and twelve samples. Variance partitioning was then performed for analytical and between-sample variance. Between sample variance was always lower than 30% of the assigned standard deviation for each variable, as suggested by Thompson et al. (2006). A single outlier was detected for calcium analysis of sample C2. Variable Sample Chloride C1 C2 C1 C2 C1 C2 C1 C2 C1 C2 C1 C2 C1 C2 Cochran test for outliers Overall RSD RSD analysis RSD between samples 0.26 0.19 0.18 0.14 0.25 0.21 * 0.18 0.09 0.22 * 0.10 * 0.09 0.26 0.76 0.71 0.30 0.31 0.25 Nitrate 0.51 0.40 0.36 0.50 0.15 0.07 Sulphate 0.43 0.63 0.58 0.46 0.26 0.15 Sodium 0.19 0.19 0.19 0.44 0.19 0.08 Potassium 0.39 0.22 0.21 0.23 0.26 0.15 Magnesium 0.35 0,22 0.23 0.28 0,10 0.03 Calcium 0.34 0,30 0.30 0,09 0.04 0.65 * denotes cases in which analytical variance was higher than overall variance. -6- % limit (30% of SDa) 4.5 4.5 3.0 3.0 2.4 2.4 2.4 2.4 3.0 3.0 2.4 2.4 2.4 2.4 3.3. Stability Sample stability was evaluated by repeating the analysis of a bottle for each sample (C1 and C2) preserved in the dark at 4°C. Analyses were performed on September 27th (two replicates), October 12th and 28th and November 9th (three replicates). The resulting variability was again compared with a limit set to 30% of SDa. During each analytical session, a second bottle, freshly opened, was analyzed. The results are compared in the following table. In general, stability was excellent and the relative standard deviation was lower than the limit. However, in the case of sample C2, in November 8th the nitrate concentration in one of the three analyzed samples was markedly lower than in the previous analyses. Variable Chloride Nitrate Sulphate Sodium Potassium Magnesium Calcium Sample C1 C2 C1 C2 C1 C2 C1 C2 C1 C2 C1 C2 C1 C2 RSD sample preserved in the dark at 4 °C 1.2 1.2 1.4 1.7 1.5 1.4 1.2 0.8 1.2 1.6 1.0 0.6 1.3 1.3 -7- RSD freshly opened samples 1.0 0.9 1.3 3.7 1.2 1.2 0.2 0.3 0.4 0.5 0.2 0.2 0.2 0.5 % limit (30% of SDa) 4.5 4.5 3.0 3.0 2.4 2.4 2.4 2.4 3.0 3.0 2.4 2.4 2.4 2.4 3.4. Expected values Expected values were obtained as robust average of the submitted results (consensus value), following ISO rules for proficiency test (PT) (ISO/IEC 43-1:1997, UNI ISO 5725:1998, ISO 13528:2005, ISO/IEC 17025:2005, ISO/IEC 17043:2010). Robust averages and standard deviations are reported in the following tables, together with assigned standard deviation. Expected values for sample 1, concentrations in mg/L SAMPLE 1 Analyte Number of participating laboratories Robust average Robust SD Assigned SD Robust RSD Assigned RSD Chloride 48 1.95 0.20 0.29 10.2 15 Nitrate 45 3.142 0.244 0.314 7.8 10 Sulphate 48 62.87 3.13 5.03 5.0 8 Sodium 37 3.79 0.27 0.30 7.1 8 Potassium 38 2.38 0.19 0.24 8.0 10 Magnesium 37 7.09 0.29 0.57 4.1 8 Calcium 38 45.86 3.20 3.67 7.0 8 Expected values for sample 2, concentrations in mg/L SAMPLE 2 Analyte Number of participating laboratories Robust average Robust SD Assigned SD Robust RSD Assigned RSD Chloride 46 2.00 0.19 0.30 9.7 15 Nitrate 42 3.728 0.239 0.373 6.4 10 Sulphate 46 51.11 2.40 4.09 4.7 8 Sodium 36 3.24 0.26 0.26 7.9 8 Potassium 37 1.73 0.15 0.17 8.4 10 Magnesium 36 11.78 0.46 0.94 3.9 8 Calcium 37 44.25 2.61 3.54 5.9 8 -8- 4. RESULTS 4.1. Data analysis Laboratory proficiency is evaluated using the z-score (zi), calculated for each analyte as the difference between the submitted result (xi) and the robust average, divided by the assigned standard deviation (SDa), the fitness-for-purpose-based “standard deviation for proficiency assessment”. zi xi average * SDa where: xi is the submitted result, average* is the robust average and SDa the assigned standard deviation. For interpreting the z-score it can be assumed that: 0<|z|<2 means that the results lye within the assigned variability 2<|z|<3 means attention level |z| >3 means that some measure is needed A score of zero implies a perfect result. Approximately 95 % of z-scores will fall between -2 and +2. The sign (i.e., - or +) of the score indicates a negative or positive error respectively. Scores in this range are commonly considered as “acceptable” or “satisfactory”. A score outside the range from -3 to 3 would be very unusual and is taken to indicate the cause for this event should be investigated and solved. Scores in this class are commonly designated as “unacceptable” or “unsatisfactory”. Scores in the ranges -2 to -3 and 2 to 3 would be expected about 1 time out of 20. Scores in this class are designated as “questionable”. Robust averages, standard deviations and z-scores were verified independently using Tool4PT Pro Cortez & Mermayde version 1.06.10 2009. 4.2. Results In the following tables the results are listed for each analyte and sample, together with the statistics and the z-scores. Distribution plots are also included, together with the Youden plots (Youden 1959; Youden & Steiner 1975). The latter compares, for each laboratory, the results of the two samples, allowing the identification of systematic and random errors: when systematic errors prevail, the results are located along a diagonal line. Results are numbered and laboratories can identify themselves on the basis of the submitted results. -9- CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: CHLORIDE Unit: mg/L Number of registered laboratories: 61 C1 48 1.95 1.96 0.21 0.29 11 15 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 R.1 2.55 1.84 1.83 2.06 1.95 1.98 1.98 1.96 1.80 2.03 1.43 1.74 1.96 0.72 1.55 1.65 2.09 1.97 2.48 2.10 1.96 0.95 1.59 1.88 C1 z-score 2.1 -0.4 -0.4 0.4 0.0 0.1 0.1 0.0 -0.5 0.3 -1.8 -0.7 0.0 -4.2 -1.4 -1.0 0.5 0.1 1.8 0.5 0.0 -3.4 -1.2 -0.2 R.2 2.60 1.91 1.89 2.07 2.04 2.08 2.08 2.05 1.86 2.10 1.51 1.69 2.06 1.46 1.78 1.71 2.16 2.06 2.58 2.19 2.07 1.06 1.81 1.99 C2 z-score 2.0 -0.3 -0.4 0.2 0.1 0.3 0.3 0.2 -0.5 0.3 -1.6 -1.0 0.2 -1.8 -0.7 -1.0 0.5 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 0.2 1.9 0.6 0.2 -3.1 -0.6 -0.0 - 10 - R.1 1.98 1.97 1.92 2.07 1.90 4.98 2.14 2.09 1.94 2.00 1.92 4.74 2.02 1.89 1.12 1.80 1.81 3.59 2.04 1.89 1.95 2.00 2.25 1.92 C2 46 2.00 2.03 0.19 0.30 10 15 C1 z-score 0.1 0.1 -0.1 0.4 -0.2 10.4 0.7 0.5 -0.0 0.2 -0.1 9.6 0.3 -0.2 -2.8 -0.5 -0.5 5.6 0.3 -0.2 0.0 0.2 1.0 -0.1 R.2 1.99 2.05 2.00 2.14 C2 z-score -0.0 0.2 -0.0 0.5 2.86 2.17 2.24 2.9 0.6 0.8 2.01 2.10 1.97 0.0 0.3 -0.1 2.10 2.02 1.14 1.86 1.87 3.53 2.11 1.95 2.03 2.00 1.82 1.84 0.3 0.1 -2.9 -0.5 -0.4 5.1 0.4 -0.2 0.1 -0.0 -0.6 -0.5 Analyte: CHLORIDE Unit: mg/L 25 mg/L Distribution plot 4.0 3.5 C2 (mg/L) 3.0 2.5 2.0 1.5 1.0 0.5 0.5 1.5 2.5 3.5 4.5 5.5 C1 (mg/L) Youden plot z-score 12 10 8 C1 C2 z +3 z +2 z 0 z -2 z -3 ClCl 6 4 2 0 -2 -6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 -4 Lab. Distribution plot of the z-scores - 11 - 3. 6 2. 8 mg/L 3. 0 2. 4 2. 6 2. 0 2. 2 1. 6 1. 8 1. 0 5.0 0 3. 3. 2 4.7 6 8 2. 2. 2. 2 4 2. 8 1. 0 1. 2. 4 6 1. 0 2 0 1. 0 1. 5 6 5 8 10 0. 10 1. 2 15 1. 4 Lab. 15 0. Lab. C2 20 3. 2 C1 20 3. 4 25 CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: NITRATE Unit: mg/L Number of registered laboratories: 51 C1 45 3.142 3.146 0.244 0.314 8 10 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 C1 R.1 z-score C2 R.2 z-score 3.339 3.289 3.090 2.923 3.055 3.200 3.270 3.217 3.146 3.090 2.245 3.187 3.002 2.490 2.840 3.230 0.6 0.5 -0.2 -0.7 -0.3 0.2 0.4 0.2 0.0 -0.2 -2.9 0.1 -0.4 -2.1 -1.0 0.3 3.953 3.785 3.596 3.454 3.676 3.764 3.785 3.848 3.690 3.660 2.701 3.781 3.653 3.340 3.280 3.800 0.6 0.2 -0.4 -0.7 -0.1 0.1 0.2 0.3 -0.1 -0.2 -2.8 0.1 -0.2 -1.0 -1.2 0.2 6.378 3.660 3.180 2.680 3.600 2.791 3.009 10.3 1.6 0.1 -1.5 1.5 -1.1 -0.4 5.173 4.270 3.667 2.910 4.300 3.487 3.494 3.9 1.5 -0.2 -2.2 1.5 -0.6 -0.6 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 - 12 - C2 42 3.728 3.767 0.239 0.373 6 10 C1 R.1 z-score 3.585 1.4 3.443 1.0 3.088 -0.2 3.058 -0.3 2.916 -0.7 3.154 0.0 3.284 0.5 C2 R.2 z-score 4.706 2.6 4.008 0.8 3.653 -0.2 3.565 -0.4 3.826 3.842 0.3 0.3 3.400 3.221 3.270 2.995 2.812 3.261 3.116 0.8 0.3 0.4 -0.5 -1.1 0.4 -0.1 4.000 3.780 3.850 3.770 0.7 0.1 0.3 0.1 3.829 3.841 0.3 0.3 3.076 3.022 3.014 3.260 3.336 3.050 -0.2 -0.4 -0.4 0.4 0.6 -0.3 3.495 3.538 3.658 3.860 4.003 3.600 -0.6 -0.5 -0.2 0.4 0.7 -0.3 4.795 2.870 5.3 -0.9 3.550 -0.5 18 16 14 12 10 8 6 4 2 0 Unit: mg/L 16 14 C1 C2 Lab. 12 10 8 6 4 2 mg/L mg/L Distribution plot 5.5 C2 (mg/L) 5.0 4.5 4.0 3.5 3.0 2.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 C1 (mg/L) Youden plot z-score 12 10 8 C1 C2 z +3 z +2 z 0 z -2 z -3 NO3 NO3- 6 4 2 0 -4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 -2 Lab. Distribution plot of the z-scores - 13 - 5. 2 5. 0 4. 8 4. 6 4. 4 4. 0 4. 2 3. 6 3. 8 3. 2 3. 4 3. 0 2. 8 2. 6 6 4 4. 4.8 & 6.4 4. 0 2 4. 3. 4. 6 8 3. 2 3. 4 3. 3. 8 0 2. 4 6 2. 2. 0 2 2. 0 2. Lab. Analyte: NITRATE CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: SULPHATE Unit: mg/L Number of registered laboratories: 61 C1 48 62.87 63.40 3.13 5.03 5 8 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 C1 R.1 z-score 66.15 0.7 67.97 1.0 59.16 -0.7 64.81 0.4 62.50 -0.1 63.00 0.0 63.30 0.1 66.42 0.7 63.50 0.1 64.53 0.3 61.99 -0.2 63.72 0.2 62.93 0.0 64.83 0.4 54.21 -1.7 67.07 0.8 61.04 -0.4 C2 R.2 z-score 54.76 0.9 55.08 1.0 46.98 -1.0 51.87 0.2 51.00 -0.0 51.82 0.2 51.65 0.1 52.50 0.3 52.10 0.2 52.08 0.2 51.00 -0.0 48.27 -0.7 51.27 0.0 52.45 0.3 48.05 -0.7 52.61 0.4 49.73 -0.3 34.34 64.96 62.51 56.80 70.00 58.70 63.82 41.46 52.02 50.60 44.70 47.00 47.80 52.26 -5.7 0.4 -0.1 -1.2 1.4 -0.8 0.2 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 -2.4 0.2 -0.1 -1.6 -1.0 -0.8 0.3 - 14 - C2 46 51.11 51.44 2.40 4.09 5 8 C1 R.1 z-score 62.05 -0.2 63.62 0.1 63.98 0.2 63.49 0.1 56.95 -1.2 62.40 -0.1 64.16 0.3 62.15 -0.1 65.50 0.5 64.32 0.3 66.45 0.7 62.31 -0.1 60.11 -0.5 64.11 0.2 63.74 0.2 51.34 -2.3 56.32 -1.3 56.11 -1.3 62.35 -0.1 63.96 0.2 64.89 0.4 62.33 -0.1 C2 R.2 z-score 50.80 -0.1 51.34 0.1 52.05 0.2 51.43 0.1 69.30 59.76 1.3 -0.6 54.20 52.12 49.74 53.50 51.60 54.68 50.43 0.8 0.2 -0.3 0.6 0.1 0.9 -0.2 51.82 51.44 60.84 44.47 44.37 50.91 50.78 51.72 49.72 0.2 0.1 2.4 -1.6 -1.6 -0.0 -0.1 0.1 -0.3 55.43 49.89 1.1 -0.3 Unit: mg/L Distribution plot C2 (mg/L) 65 55 45 (34.3; 41.5) 35 40 50 60 70 80 C1 (mg/L) Youden plot z-score 4 2 C1 C2 z +3 z +2 z 0 z -2 z -3 = SO4 SO 4 0 -2 -6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 -4 Lab. Distribution plot of the z-scores - 15 - 64 62 60 56 mg/L 58 52 54 48 50 44 mg/L 46 40 C2 38 70 68 66 64 62 60 58 56 54 52 50 48 436 6 Lab. C1 18 16 14 12 10 8 6 4 2 0 42 20 18 16 14 12 10 8 6 4 2 0 434 4 Lab. Analyte: SULPHATE CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: SODIUM Unit: mg/L Number of registered laboratories: 61 C1 37 3.79 3.82 0.27 0.30 7 8 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 C1 R.1 z-score C2 R.2 z-score 3.82 4.05 0.1 0.9 3.23 3.45 -0.0 0.8 3.85 3.73 3.59 3.88 4.12 3.09 3.55 3.98 3.85 3.83 3.74 3.27 3.68 1.53 0.2 -0.2 -0.6 0.3 1.1 -2.3 -0.8 0.6 0.2 0.1 -0.2 -1.7 -0.3 -7.4 3.27 3.17 3.05 3.26 3.47 3.67 2.95 3.79 3.24 3.19 3.18 2.87 3.10 1.23 0.1 -0.3 -0.7 0.1 0.9 1.7 -1.1 2.1 0.0 -0.2 -0.2 -1.4 -0.5 -7.8 2.62 4.30 3.51 -3.8 1.7 -0.9 1.83 3.37 2.99 -5.4 0.5 -1.0 5.50 5.7 5.00 6.8 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 - 16 - C2 36 3.24 3.23 0.26 0.26 8 8 C1 R.1 z-score C2 R.2 z-score 3.85 3.67 4.03 0.2 -0.4 0.8 3.20 3.06 3.45 -0.1 -0.7 0.8 3.95 3.58 0.5 -0.7 3.30 3.02 0.2 -0.8 3.90 3.67 3.81 3.91 3.50 0.4 -0.4 0.1 0.4 -0.9 3.20 3.09 3.80 3.34 -0.1 -0.6 2.2 0.4 3.50 4.33 3.89 3.78 -0.9 1.8 0.3 -0.0 2.92 3.71 3.23 3.22 -1.2 1.8 -0.0 -0.1 3.80 3.89 0.0 0.3 3.22 3.31 -0.1 0.3 4.07 0.9 3.38 0.6 8 6 4 -8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 z-score C2 (mg/L) 2 3. 1 2 4 4. mg/L mg/L Distribution plot 6 5 4 3 2 1 3 - 17 4 5 C1 C2 z +3 z +2 z 0 z -2 z -3 Lab. Distribution plot of the z-scores 6 C1 (mg/L) Youden plot Na+ Na 2 0 -2 -4 -6 5.0 4. 0 4.8 3. 8 3. 6 3. 4 0 3. 0 2 0 3. 2 4 2 2. 8 4 2. 6 6 2. 2 10 2. 4 12 2. 0 C1 1. 8 8 Lab. 14 1. 6 1.4 1. 4 1.2 5.6 45.4 .6 0 2 4. 3. 4. 6 8 3. 4 3. 3. 8 0 2. 4 6 10 2. 12 2. 2. 0 1.4 21.6 .2 Lab. Analyte: SODIUM Unit: mg/L 14 C2 8 6 CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: POTASSIUM Unit: mg/L Number of registered laboratories: 61 C1 38 2.38 2.39 0.19 0.24 8 10 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 R.1 2.25 2.46 2.52 C1 z-score -0.6 0.3 0.6 R.2 1.62 1.81 1.85 C2 z-score -0.6 0.5 0.7 2.43 2.25 2.42 2.34 2.50 2.17 2.34 2.22 2.39 2.49 2.38 6.14 2.08 2.55 0.2 -0.6 0.1 -0.2 0.5 -0.9 -0.2 -0.7 0.0 0.4 -0.0 15.8 -1.3 0.7 1.85 1.61 1.77 1.62 1.80 1.51 1.75 1.50 1.71 1.75 1.74 1.89 2.08 1.86 0.7 -0.7 0.2 -0.6 0.4 -1.3 0.1 -1.3 -0.1 0.1 0.1 0.9 2.0 0.8 2.57 2.28 2.84 0.8 -0.4 1.9 1.82 1.66 2.04 0.5 -0.4 1.8 2.96 2.4 1.92 1.1 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 - 18 - C2 37 1.73 1.74 0.15 0.17 8 10 C1 R.1 z-score C2 R.2 z-score 2.18 2.35 2.47 -0.9 -0.1 0.4 1.54 1.71 1.85 -1.1 -0.1 0.7 2.57 2.10 0.8 -1.2 1.86 1.57 0.8 -0.9 2.50 2.33 2.36 2.53 1.77 0.5 -0.2 -0.1 0.6 -2.6 1.80 1.69 1.69 1.89 0.4 -0.2 -0.2 0.9 2.18 2.41 2.39 2.33 -0.9 0.1 0.0 -0.2 1.58 1.70 1.74 1.63 -0.9 -0.2 0.1 -0.6 2.34 2.13 -0.2 -1.1 1.70 1.05 -0.2 -3.9 2.89 2.1 1.58 -0.9 Analyte: POTASSIUM Unit: mg/L 10 12 C1 10 Lab. 6 6 4 4 2 2 mg/L mg/L Distribution plot 2.4 2.2 C2 (mg/L) 2.0 (6.14; 1.89) 1.8 1.6 1.4 1.2 1.0 1.5 2.0 2.5 3.0 3.5 C1 (mg/L) Youden plot z-score KK+ 18 15 12 9 6 C1 C2 z +3 z +2 z 0 z -2 z -3 3 0 -6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 -3 Lab. Distribution plot of the z-scores - 19 - 2. 1 2. 2 2. 0 1. 9 1. 7 1. 8 1. 6 1. 5 1. 3 1. 4 1. 2 1. 1 0. 9 6.2 3. 1 6.1 3. 0 2. 9 2. 7 2. 8 2. 6 2. 5 2. 4 2. 2 2. 3 2. 1 2. 0 1. 9 1. 8 1. 0 0 0 1. 7 Lab. C2 8 8 CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: MAGNESIUM Unit: mg/L Number of registered laboratories: 61 C1 37 7.09 7.08 0.29 0.57 4 8 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 R.1 7.09 7.04 7.86 C1 z-score 0.0 -0.1 1.4 C2 R.2 z-score 11.85 0.1 11.63 -0.2 11.96 0.2 6.40 6.94 6.92 6.82 7.74 6.96 6.90 5.96 7.21 7.38 7.10 7.47 -1.2 -0.3 -0.3 -0.5 1.1 -0.2 -0.3 -2.0 0.2 0.5 0.0 0.7 10.85 11.53 11.50 11.40 12.86 11.52 11.43 8.77 11.95 12.29 11.60 13.18 -1.0 -0.3 -0.3 -0.4 1.2 -0.3 -0.4 -3.2 0.2 0.5 -0.2 1.5 6.68 -0.7 10.78 -1.1 7.23 7.30 7.30 0.2 0.4 0.4 13.30 12.37 12.05 1.6 0.6 0.3 6.87 -0.4 11.45 -0.3 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 - 20 - C2 36 11.78 11.74 0.46 0.94 4 8 C1 R.1 z-score C2 R.2 z-score 7.11 7.07 7.12 0.0 -0.0 0.1 11.92 11.74 11.74 0.2 -0.0 -0.0 7.28 7.08 0.3 -0.0 12.06 12.03 0.3 0.3 7.10 7.19 7.02 6.81 12.69 0.0 0.2 -0.1 -0.5 9.9 11.80 12.03 12.14 11.57 0.0 0.3 0.4 -0.2 7.97 7.06 7.06 6.87 1.6 -0.1 -0.1 -0.4 13.03 11.65 11.43 11.60 1.3 -0.1 -0.4 -0.2 6.90 7.37 -0.3 0.5 11.36 11.96 -0.4 0.2 6.19 -1.6 11.23 -0.6 10 8 6 -4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 z-score C2 (mg/L) 8 6. 5 0 8. 6 9. 7 8.5 10 .0 8.8 10 .3 10 .6 10 .9 11 .2 11 .5 11 .8 12 .1 12 .4 12 .7 13 .0 13 .3 13 .6 12.8 8. 2 12.6 6 8 7. 7. 7. 2 4 7. 0 6. 7. 4 6 6. 0 2 6. 8 6 Lab. 10 6. 6 8 5. 5. Lab. Analyte: MAGNESIUM Unit: mg/L 14 12 12 C1 10 8 4 2 0 7 - 21 - C2 6 4 2 0 mg/L mg/L Distribution plot 15 14 13 12 11 10 9 8 8 C1 C2 z +3 z +2 z 0 z -2 z -3 Lab. Distribution plot of the z-scores 9 C1 (mg/L) Youden plot Mg++ Mg 4 2 0 -2 CNR-ISE Verbania – Dionex S.p.A. Milano Quality Control IC Proficiency Test - QC-IC 2010-2 Analyte: CALCIUM Unit: mg/L Number of registered laboratories: 61 C1 38 45.86 45.75 3.20 3.67 7 8 Number of participating laboratories: Robust average mg/L Median mg/L Robust standard deviation mg/L Assigned standard deviation mg/L Robust relative standard deviation Assigned relative standard deviation Lab. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 C1 R.1 z-score 45.92 0.0 45.06 -0.2 46.84 0.3 C2 R.2 z-score 44.12 -0.0 43.37 -0.2 46.11 0.5 41.40 46.64 45.31 44.28 50.93 45.81 45.98 49.01 45.17 46.34 44.50 50.10 43.99 38.52 -1.2 0.2 -0.2 -0.4 1.4 -0.0 0.0 0.9 -0.2 0.1 -0.4 1.2 -0.5 -2.0 42.30 45.55 43.62 43.20 49.08 44.03 44.25 50.10 43.51 44.68 42.80 50.73 42.13 37.00 -0.6 0.4 -0.2 -0.3 1.4 -0.1 -0.0 1.7 -0.2 0.1 -0.4 1.8 -0.6 -2.0 58.60 36.93 44.94 3.5 -2.4 -0.3 58.00 39.82 43.27 3.9 -1.3 -0.3 41.77 -1.1 41.31 -0.8 Lab. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 - 22 - C2 37 44.25 44.03 2.61 3.54 6 8 C1 R.1 z-score C2 R.2 z-score 43.83 45.72 43.10 -0.6 -0.0 -0.8 43.25 44.07 41.99 -0.3 -0.1 -0.6 48.07 51.30 0.6 1.5 46.02 50.00 0.5 1.6 47.20 46.05 48.07 45.36 51.00 0.4 0.1 0.6 -0.1 1.4 45.40 44.24 46.85 43.86 0.3 -0.0 0.7 -0.1 52.20 44.65 47.06 45.78 1.7 -0.3 0.3 -0.0 50.00 42.87 44.39 44.53 1.6 -0.4 0.0 0.1 45.33 40.10 -0.1 -1.6 43.75 35.12 -0.1 -2.6 42.85 -0.8 41.94 -0.7 Analyte: CALCIUM Unit: mg/L 14 16 C1 14 10 Lab. 10 8 6 8 6 mg/L mg/L Distribution plot 60 55 C2 (mg/L) 50 45 40 35 30 30 35 40 45 50 55 60 65 C1 (mg/L) Youden plot z-score 6 4 C1 C2 z +3 z +2 z 0 z -2 z -3 Ca++ Ca 2 0 -4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 -2 Lab. Distribution plot of the z-scores - 23 - 58 54 56 52 50 48 46 44 42 38 40 36 32 60 58 56 52 54 48 50 44 46 40 0 42 0 38 2 36 2 34 4 4 34 Lab. C2 12 12 5. DISCUSSION Fifty out of 61 registered laboratories submitted their results. The number of results received for samples C1 and C2 for anions and cations is 42-48 and 36-38, respectively. Ion chromatography was the most used analytical methods for anions (92-98%) and cations (ca. 70%). No comparison among the methods was performed. Analytical technique Specific method Chloride Nitrate Sulphate Ion chromatography APAT IRSA 4020 11 13 13 Ion chromatography EPA 300.1 A 3 3 3 Ion chromatography ISO 10304-1 8 7 8 Ion chromatography ISS CBB 037 7 7 7 Ion chromatography A.P.H.A. St. Methods. 2005 4110 B 3 3 3 Ion chromatography Not available 13 11 13 Titration APAT IRSA 4090 A1 1 Titration ASTM D 512-04 1 Spectrophotometry Not available Not available Not available 1 2 1 Analytical technique Method Sodium Potassium Calcium Magnesium Ion chromatography APAT IRSA 3030 10 10 10 10 Ion chromatography ISO 14911 6 6 6 6 Ion chromatography ISS CBB 038 3 3 3 3 Ion chromatography Not available 9 9 10 9 ICP OES APAT IRSA 3020 1 1 1 1 ICP OES ISO 11885 2 2 2 2 ICP OES Standard Methods 2005 3120 B 1 1 1 1 ICP OES Not available 1 2 2 2 Atomic absorption APAT IRSA 3270 1 Atomic absorption APAT IRSA 3240 Atomic absorption APAT IRSA 3180 Atomic absorption APAT IRSA 3130 Atomic absorption Not available 1 1 1 2 - 24 - 2 1 1 Anion and cation concentrations were representative of most drinking waters. Values were low, but in the usual analytical range of ion chromatography. Proficiency was good for nitrate, sulphate, calcium and magnesium, with only 1-2 laboratories obtaining z-scores outside the range -3/+3. In the case of chloride, calcium and magnesium variability was higher and 3-5 laboratories obtained z-scores higher than 3 or lower than -3. Systematic errors prevailed, mainly for chloride, sodium and potassium, and may be related to problems in standard preparation or in calibration, or to the contamination of standard solutions or samples. Sodium contamination may also be due to sodium release from glass containers and vials, while potassium may suffer contamination from the potassium hydroxide used for the determination of anions. 6. CONCLUSIONS The overall proficiency of the laboratories was good, with a small number of z-scores outside the range -3/+3, and robust standard deviations smaller than the assigned values, , which were evaluated on the basis of the official analytical methods. In particular for sulphate and magnesium. Youden plots showed that systematic errors prevail, indicating that further improvement is possible by controlling the quality of standards, sample handling and instrument calibration. The analysis of certified samples and the use of control charts may help to improve analytical proficiency. The use of a website for uploading the results allowed an immediate check of the results by each lab, increasing the utility of the proficiency tests for improving analytical quality. - 25 - 7. REFERENCES A.P.H.A., A.W.W.A., W.E.F. 2005. Standard Methods for the examination of water and wastewater. Method 4110 B. Determination of anions by ion chromatography. Am. Publ. Health Ass., Washington. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 3030. Determinazione di cationi (sodio, ammonio, potassio, magnesio, calcio) mediante cromatografia ionica. Vol. 1. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 3020. Determinazione di elementi chimici mediante spettroscopia di emissione con sorgente al plasma (ICP-OES). Vol. 1. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 3180. Magnesio (F-AAS). Vol. 1. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 3130. Calcio (F-AAS). Vol. 1. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 3270. Sodio (F-AAS). Vol. 1. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 3240. Potassio (F-AAS). Vol. 1. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 4020. Anioni (fluoruro, cloruro, nitrito, bromuro, nitrato, fosfato e solfato) in cromatografia ionica. Vol. 2. 1153 pp. APAT IRSA-CNR. 2003. Metodi analitici per le acque. 4030. Azoto ammoniacale. Metodo A1. Determinazione spettrofotometrica all’indofenolo. Vol. 2. 1153 pp. EPA 300.1 1997. Determination of inorganic anions in drinking water by ion chromatography. USEPA Office of Water. ILAC-G13:08/2007. Guidelines for the Requirements for the Competence of Providers of Proficiency Testing Schemes. ISO/IEC 43-1:1997. Proficiency testing by interlaboratory comparisons. Part 1: Development and operation of proficiency testing schemes. ISO 14911:1998. Water quality. Determination of dissolved Li+, Na+, NH4+, K+, Mn2+, Ca2+, Mg2+, Sr2+ and Ba2+ using ion chromatography. Method for water and waste water. ISO 13528:2005. Statistical methods for use in proficiency testing by interlaboratory comparisons. ISO/IEC 17025:2005. General requirements for the competence of testing and calibration laboratories. ISO 10304-1: 2007. Water quality. Determination of dissolved anions by liquid chromatography of ions. Part. 1. Determination of bromide, chloride, fluoride, nitrate, phosphate and sulphate. ISO/IEC 17043:2010. Conformity assessment. General requirements for proficiency testing. ISS CBB 037. Istituto Superiore di Sanità. 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