Accred Qual Assur (2000) 5 : 331–338 Q Springer-Verlag 2000
Ioannis Papadakis Philip D.P. Taylor
Received: 31 December 1999 Accepted: 7 March 2000
I. Papadakis (Y) Institute for Reference Materials and Measurements, European Commission – JRC, Retieseweg, 2440 Geel, Belgium e-mail:
[email protected] Tel.: c32-14-571682 Fax: c32-14-591978 P.D.P. Taylor Institute for Reference Materials and Measurements, European Commission – JRC, Retieseweg, 2440 Geel, Belgium
PRACTIONER’S REPORT
Establishment of SI-traceable reference ranges for the content of various elements in the IMEP-9 water sample
Abstract The International Measurement Evaluation Programme (IMEP) attempts to shed light on the current state of the practice in chemical measurements. The main tool, which assists this attempt and also differentiates IMEP from similar projects, is the establishment of SI-traceable reference ranges (where possible) for the elements offered for measurement to the participants for every IMEP round. The Institute for Reference Materials and Measurements (IRMM), as the founder and co-ordinator of IMEP has the responsibility of establishing the SI-traceable reference ranges. This is a large task that requires knowledge, skill and resources. IRMM collaborates with a network of reference laboratories in order to achieve the establishment of SI-traceable reference ranges in a transparent and reliable way. The IMEP reference laboratories must have demonstrated experience and have a proven and successful record in the use of pri-
Introduction The International Measurement Evaluation Programme (IMEP) was established and is operated by the Institute for Reference Materials and Measurements (IRMM) in order to shed light on the current state of the practice in chemical measurements. Previous IMEP rounds have focused on trace elements in human se-
mary methods of measurements (mainly isotope dilution mass spectrometry) and the application of uncertainty evaluation according to ISO/BIPM guidelines. In IMEP-9 “trace elements in water”, results from 7 reference laboratories (including IRMM) were combined by IRMM to establish SI-traceable ranges for the 15 elements, which were then offered for measurement to the 200 participants worldwide. This paper does not discuss the individual contribution of the reference laboratories (this could be the subject of individual papers) but describes the procedures and criteria used in order to establish the reference ranges for the IMEP-9 samples by combining the individual contributions. All results submitted to IRMM are included, so as to make this publication as realistic as possible. Key words IMEP 7 Certification 7 Uncertainty calculation 7 Metals in water 7 Interlaboratory comparison
rum, water samples and polyethylene. IMEP participants can assess the quality of their results against the IMEP reference ranges and also compare them against those obtained by an international measurement community (the IMEP-6 round trace elements in water had almost 200 participants from 26 countries and 4 different continents). IMEP is open to all laboratories and full confidentiality is guaranteed with respect to the link between
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results and the participants’ identity. Participating laboratories receive a certified test sample (CTS) (with undisclosed content values), to be analysed using routine analytical procedures. The measurement results of participants are evaluated against metrological reference ranges, traceable to the SI if possible. IRMM launched the IMEP-9 measurement round focusing on trace elements in water in March 1998. The CTS for the participants were bottled in polyethylene containers, containing F60 ml of a river water. The elements B, Ca, Cd, Cr, Cu, Fe, K, Li, Mg, Ni, Pb, Rb, Sr, U and Zn were offered for measurement in IMEP-9. For most of these elements, IMEP reference ranges for amount content were available, obtained using primary methods of measurements where possible. The production of these IMEP reference ranges is discussed in this paper.
Table 1 IMEP-9 reference laboratories Institution
Elements measured
IRMM (European Union) Institute for Reference Materials and Measurements NRC (Canada) National Research Council of Canada KRISS (South Korea) Korean Research Institute of Standards and Science CIEMAT (Spain) Centro de Investigaciones Energeticsas Medioambientales y Tecnologies University of Mainz (Germany) Institute for Inorganic and Analytical Chemistry NIES (Japan) National Institute for Environmental Studies NRCCRM (China) National Research Centre for Certified Reference Materials
10 9 4 3 3 2 2
Laboratories contributing to the certification Seven laboratories all over the world collaborated under the co-ordination of IRMM, to produce the reference ranges for the IMEP-9 samples. The experience and proven successful record in the application of isotope dilution mass spectrometry (IDMS) as a primary method of measurement (PMM) were the criteria for selecting the reference laboratories. Table 1 contains all the laboratories that contributed to the certification of the IMEP-9 samples as well as the number of different elements they measured in their contribution to the establishment of the IMEP-9 reference ranges.
Sample The water samples, were sampled from the Clear Creek river (Colorado, USA) by the United States Geological Survey, Colorado, USA (USGS) and further treated by the National Institute for Standards and Technology, Gaithersburg, USA (NIST). The water, after sampling, was submitted to ultra filtration, sterilisation and stabilisation with nitric acid to pH~1.2. It was then bottled in pre-cleaned polyethylene bottles. The bottles used by the certification laboratories contained F100 ml of water, whereas the bottles for the participants F60 ml. The storage temperature of the samples was 5 7C (normal refrigerator). The samples were available to the reference laboratories from February 1998 and the initial deadline for reporting the reference results was the end of May 1998. The intention was to have the reference ranges available before the results of the participants in order to communicate them to the participants shortly after their result report. Due to several problems the deadline was extended to the end of August 1998.
A first feedback to the participants, concerning the reference ranges (the IMEP-9 certificate) was sent out at the end of November 1998. The full participants’ report [1], containing several IMEP graphs, was distributed at the end of March 1999.
Certification campaign Prior to the distribution of samples, the candidate reference laboratories gave an input as to their ability to provide data, taking into account the time schedule, their expertise and the requirements of IMEP-9. All the reference laboratories responded and a list of the 15 elements to be measured was prepared. Then the 100 ml samples were distributed to the reference laboratories. The instructions were to provide reference results (including expanded uncertainty kp2) for the elements of their responsibility using IDMS as a PMM. The Comité Consultatif pour la Quantité de Matière (CCQM) of the Bureau International des Poids et Mesures (BIPM) recognised IDMS [2] as a PMM. The reference laboratories were asked to evaluate the uncertainty of their results according to the ISO/BIPM Guide to the Expression of Uncertainty in Measurement [3]. By the end of the certification campaign, the reference laboratories were asked to inform IRMM (briefly) about the measurement procedure they followed as well as their uncertainty statement and provide an uncertainty budget (a template was sent to the reference laboratories).
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Establishing reference ranges The final “reference ranges” were characterised as “certified” or “assigned”. According to IMEP policy, only ranges derived from results traceable to the SI, can be characterised as “certified ranges”. All other (non-SItraceable) ranges are characterised as “assigned”. Calculating the reference value from a set of SItraceable reference values also includes some weighing of the available data which is not a simple process. This was not treated as a statistical procedure, but the measurement procedures used were screened in detail. The wide range of data (typically B50%) reported by the IMEP-9 participants (field laboratories) should be kept into perspective, when discussing the reference ranges. The uncertainty statements made on these values are adequate for this purpose. Due to workload or to technical problems, reference laboratories may be unable to report all their results. For that reason, some redundancy was build up: the content of most of the elements was measured by more than one reference laboratory. In case of non-concordant 1 data, the reference laboratories were contacted so as to discuss and obtain more information. On occasion, some results were eliminated. The metrological merits and disadvantages of the various methods were also taken into consideration in this evaluation process (e.g. use of different instrumental techniques). On some occasions, individual values with larger uncertainty were only used as “supportive” data. Preference was given to data produced using methods yielding small but provable combined uncertainty.
Uncertainty evaluation of the reference ranges Evaluation of the uncertainty of the reference values was performed as follows. For four elements (Cd, Pb, Sr and U) the reference values were derived from more than one reference result. For these elements an approach of uncertainty evaluation inspired by a publication from Pauwels et. al. [5] was used which takes into account the concepts of the ISO/BIMP guide [3] This approach consists of three steps: I In the first step all the uncertainties (kp2) of the different reference results are taken into account. The uncertainty is calculated by taking the average from the different reported uncertainties.
1 Proposed definition for “concordance”: the property of measurements results, obtained on identical samples (sub-samples of the same material), to agree within their uncertainties, when these results are measured on the same measurement scale (i.e. expressed in the same unit) [4].
I In the second step, the spread of the results reported by the different laboratories is taken into account. The standard deviation of the mean of the different reported results is doubled to be equivalent with the first step uncertainty (so using a coverage factor kp2). I Finally in the third step, the results of the two previous steps are combined quadratically (square root of the sum of the squares) to result in the final uncertainty of the reference range. The remaining elements can be divided into two categories: I For the elements B, Ca, Cr, Fe, Li, Mg, K, Rb and Zn the certification is based on result of one reference laboratory. For these elements the original uncertainty of the set of data was taken as the reference range uncertainty (applying the minimum uncertainty of 2% for the “certified” values and 10% for the “assigned” values). I For the elements Cu and Ni the certification is based on two contributing non-concordant results, therefore the reference range was characterised as “assigned” and a minimum 10% uncertainty was applied.
Results and discussion on the individual contributions as well as on the reference values The available reference measurement data for the IMEP-9 water samples are grouped by element. The results are coded according to the laboratory origin for reasons of confidentiality. In the second column of each table, the abbreviation of the instrumental technique used for the measurement of the isotope amount ratios is presented (thermal ionisation mass spectrometry – TIMS, inductively coupled plasma mass spectrometry – ICP-MS, high resolution ICP-MS – HR-ICP-MS and ko Neutron Activation Analysis – ko NAA). The third column contains the measurement result reported to IRMM and the fourth column the reported uncertainties. The uncertainties presented in the tables are expanded uncertainties (kp2). Boron Results from two different laboratories were provided for B (Table 2). The two results are not concordant, however the TIMS measurements suffer from severe background problems. For this reason the reference “certified” value is based only on the ICP-MS data. Calcium For Ca, data were only produced by means of ko NAA. The method was not considered as a PMM. For this
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reason the reference range (presented in Table 3) is characterised as “assigned”.
Table 2 Reference results for boron in the IMEP-9 water sample: inductively coupled mass spectrometry (ICP-ME), thermal ionisation mass spectrometry (TIMS) Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
A G Certified value
ICP-MS TIMS
4.39 4.92 4.39
0.16 0.15 0.16
Cadmium Reference results from four different laboratories were provided for Cd (Table 4, Fig. 1). One institute (code A) provided two different results, produced by two individual analysts on different samples. All five available results are concordant, thus the established reference range is characterised as “certified”.
Table 3 Reference results for calcium in the IMEP-9 water sample: neutron activation analysis (ko NAA) Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
A Assigned value
ko NAA
780 780
119 119
Chromium Two different laboratories provided results for Cr (Table 5). The established reference value is based on TIMS data, because Cr measurements can suffer from severe problems (mainly interferences) using ICP-MS. The reference value is characterised as “certified”.
Table 4 Reference results for cadmium in the IMEP-9 water sample: high resolution ICP-MS (HR-ICP-MS) Lab code
Technique
Reported value (nmol/kg)
Uncertainty (nmol/kg)
A A B C E Certified value
ICP-MS ICP-MS HR-ICP-MS ICP-MS ICP-MS
83.5 83.4 82.9 83.7 83.4 83.4
2.1 2.5 1.3 2.2 2.8 2.2
Copper Two different laboratories provided results for Cu (Table 6). The results were not concordant and for that reason the established reference value is characterised as “assigned”. Iron Two different laboratories provided results for Fe (Table 7, Fig. 2). The results were concordant, thus the established reference value is characterised as “certified”. However for Fe, TIMS measurements are known to have lower combined uncertainties than ICP-MS measurements and for that reason the “certified value” is based only on the TIMS data and the ICP-MS data is used as supportive information only.
Table 5 Reference results for chromium in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
C D Certified value
ICP-MS TIMS
0.269 0.302 0.302
0.010 0.011 0.011
Table 6 Reference results for copper in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
B C Assigned value
HR-ICP-MS 0.185 ICP-MS 0.167 0.176
Uncertainty (mmol/kg) 0.007 0.007 0.018
Table 7 Reference results for iron in the IMEP-9 water sample
Fig. 1 Graphical display of reported results and certified value for cadmium
Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
A C Certified value
TIMS ICP-MS
1.894 1.80 1.894
0.060 0.11 0.060
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Table 8 Reference results for lead in the IMEP-9 water sample Lab code
Technique
Reported value (nmol/kg)
Uncertainty (nmol/kg)
A A B C D E Certified value
ICP-MS ICP-MS HR-ICP-MS ICP-MS ICP-MS ICP-MS
62.73 62.4 62.3 62.4 62.5 62.1 62.4
0.52 1.8 0.9 3.4 0.1 1.2 1.3
Table 9 Reference results for lithium in the IMEP-9 water sample Fig. 2 Graphical display of reported results and certified value for iron
Lead Results from six different laboratories were provided for Pb (Table 8, Fig. 3). One institute (coded A) provided two different results, produced by two individual analysts on different samples. The agreement between the results was satisfactory and thus the established reference value is characterised as “certified”.
Lithium Two different laboratories provided results for Li (Table 9, Fig. 4) and the results were concordant. However TIMS measurements on Li are known to have smaller uncertainties than ICP-MS measurements, which suffer from large memory effects and large mass bias (up to 15%). This also results in the large difference on the magnitude of the uncertainty of the two results. For these reasons, the reference value is based only on the TIMS data, the ICP-MS data are used as supportive in-
Fig. 3 Graphical display of reported results and certified value for lead
Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
A C Certified value
TIMS ICP-MS
3.952 3.78 3.952
0.068 0.30 0.079
Table 10 Reference results for magnesium in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
A F Certified value
ICP-MS TIMS
349 356.1 356.1
10 1.2 7.1
formation and the established reference value is characterised as “certified”. Magnesium Two different laboratories provided results for Mg (Table 10, Fig. 5). The results were concordant, however,
Fig. 4 Graphical display of reported results and certified value for ithium
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Table 11 Reference results for nickel in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
B C Assigned value
HR-ICP-MS 0.201 ICP-MS 0.181 0.191
Uncertainty (mmol/kg) 0.006 0.009 0.019
Table 12 Reference results for potassium in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
F Certified value
TIMS
55.94 55.9
0.28 1.1
Fig. 5 Graphical display of reported results and certified value for magnesium
the TIMS measurements for Mg are known to have smaller uncertainties than ICP-MS measurements. For this reason the established reference value, which is characterised as “certified”, is based only on the TIMS data and the ICP-MS data are used as supportive information. Nickel
Table 13 Reference results for rubidium in the IMEP-9 water sample Lab code
Technique
Reported value (nmol/kg)
Uncertainty (nmol/kg)
A G Certified value
ICP-MS TIMS
25.89 28.9 25.89
0.36 1.6 0.52
Two different laboratories provided results for Ni (Table 11). The results were not concordant and for that reason the established reference value is characterised as “assigned”. Strontium Potassium Only one laboratory provided results for K (Table 12). The only available data were obtained using TIMS measurements. The data were considered to be sufficient to characterise the established reference value as “certified”. Rubidium Two different laboratories provided results for Rb (Table 13). The results were not concordant. However, TIMS measurements are known to be potentially severely interfered by K. The variation between the values for five different blends was 2%, which is much larger than what would be expected from a well-controlled IDMS experiment. For these reasons the reference value is based only on the ICP-MS set of data and the established reference value is characterised as “certified”.
Two different laboratories provided results for Sr (Table 14, Fig. 6). The results were concordant, thus the established reference value is characterised as “certified”. Uranium Three different laboratories provided results for U (Table 15, Fig. 7). The results were concordant, thus the established reference value is characterised as “certified”. Zinc Only one laboratory provided results for Zn (Table 16). Zn measurements using ICP-MS are known to suffer severe problems, thus because of the absence of additional data, the established reference value is characterised as “assigned”.
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Table 14 Reference results for strontium in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
A G Certified value
ICP-MS TIMS
2.241 2.26 2.251
0.051 0.03 0.044
Table 16 Reference results for zinc in the IMEP-9 water sample Lab code
Technique
Reported value (mmol/kg)
Uncertainty (mmol/kg)
C Assigned value
ICP-MS
0.146 0.146
0.015 0.015
Final reference ranges
Fig. 6 Graphical display of reported results and certified value for strontium
Table 15 Reference results for uranium in the IMEP-9 water sample Lab code
Technique
Reported value (nmol/kg)
Uncertainty (nmol/kg)
A C D Certified value
ICP-MS ICP-MS ICP-MS
4.68 4.95 4.45 4.69
0.04 0.28 0.17 0.33
The reference ranges should finally be converted into the unit which the participants of IMEP-9 where instructed to report their results and uncertainties. This unit is mol/l whereas the reference laboratories reported their results in mol/kg. The reason for this is that most of the field laboratories (participants), working on water samples, operate volumetrically, whereas the reference laboratories, which perform IDMS, operate gravimetrically. The Mass Metrology Group of IRMM performed the density measurements on IMEP-9 samples. The density was measured in six different bottles, as presented in Table 17, and it was found to be 0.998B0.001 kg/l at a temperature of 22 7C. After the transformation to mol/l Table 18 presents the final reference ranges (reference values and the associated uncertainty) for all the elements which were subject to analysis in the water samples of IMEP-9.
Conclusions From the establishment of the SI-traceable reference ranges of IMEP-9 several conclusions can be drawn. The results produced during the “certification campaign” yielded very small uncertainties for the reference values, especially when comparing these with the spread of the participants’ data [1, 6], which is often larger than 50%. Concerning the PMM: PMMs are methods which when applied in a primary way can lead to SI-traceable results with very small combined uncertainties (compared with other methods used for the same analysis). Table 17 Density measurement results at 22 7C
Fig. 7 Graphical display of reported results and certified value for uranium
Bottle
Density (kg/l)
Uncertainty (kg/l)
Q 524 562 Q 522 432 Q 523 269 Q 520 865 Q 523 923 Q 524 819 Final value
0.998 0.998 0.998 0.998 0.998 0.998 0.998
0.001 0.001 0.001 0.001 0.001 0.001 0.001
3 2 1 0 1 1
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Table 18 Reference values in IMEP-9 water samples Element
Reference value
Uncertainty
Characterisation
Boron Calcium Cadmium Chromium Copper Iron Lead Lithium Magnesium Nickel Potassium Rubidium Strontium Uranium Zinc
4.38 mmol/l 778 mmol/l 83.2 nmol/l 0.301 mmol/l 0.176 mmol/l 1.890 mmol/l 62.3 nmol/l 3.944 mmol/l 355.4 mmol/l 0.191 mmol/l 55.8 mmol/l 25.84 nmol/l 2.246 mmol/l 4.68 nmol/l 0.146 mmol/l
0.16 mmol/l 119 mmol/l 2.2 nmol/l 0.011 mmol/l 0.018 mmol/l 0.060 mmol/l 1.3 nmol/l 0.079 mmol/l 7.1 mmol/l 0.019 mmol/l 1.1 mmol/l 0.52 nmol/l 0.044 mmol/l 0.33 nmol/l 0.015 mmol/l
Certified Assigned Certified Certified Assigned Certified Certified Certified Certified Assigned Certified Certified Certified Certified Assigned
Nevertheless the application of a PMM does not “automatically” lead to the “correct” result. On some occasions described in this paper the realisation of IDMS (PMM) was incomplete, thus accordingly after critical evaluation and discussion these results were not taken into account. Concerning the uncertainty evaluation: all contributions were accompanied by detailed uncertainty bud-
gets. Unfortunately the ISO/GUM document [3] does not include guidelines for uncertainty evaluation of the uncertainty of a result derived from a number of other independent results (i.e. a value assignment process). The method proposed in this paper for the uncertainty calculation of the assigned value attempts to take into account both the spread of the contributing results and their individual uncertainties. Moreover this can be a start for a constructive interaction towards a method, which can also be used in other cases such as the establishment of reference values (and their uncertainties) for certified reference materials. Acknowledgements Special thanks are owed to the scientists who contributed experimentally to the establishment of the IMEP-9 reference ranges: Dr. M. Berglund, Dr. C. Quétel, Dr. M. Ostermann, Dr. I. Papadakis, Dr. P. Robouch, Mrs L. Van Nevel and Dr. J. Vogl from IRMM, Dr. A. Quejido Cabezas from CIEMAT, Dr. C. J. Park and Dr. H.Y. So from KRISS, Dr. G. Marx and Prof. K. Heumann from the University of Mainz, Dr. J. Yoshinara from NIES, Dr. J.W.H. Lam and Ms Lu Yang from NRC and Dr. Z. Motian from NRCCRM. We are also very grateful to Prof. P. De Bièvre for his constructive advice, to Dr. J. Moody (former NIST staff member) who helped in providing the IMEP-9 samples and supervised the sample bottling, E. Poulsen and G. Verborgt from IRMM for secretarial support and Mr F. Hendrickx and Mrs B. Dyckmans of the Mass Metrology Group of IRMM for their contribution to the weighing of the IDMS and the density measurements.
References 1. Papadakis I et al. (1999) IMEP-9 participants report, EUR 18724 EN. IRMM, Geel 2. Quinn TJ (1997) Metrologia 34 : 61–65
3. ISO (1993)Guide to the expression of uncertainty in measurement. International Organisation for Standardisation (ISO), Geneva 4. Papadakis I, De Bièvre P (1997) Accred Qual Assur 2 : 347–348
5. Pauwels J et al. (1998) Accred Qual Assur 3 : 180–184 6. Van Nevel L et al. (1998) Accred Qual Assur 3 : 56–68
