International Measurement Evaluation Programme Research Articles

State of the art in the determination of trace elements in seawater: a worldwide proficiency test.

This manuscript presents the results of the International Measurement Evaluation Programme 40 (IMEP-40) study, a proficiency test (PT) which was organised to assess the worldwide performance of laboratories for the determination of trace elements in seawater. This PT supports the implementation of the European Union Water Framework Directive 2000/60/EC, which aims at achieving a long-term high level protection of the aquatic environment, covering lakes, ground water and coastal waters. Forty-six participants reported results. The test item was seawater containing the trace elements As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Se and Zn. The trace elements in the test item were present in very low concentrations to mimic natural levels. The results of the participants were rated with z and zeta (ζ) scores in accordance with ISO 13528 and ISO 17043. The standard deviation for proficiency assessment, widehat{upsigma} , was set at 25 % of the respective assigned values for the 12 measured elements based on previous experience with similar PTs. The low levels of the trace elements combined with the high salt concentration of the seawater made the measurements challenging. Many laboratories were unable to detect or quantify the elements and reported “lower than X” values. The percentage of satisfactory performances (expressed as z scores) ranged from 41 % (Cr, Fe) to 86 % (Mo). The PT study showed that the use of proper standard methods, like ISO 17294-2, and sensitive techniques, like inductively coupled plasma mass spectrometry (ICP-MS), contributed to performing well in this PT round.

Quality means decency-on the quality of teaching through accreditation of the course?

The term "accreditation" applies to the attempts in various fields of scientific work and administration. It refers to an acknowledgement of some particular positive (wanted) property being present in or brought about by either a procedure or technical system, an acknowledgement which is then formally issued by some authority generally accepted to be able to prove and file such statements. Bioindication & Biomonitoring (B&B) technologies are highly influenced by accreditation procedures, because both B&B technologies are often related to accreditation during the overall instrumental measurement processes applied. As an example, tuna fish used for (bio) monitoring salt water systems on pollution by Pb will be presented. Analytical measurement results from the International Measurement Evaluation Programme (IMEP 20) and additionally the construction of the university course "Integrative sustainable management: quality, environmental, health, safety and risk management (QUAR)" are reviewed in this respect. It was a serious finding by the multi-participant IMEP study that neither quality control measures nor accreditation actually ensure improved quality of measurement results, i.e. data produced by analytical laboratories. Accordingly, even though the accreditation of study curricula is often done with utmost diligence and precision, and while it is fairly important for making study curricula throughout Europe (and the World) fitting and comparable to each other to get study results produced and tested at university X also be accepted when changing to university Y, we cannot assume or take for granted that the mere protocol of accreditation will do anything positive to improve the chance to get better or "more correct" results. A single member of lab staff or professor changing to some other position might seriously compromise the turnout of some analytical lab or university course at least in the short term regardless of whether accreditation was done or not. As consequence, data obtained by B&B technologies have to be discussed and handled with the highest care to reach acceptable and scientifically sound goals of quality.

Proficiency test on the determination of pesticide residues in grapes with multi-residue methods

This manuscript presents the results of the International Measurement Evaluation Programme 37 (IMEP-37) study, a proficiency test (PT) which was organised to assess the world-wide performance of food control laboratories on the determination of pesticide residues in grapes. This PT supports the implementation of Regulation (EC) No 396/2005 on maximum residue levels of pesticides in or on food and feed of plant and animal origin. Eighty-one participants reported results, forty from EU Member States and forty-one from outside the EU. The test item was a grape sample spiked with 20 selected pesticides. The results of the participants were rated with z- and zeta (ζ-) scores in accordance with ISO 13528 and ISO 17043. The standard deviation for the proficiency assessment, σˆ, of this PT was set at 25% for the 20 measured pesticides based on previous experience with similar measurands. The results reported to IMEP-37 showed that the participants performed satisfactorily, ranging from 81% (carbendazim) to 97% (azoxystrobin, penconazole, pyrimethanil) of the participating laboratories. However, only 30% of the participants managed to analyze all pesticides satisfactorily. Overall, the performance of the participants in this PT was good but there is room for improvement in the development of multi-residue methods for the simultaneous analysis of a large number of pesticides with an increased accuracy.

Determination of total cadmium, lead, arsenic, mercury and inorganic arsenic in mushrooms: outcome of IMEP-116 and IMEP-39

The Institute for Reference Materials and Measurements (IRMM) of the Joint Research Centre (JRC), a Directorate General of the European Commission, operates the International Measurement Evaluation Program (IMEP). IMEP organises inter-laboratory comparisons in support of European Union policies. This paper presents the results of two proficiency tests (PTs): IMEP-116 and IMEP-39, organised for the determination of total Cd, Pb, As, Hg and inorganic As (iAs) in mushrooms. Participation in IMEP-116 was restricted to National Reference Laboratories (NRLs) officially appointed by national authorities in European Union member states. IMEP-39 was open to all other laboratories wishing to participate. Thirty-seven participants from 25 countries reported results in IMEP-116, and 62 laboratories from 36 countries reported for the IMEP-39 study. Both PTs were organised in support to Regulation (EC) No. 1881/2006, which sets the maximum levels for certain contaminants in food. The test item used in both PTs was a blend of mushrooms of the variety shiitake (Lentinula edodes). Five laboratories, with demonstrated measurement capability in the field, provided results to establish the assigned values (Xref). The standard uncertainties associated to the assigned values (uref) were calculated by combining the uncertainty of the characterisation (uchar) with a contribution for homogeneity (ubb) and for stability (ust), whilst uchar was calculated following ISO 13528. Laboratory results were rated with z- and zeta (ζ)-scores in accordance with ISO 13528. The standard deviation for proficiency assessment, σp, ranged from 10% to 20% depending on the analyte. The percentage of satisfactory z-scores ranged from 81% (iAs) to 97% (total Cd) in IMEP-116 and from 64% (iAs) to 84% (total Hg) in IMEP-39.

Setting maximum limits for trace elements in baby food in European legislation: the outcome of International Measurement Evaluation Programme®–33

The Institute for Reference Materials and Measurements (IRMM) of the Joint Research Centre (JRC), a Directorate-General of the European Commission, operates the International Measurement Evaluation Programme® (IMEP). It organises various types of inter-laboratory comparisons in support of European Union policies. This paper presents the results of a proficiency testing exercise (PT) focusing on the determination of total cadmium (Cd) and total lead (Pb) mass fractions in baby food in support to Commission Regulation (EC) 1881/2006 of 19 December 2006 setting maximum levels for certain contaminants in foodstuffs. The test material used in this exercise was soya-based baby food formula purchased in a local pharmacy and prepared by the Reference Materials Unit of the IRMM for this exercise. Sixty-six laboratories from 23 countries registered to the exercise and 61 of them reported results. Each participant received one bottle containing approximately 15 g of test material. Participants were asked to quantify the measurands in the powder and in the reconstituted formula. Reference values independent from the participants’ results were established using isotope dilution inductively coupled plasma mass spectrometry. The total Cd mass fraction was determined by IRMM and LGC Ltd (UK), while the total Pb was determined by IRMM. The standard deviation for proficiency assessment was set at 22% of the assigned value for all measurands. Laboratories were rated with z- and ζ- (zeta) scores in accordance with ISO 13528. The outcome of this exercise is clearly influenced by the very low level of Cd and Pb content in the test material which triggered: a high number of ‘less than’ values; overestimated values especially for Pb very likely due to contamination; and a visible method influence in the case of Pb (methods based on atomic absorption were not sensitive enough to attain such low limits of detection). The results were also evaluated with regard to the reported limit of detection and some incoherencies were observed.

Determination of total arsenic, total inorganic arsenic and inorganic arsenic species in rice and rice flour by electrothermal atomic absorption spectrometry

The determination of inorganic arsenic species in food has become very important during the last few years, since As(III) and As(V) are considered carcinogenic and found at high concentrations in food products, such as rice. The present work describes the development of three different methods for the determination of total arsenic, total inorganic arsenic, and As(III)–As(V) in rice and rice flour food products, purchased from the local market by electrothermal atomic absorption spectrometry (ETAAS). The methods were based on the use of different selective extraction procedures and the optimization of all crucial instrumental and methodological parameters. By these new preparation procedures for the determination of arsenic species, the loss and transformation of analytes during the extraction and digestion steps was prevented. For the validation of each method, precision, accuracy, and selectivity have been assessed, as performance criteria. All developed methods were accurate and precise. The calculated recoveries ranged between 92% and 105% and the (%) relative standard deviation values, under repeatability or reproducibility conditions, were lower than 15% for all different concentration levels tested. The validated methods were applied successfully for the determination of total arsenic and total inorganic arsenic in a proficiency test organized by the International Measurement Evaluation Program (IMEP-107). The methods were also applied in rice and rice flour samples purchased from the Greek market and the obtained results indicated that in almost all samples total arsenic and total inorganic arsenic were detected at ng/g levels.

Proficiency test for the determination of heavy metals in mineral feed. The importance of correctly selecting the certified reference materials during method validation

In 2008, the International Measurement Evaluation Programme, IMEP®, organised two proficiency tests (PTs) for the determination of heavy metals in mineral feed, named IMEP-105 and IMEP-27, respectively. IMEP-105 was organised in support of the activities of the European Union Reference Laboratory for Heavy Metals in Feed and Food, and participation was restricted to the officially nominated National Reference Laboratories (NRLs). IMEP-27 was open to all interested control laboratories in the field. The test material used in the two PTs was the same and the timeframe of the two exercises overlapped. The measurands in both exercises were total Cd, Pb and As and extractable Cd and Pb, as defined in Directive 2002/32/EC of the European Parliament and the Council on undesirable substances in animal feed. Forty-nine laboratories from 25 countries reported results to the two exercises, 29 to IMEP-105 and 20 to IMEP-27. In both PTs, external reference values were used instead of consensus values derived from the participants' results. It was shown that the concentration of total and extractable Cd according to Directive 2002/32/EC were identical, while the concentration of extractable Pb was ∼80% of total Pb. The main outcome of these comparisons was that an underestimation of the concentrations of the addressed measurands, in particular total Pb, took place due to an erroneous estimation of the bias of the analytical methods used by most of the participants. It appeared that the nature of the certified reference materials chosen for method validation and recovery estimation was the cause of the problem (insufficient matching of the matrix characteristics between these materials and the test sample). No significant difference between the results reported to IMEP-105 and to IMEP-27 was observed.

Performance of laboratories in speciation analysis in seafood – Case of methylmercury and inorganic arsenic

The international measurement evaluation program (IMEP) has together with the European Reference Laboratory for Heavy Metals in Feed and Food (EU-RL-HM) carried out two interlaboratory comparisons (ILC) in 2010 on the measurement of trace metals, as well as methylmercury and inorganic arsenic in seafood. In IMEP-109 only EU National Reference Laboratories (NRL) took part, while IMEP-30 was open to all laboratories. In this article only methylmercury and inorganic arsenic analysis will be discussed, as these appear generally to be more problematic measurands. They are also particularly interesting to legislators, as no maximum limits have been set yet for them in European legislation. The aim of the two ILCs was to produce more information to help tackling this issue. The results of the two exercises were pooled together, evaluated, and compared with former ILC projects for methylmercury and inorganic arsenic analysis. Results for inorganic arsenic were spread, but not method dependant. The measurand seems to be difficult to analyse in this matrix and possible method issues were identified. Methylmercury results were satisfactory, but not many laboratories perform this type of analysis because it is generally believed that specific instrumentation is needed. As an answer to this presumption, alternatives are suggested.

IMEP Interlaboratory Comparisons – A Tool to Establish Confidence in Measurements

The International Measurement Evaluation Programme (IMEP) is owned by the European Commission's Joint Research Centre. IMEP organises interlaboratory comparison (ILC) in support of the policies of the European Union, covering a wide range of measurement tasks from food safety to environmental pollution. IMEP can be considered as a whole metrological process underlining the need for reliability and comparability of measurements. The article describes shortly the history and purpose of IMEP, and furthermore sums up its activities, links and impacts on the European measurement community.

IMEP-12: trace elements in water; objective evaluation of the performance of the laboratories when measuring quality parameters prescribed in the European Directive 98/83/EC

The International Measurement Evaluation Programme (IMEP) is an interlaboratory comparison scheme, founded, owned and co-ordinated by the Institute for Reference Materials and Measurements (IRMM) since 1988. IMEP-12, for the fourth time in the series, focused on trace elements in water and it was designed specifically to support European Commission directive 98/83/EC. Reference values for the concentration of ten elements were established with expanded uncertainties according to GUM. In total, 348 laboratories from 46 countries in five continents participated in the comparison and the degree of equivalence between the results of the laboratories and the reference values is presented graphically. Samples from the same batch were distributed to ten laboratories from European Countries, which represented their country in the framework of the EUROMET project 528. Participation in this comparison was offered to the European Co-operation for Accreditation (EA) for participation of accredited laboratories from all over Europe in the framework of the collaboration between IRMM and EA and to laboratories from the EU new member states and acceding countries in the frame of IRMM’s ‘Metrology in Chemistry support program for EU new member states and acceding countries.’

IMEP in support of the comparability of measurement results across the Romanian chemical metrology infrastructure

On the basis of quantitative chemical measurements many important decisions are made in support of legislation or in industrial processes or social aspects. For this reason it is important to improve the quality of chemical measurement results and thus make them comparable and acceptable everywhere. The measurement quality is important to enable an equivalent implementation of the European Union regulations and directives across an enlarged EU. In this context, the European Commission–Joint Research Centre–Institute for Reference Materials and Measurement (EC-JRC-IRMM) set up a programme to improve the scientific basis for metrology in chemistry (MiC) in EU candidate countries in the framework of EU enlargement. Several activities were initiated, such as training, fellowships, sponsoring of seminars, conferences and participation in interlaboratory comparisons. To disseminate measurement traceability, IRMM provides through its International Measurement Evaluation Programme (IMEP) an interlaboratory tool to enable the benchmarking of laboratory performance. IMEP emphasizes the metrological aspects of measurement results, such as traceability and measurement uncertainty. In this way it has become a publicly available European tool for MiC. The Romanian Bureau of Legal Metrology – National Institute of Metrology (BRML-INM) actively supports the participation of Romanian authorized and field laboratories in IMEP interlaboratory comparisons. This paper describes the interest of Romanian laboratories participating in this programme, the analytical and metrological problems that became relevant during these exercises and some actions for improvement. The results from Romanian laboratories participating in IMEP-12 (water), IMEP-16 (wine), IMEP-17 (human serum) and IMEP-20 (tuna fish) are presented. To conclude, the educational and training activities at national level organized jointly by the Romanian National Institute of Metrology (INM) and IRMM are also mentioned.

SI-traceable certification of methylmercury amount content in a tuna material

Speciation analysis has an ever-increasing relevance in environmental studies, for risk assessment and, eventually, for legislation. There is a concomitant need for reliability and comparability of speciation measurements made at different places and at different times, to assist with which the concepts of traceability (to a stated system of reference) and combined uncertainty budgetting can be applied. Traceability can be visualised as a chain, or a combination of chains, between a measurement result and a reference where each link in the chain(s) is a mathematical expression representing our best understanding of a measurement process, within a declared level of uncertainty. The combined uncertainty of the measurement result is calculated by combination of the uncertainties associated with each parameter of the traceability chain. This also helps method optimisation and in the diagnosis of possible errors in results by highlighting components that contribute significant uncertainty, provided that an appropriate uncertainty was attached to each parameter affecting the result. This work shows how a methylmercury, MeHg, measurement was made, and how understanding of the measurement process could make the result reliable and comparable to results between laboratories. The measurement process was validated according ISO 17025 through the use of reference materials for calibration, comparison of results with different methods, a systematic assessment of factors influencing the result, by calculating the combined measurement uncertainty and by participation in an inter-laboratory comparison. A tuna material was distributed for the laboratory inter-comparison studies for the Comite Consultatif pour la Quantite de Matiere (CCQM) pilot study 39 and International Measurement Evaluation Programme (IMEP) comparison 20. MeHg was measured using species-specific isotope dilution, SSID, by blending samples with an isotopic certified reference material, ERM®-AE670, which contains CH3HgCl isotopically enriched in 202Hg, followed by extraction and derivatisation of Hg species and detection by gas chromatography-inductively coupled plasma mass spectrometry. SSID was applied to provide mass fractions traceable to the SI through the comparison of isotope ratios measured in blends prepared by metrological weighing, with reference materials carrying values traceable to the SI. Uncertainties associated with each of the experimental steps were investigated and combined according to ISO guidelines. The mass fraction of Hg in the form of MeHg in the tuna sample carried an expanded combined uncertainty estimated at 5.6% (k = 2). The greatest uncertainty contributions stemmed from the degree to which sample + spike equilibration could be proved, the certified MeHg content of the spike material and (for the IMEP-20 value) sample homogeneity. Uncertainty contributions from the known natural Hg isotopic composition, isotope ratio measurement and instrumental correction factors were minor in comparison. The level of equivalence was demonstrated by CCQM-P39 in that the standard deviation of mass fractions reported by 9 participating laboratories that applied SSID was 2.6%, and the IRMM value lay 0.24% from the mean.

International Measurement Evaluation Programme (IMEP); IMEP-14: Picturing the performance of analytical laboratories measuring trace elements in sediment

The International Measurement Evaluation Programme (IMEP) is an interlaboratory comparison scheme founded, owned and co-ordinated by the Institute for Reference Materials and Measurements (IRMM) since 1988. IMEP-14, for the first time in the series, is focusing on trace elements in sediment. Reference values for ten elements stating total concentrations and expanded uncertainties according to GUM were established. In total, 239 laboratories from 43 countries in five continents participated in the comparison, and the degree of equivalence between the results from the laboratories and the reference values is presented graphically. Identical samples were distributed to nine National Measurement Institutes within the frame of the CCQM-P15 pilot study. Participation in the IMEP-14 interlaboratory comparison was offered to laboratories in the EU new member states and acceding countries according to the IRMM’s “Metrology in Chemistry support programme for EU new member states and acceding countries”.

Certification measurement of the cadmium, copper and lead contents in rice using isotope dilution inductively coupled plasma mass spectrometry

This paper describes the certification of the Cd, Cu and Pb amount contents in a rice material, that were used as reference values for round 19 of the international measurement evaluation programme (IMEP).The protocol developed in this study was based on isotope dilution associated to inductively coupled plasma mass spectrometry (ID-ICP–MS). A multiple spiking approach was applied to reduce significantly the number of analytical steps. For the decomposition of the sample, three different microwave assisted digestion procedures were tested and compared. The use of hydrofluoric acid was found necessary to ensure full solubilisation and complete isotopic equilibration. Estimation of the combined uncertainty attached to the measurement results was performed according to the ISO guidelines. Contributions from the correction for moisture content, sample homogeneity, procedural blank, spike impurities, spectral interferences, instrumental background and dead-time effects were evaluated.SI-traceable values with less than 2% combined uncertainty (k = 2) were obtained for Cd, Cu and Pb, respectively, (14.39 ± 0.21) × 10−6molkg−1, (44.31 ± 0.42) × 10−6molkg−1 and (2.034 ± 0.034) × 10−6molkg−1.The rice powder was observed to be highly hygroscopic and a second series of isotope dilution mass spectrometry measurements was carried out on samples in equilibrium with the ambient moisture conditions (‘saturated’ samples). The Comité Consultatif pour la Quantité de Matière (CCQM) requested this approach for the participation to the key comparison 24 on the same rice test material. The excellent agreement for Cd between the IMEP-19 reference value, the value submitted by the institute for reference materials and measurements (IRMM) to CCQM-K24 and the corresponding reference value obtained as the arithmetic mean from the results of the 11 participating National Measurement Institutes (less than 0.27% difference) further validated this work. Eventually, this series of results on ‘saturated’ samples was submitted, for the 3 elements, as the IRMM contribution to the certification of the IRMM-804 rice candidate certified reference material (CRM).

SI-traceable certification of the IMEP-12 water sample combining contributions from different reference laboratories

In the International Measurement Evaluation Programme (IMEP-12) comparison, a synthetically prepared water sample was offered to the analytical laboratories to perform measurements of As, B, Cd, Cr, Cu, Fe, Mg, Mn, Ni and Pb. The choice of elements to be measured was based on EU legislation, which the comparison was aiming to support. As to the IMEP policy, the laboratories’ results were presented according to the certified/assigned reference values established by several reference laboratories all around the world. The performed certification campaign is described in detail in this paper. Isotope dilution mass spectrometry (IDMS) was applied as a primary method of measurement (PMM), whenever possible, to achieve SI-traceable results. Apart from IDMS for reference measurements of some elements, ko-neutron activation analysis (ko-NAA) and external calibration (Ext-Calib) using inductively coupled plasma-mass spectrometry (ICP-MS) were applied. The reference values were characterised as “certified” (for B, Cd, Cr, Cu, Fe, Mg, Ni and Pb) or “assigned” (for As and Mn) according to the IMEP policy. Measurement uncertainty of the certified/assigned reference values was calculated according to the ISO/BIPM guide using the specialised software GUM Workbench.

Development of a liquid chromatography–mass spectrometry method for the high-accuracy determination of creatinine in serum

An LC–MS method for the high-accuracy determination of creatinine in serum has been developed and used to provide results for an international measurement evaluation programme (IMEP) and the Comité Consultatif pour la Quantité de Matière (CCQM) international inter-laboratory studies. An assessment of different sample preparation methods, including ion-exchange chromatography, solid-phase extraction, plasma ultrafiltration and ethanol protein precipitation, revealed that no bias or reduced precision was associated with the quicker less extensive clean-up methods, when using liquid chromatography–isotope dilution mass spectrometry (LC–IDMS) for quantitation. A number of different calibration regimes were also investigated. External calibration was shown to provide adequate calibration for most routine analysis with a relative associated expanded uncertainty ( k=2) of 6% at the 95% confidence level. The use of a non-isotopically labelled internal standard was shown to improve the relative expanded uncertainty ( k=2) to 4%. However, the difference in retention time between the internal standard and the creatinine was such that a matrix interferent produced an observed bias of over 16%. The use of an isotopically labelled internal standard was shown to reduce any bias to less than 0.2% with an expanded uncertainty ( k=2) of less than 0.3%. The developed method was then used, in a blind trial organised jointly by IMEP and CCQM, to determine the amount of creatinine in human serum. The method performed well against the established reference method of ion-exchange chromatography followed by derivatisation gas chromatography (GC)–IDMS. The observed difference between the values determined by LC–IDMS and the key comparison reference value (average of all the submitted results) was less than 0.3%. The biggest advantage of the described method is in the speed of analysis. With a chromatographic run time of less than 10 min and sample preparation consisting of a simple protein precipitation, without the need for a derivatisation stage, the analysis is vastly simpler then the conventional GC–IDMS reference method.

A participatory improvement activity of the EC-JRC to improve metrology in chemistry in EU candidate countries

In 2000, the European Commission-Joint Research Centre, Institute for Reference Materials and Measurements (EC-JRC-IRMM) set up a project to improve the scientific basis for metrology in chemistry (MiC) in EU candidate countries. Several activities were initiated (training, fellowships, sponsoring seminars, conferences and participation in the International Measurement Evaluation Programme – IMEP). A particular initiative, discussed here, was to assist each of the national measurement institutes of these countries to write a status report on MiC. Most importantly, the report was intended to be the end-product of a process to document the current status of MiC. Intentionally, this process involved not only the providers of services in MiC but all the major stakeholders in this activity (e.g. major field laboratories, accreditation bodies, regulatory bodies, educators, professional bodies). The status reports are intended to be the first step of the future improvement process. In those countries where writing of the status reports has been started (Slovenia, Poland, Bulgaria and Estonia) the first signs are that this process leads to better co-operation and particularly a better understanding of what the future role and activities of each of these stakeholders should be.

PIXE determination of Pb in wine

The permissible level of the Pb content in wine varies in the range of 200–300 μg/l in different countries. These values are below the detection limit for standard PIXE spectrometry. Initiated by an intercomparison action of The International Measurement Evaluation Programme (IMEP, Geel, Belgium), an attempt was made to lower the detection limit of PIXE without introducing any additional chemical steps. Wine samples of 5 ml were gradually condensed below an infrared lamp and 8 μl aliquots were taken from time to time and pipetted onto filter paper. The samples were analysed by standard PIXE technique, with the analyte addition method used for calibration. A detection limit of about 50 ppb was finally achieved. Comparing this value to the permissible levels, this simple and fast technique can be used for screening purposes. As an application, the lead content of three Hungarian market wines was also measured.

Establishment of SI-traceable reference values for the content of various elements in the IMEP-14 sediment sample

For the first time in the International Measurement Evaluation Programme (IMEP)-14, a sediment sample was offered to analytical laboratories to perform measurements of As, Cd, Cr, Cu, Fe, Pb, Hg, Ni, U and Zn. In line with IMEP policy, the results were presented according to the certified / assigned reference values established by several reference laboratories around the world. The certification campaign is described in detail. Isotope dilution mass spectrometry was applied as a primary method of measurement, whenever possible, to achieve SI-traceable results. For reference measurements of As, Fe, Hg and Zn, k0-neutron activation analysis and Zeeman atomic absorption spectrometry were applied. The reference values (ranges) were characterised as ”certified” (for Cd, Cr, Pb, Ni and U) or ”assigned” (for As, Cu, Fe, Hg and Zn) according to IMEP policy. The measurement uncertainty of the certified / assured reference values was calculated according to the ISO/BIPM Guide.

International Measurement Evaluation Programme: IMEP-9, trace elements in water.

The International Measurement Evaluation Programme (IMEP) is an interlaboratory comparison scheme, founded, owned and coordinated by the Institute for Reference Materials and Measurements (IRMM) since 1988. IMEP-9 is the third round of trace elements in water evaluation following IMEP-3 and IMEP-6. Reference values for 15 elements stating total concentrations and combined uncertainties (according to GUM) were established. The reference values were established mainly by isotope dilution mass spectrometry (IDMS) as a primary method of measurement, and values traceable to the SI were obtained. The four elements that could not be certified by IDMS were assigned values by means of other measurement techniques. Results from 201 laboratories from 35 countries and four continents were evaluated against the reference values and the comparability between the laboratories is presented graphically.