The IRMM – International Measurement Evaluation Programme,IMEP

 The aim of the International Measurement Evaluation Programme (IMEP) is to give an objective picture of the state-of-the-practice (SoP) of chemical measurements in field laboratories by comparing them to a reference range that contains a value that is as traceable to the SI system of measurements (in this case to the Avogadro Constant, one of the best realizations of the mole so far), as can presently be achieved, but which in any case is independent of human or political decision. Thus, a large-scale field test has been made to realize (international) comparability of these measurements by providing them with an independent scientific common basis. In the third measurement round, IMEP-3, ten trace elements, B, Ca, Cd, Cu, Fe, K, Li, Pb, Rb and Zn were measured in a synthetic and in a natural water by about 155 participating laboratories using their routine methods. The (coded) results are graphically reported and compared to certified reference values, established by IRMM and NIST, using isotope specific methods (isotope dilution mass spectrometry and neutron activation analysis), wherever possible. One of these methods (IDMS) has recently been defined as a primary method of measurement by the Consultative Committee on Amount of Substance (Comité Consultatif pour la Quantité de Matière: CCQM) in its founding meeting in April 1995 at BIPM, Paris. Results indicate a spread of more than ±50%, asymmetrically distributed around the reference range, although the declared accuracy was 5–10%. Self-assessment by participants of their analytical capabilities does not show a high correlation between a self-rating ("more experienced" or "less experienced") and actual performance. In the way they have been applied, all methods seem to produce results of approximately the same quality. There is little reduction in the spread of the measurements if the results obtained for one element in the natural water B are divided by the results obtained for the same element in the synthetic water A (which was unknowingly, a reference material). Index entries International measurement evaluation programme (IMEP).     

Contribution to the certification of B,Cd, Cu,Mg and Pb in a synthetic water sample,by use of isotope-dilution ICP-MS,for Comparison 12 of the International Measurement Evaluation Programme

The contribution of the Institute for Reference Materials and Measurements to the certification of the B, Cd, Cu, Mg, and Pb content of a synthetic water sample used in Comparison 12 of the International Measurement Evaluation Programme (IMEP-12) is described. The aim of the IMEP programme is to demonstrate objectively the degree of equivalence and quality of chemical measurements of individual laboratories on the international scene by comparing them with reference ranges traceable to the SI (Système International d'Unités). IMEP is organized in support of European Union policies and helps to improve the traceability of values produced by field chemical measurement laboratories. The analytical procedure used to establish the reference values for the B, Cd, Cu, Mg, and Pb content of the IMEP-12 sample is based on inductively coupled plasma-isotope-dilution mass spectrometry (ICP-IDMS) applied as a primary method of measurement. The measurements performed for the IMEP-12 study are described in detail. Focus is on the element boron, which is particularly difficult to analyze by ICP-MS because of potential problems of low sensitivity, high mass discrimination, memory effects, and abundance sensitivity. For each of the certified amount contents presented here a total uncertainty budget was calculated using the method of propagation of uncertainties according to ISO (International Organization for Standardization) and Eurachem guidelines. For all investigated elements with concentrations in the low micro g kg(-1) and mg kg(-1) range (corresponding to pmol kg(-1) to the high micro mol kg(-1) level), SI-traceable reference values with relative expanded uncertainties ( k=2) of less than 2 % were obtained.     

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. Received: 7 June 2001-10-27 Accepted: 19 August 2001     

Nuclear forensics—metrological basis for legal defensibility

The admissibility of nuclear forensics measurements and opinions derived from them in US Federal and State courts are based on criteria established by the US Supreme Court in the case of Daubert v. Merrell Dow and the 2000 Amendment of Rule 702 of the Federal Rules of Evidence. These criteria are being addressed by new efforts that include the development of certified reference materials (CRMs) to provide the basis for analytical method development, optimization, calibration, validation, quality control, testing, readiness, and declaration of measurement uncertainties. Quality data is crucial for all stages of the program, from R&D, and database development, to actual casework. Weakness at any point in the program can propagate to reduce the confidence of final conclusions. The new certified reference materials will provide the necessary means to demonstrate a high level of metrological rigor for nuclear forensics evidence and will form a foundation for legally defensible nuclear chemical analysis. The CRMs will allow scientists to devise validated analytical methods, which can be corroborated by independent analytical laboratories. CRMs are required for ISO accreditation of many different analytical techniques which may be employed in the analysis of interdicted nuclear materials.     

Ion chromatographic precision measurement procedure for electrolytes in human serum: validation with the aid of primary measurement procedures

In order to make analytical measurement results traceable to the SI units in the field of clinical chemistry, an ion chromatographic (IC) measurement procedure has been developed which allows the amount of substance of the four so-called electrolytes Na, K, Mg and Ca as well as that of Li to be determined efficiently in human serum and with high accuracy. The IC measurement procedure was validated using primary measurement procedures confirmed by international comparison measurements and is proposed for use as a transfer standard when comparing measurements with clinical reference laboratories. The solutions used for calibration were gravimetrically prepared from pure substances (salts). Their chemical compositions had been iteratively fitted to those of the samples. The serum samples were mineralized by microwave-assisted digestion. The following relative expanded uncertainties for the average elemental contents were obtained: Li 0.4%, Na 0.14%, K 0.6%, Mg 0.8% and Ca 0.4%.

     

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, k _o-neutron activation analysis (k _o-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.

     

A new certified reference material for benzene measurement in air on a sorbent tube: development and proficiency testing

A certified matrix reference material (CRM) for the measurement of benzene in ambient air has been developed at Laboratoire National de Métrologie et d’Essais. The production of these CRMs was conducted using a gravimetric method fully traceable to the International System of Units. The CRMs were prepared by sampling an accurate mass of a gaseous primary reference material of benzene, using a high-precision laminar flowmeter and a mass flow controller, with a PerkinElmer sampler filled with Carbopack™ X sorbent. The relative standard deviations obtained for the preparation of a batch of 20 tubes loaded with 500 ng of benzene were below 0.2%. Each CRM is considered independent from the others and with its own certified value and an expanded uncertainty estimated to be within 0.5%, lower than the uncertainties of benzene CRMs already available worldwide. The stability of these materials was also established up to 12 months. These CRMs were implemented during proficiency testing, to evaluate the analytical performances of seven French laboratories involved in benzene air monitoring.     

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, k _o-neutron activation analysis (k _o-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.     

Plutonium metal standards exchange program for actinide measurement quality assurance (2001–2007)

Plutonium metal exchange programs operated by the Rocky Flats Plant were conducted from 1956–1989 to ensure quality and to compare measurements in a plutonium metal matrix. Los Alamos National Laboratory (LANL) re-established the program in 2001 to assess the quality of analytical chemistry capabilities that support special nuclear material characterization. It is the only program of its kind for the preparation and distribution of plutonium metal reference materials with a range of impurity contents to multiple laboratories for destructive measurements of elemental concentration, isotopic abundance, and both metallic and non-metallic impurity levels. This program provides independent verification of analytical measurement capabilities for each of the seven currently participating laboratories, and allows any technical problems with analytical measurements to be identified and corrected. This paper focuses on basic program elements and presents a summary of methods and results for plutonium, uranium, neptunium, and americium, measurements.     

Ion chromatographic precision measurement procedure for electrolytes in human serum: validation with the aid of primary measurement procedures

In order to make analytical measurement results traceable to the SI units in the field of clinical chemistry, an ion chromatographic (IC) measurement procedure has been developed which allows the amount of substance of the four so-called electrolytes Na, K, Mg and Ca as well as that of Li to be determined efficiently in human serum and with high accuracy. The IC measurement procedure was validated using primary measurement procedures confirmed by international comparison measurements and is proposed for use as a transfer standard when comparing measurements with clinical reference laboratories. The solutions used for calibration were gravimetrically prepared from pure substances (salts). Their chemical compositions had been iteratively fitted to those of the samples. The serum samples were mineralized by microwave-assisted digestion. The following relative expanded uncertainties for the average elemental contents were obtained: Li 0.4%, Na 0.14%, K 0.6%, Mg 0.8% and Ca 0.4%.     

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.     

A natural matrix (pureed tomato) candidate reference material containing residue concentrations of pesticide chemicals

NARL (the Australian National Analytical Reference Laboratory) is preparing a pureed tomato reference material spiked with residue concentrations of a range of pesticide chemicals relevant to the Australian horticultural industry. Traceable certified pesticide concentration values will be established using both isotope dilution mass spectrometry primary methods (developed within NARL) and measurements carried out by a number of experienced laboratories. As far as we are aware, there is no other similar CRM available anywhere in the world. The need for such a material is evident from the results of interlaboratory proficiency studies conducted by NARL among Australian and Asia-Pacific residue testing laboratories. Many participants are experiencing difficulties with the analysis of pesticide residues in fruit and vegetables at concentrations of regulatory significance. Chemicals such as methamidophos (an organophosphorus pesticide widely used on tomato crops) are causing particular problems. In a pilot study, a number of units of control (unspiked tomato) and the candidate reference material were prepared and packaged into lacquered steel cans which were sealed and sterilised by immersion in boiling water. Accelerated stability testing of the packaged material was conducted using isochronous measurement. All of the pesticides showed some degree of degradation after 4 weeks of storage at 50 degrees C, and after 168 days of storage at room temperature. However, all appeared to be stable after 168 days of freezer storage. Homogeneity testing involved duplicate test portions taken from every 50th unit of reference material. An experimental protocol was devised with the aim of minimising the analytical variability and assuring the quality of the data generated. There is some degree of inhomogeneity in the prepared material and a small fill trend is also indicated. Potential improvements to the spiking and preparation procedure have been identified and it is planned to prepare a second batch of both control and spiked material for further homogeneity trials and for certification of the pesticide concentrations.     

Value assignment of retinol, retinyl palmitate, tocopherol, and carotenoid concentrations in Standard Reference Material 2383 (Baby Food Composite)

In 1997, the National Institute of Standards and Technology (NIST) released Standard Reference Material (SRM) 2383 Baby Food Composite. This SRM can be used as a control material when assigning values to in-house control materials and when validating analytical methods for the measurement of proximates, vitamins, minerals, and trace elements in baby foods and similar matrixes. The Certificate of Analysis for SRM 2383 provides certified and reference values for concentrations of lutein, zeaxanthin, beta-cryptoxanthin, lycopene, alpha-carotene, beta-carotene, delta-tocopherol, gamma-tocopherol, alpha-tocopherol, retinol, and retinyl palmitate for 2 types of sample preparation--extraction and saponification. The assigned values were based on the agreement of measurements made by NIST and collaborating laboratories. The Certificate of Analysis also provides reference and information values for concentrations of proximates, minerals, and additional vitamins; assignment of these values is discussed in a companion paper (this issue, page 276).     

Monitoring the precision of routine analyses by using duplicate determinations

To ensure the reliability of results, analytical laboratories require a continuous qualitycontrol program which must take account of both systematic and random errors. Analyses of reference materials can be used to estimate systematic errors but estimates of random errors (precision) tend to be optimistic, mainly because reference materials cannot be put through the whole analytical process (e.g., primary sampling is often a major source of error). Estimates of precision must be based on routine samples. If duplicate determinations are done on routine samples, the precision can be estimated reliably. Within the optimum concentration range of analytical method (usually starting from 5-10 times the detection limit), the relative standard deviation (s_r can be regarded as being almost constant or independent of concentration. The precision can then be estimated by first calculating the s_r value of each pair of results. Individually, these are not reliable estimates of the true s_r, but they can be regarded as independent measurements of the same s_r and so can be pooled to obtain a more reliable estimate of precision with the number of duplicates as the degrees of freedom. The applicabiilty of the method is tested on soil, rock and ore samples.     

UK delivery of traceable chemical measurements in the 21st century: building on the foundation of the VAM programme

The paper discusses the requirements for achieving traceable chemical measurements in the UK. It is emphasised that success will depend on establishing an appropriate UK chemical measurement infrastructure and encouraging reference and field laboratories to make use of it. The demanding requirements of the BIPM Mutual Recognition Arrangement (MRA) also require a point of focus to link UK reference laboratories into international metrology. Two key factors are described which have provided the UK with the means to meet these requirements and which have established a sound basis on which to build a system of traceable chemical measurements in the 21st century. These two factors are LGC's long-standing role as the UK's national centre for analytical chemistry and the development and delivery over many years of the UK's Valid Analytical Measurement (VAM) Programme.     

The practical realization of the traceability of chemical measurements standards

Metrology is based on the concept of traceability. Traceability provides a means of relating measurement results to common standards thereby helping to ensure that measurements made in different laboratories are comparable. Good progress has been made in the application of metrological principles to chemical measurement, but there remains confusion about how you actually achieve traceability in a practical way. This paper elaborates on the meaning and application of much used phrases such as 'the value of a standard', 'stated references', 'unbroken chain of comparisons', and 'stated uncertainties'. It also explains how traceability can be established in a practical way for different types of stated references, namely pure substance reference materials, matrix reference materials, and primary and reference methods. Finally, traceability chains for some typical examples of chemical measurement are described.     

A natural matrix (pureed tomato) candidate reference material containing residue concentrations of pesticide chemicals

NARL (the Australian National Analytical Reference Laboratory) is preparing a pureed tomato reference material spiked with residue concentrations of a range of pesticide chemicals relevant to the Australian horticultural industry. Traceable certified pesticide concentration values will be established using both isotope dilution mass spectrometry primary methods (developed within NARL) and measurements carried out by a number of experienced laboratories. As far as we are aware, there is no other similar CRM available anywhere in the world. The need for such a material is evident from the results of interlaboratory proficiency studies conducted by NARL among Australian and Asia-Pacific residue testing laboratories. Many participants are experiencing difficulties with the analysis of pesticide residues in fruit and vegetables at concentrations of regulatory significance. Chemicals such as methamidophos (an organophosphorus pesticide widely used on tomato crops) are causing particular problems. In a pilot study, a number of units of control (unspiked tomato) and the candidate reference material were prepared and packaged into lacquered steel cans which were sealed and sterilised by immersion in boiling water. Accelerated stability testing of the packaged material was conducted using isochronous measurement. All of the pesticides showed some degree of degradation after 4 weeks of storage at 50?°C, and after 168 days of storage at room temperature. However, all appeared to be stable after 168 days of freezer storage. Homogeneity testing involved duplicate test portions taken from every 50th unit of reference material. An experimental protocol was devised with the aim of minimising the analytical variability and assuring the quality of the data generated. There is some degree of inhomogeneity in the prepared material and a small fill trend is also indicated. Potential improvements to the spiking and preparation procedure have been identified and it is planned to prepare a second batch of both control and spiked material for further homogeneity trials and for certification of the pesticide concentrations.     

The history and development of a rigorous metrological basis for pH measurements

This paper discusses the basis and historical development of the traceability chain for pH. The quantity pH, first introduced in 1909, is among the most frequently measured analytical quantities. The practical measurement of the pH value of a sample is inexpensive, easy to perform, and yields a rapid result. However, the problems posed by the traceability of pH are not easy to solve. Most pH measurements are performed by potentiometry, using a glass electrode as the pH sensor. Such pH electrodes must be calibrated at regular intervals. Confidence in the reliability of pH measurements requires establishment of a metrological hierarchy including an uncertainty budget for calibration that links the pH measured in the sample to an internationally agreed and stated reference. For pH, this reference is the primary measurement of pH. A traceability chain can be established that links field measurements of pH to primary buffer solutions that are certified using this primary method. This allows the user in the field to estimate the measurement uncertainty of the measured pH data. As the realization of the primary measurement is sophisticated and time-consuming, primary standards are generally realized at national metrology institutes. A number of potentiometric methods are suitable for the determination of the pH of reference buffer solutions by comparison with the primary standard buffers. The choice between the methods should be made according to the uncertainty required for the application. For reference buffer solutions that have the same nominal composition as the primary standard, the differential potentiometric cell, often called the Baucke cell, is recommended.     

Reliability of measurement results in food microbiology: the contribution of reference laboratories in the European Union and of international/European standardization

The contribution of reference laboratories in the European Union and of European/international standardization to the reliability of food microbiology measurement results is discussed. A set of European Union reference laboratories has been established. Each of them coordinates a network of national reference laboratories which, in turn, coordinate networks of laboratories in charge of official testing and sometimes own checks in each European Union country. Their contribution to the reliability of food microbiology measurement results is illustrated by three food safety cases: Listeria monocytogenes, coagulase positive staphylococci and milk/milk products. The contribution of European/international standardization focuses on two topics: method validation and measurement uncertainty. The standards covering these topics—EN ISO 16140 and ISO/TS 19036—are briefly discussed, and an update given on their ongoing revision.