Accreditation and Quality Assurance (v.18, #6)

Method validation by H. Emons (457-458).

House-of-security approach to measurement in analytical chemistry: quantification of human error using expert judgments by Ilya Kuselman; Elena Kardash; Emil Bashkansky; Francesca Pennecchi; Stephen L. R. Ellison; Karen Ginsbury; Malka Epstein; Aleš Fajgelj; Yury Karpov (459-467).
A new technique for quantification of human errors in chemical analysis using expert judgments is described. This technique is based on the house-of-security approach developed recently in the field of safety and security for prevention of terrorist and criminal attacks against an organization. The following relative quantification parameters (expressed in %) are proposed in the technique: (a) likelihood score of human error in a chemical analytical measurement/testing method, (b) severity score of human error for reliability of the test results, (c) importance score of a component of a laboratory quality system, and (d) effectiveness score of the quality system as a whole in preventing/blocking human error. As an example, 34 scenarios of human error in pH measurement of groundwater are discussed and quantified.
Keywords: Human error quantification; Analytical chemistry; Metrology; Quality assurance; Reliability; Atypical test results; House-of-security approach

Pitzer ion activities in mixed electrolytes for calibration of ion-selective electrodes used in clinical chemistry by Frank Bastkowski; Petra Spitzer; Ralf Eberhardt; Beatrice Adel; Samuel Wunderli; Daniel Berdat; Hanspeter Andres; Olivier Brunschwig; Michal Máriássy; Roger Fehér; Caspar Demuth; Fabiano Barbieri Gonzaga; Paulo Paschoal Borges; Wiler Batista da Silva Junior; Alena Vospělová; Martina Vičarová; Sirinapha Srithongtim (469-479).
Metrological comparability as well as reliability of ion activity results measured with ion-selective electrodes (ISE) was investigated within the framework of an interlaboratory comparison between eight partners from national metrology institutes and expert laboratories. Two electrolyte solutions containing the clinically most relevant ions sodium, potassium, magnesium, calcium and chloride having ion activities near the physiological range served as samples. The calibration of the measurement set-ups of the participants was carried out using gravimetrically prepared aqueous electrolyte solutions. The ion activities of these calibration standards were calculated by means of the semi-empirical Pitzer model. The measurement uncertainty of the measurement results was calculated according to the guide to the expression of uncertainty in measurement, GUM. Based on a new scale for ion activities traceable to the SI system of units, comparability and reliability of ISE measurement results of clinically relevant ions is realised.
Keywords: Ion activity; Ion-selective electrode; Traceability; Uncertainty; Pitzer model

The determination of amino acid (AA) impurity is important for the development of AA high-purity certified reference materials (CRMs), which play a key role in metrological traceability. We performed three analytical methods for the determination of AAs, such as postcolumn method derivatized with o-phthalaldehyde (OPA method), precolumn method derivatized with 6-aminoquinolyl-N-hydroxylsuccinimidyl carbamate (AQC method), and liquid chromatographymass spectrometry combined with ion-pairing reverse-phase chromatography (IP-RP-LCMS method), comparing their separation and sensitivity. We also applied them onto the AA impurity analysis in the candidate CRMs of l-isoleucine (Ile) and l-valine (Val), comparing their qualitative and quantitative performance. The OPA method was superior to others in separation and sensitivity, but several methods seemed to be necessary for the identification of the majority of AA impurities. In relation to the quantitative performance, the analytical results obtained by the different methods were equivalent within their expanded uncertainty for the detected AA impurities. On the other hand, the OPA method could detect the largest number of AA impurities and, consequently, obtained the largest sum of the mass fraction of AA impurities. The expanded measurement uncertainty of the OPA method was sufficiently smaller than the target standard uncertainty, 600 mg kg−1, where the expanded uncertainty (k = 2) of the assigned value of high-purity AA CRM should be approximately <0.002 kg kg−1.
Keywords: Amino acids; Purity; Certified reference material; Amino acid impurity; Amino acid analysis

Complexometry with EDTA as a quality control tool for certified single-element aqueous standard solutions by Martin Vlasák; Zuzana Luxemburková; Václav Sychra; Miloslav Suchánek (491-499).
The capability of complexometric EDTA titrimetry to determine the nominal concentration of the analyte in aqueous single-element standard solutions with an expected expanded uncertainty U of <0.2 % is investigated. A calibration solutions Astasol® and a standard solutions from internationally recognized producers are tested. The results covering about 40 analytes show that EDTA titrations can be effectively employed for this purpose. Nominal concentrations and their uncertainties are calculated by using numerical Kragten spreadsheets. The metrological traceability of the results is realized through NIST SRM 928 (lead nitrate), which is used for the standardization of the EDTA.
Keywords: Certified reference materials; Aqueous standard calibration solutions; Complexometry EDTA titrations; Kragten spreadsheets; Metrological traceability; Metrological compatibility

In this work, a procedure for total hardness determination in drinking and natural waters, expressed by the equivalent mass concentration of CaCO3, $$ gamma_{ ext{s}} $$ γ s , using an in-house complexometric titration with ethylenediaminetetraacetic acid based on standardized procedure was detailed. Compliance of the procedures requirements with legislation is discussed. Validation and quality control actions were implemented, and a limit of detection of 0.6 mg L−1 was determined fulfilling the target limit of detection of 15 mg L−1 imposed by specific legislation. Measurement precision was estimated under repeatability conditions, where relative standard deviations close to 1 % were estimated, and in within-laboratory reproducibility, where relative standard uncertainties of 4 % and 0.5 % were obtained for $$ gamma_{ ext{s}} $$ γ s lower and higher than 100 mg L−1, respectively. The results were considered unbiased whenever recoveries were within the range 0.90–1.10, fulfilling the target trueness value stated as 10 % of parametric value. The performance of the measurement procedure was also assessed by proficiency testing and z scores less than 2 were obtained. Measurement uncertainty was estimated by a modeling approach, using the law of propagation of uncertainty for variables not independent, and by empirical ones, such as single-laboratory validation and quality control data and proficiency testing. Measurement uncertainty computed from each model was assessed by ξ score performance indicator showing a good estimated uncertainty (|ξ| < 2). Finally, a way is proposed to assess the suitability of the combined uncertainty by the different models, comparing them to a target expanded measurement uncertainty within the analytical range, calculated from maximum decreed precision and trueness criteria since no target uncertainties were stated in the legislation.
Keywords: Uncertainty; Validation; Total hardness; Target uncertainty; Water

Certified reference materials (CRMs) play an important role in tracing results to the International System of Units through an unbroken chain of comparisons. Demand for new certified reference materials is steadily increasing in all areas. In Brazil, the demand for CRMs exceeds their availability by far, and the needs of the scientific community are not met. Food production is one area where CRMs are required, and they play an important role for export and local products. This paper describes the preparation and certification of a reference material for the content of mercury and methylmercury in fish samples. The material selection, preparation, homogeneity and stability studies, and characterization are described. Certification was performed by flow injection analysis–cold vapor atomic absorption spectrometry (FIA–CV-AAS) and isotope dilution–inductively coupled plasma mass spectrometry (ID–ICP-MS), which is a primary method. The standards ISO 30 (ABNT 30–34) and ISO Guide 35 were used as a basis for the preparation and characterization of the material. The material was certified for the mass fractions of total mercury w(total Hg) = (0.271 ± 0.059) μg g−1 and methylmercury w(MeHg) = (0.245 ± 0.053) μg g−1.
Keywords: Certified reference material; Fish; Mercury; Methylmercury

State of UK emissions monitoring of stacks and flues: an evaluation of proficiency testing data for SO2, NO and particulates by Marc D. Coleman; Rod A. Robinson; Matthew B. Williams; Martin J. Clack; David M. Butterfield (517-524).
We report an examination of the UK stack testing industry’s proficiency for monitoring industrial emissions of SO2, NO and particulates from 2000 to 2011. Data were taken from three proficiency testing schemes run by the National Physical Laboratory (NPL), UK; Calibration Gas Scheme (gas bottle certified reference materials), Gas Measurement Scheme (using a Stack Simulator Facility to test the entire measurement system) and Particulate Scheme (foil shims and salt solutions—i.e., filter and probe washing simulants). In each round of each scheme, participants’ deviations from assigned value were normalised to an allowable deviation based on the required uncertainty for stack emission measurements stipulated in the European Union’s Industrial Emissions Directive. This normalisation produced a z-score and limits were set to define satisfactory, warning and unsatisfactory participant performance. As a function of time, it was found that across all schemes, the number of unsatisfactory/outlier scores decreased, evidencing an overall improvement in industry proficiency. With regard to the gas schemes, it was found that the industry had a poorer proficiency for SO2 than NO and that there was a distribution bias toward negative scores in the Gas Measurement scheme consistent with SO2 sample losses in drying units. It was evident that this industry bias was insufficient to force the vast majority of the industry outside of the satisfactory z-score limits; however, it was noted that this issue should be carefully monitored in the future.
Keywords: NPL PT schemes; Calibration Gas scheme; Gas Measurement scheme; Particulate scheme; z-scores; Stack testing proficiency

Estimation of uncertainty in the determination of nitrogen and oxygen in U(Al, Si)3 by P. S. Ramanjaneyulu; A. S. Kulkarni; Komal Chandra; C. S. Yadav; M. K. Saxena; B. S. Tomar; K. L. Ramakumar (525-531).
U(Al, Si)3, an inter-metallic compound, is a proposed fuel in advanced nuclear reactors. Determination of trace amounts of nitrogen and oxygen in this material is an essential requirement for quality assurance. Mass fraction of nitrogen and oxygen were determined in this compound employing inert gas fusion in conjunction with thermal conductivity and infrared detection techniques, respectively. Uncertainty in the measured data has been estimated employing bottom-up approach as per ISO guidelines. Uncertainty in the calibration of the measurement procedure employing standards with own uncertainty of the measurement was estimated following linear regression equation and uncertainty propagation laws. The relative combined uncertainty of measurement has been calculated employing law of uncertainty propagation. Finally, the expanded uncertainty of measurement (coverage factor k = 2) for the determination of nitrogen and oxygen was estimated, and they were found to be 10 and 12 %, respectively.
Keywords: U(Al, Si)3, determination of nitrogen and oxygen; Inert gas fusion, thermal conductivity and infrared detection; Estimation of uncertainty

Metrology in analytical chemistry by Kaj Heydorn (533-535).
It is proposed to amend the definition of a measurand to include exact identification of the entity to be determined and specification of the system to which the measurement is intended to apply.
Keywords: Measurand; Analyte; Determinand; Indicator