Uncertainty Of Analytical Results Research Articles

A probabilistic approach to assessment of the quality of drinking water

Abstract Knowledge of uncertainty in analytical results is of prime importance in assessments of compliance with requirements set out for the quality of water intended for human consumption. Assessments of drinking water quality can be performed using either a deterministic or a probabilistic method. In the former approach, every single result is referred directly to the parametric value, while in the probabilistic method uncertainty related to analytical results is taken into account during the decision-making process. In the present research, laboratory uncertainty and uncertainty determined on the basis of results of analyses of duplicate samples collected in two Polish cities were compared and used in the probabilistic approach of water quality assessment. Using the probabilistic method, more results were considered to be “above the parametric value”. Most excesses were observed when the maximum allowable uncertainty as set out in the Regulation of the Minister of Health of 7 December 2017 was used, which is due to the highest values of these uncertainties. The lowest values above parametric values in the probabilistic approach were observed when measurement uncertainty was considered.

Reference Measurements for Priority and Essential Trace Elements and Methyl Mercury with Isotope Dilution Inductively Coupled Plasma-Mass Spectrometry for Seafood Safety Assessment and CRM Production

The continuous release of anthropogenic pollutants into marine environment increases the needs for the development of analytical procedures for their accurate determination in many sample types. Sound strategies for seafood safety monitoring call for measurement systems capable of producing comparable analytical results with demonstrated quality. Method validation, traceability, and uncertainty of analytical results are the three milestones to assess data quality. Some trace elements are essential for biological structures, but they can become toxic at concentrations beyond those necessary for their functions; others are toxic even at very low levels. Their accurate determination in reference samples serves as an important step in seafood safety control and pollution monitoring. Isotope dilution inductively coupled plasma-mass spectrometry (ID ICP-MS) has been applied for the determination of the total mass fractions of five trace elements (Cd, Cu, Hg, Pb, and Zn) and methyl mercury (MeHg) in marine biota candidate reference material IAEA-476. Because of the complex matrix of the sample and the expected spectral interferences, special care was taken for the validation of the applied methodology, particularly for its measurement step. Reference isotopic measurements were carried out by sector field inductively coupled plasma-mass spectrometer (ICP-SFMS). The entire ID ICP-SFMS measurement procedure was described by mathematical modeling and the combined uncertainty of measurement results was estimated. All factors influencing the final results and isotopic equilibrium were systematically investigated. This included the procedural blank, the moisture content in biota samples, and all factors affecting the blend ratio measurements (i.e., instrumental background, spectral interferences, dead time, mass discrimination effects, and the repeatability of measured isotope ratios). Modeling of the entire measurement procedure and the use of appropriate certified reference materials (CRM) enable to assure the traceability of obtained values to the International System of Units (SI). Finally, the developed procedure has been successfully applied in the process of the certification of the International Atomic Energy Agency (IAEA) certified reference material for trace elements and MeHg mass fractions in fish homogenate sample. The excellent agreement between obtained results in the present study and those derived from the IAEA certification campaign for trace elements and MeHg in the same sample matrix further validated the reference methods developed in the IAEA.

Uncertainty estimation for the determination of Fe, Pb and Zn in natural water samples by SPE-ICP-OES

In this paper we estimate measurement of uncertainty for determination of Fe, Pb and Zn in natural water samples by solid-phase extraction and inductively coupled plasma optical emission spectrometry (SPE-ICP-OES). The procedure is based on the retention of analytes in the form of 8-hydroxyquinoline (8-HQ) complexes on a mini column of XAD-4 resin and subsequent elution with nitric acid. The influence of various analytical parameters including the amount of solid phase, pH, elution factors (concentration and volume of eluting solution), volume of sample solution, and amount of ligand on the extraction efficiency of analytes was investigated. To estimate the uncertainty of analytical result obtained, we propose assessing trueness by employing spiked sample. Two kinds of bias (proportional bias and constant bias) estimated. We applied Nested design for calculating proportional bias and Youden method to calculate the constant bias. The results we obtained for proportional bias are calculated from spiked sample...

Use of the ‘characteristic function’ for modelling repeatability precision

The 'characteristic function' is a two-parameter function relating precision or uncertainty in analytical results to the concentration of the analyte. In previous papers, in this series, it has been shown to provide a good model of precision measured: (a) under reproducibility conditions and (b) under 'instrumental' conditions. The present study shows that it is also a valuable model for precision estimated under repeatability conditions. The study data were large sets of duplicated results obtained for the purposes of quality control on typical test materials in routine analysis. As the analytes exhibited concentration ranges encompassing between one and three orders of magnitude, there was ample scope to demonstrate goodness of fit to the function under different circumstances.

Dry mass determination: what role does it play in combined measurement uncertainty?

Elemental mass fractions are normally reported on a dry-mass basis. In addition, producers of reference material state their property values for dry masses and relate them to a defined dry mass determination method. This paper describes how biases and uncertainties in dry mass correction factors may affect values reported as ‘concentration per dry mass’. The influence of the use of different drying methods, accuracy of drying temperature, length of drying, environmental humidity, and the sample mass used for dry mass determination have been evaluated using two candidate International Atomic Energy Agency reference materials. Deviation from proposed drying methods was observed to lead to differences of up to 5% in dry mass correction factors. In addition, the heterogeneity of sample moisture as well as hygroscopic behaviour during weighing play an important role in the uncertainty estimation. Proposals are made regarding which sources of uncertainty arising from dry mass determination should be considered and included in the combined uncertainty of analytical results.

Estimation of sample processing uncertainty for chlorpyrifos residue in cucumber

Sample processing is a very important component of uncertainty in analytical results. In order to have reliable results, the laboratory sample should be properly processed to obtain statistically homogenous matrix—before the representative test portions are withdrawn for analysis. The use of 14C-labeled compound is preferable because the analyte can be quantified without cleanup. The method is based on surface treatment of cucumber with 14C-chlorpyrifos, determination of 14C-chlorpyrifos activity in the replicate test portions of different size, and determination of the uncertainty of sample processing.

Trends in quality in the analytical laboratory. I. Traceability and measurement uncertainty of analytical results

Trends in quality in the analytical laboratory. I. Traceability and measurement uncertainty of analytical results

The system MgO-Al2O3-SiO2-Cr2O3 revisited: reanalysis of Doroshev et al.'s (1997) experiments and new experiments

New experimental data are presented on the partitioning of Cr and Al between spinel-picrochromite and corundumeskolaite solid solutions at 5.0 GPa and 1300, 1500°C and garnet-spinel-orthopyroxene equilibria at 3.0 and 4.0 GPa and 1000°C. The experimental products of Doroshev et al. 9s (1997) experiments on mineral equilibria in the MASCr system were reanalysed in order to clarify the direction and mechanism of approach to equilibrium. Detailed analysis of mineral compositions was carried out by electron microprobe with high spatial resolution, which demonstrated complex zoning patterns of minerals. In experiments with pure pyrope in the starting material, garnet shows simple zoning with Cr-rich seams mantling relicts of aluminous garnet. In experiments with Cr-rich garnets in the starting mixture (Cr/(Cr+Al) = 0.3–0.8), more complex patterns were observed. Knorringiterich garnet cores are overgrown in sequence by Al-rich (X Cr = 0.1–0.2) and then Cr-rich zones. The analytical data allowed us to establish the direction of approach to equilibrium for particular grains, which eliminated the path-looping problem. The results were compared to published experimental data on garnet, orthopyroxene, spinel, and eskolaite equilibria in the MAS and MASCr systems. It was shown that all the data can be described by a simple thermodynamic model, if the uncertainty of analytical results is taken into account.

Evaluation of accuracy and uncertainty of ELISA assays for the determination of interleukin-4, interleukin-5, interferon-gamma and tumor necrosis factor-alpha.

The comparison of analytical results calls for validation of the assays used, i.e. for the documentation of accuracy (trueness and precision), linearity and specificity. In addition, there is a growing demand for evaluation and documentation of traceability and uncertainty of analytical results. However, models for establishing the traceability and uncertainty of immunoassay results are lacking. Sandwich enzyme-linked immunosorbent assays (ELISAs) were developed for determination of the human cytokines interleukin-4 (IL-4), interleukin-5 (IL-5), interferon-y (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). The accuracy of each of the assays was evaluated in the ranges of 1-15 microg/l (IL-4), 0.001-1 microg/l (IL-5), 0.5-2.5 microg/l (IFN-T) and 0.14-2.2 microg/l (TNF-alpha). Other evaluated performance characteristics were the limit of detection (LOD), immunological specificity, and robustness. Traceability was ensured by the use of World Health Organization International Standards (WHO IS). An uncertainty budget, which combined the contribution from all known uncertainty components, was established for each cytokine ELISA. The between-run relative analytical standard deviation (RSDA) of the assessed ELISAs was found to be in the range of 11-18%, except for IL-5 where RSDA increased at decreasing concentrations. The LOD was 0.12 microg/l, 0.0077 microg/l, 0.0069 microg/l and 0.0063 microg/l for IL-4, IL-5, IFN-gamma and TNF-alpha, respectively. Traceability to the WHO IS was established for each of the cytokines. The combined relative standard uncertainty (U(result)/C(result)) was 28%, 22-62%, 28% and 24% for the IL-4, IL-5, IFN-gamma and TNF-alpha results, respectively. The major contributions to uncertainty came from the relative analytical standard deviation and from the uncertainty of the mass concentration of the WHO IS. The uncertainty of the WHO IS was not stated in the accompanying certificate and was evaluated by other means. The largest sources of uncertainty were located outside our laboratory. This means that the possibilities to improve the reliability of the results produced by the ELISAs are very limited. The task of evaluating measurement uncertainty would be much easier if producers of international reference standards reported the uncertainty of the value of standards. The model for evaluating uncertainty presented in this paper is applicable to other types of assays and to most analytical methods.

Ménage À Trois: Activation, Analysis and Uncertainty, 10 Projections into the Impending Millenium

For more than half a century we have been using activation of stable isotopes to determine chemical elements, and significant contributions have been made to a variety of scientific subjects. Nevertheless, activation analysis has not yet become integrated in the field of chemical analysis, and therefore the special features characteristic of activation analysis have not been fully realised in analytical chemistry. At the same time basic chemical knowledge has only to a limited extent been utilised in the development of analytical methods based on activation. This situation has only now become painfully clear, when a world-wide requirement is being made to express the uncertainty of analytical results in accordance with the BIPM philosophy. The identification and estimation of all uncertainty components needed to produce a reliable uncertainty budget requires the combined efforts of all parties. An attempt is here made to extrapolate current trends for the expression of uncertainty in activation and analysis into the future and to show, how the implementation of the BIPM Guidelines with respect to correction for all known or suspected biases, achievement of statistical control, and full traceability, can help bringing analytical chemistry into its rightful position as a scientific discipline in the field of metrology.

Computer simulation of the uncertainty of analytical results

Uncertainty is an important quality parameter of any analytical result. Estimating the uncertainty of analytical procedures can be rather difficult in many instances. Computer simulation based on multiple repetition of calculation of the resulting quantity while varying slightly the input parameters is an alternative option. The input parameters for the various computation runs are composed of two terms: the constant mean value and the error, obtained as the product of the standard uncertainty of the parameter in question and a random number with the normal distribution N(0.1). This approach can also serve to optimize measuring procedures, as demonstrated on an example of the determination of lead in biological materials by isotope dilution mass spectrometry. The accuracy of the optimized method was tested by analysis of some CRMs, the robustness of the method was examined, and the detection limit (10 ng/g Pb) and repeatability (4 ng/g Pb) were determined.

Uncertainty in analytical results from solid materials with electrothermal atomic absorption spectrometry: a comparison of methods

The combined uncertainty in the analytical results of solid materials for two methods (ET-AAS, analysis after prior sample digestion and direct solid sampling) are derived by applying the Guide to the Expression of Uncertainty in Measurement from the International Standards Organization. For the analysis of solid materials, generally, three uncertainty components must be considered: (i) those in the calibration, (ii) those in the unknown sample measurement and (iii) those in the analytical quality control (AQC) process. The expanded uncertainty limits for the content of cadmium and lead from analytical data of biological samples are calculated with the derived statistical estimates. For both methods the expanded uncertainty intervals are generally of similar width, if all sources of uncertainty are included. The relative uncertainty limits for the determination of cadmium range from 6% to 10%, and for the determination of lead they range from 8% to 16%. However, the different uncertainty components contribute to different degrees. Though with the calibration based on reference solutions (digestion method) the respective contribution may be negligible (precision < 3%), the uncertainty from a calibration based directly on a certified reference material (CRM) (solid sampling) may contribute significantly (precision about 10%). In contrast to that, the required AQC measurement (if the calibration is based on reference solutions) contributes an additional uncertainty component, though for the CRM calibration the AQC is “built-in”. For both methods, the uncertainty in the certified content of the CRM, which is used for AQC, must be considered. The estimation of the uncertainty components is shown to be a suitable tool for the experimental design in order to obtain a small uncertainty in the analytical result.