Shall we consider covariances?

 The impact of correlation between the reference points used in determination of a functional relationship by application of least-squares techniques is discussed. Calculations are based on an improved effective-variance approach to the general least-squares problem. From a simulation study, general conclusions about the impact of correlation are drawn. Standard addition as an example of a method using correlated data is discussed in detail. Received: 29 October 1997 · Accepted: 26 November 1997     

How accurate must a reference material be?

 This article deals with reference materials (RMs) used for calibration purposes. For most common spectroscopic and chromatographic methods, RMs which have a relative content uncertainty of up to 3% can be used without considerably increasing the overall measurement uncertainty of the complete procedure. It is vital that the correct amount of the RM is used to ensure the quality of the quantitative chemical analyses. This can only be guaranteed if, in addition to the correct determination of the amount, the stability of the RM in the sales packaging is considered in the quoted uncertainty. For instance when the RM is packed in a container made of polyvinylidene fluoride water losses in hot climates can result in an increase in concentration which cannot be neglected. This is also true for high density polyethylene containers (PE-HD) although to a lesser degree. When mercury solutions of 1000 mg/l to 10 000 mg/l are kept in diluted nitric acid in PE containers, a relevant reduction in amount can be measured despite the water losses. However, the solutions remain stable in glass containers.     

Influence of buffer quality on pH measurement uncertainty: prediction and experimental evaluation

Dependence of the uncertainty of a pH measurement result on the quality of buffers (i.e. the uncertainty of their certified pH values) at different levels of instrumental uncertainty (pH-meter, etc.) was simulated using the Monte Carlo method and regression analysis. The contribution of the instrumental standard uncertainty (in the studied range from 0.1 to 1 mV) to the uncertainty of the pH measurement result is negligible, if the standard uncertainties of the pH buffers exceed 0.04 pH (e. g. for in-house buffers). It is shown how the choice of pH-meter and buffers should be correlated in order to meet the required uncertainty of a pH measurement result in a sample under analysis. The results of the simulation were compared with experimental data obtained from calibrations of a pH/ion-meter with a hydrogen working electrode (Radiometer PHM-240) and with a glass electrode (Metrohm 744). Buffers of different quality (National Institute for Standards and Technology standard reference materials, certified Radiometer buffers and Merck CertiPUR buffers) were used for the calibrations. The uncertainties of the experimental results are close to the predicted ones obtained by the simulation. Received: 16 June 2002 Accepted: 19 July 2002 Presented at the 14^th International Conference of the Israel Society for Quality, 18–21 November 2002, Jerusalem, Israel Correspondence to I. Kuselman     

Calibrated Estimators of the Population Covariance

We introduce some calibrated estimators of a finite-population covariance. The estimators are defined by using different calibration equations and different loss functions. The estimators derived are compared by simulation. The study is supported by the Nordplus Neighbour program of the Nordic Council of Ministers.     

Calibration by the gradient ratio-standard addition method in flow injection flame atomic absorption spectrometry

A calibration method has been developed which is realised in the flow injection analysis (FIA) by the gradient technique. According to this method two transient peaks, one for a sample and the other for a sample with standard addition, are recorded and compared point by point in the entire defined time range. The analytical result is estimated on the basis of information gained about the local analyte concentrations in the sample zone. The method allows the results to be reliable when both the non-linear calibration dependence and the interference effect occur. As an example, calcium in synthetic samples containing silicon, phosphate, aluminium, vanadium and titanium, and also in iron ore sample, were determined by flame atomic absorption spectrometry (FAAS). It has been proved, that the method can be effective in overcoming even extremely strong interferences, providing analytical results with average accuracy equal to ca. 5% and with precision 2–3 times inferior to that obtained by conventional FI calibration.     

Evaluation of uncertainty in measurements—problems and tools

The ISO “Guide to the Expression of Uncertainty in Measurement” (GUM) establishes a unified method for evaluating and stating measurement uncertainties that has been accepted by nearly all calibration services and most test co-operations in all parts of the world. In measurements of mechanical quantities, such as acceleration and derived motion quantities, the application of the GUM may be difficult and very time-consuming unless some possibilities of simplification are made use of. After a brief introduction to the basic procedure specified in the GUM for the calculation of the measurement uncertainty, a survey of the problems typically encountered in uncertainty calculations when, for example, vibrations are measured or accelerometers calibrated by laser interferometry is given. It is shown how a model function of simple structure can be established for the usually complex relationship between the output quantity (e.g. sensitivity of an accelerometer), the quantity to be measured (e.g. acceleration) and various influence quantities (noise, transverse motion, base strain, etc.). Among other things, non-linear effects such as the influences of distortion, hum and noise can be properly taken into account.     

Improving quantitative precision and throughput by reducing calibrator use in liquid chromatography-tandem mass spectrometry

To improve efficiency in our mass spectrometry laboratories we have made efforts to reduce the number of calibration standards utilized for quantitation over time. We often analyze three or more batches of 96 samples per day, on a single instrument, for a number of assays. With a conventional calibration scheme at six concentration levels this amounts to more than 5000 calibration points per year. Modern LC-tandem mass spectrometric instrumentation is extremely rugged however, and isotopically labelled internal standards are widely available. This made us consider whether alternative calibration strategies could be utilized to reduce the number of calibration standards analyzed while still retaining high precision and accurate quantitation.     

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采用了新的放电方案,消除了环向场对磁测量的影响。通过修改信号调理电路,解决了积分器不能响应磁通快速变化的问题;通过改进磁测量的标定方法,在准确标定磁通和极向磁探针测量的同时,有效地标定了径向磁探针的测量信号。输入这些有效标定数据后,EFIT能快速收敛,获得了准确可靠的位形重建结果,与探针、氢阿尔法和CCD等诊断结果很好符合。同时介绍了自行研制的sFIT位形重建代码,该代码得到的位形重建结果,与EFIT反演结果一致。     

On the expression “external calibration” in atomic spectrometry

The expressions “calibration" and “external calibration" appear together in the present literature of atomic spectrometry resulting in a dilemma of understanding and correct use. It is examined how the IUPAC can provide a guidance to the solution of this problem by recalling the definitions of related terms of optical, mass and X-ray atomic spectrometry techniques. The introduction and definition of these expressions in widely used text books are investigated and statistically evaluated for the articles published during the last 30 years in the periodical Spectrochimica Acta Part B. For the elimination of the literary difficulties with the use of the term “calibration", attributes are proposed to express the degree of matrix matching of standards and samples.     

On the specific experience of national metrology institutes (NMIs) with national accreditation bodies (NABs)

The following paper addresses the experience of certain national metrology institutes (NMIs) with national accreditation bodies (NABs), in particular in small countries with emerging economies in new EU or candidate member states. Specific cases have been analysed primarily in some Central and South-East European countries. NABs under consideration are, in principle, members of European Co-operation for Accreditation (EA). EA cross-frontier policy in these cases will be discussed and resulting dilemmas outlined. Focus will be given to the problems of NMIs also performing calibrations, commercial calibration laboratories, and their relations with NABs. Due to numerous problems of smaller NABs, cross-frontier policy should stimulate cooperation between foreign and local accreditation bodies not only at the request of the calibration laboratory or some other conformity assessment body, but at the request of the local accreditation body in order to provide the best service for their local economy. As a solution to the accreditation problems, this paper proposes a process of gradual formation of a common European pool of experts, real-life common practices and, finally, maybe even some form of joint European accreditation service. The opinions expressed in the paper are personal statements of the author, and do not necessarily reflect the official opinions of the institutions of the author. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher.