Calibration in chemical analysis

Hasselbarth W (1995) Traceability of measurement and calibration in chemical analysis. Fresenius J Anal Chem 352 400... 

Examples are water for the calibration of viscometers, sapphire as a heat-capacity calibrant in calorimetry, and solutions used for calibration in chemical analysis... 

Element solutions with mass concentrations of elements of nominally 1 g/1, either as single or multi-element solutions are among the most frequently used calibrators in chemical analysis, and traceability to the SI units of the concentrations stated by the manufacturers is increasingly required. In response to this growing demand a trace-ability structure for elemental analysis is at present being set up. 

There are two uses of chemical standards in chemical analysis. In the first place, they may be used to verify that an instrument works correctly on a day-to-day basis - this is sometimes called System Suitability checking. This type of test does not usually relate to specific samples and is therefore strictly quality assurance rather than quality control. Secondly, the chemical standards are used to calibrate the response of an instrument. The standard may be measured separately from the samples (external standardization) or as part of the samples (internal standardization). This was dealt with in Section 5.3.2. 

Reference Material Material or substance whose property values are sufficiently homogeneous and well-established so as to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials. (Note A reference material may be in the form of a pure or mixed gas, liquid, or solid. Examples include synthetic mixtures such as calibration solutions used in chemical analysis as well as materials based on natural environmental samples such as sediments.)... 

Statistics have been used in chemical analysis in increasing amounts to quantify errors. The focus shifts now to other areas, such as in sampling and in measurement calibrations. Statistical and computer methods can be brought into use to give a quantified amount of error and to clarify complex mixture problems. These areas are a part of chemometrics as we use the term today. 

Abstract Since the uncertainty of each link in the traceability chain (measuring analytical instrument, reference material or other measurement standard) changes over the course of time, the chain lifetime is limited. The lifetime in chemical analysis is dependent on the calibration intervals of the measuring equipment and the shelf-life of the certified reference materials (CRMs) used for the calibration of the equipment. It is shown that the ordinary least squares technique, used for treatment of the calibration data, is correct only when uncertainties in the certified values of the measurement standards or CRMs are negligible. If these uncertainties increase (for example, close to the end of the calibration interval or shelf-life), they are able to influence significant-... 

Stability of the traceability chain in chemical analysis is dependent on the calibration intervals of measuring equipment and shelf-life of CRMs used for the calibration of the equipment. 

In analytical chemistry, several calibration methods have been developed and introduced into laboratory practice [1], Most of them can be recommended for use in trace analysis. The methods differ from each other in terms of (a) preparation of the calibration solutions, (b) interpretation of the measurement results and construction of calibration graphs, and (c) calculation of the final analytical results. We present the calibration methods exploited most often in chemical analysis and point out their advantages and drawbacks, which are especially significant from the point of view of the specific conditions characterizing trace analysis. 

As shown, various calibration methods can be applied in chemical analysis. The choice of method depends on the kind of analytical problems and sources of random errors expected in the course of analysis. Nevertheless, it is hard to say that any of the discussed methods is especially adapted to trace analysis. However, because of its specificity, trace analysis does require special attention in the choice of calibration method, as well as special care in realization of the selected method at every step of the calibration procedure. 

Vol. 117. Applications of Fluorescence in Immunoassays. By Ilkka Hemmila Vol. 118. Principles and Practice of Spectroscopic Calibration. By Howard Mark Vol. 119. Activation Spectrometry in Chemical Analysis. By S. J. Parry Vol. 120. Remote Sensing by Fourier IVansform Spectrometry. By Reinhard Beer Vol. 121. Detectors for Capillary Chromatography. Edited by Herbert H. Hill and Dennis McMinn... 

This book is intended to introduce a student or practitioner of analytical chemistry to the technical elements and practical benefits of the Raman revolution. It is not intended to describe high-end Raman techniques such as nonlinear or time-resolved Raman spectroscopy, nor does it attempt to describe the many theoretical treatments of Raman scattering. The book emphasizes the concepts and technology important to applications of Raman spectroscopy in chemical analysis, with attention to calibration, performance, and sampling modes. While many recent innovations in analytical Raman spectroscopy are... 

It is often the case in chemical analysis that the independent variable, standard solution concentrations in the above example, is said to be fixed. The values of concentration for the calibration solutions can be expected to have been chosen by the analyst and the values to be accurately known. The errors associated with X, therefore, are negligible compared with the uncertainty in y due to fluctuations and noise in the instrumental measurement. 

Chapter 3 Using Spreadsheets in Analytical Chemistry 54 Chapter 4 Calculations Used in Analytical Chemistry 71 Chapter 5 Errors in Chemical Analyses 90 Chapter 6 Random Errors in Chemical Analysis 105 Chapter 7 Statistical Data Treatment and Evaluation 142 Chapter 8 Sampling, Standardization, and Calibration 175... 

In chemical analysis, the substances to be determined are rarely directly measurable and sample pretreatment is in most cases necessary to convert or separate the analyte in a form that is compatible with the measurement system. This may imply that the initial physical or chemical composition of the sample is changed without loosing control of this change so that the traceability to a determined reference (e.g. fundamental units) is maintained. Typical pretreatment steps are e.g. digestion, extraction, purification these are frequently followed by intermediate steps such as derivatisation or separation, calibration and final detection. Each action undertaken in one of these steps represents a possible source of error, which adds to the total uncertainty of the final determination. 

In chemical analysis, as in many other sciences, statistical methodologies are unavoidable. The calibration curve constitutes an everyday application, just as an analytical result can only be ascertained if an estimation of the possible error has been considered. Once a measurement has been repeated, a statistical exploitation becomes possible. However, the laws of sampling and tests based upon hypotheses must be understood to avoid non-value conclusions, or to ensure the meaningful quality tests. Systematic errors (user-based, instrumental) or gross errors which lead to results beyond reasonable limits do not enter into this chapter. For the tests most frequently met in chemistry only indeterminate errors are considered here. 

Up to this point, regression has been restricted to two blocks of two-way data Y and X. In chemical analysis, however, a growing number of problems can be cast as three-way regression analysis. Consider the calibration of chemical constituents on the basis of their fluorescence excitation/emission spectrum or of gas chromatography/mass spectrometry (GC/MS) data. For each sample, two-dimensional measurements are available that constitute a three-way data array, X. This data array has to be related to sample concentrations of one, vector y, or several analytes, matrix Y. Cases can be imagined where even the matrix Y constitutes a three-way data array. 

The basis of the use of radioactive kryptonates in chemical analysis is that during the chemical reaction the crystal lattice of the kryptonated carrier is destroyed, the carrier consumed, and the radioactive krypton released. The determination can be done with a calibration graph or by comparison with a standard. 

The most important applications of pyroelectric effect are in the field of detectors of infrared radiation, thermal imaging and pyroelectric vidicons and photoferro-electric detectors. Pyroelectric materials are also used in electrically calibrated pyroelectric radiometers, for measurement of energy, in chemical analysis and in... 

Standards with known additive loadings are required to calibrate in-polymer analysis techniques. In the laboratory preparation of standards, it is extremely important to refer to the actual materials used in the formulation of the final product rather than ultra pure grades of chemicals. In the selection of mixtures to be used as standards all component levels should be evenly represented in the calibration blends, in order to avoid one component from dominating the spectral information for quantitative measurements. The precision of standard mixtures generally needs to be better than that of the analytical system being developed. Preparation of good calibration standards and the choice of a suitable internal standard are of cmcial importance for quantitation of polymer additives. 

In many situations in chemical analysis, a full calibration curve is not prepared because of the complexity, time, or cost. In such situations, abbreviated external standard methods are often used. Under no circumstances can an abbreviated method be used if the raw signal response is nonlinear. Moreover, these methods are not generally appropriate for analyses in regulatory, forensic, or health care environments where the consequences can be far-reaching. 

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