Multi-range calibration

Calibration curves must be made from chemicals with the highest purity as possible. To avoid dilution errors a multi-level calibration curve (six points) based on three stock solutions is recommended. One must also be aware that low concentrations of for example, PAHs (2 ppm) may be adsorbed by the vials up to -90% (Pinto, Jose and Cordero, 1994). A calibrated and traceable balance or a traceable pipette must be used for accurate preparation and dilution of the standards. The calibration curve must cover the concentration range that is needed for the analysis. Both the slope and the intercept must be used to calculate the concentration in the sample, especially if the intercept is different from zero. 

Multi point calibration will be recommended if minimum uncertainty and maximum consistency are required over a wide range of pH(X) values [21, 22]. The calibration function of the electrode is then calculated by linear regression of the difference in cell voltage results from the standard pH values. This calibration procedure is also recommended for characterising the performance of electrode systems. 

Metals were analyzed using dispersive X-ray fluorescence spectroscopy (XRF) on a SPECTRACE 6000. 2 g of sample were used. For metals in char multi-level calibrations within a range of 10-20,000 ppm were carried out. For the liquid samples the range was set between 1 and 10,000 ppm. 

There are various ways to derive a calibration curve. Multi-point calibration curves, for example, include a minimum of three different concentrations of the analyte. For semiquantitative assays, a single-point calibration is common. The single point is usually the threshold concentration used to determine whether a specimen is positive or negative for the analyte of interest. Depending on the validation process and performance characteristics of the assay, a single-point calibration curve may also be used in quantitative applications over a limited range of linearity. A historical (pre-established) multi-point calibration curve may also be used, but only if the stability of the analytical method over time has been well established (Goldberger et al., 1997). 

As soon as a larger number of different gas mixtures is necessary (either because characteristic curves of analyzers are to be taken at several concentration points, or because a wide variety of analyzers with different gas compositions in differing concentration ranges are to be calibrated), a large number of gas mixture cylinders must be kept on stock. As gas mixtures are expensive, and a large number of gas cylinders must be stored and handled, it is worthwhile to use multi-functional calibration gas generators. 

The transition used to calibrate the temperature scale of a thermobalance should have the following properties [1] (i) the width of the transition should be as narrow as possible and have a small energy of transformation (ii) the transition should be reversible so that the same reference sample can be used several times to check and optimize the calibration (iii) the temperature of the transition should be independent of the atmospheric composition and pressure, and unaffected by the presence of other standard materials so that a multi-point calibration can be achieved in a single run and (iv) the transition should be readily observable using standard reference materials in the milligram mass range. Transitions or decompositions which involve the loss of volatile products are usually irreversible and controlled by kinetic factors, and are unsuitable for temperature calibration. Dehydration reactions are also unsuitable because the transition width is strongly influenced by the atmospheric conditions. 

Prepare the aforementioned standards and suitable dilutions for the calibration graphs to cover the three ranges. A computer included with the analyzer may be used, if an internal multi-point calibration curve is provided. 

If analytical methods are validated in inter-laboratory validation studies, documentation should follow the requirements of the harmonized protocol of lUPAC. " However, multi-matrix/multi-residue methods are applicable to hundreds of pesticides in dozens of commodities and have to be validated at several concentration levels. Any complete documentation of validation results is impossible in that case. Some performance characteristics, e.g., the specificity of analyte detection, an appropriate calibration range and sufficient detection sensitivity, are prerequisites for the determination of acceptable trueness and precision and their publication is less important. The LOD and LOQ depend on special instmmentation, analysts involved, time, batches of chemicals, etc., and cannot easily be reproduced. Therefore, these characteristics are less important. A practical, frequently applied alternative is the publication only of trueness (most often in terms of recovery) and precision for each analyte at each level. No consensus seems to exist as to whether these analyte-parameter sets should be documented, e.g., separately for each commodity or accumulated for all experiments done with the same analyte. In the latter case, the applicability of methods with regard to commodities can be documented in separate tables without performance characteristics. 

Geological and geochemical applications of PXRF generally require multi-element analysis however, the more elements that are included within an analytical test, the greater the likelihood of problems such as peak overlaps or interferences, and manufacturers typically will provide machine calibrations for 20-30 elements in a particular analytical mode (see below). Our instruments have been calibrated for a range of elements for characterization of lithological units, different mineralization types and associated hydrothermal alteration, and other geochemical exploration vectors. 

The advantages of using plasma emission sources include the ability to perform multi-element analysis, a calibration linear dynamic range of more than three orders of magnitude and for some elements the limits of detection are comparable to those found by GFAAS. The ability to perform multi-element analysis is essential when the purpose of the experiments is to study element interaction effects. 

For the materials and concentration ranges we have studied, comparative batch tests have supported the use of linear isotherms (e.g., Khandelwal et al., 1998 Khandelwal and Rabideau, 2000), although the assumption of a linear isotherm may not be appropriate for all systems. In particular, Mott and Weber (1992) and Gullick (1998) have utilized nonlinear isotherms to describe the behavior of various sorbing additives, including flyash and organoclays. However, it is important to note that multi-parameter nonlinear isotherms are not easily calibrated from column data, particularly if a sorption rate constant must also be estimated. 

Multi-component hydrocarbon standards to provide accurate calibration of instruments (generally gas chromatographs) used to monitor the concentrations of a wide range of volatile organic hydrocarbon compounds (VOCs) in ambient air. These standards currently contain 30 different hydrocarbon species that are important to photochemical ozone formation, with concentrations ranging down to a few parts per billion by molar value. They are disseminated widely in the United Kingdom and the rest of Europe as calibration standards, and as test mixtures for assessment of the quality of international ambient hydrocarbon measurements (often under the auspices of the European Commission - EC). 

Linearity needs to be proven by running a multi-point—usually five—calibration. The range will depend on detector and should cover at least two orders of magnitude. 

Abstract. Recent experiments are aiming at an accuracy of 1 ppm for the mass of the charged pion using the characteristic X-rays from exotic atoms. Once the pion mass is established with that precision, the narrow lines from medium Z pionic atoms can be used as a calibration standard in the few keV range. The precision of this new standard is not limited by the large natural line width of fluorescence X-rays and their complex structure due to multi-hole excitations. 

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