Calibration external bracketing

The calibration mode selected by the laboratory should also be carefully considered, i.e. standard additions, calibration curve and/or use of bracketing standards. All calibration methods suffer from typical sources of error or drawbacks, e.g. for standard additions non-linearity of the calibration curve, extrapolation difficulties, chemical form of calibrant added, etc. for external calibration (calibration curve) changes of the matrix affecting the linearity of the curve for bracketing standards time-consuming procedures for many routine laboratories, etc. (Quevauviller et al., 1996a Quevauviller, 1998b). 

External standardization methods are used for most quantitative assays. Solutions containing known concentrations of reference standards of the analytes are required to calibrate (standardize) the HPLC system. Bracketed standards injected before and after the samples set are preferred for improved accuracy. 

Determining the Relationship between Absorbance and Concentration 1 he method of external standards (sec Section ID-2) is most often used to establish the absorbance versus concentration relationship. After deciding on the conditions for the analysis, the calibration curve is prepared from a series of standard solutions that bracket the concentration range expected for the samples. Seldom, if ever, is it safe to assume adherence to Beer s law and use only a single standard to determine the molar ab,soq)tivity. It is never a good idea to base the results of an analysis on a literature value for the molar absorptivity. 

Standard addition calibration is more robust and reliable than conventional external calibration, but is more time consuming and costly if it is applied separately for each sample. A major advantage of standard addition is the correction of multiplicative matrix effects, for example alteration of nebuli-sation efficiency. The intensities of all samples (and spiked samples) change by the same factor, which leads to an altered cahbration slope. However, for additive effects, such as interferences caused by the matrix, the calibration line is shifted parallel and the intercept changes, which results in biased analyte concentrations. In some cases, this bias can be avoided (or indeed identified) by choosing another isotope and comparing the results for each. Standard addition has no inherent compensation for instrumental drift in the ICP-MS system. However, reduction of the drift, whieh limits the applicability of standard addition for ICP-MS, has been achieved by applying a chemometric method (a bracket approach, where the spiked sample is measured between two different measurements of the sample). ... 

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