Atomic external calibration

Quantitation by external calibration The most common and straightforward method of calibration in atomic absorption spectrometry is the use of an external calibration with suitable standard solutions. It is based on the assumption that the standard solution matches the composition of the sample sufficiently well. This is an assumption that must always be examined with care, since, for example, samples of different viscosity may be aspirated at different rates in flame AAS,... 

The isotopic abundances of the tracee or the tracer can be obtained from measured isotope ratios using Eqs. (16.2) and (16.3), while atomic masses can be taken from the literature (10]. Based on IDMS principles, measured isotope ratios can be converted into tracer to tracee ratios in the sample analyzed. The amount of tracee can then be calculated if the amount of tracer in the sample is known. The latter can be determined either by IDMS using an additional tracer or by non-IDMS techniques for quantitative analysis employing external calibration or standard addition techniques. 

The standard addition method is commonly used in quantitative analysis with ion-sensitive electrodes and in atomic absorption spectroscopy. In TLC this method was used by Klaus 92). Linear calibration with R(m=o)=o must also apply for this method. However, there is no advantage compared with the external standard method even worse there is a loss in precision by error propagation. The attainable precision is not satisfactory and only in the order of 3-5 %, compared to 0.3-0.5 % using the internal standard method 93). 

Frequently, quantitative analyses are based on external-standard calibration (see Section 8C-2). In atomic absorption, departures from linearity are encountered more often than in molecular absorption. Thus, analyses should never be based on the... 

Similar to mass calibration, quantity calibration can be done internally or externally by using appropriate calibration compounds. The calibration curve covers the intensity range corresponding to the intensities of analyte peaks. In order to obtain the highest accuracy, the calibrants used in quantity calibration should have similar atomic composition, molecular structure, and mass as analytes. This requirement is because ionization efficiency depends on molecular structure, and detection efficiency is mass-dependent. In fact, ion detectors rely on secondary electrons produced by the impact of primary ions with detector surfaces, including microchannel plates, electron multipliers, and many others. 

Related to the above-described inter-elemental correction method, the need for a proxy element can be obviated at the expense of resorting to sequential analysis of the sample and the calibrator, that is, with external gravimetric calibration. Although this technique is not favored by all, it has played a major role, for example, in the recent re-evaluation of the atomic weight of zinc [13]. 

Similar to other quantitative analytical approaches, the area under the spectral lines is proportional to the number of Mossbauer atoms detected. Practically, in one approach, a calibration graph of the area value of a series of external analogous samples with known concentrations would be constructed and compared with that of the unknown samples. In another approach a graph of the area value of unknown sample added with analogous samples with a series of known concentration can be drawn for calibration. Both approaches are common methods employed in instrumental analysis. 

Matrix effects frequently lead to varying extraction yields. The headspace and purge and trap techniques in particular are affected by this type of problem. If measures for standardizing the matrix are unsuccessful, the standard addition procedure can be used, as in, for example, atomic absorption (AAS) for the same reason. Here the calibration is analogous to the external standardization described earher and involves addition of known quantities of the analyte to be determined (Miller, 1992). The calibration samples are prepared with constant quantities of the sample material by addition of corresponding volumes of the standard solution. One sample is left as it is, that is, no standard is added. 

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