Addition-calibration method

A well-known drawback is that the calibration derived from SAM cannot be used for more than one test solution. Hence, a new standard additions calibration has to be developed for each test solution. This implies usually very large workloads in laboratories. Despite this, Perkin-Elmer tried to simplify routine measurements and implemented the so-called "addition-calibration method. It consists in developing a unique SAM calibration for several test solutions for each one its own ordinate is considered to get a new prediction by adapting the regression equation.However, one must be certain of the straight line behaviour of the calibration and, moreover, of the true similarity in the composition of the samples otherwise it should not be applied. 

Since a standard additions calibration curve is constructed in the sample, it cannot be extended to the analysis of another sample. Each sample, therefore, requires its own standard additions calibration curve. This is a serious drawback to the routine application of the method of standard additions, particularly in laboratories that must handle many samples or that require a quick turnaround time. For example, suppose you need to analyze ten samples using a three-point calibration curve. For a normal calibration curve using external standards, only 13 solutions need to be analyzed (3 standards and 10 samples). Using the method of standard additions, however, requires the analysis of 30 solutions, since each of the 10 samples must be analyzed three times (once before spiking and two times after adding successive spikes). 

The method of standard additions can be used to check the validity of an external standardization when matrix matching is not feasible. To do this, a normal calibration curve of Sjtand versus Cs is constructed, and the value of k is determined from its slope. A standard additions calibration curve is then constructed using equation 5.6, plotting the data as shown in Figure 5.7(b). The slope of this standard additions calibration curve gives an independent determination of k. If the two values of k are identical, then any difference between the sample s matrix and that of the external standards can be ignored. When the values of k are different, a proportional determinate error is introduced if the normal calibration curve is used. 

Calibration by Standard Addition Method (SAM). When matrix effects appear or are to be expected and matrix-matched calibration samples are not available, the standard addition method (SAM) can be the calibration method of choice. 

Although standard addition calibration is an unreliable method if linearity in the range x < x0 is not experimentally verified but only supposed, there is scarcely an alternative in trace and ultra trace analysis when matrix effects are seriously suspected. 

Accuracy The degree of accuracy needed is determined by the question being asked. In the Bulging Drum Problem, does it matter if the hydrogen gas was found to be 12.0% or 12.1% of the headspace Which calibration method—external standards, internal standards, or standard addition—is appropriate for your analysis ... 

PCR or PLS establish a mathematical relationship (calibration) between the matrix that is formed by the spectra taken of a collection of samples and the vector of properties or qualities for these same samples. Additionally, both methods allow the prediction of the quality for new samples, just based on their spectra. In contrast to most methods discussed so far, PCR and PLS do not require any order in the data set. 

Simultaneous Spectrophotometric Determination of Food Additives (Benzoie Aeid, Caffeine, Aspartame and Aeesulfame-K) in Cola Drinks by PLS Multivariate Calibration Method... 

There is no standard report form offered because the standard requires it be completed and in accordance with any specific instructions in the test or calibration methods (subsection 5.10.1). It is noteworthy that the general notes to subsection 5.10 do allow a simplified report to be issued to internal clients, as long as the full information is readily available in the laboratory. There are 11 items that are required on test reports and calibration certificates (unless there are valid reasons for their exclusion) plus five additional items that cover aspects of interpretation and six items when sampling has also been carried out as part of the test. Increasingly test reports are submitted electronically. This is referred to in the standard (subsection 5.10.7) and is allowed as long as the requirements of the standard are met. Of importance is the integrity of the report and the nature of the signature when the report is sent by e-mail or via the Internet. 

Although PCR and PLS are powerful methods, they are not without their limitations. In particular, implicit calibration methods can be susceptible to chance correlations. Thus, when the calculated b is applied to a future spectrum in which those correlations are not present, increased error is likely. It may be possible to improve implicit calibration and limit spurious correlations by incorporating additional information about the system or analytes. This combination of features from implicit and explicit calibration methods is termed hybrid calibration. 

Multivariate calibration methods are in general not analyte specific. Calibration models are built based on correlations in the data, which may be owing to the analyte or to systematic or spurious effects. One way to effectively boost the model specificity is through incorporation of additional analyte-specific information such as its pure spectrum. Hybrid methods merge additional spectral information with calibration data in an implicit calibration scheme. In the following section, we present two of these methods developed in our laboratory. 

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). 

One of the important aspects of any analytical method is its calibration and, therefore, much effort has been put into SPME calibration. As it is not always practicable to employ traditional calibration methods (external standards, internal standards, and standard addition) owing to the sometimes significant matrix effects in complex samples, equilibrium calibration has been suggested as an alternative. In SPME, however, it would normally take rather a long time to achieve equilibrium calibration. If sensitivity were not a concern in an analysis, reduction of extraction time would be desirable, that is, the extraction could be stopped before equilibrium but this would thus demand a new approach to calibration. In this regard, as a way of circumventing matrix effects in environmental analysis, several diffusion-based calibration methods have been recently developed for quantification in SPME.30... 

Internal standards could be used in external calibration, matrix-matched external calibration, and standard addition calibration [2], However, the use of internal standards in LC-MS quantitative methods should not be confused with internal calibration in which an internal standard is employed as a calibrant and the concentration of a unknown sample is calculated from the concentration of this internal standard and its analyte/IS signal ratio, i.e., the concentration of the unknown sample is calculated without the need for a calibration curve [3], The use of internal standards in most LC-MS quantitative methods belongs to signal-ratio calibration or internal standardization [2,4], In fact, the majority of bioanalytical LC-MS methods use matrix-matched signal-ratio external calibration. 

The principle of this method is that the extra signal produced by the addition of standard is proportional to the original signal. Equations can be used to make the necessary calculations, but the principle is more easily seen graphically. Figure 7.11 shows a typical standard addition calibration plot. Note that a signal is present when no standard is added it represents the original concentration, which is to be determined. As... 

Lelievre et al. [7] developed a rapid method for the direct determination of Cu and Fe in butter by ET-AAS. Butter was dissolved in a butylamine (BTA)D waterDtetrahydrofuran (THF) mixture. The solution (10 xL) was pipetted directly into the graphite tube. The calibrations were compared with the matrix-matched standard and standard addition calibration. 

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