Standard-additions method, calibration disadvantages

Although it is an elegant approach to the common problem of matrix interference effects, the method of standard additions has a number of disadvantages. The principal one is that each test sample requires its own calibration graph, in contrast to conventional calibration experiments, where one graph can provide concentration values for many test samples. The standard-additions method may also use larger quantities of sample than other methods. In statistical terms it is an extrapolation method, and in principle less precise than interpolation techniques. In practice, the loss of precision is not very serious. 

The known addition method has an advantage in greater rapidity and somewhat improved precision compared with calibration with two standards. It is claimed that it requires about half the time necessary for two-standard calibration [17] and that the relative error is about 2% [22]. A disadvantage is the necessity of determining the value for most systems. The method should be used only for concentrations corresponding to the linear part of the calibration curve. To eliminate the effect of a change in the ionic strength, it has been recommended [19] that a mixture of a standard with the test solution be added to the sample. The known subtraction method is much less used in practice. 

The method of standard additions is quite powerful when used properly. First, there must be a good blank measurement so that extraneous species do not contribute to the analytical response. Second, the calibration curve for the analyte must be linear in the sample matrix. The multiple additions method provides a check on this assumption. A significant disadvantage of the multiple additions method is the extra time required for making the additions and measurements. The major benefit... 

A new Chapter 8 consolidates material on sampling and integrates material on calibration and standardization. Methods such as external standards, internal standards, and standard additions are presented in this chapter, and their advantages and disadvantages are discussed. 

Unfortunately, the CLS method has some practical and technical disadvantages and limitations. From a practical viewpoint, it is only applicable for concentration properties, rather than nonconcentration properties (such as viscosity, octane number, etc.). In addition, one must be sure that all of the spectrally active analytes that could be present in a process sample have been identified, in order to build a sufficiently relevant model. Furthermore, if one wants to use estimated pure component spectra as a basis for the CLS method, one must be able to obtain or prepare calibration standards in which the concentrations of all spectrally active analytes in all of the calibration standards are known. This requirement can make the CLS method rather resourceintensive for many PAT applications. 

The internal standard method is based on the use of the relative response factor of each component to be measured with respect to a marker introduced as reference. This avoids the imprecision related to the injected volumes, which is a disadvantage of the previous method. However, it requires the addition of a component to a sample dilution. In general, a calibration curve is built by ap>plying different solutions of increased concentrations of the standard analyte with a constant quantity of internal standard. When injecting such samples, we obtain the relation between the areas of the analyte and the internal standard then, it is marked in a graph according to the concentration of analyte in each solution. By means of interpolation in the graphic, we get the relation of the areas of an unknown sample, which has to contain the same quantity of internal standard. 

A disadvantage of the iterative least-squares method is that it involves inverting a matrix whose size depends on the number of digitized frequencies used. In addition, the number of frequencies used cannot exceed the number of calibration standards used. Thus, in practical cases the number of frequencies that can be employed is limited. 

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