The Standard Additions Method SAM

Standards used to constmct a cahbration curve must be prepared such that the matrix of the standard is identical to the sample s matrix because the values of the parameters k and b associated with a linear cahbration curve are matrix dependent. Many areas of chemical analysis are plagued by matrix effects, and it is often difficult to duphcate the sample matrix when preparing external standards. Because it is desirable to eliminate matrix effects, cahbration in the sample matrix itself can be performed. This approach is called the standard addition method (SAM) (14). In this method, the standards are added to the sample matrix and the response of the analyte plus the standard is monitored as a function of the added amount of the standard. The initial response is assumed to be Rq, and the relationship between the response and the concentration of the analyte is... 

In addition to statistical peculiarities, special features may also result from certain properties of samples and standards which make it necessary to apply special calibration techniques. In cases when matrix effects appear and matrix-matched calibration standards are not available, the standard addition method (SAM, see Sect. 6.2.6) can be used. 

When the analytical procedure is susceptible to matrix effects, the standard addition method, SAM, can be applied to improve accuracy. SAM involves several additions of known amounts of the analyte (AAdd — Fig. 8.25) to the sample and measurement of the analytical signal (M — Fig. 8.25) after every addition. A specific "analytical curve" (M vs AAdd) is obtained for each assayed sample and the analytical result (C) is obtained by extrapolation. 

Perhaps the greatest problem in trace analysis is assurance of the accuracy of the results (i.e., the avoidance of systematic errors). Systematic sources of error are possible in every step of an analytical process. The most reliable method for detecting systematic errors is continuous and comprehensive quality assurance, particularly by occasional analysis of (certified) standard reference materials. Strictly speaking, an analytical method cannot be calibrated if suitable (i.e., representative) standard reference materials adequately representing the matrix of the expected test samples are not available. However, internal laboratoiy reference materials can then usually be prepared, whose matrix largely resembles the matrix of the test portions expected. If problems occur in the preparation of such reference samples, the standard addition method (SAM) can be applied, in which internal laboratory standards are added stepwise to the test sample (analyte and matrix)... 

This same univariate calibration can be used with the standard addition method (SAM) wherein known amounts of pure compound are added to samples. This allows matrix interferences to be accounted for in generating the calibration. 

The standard additions method (SAM) is a calibration technique devised to overcome the problem of the matrix effects that modify the analytical signal. It enables the analyst to obtain unbiased results when the matrix of the test solution varies unpredictably among test materials in a run. This difficulty renders matrix matching of the calibrators to the test solutions impossible to apply. The sensitivity of the method is affected unpredictably from solution to solution. ... 

Another inference system commonly used is the Standard Additive Model (SAM). The SAM inference approach is based on correlation-product inference. This overcomes the loss of information associated with the conelation-min inference in the Mamdani inference method. Also, the additive-combiner in the SAM makes defuzzification simpler than the max-combiner of Mamdani inference. The additive-combiner also accounts for the information in the overlap of the fired output sets that the max-combiner ignores. 

Analysis of synthetic solutions using a diode array spectrophotometer. This application of SAM involved gradient exploitation. The sample was kept undiluted ("infinite volume situation") and the standard zone was added by stream confluence with two different standard zones, multi-parametric determinations involving the generalised standard addition method were achieved [354],... 

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. 

---