Standard-additions method, calibration effects

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. 

Yamamoto et al. [6] studied preservation of arsenic- and antimony-bearing samples of seawater. One-half of the sample (201) was acidified to pH 1 with hydrochloric acid immediately after sampling, and the remaining half was kept without acidification. In order to clarify the effect of acidification on storage, measurements were made over a period of a month after sampling. Results are given in Table 1.1. In this study, a standard addition method and calibration curve method were used for comparison and it was proven that the two gave the same results for the analyses of seawater. 

A particular issue that must be considered for all calibration procedures is the possibility of matrix effects on the analyte signal. If such effects are present they may be allowed for in many cases by matrix matching of the standard to the sample. This of course requires an accurate knowledge of the sample matrix. Where this is not available, the method of standard addition is often effective. This involves spiking at least three equal aliquots of the sample with different amounts of the analyte, and then measuring the response for both spiked and unspiked aliquots. A plot of response vs analyte, extrapolated back, will give abscissae intercepts from which the amount of analyte in the sample may be deduced (Figure 2.8). 

The standard addition method of calibration (see Chapter 1) is often used to combat the uncertainties of varying interference effects in electrothermal atomization. However, care should be taken with this approach, as errors from spurious blanks and background may go undetected. It must also be emphasized that the technique of standard additions does not correct for all types of interference. 

Copper, Chromium, Manganese, and Nickel. The analytical method for determining copper, chromium, manganese, and nickel involves digesting the coal with nitric and perchloric acids, fusing the residue with lithium metaborate, and determining the combined digestion and leach solutions by atomic absorption spectrophotometry. Since there is no standard material to analyze for the construction of calibration curves, the standard additions method is used for the assay. While this method increases the time required for analysis, it helps to eliminate the effect of the matrix. 

Physical and chemical effects can be combined for identification as sample matrix effects. Matrix effects alter the slope of calibration curves, while spectral interferences cause parallel shifts in the calibration curve. The water-methanol data set contains matrix effects stemming from chemical interferences. As already noted in Section 5.2, using the univariate calibration defined in Equation 5.4 requires an interference-free wavelength. Going to multivariate models can correct for spectral interferences and some matrix effects. The standard addition method described in Section 5.7 can be used in some cases to correct for matrix effects. Severe matrix effects can cause nonlinear responses requiring a nonlinear modeling method. 

The most important advantage of this calibration approach is that each calibration solution contains the analyte in the environment of all sample components, including potential interferents. Thus, if the calibration dependence is distorted as a result of interferents, there is still a chance to reconstruct it accurately by means of the calibration graph. From this point of view, the standard addition method is one way to eliminate (or rather compensate for) the interference effect. Furthermore, it gives a chance to compensate for the effects of all interferents, independently of their kind, number, and concentration in the sample. This feature supports application of the standard addition method to trace analysis. 

Because of the problems described above, the standard addition method can only be recommended in trace analysis with serious reservation. However, the possibility of compensating for the interference effect, even when unexpected or caused by unknown sample components, is so great an advantage that this calibration approach deserves greater interest in analytical practice than it is given at present. 

In order to improve the procedure described above it was adapted to flow mode with the use of an original flow-injection manifold [11], Once a sample and a single standard solution are introduced to the system they can be gradually diluted, and interpolative and extrapolative estimations of analyte concentration in the sample can be obtained for every dilution degree. This procedure allows examination of possible interferences, and is, in effect, a sterling, integrated calibration method that combines in one calibration procedure the set of standards method and the standard addition method with the dilution method. Moreover, it has been demonstrated that any of the calibration methods described above can be integrated with the use of a constructed manifold. 

There is one more, very important and relatively simple method to use when an interference effect is difficult to explore but its occurrence is probable and poses a threat to the reliability of analytical results. This refers to the case when an analyzed series of samples have similar chemical composition (at least in terms of the composition of interferents) and the determined component is present in all samples in a similar quantity. In this situation, the standard addition method can be used for analysis of one selected sample and the constructed calibration graph employed for interpolative determination of analyte in the remaining samples. This combined procedure is depicted in Fig. 3.16. Thus obtained results are, as a mle, more accurate than those obtained after application of the set of standards method to all the samples. In addition, the analyses are conducted faster than when all the samples are analyzed using the standard addition method. 

The instrument is calibrated for a given element for each series of samples. Direct calibration requires detailed knowledge of the milieu to be analysed. Precise results depend on the composition of the calibration solutions being as close as possible to that of the solutions to be analysed. The standard addition method can be u.sed in cases where it is not possible to produce external reference solutions similar to the solutions to be analysed. This method should be used with considerable caution because it assumes that the absorption is due solely to the element under analysis and, in particular, that the non specific absorption is fully corrected for. If this is not the case, any interfering absorption leads to an overestimate of the values observed. On the other hand, this method does have the advantage of eliminating the matrix effect. 

The active ingredients are commonly divided into chemical classes, such as pyrethroids, chloroorganic, and organophosporus. Coextracted substances may affect the analyte signal. To avoid this matrix effect, standard solutions should be prepared by using an extract from a non-contaminated sample, or a calibration curve calculated by the standard addition method. 

When the analytical signal is affected by the sample matrix, serious errors in the analytical results can arise due to improper calibration, especially for sample lots with highly variable matrix composition. The standard addition method is a powerful tool for compensating matrix effects, but its implementation in flow analysis usually involves a complex manifold and can impair the sampling rate (see 8.6.3). Matrix matching, i.e. matching the bulk compositions of the standard and sample solutions, is another way to achieve accurate results, but requires information about the sample matrix, which is not always available. To address this issue, expert systems have been exploited. 

Analytical Chemistry" of the "Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area" of the Deutsche Forschungsgemenschaft (Henschler), Since GF-AAS analysis of diluted blood is strongly influenced by matrix effects it is recommended to use the internal standard addition method for calibration. For routine analyses, however, calibration may also be performed by the use of lead-spiked blood. The method subsequently described is based on the internal standard addition method ... 

Matrix effects can be important during the analysis of petroleum products. They generally disturb intensities emitted by a detected isotope [10]. Organic matrix may modify element ionization in the plasma and consequently cause a variation of sensitivity. For these reasons, a standard addition method was performed for the calibration procedure to control the matrix effects. Concentrations of metals in analyzed organic samples must be compatible with ICP-MS potential in order to obtain reliable results. Experience shows that concentrations detected by the mass spectrometer ideally should be in the range of 1-100 ng/g. Thus, for optimal detection conditions, samples were diluted in xylene according to their pre-estimated element concentration range. 

The main drawback is flic existence of matrix effects and the possible non- ideahty of the solvents mixture. These imply that ideally one should determine the cahbration curve by adding standard solutions of the solvents of interest to the sample matrix, fiee of solvents. Because it is very difficult to obtain such a sample matrix, the classical standard addition method is recommended. It consists of adding to the sample matrix to be analyzed a known amount of the solvents to be determined. This method requires two analyses for the final calculation but the main advantage is that the matrix effect is overcome. The linearity of the response has, of course, to be demonstrated before the use of the simplified version mentioned above. Nevertheless, if based on a sound validation, external calibration can be used. " ... 

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 standard addition method [35] represents a combination of calibration with the aid of both external and internal standards. In ion chromatography, it is used predominantly for the analysis of samples with difficult matrices. Matrix problems may lead to an increase in nonprecision and/or express themselves as constant or proportional systematic deviations of the analytical results. Matrix influence can be identified via calculation of the recovery function. In constant systematic deviation, the error is independent of the analyte component. Such a deviation will cause a parallel shift of the calibration line. A possible origin for this deviation might be a codetection of a matrix component. In proportional systematic deviations, the error depends on the concentration of the analyte component. This type of deviation results in a change of the slope of the calibration line. Deviations of this kind can be caused by individual sample preparation steps such as sample digestion and sample extraction, and also by matrix effects. Systematic deviations can be identified by standard addition and/or calculation of the recovery function. 

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. ... 

---