Standard additions, methods

Method. Add known and similar weights of cyanoacrylate sample to four grade B plastic volumetric flasks followed by increasing concentrations of the analyte Nb (0.0, 0.5, 1.0 and 2.0 ppm) and dilute to volume with 50 50 acetic acid and propylene carbonate. Measure the intensity of each concentration and plot against the analyte concentration. The straight line is extrapolated to the negative concentration axis and the point where the calibration line cuts the concentration line is the concentration of analyte in the sample. Similarly, prepare the same concentration of standards without the sample and measure intensity for each concentration. Plot a calibration curve of intensity versus concentration and if this line is parallel to the sample curve then the standard addition method can be used to quantify the level of metal present. The control standard is also prepared in the same manner as for the sample. 

The method of internal standard can also be used as it corrects for variable sample viscosities for both standards and sample. 

It is not possible every time to overcome matrix effects by matching standards with the sample, especially when the full composition of the sample is unknown. For complex samples, where the matrix effects cannot be 

The method of standard additions is an extrapolation method, and usually its accuracy is not as good as that of an interpolation method. 

Microprocessor-assisted spectrometers have the method of standard additions program included. 

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Franke and co-workers evaluated a standard additions method for a voltammetric determination of Tl. A summary of their results is tabulated here. 

Bader, M. A Systematic Approach to Standard Addition Methods in Instrumental Analysis, /. Chem. Educ. 1980, 57, 703-706. 

Nimura, Y. Carr, M. R. Reduction of the Relative Error in the Standard Additions Method, Analyst 1990, 115, 1589-1595. The following paper discusses the importance of weighting experimental data when using linear regression Karolczak, M. To Weight or Not to Weight An Analyst s Dilemma, Curr. Separations 1995, 13, 98-104. 

The generalized standard addition method (GSAM) extends the analysis of mixtures to situations in which matrix effects prevent the determination of 8x and 8y using external standards.When adding a known concentration of analyte to a solution containing an unknown concentration of analyte, the concentrations usually are not additive (see question 9 in Chapter 5). Conservation of mass, however, is always obeyed. Equation 10.11 can be written in terms of moles, n, by using the relationship... 

Raymond, M. Jochum, C. Kowalski, B. R. Optimal Multicomponent Analysis Using the Generalized Standard Addition Method, /. Chem. Educ. 1983, 60, 1072-1073. 

This experiment demonstrates the application of the generalized standard additions method for the analysis of mixtures of K2Cr207 and KMn04. 

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... 

Fig. 13. The standard addition method where MB is the confidence interval for the slope of the line = k, and represents 95% confidence interval (14).

Principle. The content of 1,4-dioxane in ether sulfates is determined by headspace gas chromatography according to the standard additions method. The method is suitable for all ether sulfates and gives reliable results independent of chain length distribution and water content. 

Figure 2.16. Depiction of the standard addition method extrapolation (left), interpolation (right). The data and the numerical results are given in the following example.

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

In Section 8.2.8 we have discussed the standard addition method as a means to quantitate an analyte in the presence of unknown matrix effects cf. Section 13.9). While the matrix effect is corrected for, the presence of other emalytes may still interfere with the analysis. The method can be generalized, however, to the simultaneous analysis of p analytes. Multiple standard additions are applied in order to determine the analytes of interest using many q > p) analytical sensors. It... 

More efficient estimation methods exist than the simple method described here [17]. The generalized standard addition method (GSAM) shares the strong points (e.g correction for interferences) and weak points (e.g. error amplification because of the extrapolation involved) of the simple standard addition method [18]. 

R. Sundberg, Interplay between chemistry and statistics, with special reference to calibration and the generalized standard addition method. Chemom. Intell. Lab. Syst., 4 (1988) 299-305. 

In the standard addition method the sample is analyzed twice, the second time after the addition of a known amount of the substance to be determined. The sample must be re-equilibrated after addition of the standard. 

For solid samples forming homogeneous solutions the model system may be used if pure sample matrix materials are available otherwise, the standard additions method is used. 

Most measurements include the determination of ions in aqueous solution, but electrodes that employ selective membranes also allow the determination of molecules. The sensitivity is high for certain ions. When specificity causes a problem, more precise complexometric or titri-metric measurements must replace direct potentiometry. According to the Nernst equation, the measured potential difference is a measure of the activity (rather than concentration) of certain ions. Since the concentration is related to the activity through an appropriate activity coefficient, calibration of the electrode with known solution(s) should be carried out under conditions of reasonable agreement of ionic strengths. For quantitation, the standard addition method is used. 

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. 

The accuracy of an analytical method is given by the extent by which the value obtained deviates from the true value. One estimation of the accuracy of a method entails analyzing a sample with known concentration and then comparing the results between the measured and the true value. The second approach is to compare test results obtained from the new method to the results obtained from an existing method known to be accurate. Other approaches are based on determinations of the per cent recovery of known analyte spiked into blank matrices or products (i.e., the standard addition method). For samples spiked into blank matrices, it is recommended to prepare the sample at five different concentration levels, ranging over 80-120%, or 75-125%, of the target concentration. These preparations used for accuracy studies usually called synthetic mixtures or laboratory-made preparations . 

On the other hand, for the standard addition method, the spiking concentrations are in the range of 50-150% of the label-claimed value, and are made by spiking of known analyte concentrations in matrices such as serum, plasma, etc. 

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. 

Walters [24] examined the effect of chloride on the use of bromide and iodide solid state membrane electrodes, and he calculated selectivity constants. Multiple linear regression analysis was used to determine the concentrations of bromide, fluorine, and iodide in geothermal brines, and indicated high interferences at high salt concentrations. The standard curve method was preferred to the multiple standard addition method because of ... 

Bond et al. [791 ] studied strategies for trace metal determination in seawater by ASV using a computerised multi-time domain measurement method. A microcomputer-based system allowed the reliability of the determination of trace amounts of metals to be estimated. Peak height, width, and potential were measured as a function of time and concentration to construct the database. Measurements were made with a potentiostat polarographic analyser connected to the microcomputer and a hanging drop mercury electrode. The presence of surfactants, which presented a matrix problem, was detected via time domain dependent results and nonlinearity of the calibration. A decision to pretreat the samples could then be made. In the presence of surfactants, neither a direct calibration mode nor a linear standard addition method yielded precise data. Alternative ways to eliminate the interferences based either on theoretical considerations or destruction of the matrix needed to be considered. 

For the first standard additions method, only one portion of the standard is added to the sample. In this case, the original analyte concentration X is given by the following equation ... 

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