Quantitative standard-additions 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... 

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. 

Quantification is usually achieved by a standard addition method, use of labeled internal standards, and/or external calibration curves. In order to allow for matrix interferences the most reliable method for a correct quantitation of the analytes is the isotope dilution method, which takes into account intrinsic matrix responses, using a deuterated internal standard or carbon-13-labeled internal standard with the same chemistry as the pesticide being analyzed (i.e., d-5 atrazine for atrazine analysis). Quality analytical parameters are usually achieved by participation in interlaboratory exercises and/or the analysis of certified reference materials [21]. 

Select one of the quantitation procedures we have discussed (response factor method, internal standard method, or standard addition method) and describe ... 

In atomic absorption spectroscopy (AAS) the technique using calibration curves and the standard addition method are both equally suitable for the quantitative determinations of elements. 

Quantitive analysis was based on the standard addition method colourant standards were added to each food sample in the range 1 to 5 pg/ml. Values are the mean of nine measurements. bValues are the mean of nine measurements. 

Bosch F, Domenech A, Domenech-Carbo MT, Gimeno JV (2007) H-point standard addition method applied to voltammetry of microparticles. Quantitation of dyes in pictorial samples. Electroanalysis 19 1575-1584. 

In the standard addition method, the sample is analyzed first. A small, known amount of a known compound, thought to be in the sample, is then added to the sample, and the mixture is reanalyzed. If a quantitative increase is obtained in the peak of the compound corresponding to the standard, the compound representing the peak is tentatively identified. The technique of spiking is applicable provided that the sample is known to contain certain components appropriate standards can then be added to enable each peak to be identified. If the identities of the components in the sample are totally unknown, the spiking technique is inappropriate, and additional analytical or comparative techniques will be required. 

Booksh, K., Henshaw, J.M., Burgess, L.W., and Kowalski, B.R., A second-order standard addition method with application to calibration of a kinetics-spectroscopic sensor for quantitation of trichloroethylene, J. Chemom., 9, 263-282, 1995. 

Matisova and co-workers11 have suggested that the need for a reproducible sample volume can be eliminated by combining the standard addition method with an in situ internal standard method. In the quantitative analysis of hydrocarbons in petroleum, they chose ethyl benzene as the standard for addition, but they used an unknown neighboring peak as an internal standard to which they referenced their data. This procedure eliminated the dependency on sample size and provided better quantitation than the area normalization method they were using. 

Figure 13. Quantitation of fluoranthene in SRC using standard addition method by monitoring the fragmentation process 202 - 187 while sample was desorbed from the solids probe. Inset is a typical SRC desorption profile (containing 9 fig...

The direct determination of trace elements (Al, Ba, Cu, I, Mn, Mo, Pb, Rb, Se, Sr, and Zn) by ICP-MS in powdered milk was reported [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. Mass intensity losses were not observed. The quantitative ICP-MS procedure was applied the standard additions method with a Y internal reference. This direct technique is as fast as the slurry approach without particle size effects or sensitivity losses. 

Hence, the standard additions method is unique in that it actually employs the very material under analysis as a reference matrix material, thus providing for efficient elimination of very complex matrix effects even when the final material is the result of a multi-step preparative procedure and the composition of the matrix of the original material is completely unknown. These advantageous features of the standard additions technique have been discussed and verified in context with quantitative headspace gas analysis [68]. 

While the selection of an isotopically-labelled or analogue internal standard is relatively easy in quantitative bioanalysis, the situation is more complicated in multiresidue analysis. It is difficult to select appropriate analogue standards for a wide variety of target compounds, while isotopically-labelled standards are often not available for all target compounds. In addition, if one would introduce one standard per target compound, this would seriously limit the sensitivity of the method as it doubles the number of SRM transitions that have to be monitored. Another problem in multiresidue analysis is the selection of appropriate blanks for the production of the matrix-matched standards and the number of matrices that might have to be studied. When no adequate blank matrix is available, the standard addition method is the only way to achieve sufficiently accurate and precise results [123]. This method is time-consuming and labourious. 

An important feature of modern high-performance liquid chromatography (HPLC) is its excellent quantitation capability. HPLC can be used to quantify the major components in a purified sample, the components of a reaction mixture, and trace impurities in a complex sample matrix. The quantitation is based on the detector response with respect to the concentration or mass of the analyte. In order to perform the quantitation, a standard is usually needed to calibrate the instrument. The calibration techniques include an external standard method, an internal standard method, and a standard addition method. For cases in which a standard is not available, a method using normalized peak area can be used to estimate the relative amounts of small impurities in a purified sample. 

Figure 4.24. Illustration of standard addition method of quantitative analysis. The plot of the 4, hkillb, hki intensity ratio as a function of the known amount (Ta) of added phase a. The point marked corresponds to the original two-phase mixture. The unknown amount of phase a in the original sample is X .

The standard addition method is commonly used in quantitative analysis with ion-sensitive electrodes and in atomic absorption spectroscopy. In TLC this method was used by Klaus 92). Linear calibration with R(m=o)=o must also apply for this method. However, there is no advantage compared with the external standard method even worse there is a loss in precision by error propagation. The attainable precision is not satisfactory and only in the order of 3-5 %, compared to 0.3-0.5 % using the internal standard method 93). 

Method describes the quantitative determination of salicylic acid in aspirin tablets by infra-red spectrophotometry. 200 mg samples of aspirin was dissolved in CHCI3 and centrifuged. The supernatant solution was then analysed for salicylic acid by I.R. spectrophotometry at 1160 cm 1 with the use of a standard-additions method (45). 

Quantitative Analysis Using Standard Addition Method... 

Like all classical quantitative analysis methods, NMR spectroscopy needs calibration, calibration standards and a validation procedure. The standard techniques are used for calibration external calibration, the standard addition method and the internal standard method. A fourth is a special NMR calibration method, the tube-in-tube technique. A small glass tube (capillary) containing a defined amount of standard is put into the normal, larger NMR tube filled with the sample for analysis. In most cases, there are slight differences in the chemical shift of corresponding signals of the same molecule in the inner... 

The standard addition method is used for quantitation. After determination of the phosphates with P NMR (Figure 3-48) a calibration of the F NMR spectrum is not necessary. The amount of monofluorophosphate is known from the P NMR analysis. The fluoride content is determined from the integral areas of the fluoride and the monofluorophosphate signals in the F NMR spectrum. Differences in the response of both nuclei are determined by measurement of standardized solutions. 

Quantitation by the standard addition technique Matrix interferences result from the bulk physical properties of the sample, e.g viscosity, surface tension, and density. As these factors commonly affect nebulisation efficiency, they will lead to a different response of standards and the sample, particularly with flame atomisation. The most common way to overcome such matrix interferences is to employ the method of standard additions. This method in fact creates a calibration curve in the matrix by adding incremental sample amounts of a concentrated standard solution to the sample. As only small volumes of standard solutions are to be added, the additions do not alter the bulk properties of the sample significantly, and the matrix remains essentially the same. Since the technique is based on linear extrapolation, particular care has to be taken to ensure that one operates in the linear range of the calibration curve, otherwise significant errors may result. Also, proper background correction is essential. It should be emphasised that the standard addition method is only able to compensate for proportional systematic errors. Constant systematic errors can neither be uncovered nor corrected with this technique. 

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