Quantitative analysis standard addition method

Quantitative Analysis Using the Method of Standard Additions Because of the difficulty of maintaining a constant matrix for samples and standards, many quantitative potentiometric methods use the method of standard additions. A sample of volume, Vx) and analyte concentration, Cx, is transferred to a sample cell, and the potential, (ficell)x) measured. A standard addition is made by adding a small volume, Vs) of a standard containing a known concentration of analyte, Cs, to the sample, and the potential, (ficell)s) measured. Provided that Vs is significantly smaller than Vx, the change in sample matrix is ignored, and the analyte s activity coefficient remains constant. Example 11.7 shows how a one-point standard addition can be used to determine the concentration of an analyte. 

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... 

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]. 

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. 

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. 

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. 

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). 

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. 

Overspotting can be performed (also with the Linomat), in which more than one sample can be applied to a single initial zone position. These samples can include multiple standard reference compounds from different vials to prepare an in situ mixture, sample plus spiking solution, for validation of quantitative analysis by the standard addition method, or sample plus reagent, for in situ prechroma-tographic derivatization. 

Four techniques are commonly used in quantitative analysis the normalization method, the external standard method, the internal standard method, and the standard addition method. Whatever method is used, the accuracy often depends on the sample preparation and on the injection technique. Nowadays these are two main sources of error in quantitative analysis. The quantitative results produced by PCGC and CGC are comparable. 

Quantitative analysis in ICP-MS is typically achieved by several univariate calibration strategies external standardisation, the standard additions method, or internal standardisation (see Chapter 2). Nevertheless multivariate calibration has also been applied, as will be presented in Chapters 4 to 6. 

Typical sample preparation steps include homogenization, extraction (liquid—liquid extraction, LLE, or instrumental based techniques), cleanup (usually by solid-phase extraction, SPE), and concentration of extracts. Sometimes, derivatization has to be incorporated into sample preparation (e.g., release of bormd residues or deconjugation). For quantitative analysis, the preparation of adequate calibration standards also may be a key aspect in some cases, matrix-matched standards or the standard additions method may be necessary, as well as the use of suitable internal standards (e.g., isotopically labeled compormds) [19]. Matrix-matched calibration is now preferred, as it is the best compromise in terms of speed and cost of analysis, taking into consideration the features of the MS analyzers. 

The analytical power of combining capillary PyGC with the selectivity of FID and NPD has been demonstrated for rapid quantitative and qualitative analysis of high-MW and polymer stabilisers in PP, using the standard addition method (up to 10,000 ppm) [42]. Quantitative aspects of PyGC are discussed in Chp. 2.2.1. 

Future development in GC-MS analysis will require the use of more stable GC capillary columns or multidimensional GG coupled to advanced MS techniques such as HRMS and MS-MS with El and EGNI ionization. The availability of further individual congeners for specific analysis and isotopically labeled standards will assure an accurate quantitative analysis. In addition, the development of more selective analytical methods for the isolation of these compounds from environmental and biological matrices can help to enhance the selectivity of the LRMS techniques. Further research and collaborative studies still need to be done to solve the problems associated with the analysis of PGTs and toxaphene. 

In principle, it is not necessary to calculate the partition coefficients, when quantitative analysis is performed with the standard addition method or with iso-topically labeled standards. On the other hand, calibration of the sampling device is not necessary when the analyte s Kat various temperatures is known [37,42]. In any case, it is always advisable to determine partition coefficients, since knowledge of them may help to better understand all the phenomena occurring at the various interfaces and to predict the effects of a modification of the system. 

Since the half-wave potential is characteristic of the particular reaction that is occurring at that potential, it is possible to identify the species involved. A simple case is shown in Figure 3 where a mixture of metal ions was analyzed. The two reduction waves for copper occur at -0.1 and -0.35 V, cadmium at -0.69, nickel at -1.10 and zinc at -1.35 V. This illustrates an analysis that may identify the species qualitatively and, by using a standard addition method, can also determine the ions quantitatively. 

The application of internal standards or the use of the standard addition method for quantitation is strongly recommended for achieving low relative standard deviations (Nielsson etal., 1994 Poerschmann etal., 1997). After the equilibrium is established, the fibre with the collected analytes is withdrawn from the sample and transferred into a GC injector, either manually or more convenient via an autosampler. The analyte is desorbed thermally in the hot injector from the coating. The fibre material is used for a large number of samples in automated serial analysis. Modern autosampler are capable to exchange the fibre holder to provide automated access to different fibre characteristics according to the analyte requirements. 

The static headspace method is therefore an indirect analysis procedure, requiring special care in performing quantitative determinations. The position of the equilibrium depends on the analysis parameters (e.g. temperature) and also on the sample matrix itself. The matrix dependence of the procedure can be counteracted in various ways. The matrix can be standardized, for example, by addition of Na2S04 or NajCOj. Other possibilities include the standard addition method, internal standardization or the multiple headspace extraction procedure (MHE) as published by (Kolb and Ettre, 1991 Zhu et al., 2005) (Figure 2.11). 

Carefully measured 2.000-g portions of the powdered yeast are placed into 20-mL headspace vials and diluted with either 10 mL of deionized water or 10 mL of deionized water containing 2 pg of indole. The vials are tightly capped using Teflon-faced septa and then vortexed or otherwise mixed well to suspend all of the dry material. They should then be placed in a thermostatted oven or waterbath at about 40°C before analysis for indole by headspace SPME GC/MS using the ion at m/z 117 for quantitation. At least two sets of vials are prepared for each sample—the first is used to determine the peak area of the indole in the native material and the second to measure the sum of the peak areas of the native indole plus the amount of indole added to the sample as a standard. The difference between the two sets of data can be used to calculate a response factor for the indole standard, and Anally the amount of native indole can be calculated. Indole in dry yeast can be measured accurately between 50 ppb and 10 ppm using headspace standard addition methods with a 100- J,m PDMS fiber. 

Standardization—External standards, standard additions, and internal standards are a common feature of many quantitative analyses. Suggested experiments using these standardization methods are found in later chapters. A good project experiment for introducing external standardization, standard additions, and the importance of the sample s matrix is to explore the effect of pH on the quantitative analysis of an acid-base indicator. Using bromothymol blue as an example, external standards can be prepared in a pH 9 buffer and used to analyze samples buffered to different pHs in the range of 6-10. Results can be compared with those obtained using a standard addition. 

Quantitative Analysis for a Single Analyte The concentration of a single analyte is determined by measuring the absorbance of the sample and applying Beer s law (equation 10.5) using any of the standardization methods described in Chapter 5. The most common methods are the normal calibration curve and the method of standard additions. Single-point standardizations also can be used, provided that the validity of Beer s law has been demonstrated. 

When possible, a quantitative analysis is best conducted using external standards. Unfortunately, matrix interferences are a frequent problem, particularly when using electrothermal atomization. Eor this reason the method of standard additions is often used. One limitation to this method of standardization, however, is the requirement that there be a linear relationship between absorbance and concentration. 

The technique of hydrodynamic modulation voltammetry (HMV), in which the rate of stirring is pulsed between high and low values, is demonstrated in this experiment. The application of HMV for the quantitative analysis of ascorbic acid in vitamin C tablets using the method of standard additions also is outlined. 

Bohman and colleagues described a reverse-phase HPLC method for the quantitative analysis of vitamin A in food using the method of standard additions. In a typical example, a 10.067-g sample of cereal is placed in a 250-mL Erlenmeyer flask along with 1 g of sodium ascorbate,... 

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