Quantitative Analysis Using Standard Addition Method

3 Quantitative Analysis Using Standard Addition Method 

Method for Standard Addition. The sample is divided into four equal aliquots into 100 ml volumetric flasks, all but one is spiked with volumes of standards of increasing concentrations and diluted to the mark with solvent. Under these conditions, all the solutions differ in the analyte concentration but have the same matrix composition ensuring that the influence of the matrix will be the same for total analysis. A similar series of standards at exactly the same concentrations are prepared in the solvent only without the sample matrix and the intensity for each set of samples is measured against their concentrations. The concentration of the analyte in the sample is determined by extrapolating the plot back to the negative x axis where the concentration in the sample can be determined. 

Interferences due to matrix effects can be detected by comparing the slopes of the curves for the spike sample and the pure standard solutions. In the absence of interferences both slopes should be parallel. In effect, the method is equivalent to preparing the standard calibration curve with exact matrix matching. To apply this 

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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]. 

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. 

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 external calibration, internal standard calibration, and the method of standard additions are all possible with SPME. Calibration is discussed in Section 1.5.2 and at greater length in Chapter 2. 

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. 

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. 

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. 

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. 

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,... 

It is crucial in quantitative GC to obtain a good separation of the components of interest. Although this is not critical when a mass spectrometer is used as the detector (because ions for identification can be mass selected), it is nevertheless good practice. If the GC effluent is split between the mass spectrometer and FID detector, either detector can be used for quantitation. Because the response for any individual compound will differ, it is necessary to obtain relative response factors for those compounds for which quantitation is needed. Care should be taken to prevent contamination of the sample with the reference standards. This is a major source of error in trace quantitative analysis. To prevent such contamination, a method blank should be run, following all steps in the method of preparation of a sample except the addition of the sample. To ensure that there is no contamination or carryover in the GC column or the ion source, the method blank should be run prior to each sample. 

Table 5.17 Quantitative results obtained for the determination of four diarrhetic shellfish poisons (DSPs) using external standards and the method of standard additions. Reprinted from J. Chromatogr., A, 943, Matrix effect and correction by standard addition in quantitative liquid chromatographic-mass spectrometric analysis of diarrhetic shellfish poisoning toxins , Ito, S. and Tsukada, K., 39-46, Copyright (2002), with permission from Elsevier Science...

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