Standard addition Quantitation

A fifth spectrophotometric method for the quantitative determination of the concentration of Pb + in blood uses a multiple-point standard addition based on equation 5.6. The original blood sample has a volume of 1.00 mb, and the standard used for spiking the sample has a concentration of 1560 ppb Pb +. All samples were diluted to 5.00 mb before measuring the signal. A calibration curve of Sjpike versus Vj is described by... 

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. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Quantitative Determination of Cr(lll) and Co(ll) Using a Spectroscopic H-Point Standard Addition, /. Chem. Educ. 1997, 74, 848-850. 

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. 

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,... 

Figure 5.59 Molecular structures of the diarrhetic shellfish poisons (a) pectenotoxin-6 (PTX6) (b) okadaic acid (OA) (c) dinophysistoxin-1 (DTXl) (d) yessotoxin (YTX). 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.

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... 

The method of standard additions is the least widely used method of quantitation in chromatography because it requires several chromatographic separations to yield reliable results. A portion of the unScnown sample is analyzed to estimate the... 

Internal rather than external standards are recommended for quantitative analysis. If standard additions are used for calibration all the parameters listed in this table must be held constant. 

Quantitation Poor reproducibility Comparison to spiked blank matrix or standard addition... 

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. 

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

Fig. 17 Application of the method of standard additions to the quantitation of free phenol in sodium phenolate. Thermograms are shown for (a) the initial sample, (b) the sample with 1 mg free phenol added, (c) the sample with 3 mg free phenol added, and (d) the sample with 5 mg free phenol added.

Either calibration graphs prepared from standards or the method of standard addition (p. 30) can be used. For the former, the standards should be as similar as possible in overall chemical composition to that of the samples so as to minimize errors caused by the reduction of other species or by variation in diffusion rates. Often, the limiting factor for quantitative work is the level of impurities present in the reagents used. 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

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 ... 

Obstacles of ionisation interferences in the quantitative determination of the N-containing surfactants from industrial blends dissolved together with AE compounds in methanol applying FIA-MS can be minimised or even eliminated if quantification was performed in the standard addition mode. So the standard deviations (SD) observed now reached a maximum of 7% for N-containing compounds whereas AE could be quantified with a SD of 4%. In parallel, the time investment for FIA-MS quantification in the standard addition mode, however, increased considerably and reached a factor of 3-4. 

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