Flame emission standard addition

Description of Method. Salt substitutes, which are used in place of table salt for individuals on a low-sodium diet, contain KCI. Depending on the brand, fumaric acid, calcium hydrogen phosphate, or potassium tartrate also may be present. Typically, the concentration of sodium in a salt substitute is about 100 ppm. The concentration of sodium is easily determined by flame atomic emission. Because it is difficult to match the matrix of the standards to that of the sample, the analysis is accomplished by the method of standard additions. 

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. 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

Malmstadt and Chambers (M2) with their nuUpoint instrument and standard addition method obtained high accuracies they state that no interference was seen from an excess of sodium. The latter is difficult to understand since the degree of mutual ionization depression between alkali metals at a particular flame temperature can be predicted (F6), and has been experimentally demonstrated in absorption with flames of comparable or even lower temperature (B2, Wll). On the other hand, the use of the standard addition method may have minimized the sodium interference and, since the mutual enhancement effect in absorption seems to be smaller than in emission (B2), it may have been negligible in the measurements. 

The addition of a spectroscopic buffer such as lithium carbonate has a stabilizing effect on the flame temperature and decreases the ionization of elements with low ionization potentials as well. At a constant flame temperature, the intensity of a given emission line is directly proportional to the concentration of analyte according to the equations given for emission spectrometry in the theoretical section. In practice, a standard curve is usually prepared in order to assess linearity between intensity and concentration. Alternatively, the method of standard addition (24) can be utilized. 

A serum sample is analyzed for potassium by flame emission spectrometry using the method of standard additions. Two 0.500-mL aliquots are added to 5.00-mL portions of water. To one portion is added 10.0 jxL of 0.0500 M KCl solution. The net emission signals in arbitrary Units are 32.1 and 58.6. What is the Concentration of potassium in the serum ... 

Flame emission spectrometry, p. 522 Distribution of atoms in a flame (key equation 17.1), p. 524 Atomic absorption spectrometry—aflame and electrothermal, p. 525 Internal standard and standards addition calibration— using spreadsheets (key equations 17.5, 17.11), pp. 533, 534... 

If the amount of sample is limited, as it may very well be for a patient s semm, a one-point standard addition technique may be used. The technique is outlined for a sodium determination in semm by flame AES. The emission intensity from the sample is measured, and then a known concentration of Na is added, again without significantly... 

FIGURE 9-19. Standard addition method for the determination of the concentration of an unknown. [From W. G. Schrenk, in Flame Emission and Atomic Absorption Spectroscopy, Vol. 2, Edited by J. A. Dean and T. C. Rains, Marcel Dekker, New York (1971), Chapter 12. Used by permission of Marcel Dekker Inc.]... 

Prepare calibration graphs by using standard additions. In effect, each sample is used in preparing its own calibration graph. It is very slow and has various pitfalls it is rarely used in flame emission in the NOA flame. 

For a quantitative measurement, the sample is aspirated into the flame and the intensity of radiation is measured at the emission wavelength of the analyte element to be determined. The concentration of the element in the sample is calculated from comparison with an external calibration curve or by the standard addition method. Both of these methods have been discussed in Chapter 2. A typical external calibration cnrve for lithium is shown in Figure 7.4. The calibration curve was constructed by aspirating Li standard solntions of 5 and 10 ppm Li and the blank solution (0.0 ppm Li). Each intensity was measured and a plot of intensity versns concentration made. Modem systems nse a statistical cnrve-fitting program to construct the calibration curve and to calculate the concentrations of unknowns. Note that an emission signal was detected in the standard that contained no... 

The method of standard addition is widely used, particularly when the composition of the sample matrix is variable or unknown so that the response of a reagent/matrix blank would be unreliable. At least three and preferably more spiked samples should be prepared, but if the amount of sample is limited, as few as one sample must suffice. It is especially useful with such analytical techniques as flame and plasma emission spectrometry and potentiometry (Topics E4, E5 and C8). 

The calcium level in a clinical sample was determined by flame emission spectrometry using a standard addition method, which gave the following data ... 

Fig. 4. Standard addition caiibration graph, regression line, slope and intercept values for the flame emission determination of calcium in a clinical sample.

All of these methods require that the sample be dissolved, which necessarily causes some dilution and a consequent loss of sensitivity. In addition, matrix effects can cause serious error unless standards similar in composition to the sample can be prepared. The use of organic solvents, rather than water, often improves the limits of detection, occasionally by an order of magnitude or more flame emission and absorption measurements are often especially useful in the determination of impurities following solvent extraction. 

A special class of non-reactive additives in polymer materials are CPs, which have plasticizing and flame-retarding properties. They were found in an emission test of a television set with a maximum concentration of 2.2 tgm-3 after 220 h of operation (Wensing, 2003). CP are used as a substitute for PCBs, which have been prohibited worldwide since 2001. The detection of CP in indoor air needs a complex analytical procedure because the amount of single compounds in a CP mixture is high. Therefore, this class is seldom found during standard TD-GC-MS analysis. 

The reference materials for the calibration of the spectrophotometer for flame atomic emission analysis were prepared from the sodium standard solution by its dilution with water and addition of the matrix modifier blank (CsCl 0.1% solution in water), 0.1, 0.2, 0.4, 0.6 and 1.0 ppm. A solution with sodium concentration of 0.5 ppm ( true value) was used as a test solution. 

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