Atomic emission is used for the analysis of the same types of samples that may be analyzed by atomic absorption. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomiza- tion and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization.
Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these rea- sons a plasma emission source is usually the better choice.
The choice of wavelength is dictated by the need for sensitivity and freedom from interference due to unresolved emis- sion lines from other constituents in the sample. Be- cause an atomic emission spectrum usually has an abun- dance of emission lines, particularly when using a high- temperature plasma source, it is inevitable that some overlap will occur between emission lines. For example, an analysis for Ni using the atomic emission line at 349.30 nm is complicated by the atomic emission line for Fe at 349.06 nm. Narrower slit widths provide for better resolution. The easiest approach to selecting a wavelength is to obtain an emission spectrum for the sample and then to look for an emission line for the an- alyte that provides an intense signal and is resolved from other emission lines.
Flame and plasma sources are best suited for the analysis of samples in solution and liquid form. Although solids can be analyzed by direct insertion into the flame or plasma, they usually are first brought into solution by digestion or extraction.
The most impor- tant spectral interference is a continuous source of background emission from the flame or plasma and emission bands from molecular species. This back- ground emission is particularly severe for flames in which the temperature is insufficient to break down re- fractory compounds, such as oxides and hydroxides.
Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 10.51). Because the temperature of a plasma is much higher, background interferences due to molecular emission are less prob- lematic. Emission from the plasma’s core is strong but is insignificant at a height of 10–30 mm above the core, where measurements normally are made.
Flame emission is subject to the same types of chemical interferences as atomic absorption. These interferences are minimized by adjusting the flame composition and adding protecting agents, releasing agents, and ionization suppressors. An additional chemical inter- ference results from self-absorption. Since the temperature of a flame is greatest at its center, the concentration of analyte atoms in an excited state is greater at the center than at the outer edges. If an excited state atom in the center of the flame emits radiation while returning to its ground state, then ground state atoms in the cooler, outer regions of the flame may absorb the radiation, thereby decreasing emission intensity. At high analyte concentra- tions a self-reversal may be seen in which the center of the emission band de- creases (Figure 10.52).
Chemical interferences with plasma sources generally are insignificant. The higher temperature of the plasma limits the formation of nonvolatile species. For example, the presence of PO43– in solutions being analyzed for Ca2+, which is a significant interferant for flame emission, has a negligi- ble effect when using a plasma source. In addition, the high concentration of electrons from the ionization of argon minimizes the effects of ionization interferences.
Equation 10.34 shows that emission intensity is proportional to the population of the excited state, N*, from which the emis- sion line originates. If the emission source is in thermal equilibrium, then the excited state population is proportional to the total population of analyte atoms, N, through the Boltzmann distribution (equation 10.35).
Calibration curves for flame emission are generally linear over two to three orders of magnitude, with chemical interferences due to ionization limiting linearity for lower concentrations of analyte, and self-absorption limiting linear- ity for higher concentrations of analyte. Plasma sources, which suffer from fewer chemical interferences, often yield calibration curves that are linear over four to five orders of magnitude and that are not affected significantly by changes in the matrix of the standards.
When possible, quantitative analyses are best conducted using external stan- dards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomiza- tion. 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 con- trol. 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 excita- tion 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 emis- sion lines must be monitored simultaneously. The method of standard additions also can be used.
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