Analytical Chem istry - DePauw University

626 Analytical Chemistry 2.0Se l e c t i v i t yThe selectivity of fluorescence and phosphorescence is superior to that ofabsorption spectrophotometry for two reasons: first, not every compoundthat absorbs radiation is fluorescent or phosphorescent; and, second, selectivitybetween an analyte and an interferent is possible if there is a differencein either their excitation or their emission spectra. The total emission intensityis a linear sum of that from each fluorescent or phosphorescent species.The analysis of a sample containing n components, therefore, can beaccomplished by measuring the total emission intensity at n wavelengths.Tim e , Co s t, a n d Eq u i pm e n tAs with other optical spectroscopic methods, fluorescent and phosphorescentmethods provide a rapid means for analyzing samples and are capableof automation. Fluorimeters are relatively inexpensive, ranging from severalhundred to several thousand dollars, and often are satisfactory for quantitativework. Spectrofluorimeters are more expensive, with models oftenexceeding $50,000.Energy3s5s4s1138.35p4p330.3330.21140.4818.3819.5589.0 3pFigure 10.57 Valence shell energy leveldiagram for sodium. The wavelengthscorresponding to several transitions areshown. Note that this is the same energylevel diagram as Figure 10.19.4d3d10GAtomic Emission SpectroscopyThe focus of this section is on the emission of ultraviolet and visible radiationfollowing the thermal excitation of atoms. Atomic emission spectroscopyhas a long history. Qualitative applications based on the color offlames were used in the smelting of ores as early as 1550 and were more fullydeveloped around 1830 with the observation of atomic spectra generatedby flame emission and spark emission. 18 Quantitative applications basedon the atomic emission from electric sparks were developed by Lockyer inthe early 1870 and quantitative applications based on flame emission werepioneered by Lundegardh in 1930. Atomic emission based on emissionfrom a plasma was introduced in 1964.10G.1 Atomic Emission SpectraAtomic emission occurs when a valence electron in a higher energy atomicorbital returns to a lower energy atomic orbital. Figure 10.57 shows a portionof the energy level diagram for sodium, which consists of a series ofdiscrete lines at wavelengths corresponding to the difference in energy betweentwo atomic orbitals.The intensity of an atomic emission line, I e , is proportional to the numberof atoms, N*, populating the excited state,I = kN *10.30ewhere k is a constant accounting for the efficiency of the transition. If asystem of atoms is in thermal equilibrium, the population of excited state iis related to the total concentration of atoms, N, by the Boltzmann distribu-18 Dawson, J. B. J. Anal. At. Spectrosc. 1991, 6, 93–98.