Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods627tion. For many elements at temperatures of less than 5000 K the Boltzmanndistribution is approximated asN* = N ⎛ g ⎝⎜g⎞e⎠⎟i −Ei/ kT010.31where g i and g 0 are statistical factors that account for the number of equivalentenergy levels for the excited state and the ground state, E i is the energyof the excited state relative to a ground state energy, E 0 , of 0, k is Boltzmann’sconstant (1.3807 10 –23 J/K), and T is the temperature in kelvin.From equation 10.31 we expect that excited states with lower energies havelarger populations and more intense emission lines. We also expect emissionintensity to increase with temperature.10G.2 EquipmentAn atomic emission spectrometer is similar in design to the instrumentationfor atomic absorption. In fact, it is easy to adapt most flame atomicabsorption spectrometers for atomic emission by turning off the hollowcathode lamp and monitoring the difference in the emission intensity whenaspirating the sample and when aspirating a blank. Many atomic emissionspectrometers, however, are dedicated instruments designed to take advantageof features unique to atomic emission, including the use of plasmas,arcs, sparks, and lasers as atomization and excitation sources, and an enhancedcapability for multielemental analysis.At o m i z at i o n a n d Exc i t a t i o nAtomic emission requires a means for converting a solid, liquid, or solutionanalyte into a free gaseous atom. The same source of thermal energy usuallyserves as the excitation source. The most common methods are flamesand plasmas, both of which are useful for liquid or solution samples. Solidsamples may be analyzed by dissolving in a solvent and using a flame orplasma atomizer.Fl a m e So u r c e sAtomization and excitation in flame atomic emission is accomplished usingthe same nebulization and spray chamber assembly used in atomic absorption(Figure 10.42). The burner head consists of single or multiple slots, ora Meker style burner. Older atomic emission instruments often used a totalconsumption burner in which the sample is drawn through a capillary tubeand injected directly into the flame.Pl a s m a So u r c e sA plasma is a hot, partially ionized gas that contains an abundant concentrationof cations and electrons. The plasmas used in atomic emission areformed by ionizing a flowing stream of argon gas, producing argon ions and