Analytical Chem istry - DePauw University

604 Analytical Chemistry 2.0Because atomic absorption lines are narrow,we need to use a line source instead ofa continuum source (compare, for example,Figure 10.18 with Figure 10.20). Theeffective bandwidth when using a continuumsource is roughly 1000 larger thanan atomic absorption line; thus, P T ≈ P 0 ,%T ≈ 100, and A ≈ 0. Because a hollowcathode lamp is a line source, P T and P 0have different values giving a %T < 100and A > 0.Table 10.10 Concentration of Analyte Yielding an Absorbance of 0.20Concentration (mg/L) aelement flame atomization electrothermal atomizationAg 1.5 0.0035Al 40 0.015As 40 b 0.050Ca 0.8 0.003Cd 0.6 0.001Co 2.5 0.021Cr 2.5 0.0075Cu 1.5 0.012Fe 2.5 0.006Hg 70 b 0.52Mg 0.15 0.00075Mn 1 0.003Na 0.3 0.00023Ni 2 0.024Pb 5 0.080Pt 70 0.29Sn 50 b 0.023Zn 0.3 0.00071a Source: Varian Cookbook, SpectraAA Software Version 4.00 Pro.b As: 10 mg/L by hydride vaporization; Hg: 11.5 mg/L by cold-vapor; and Sn:18 mg/L by hydride vaporizationof its greater sensitivity, it takes less analyte to achieve a given absorbancewhen using electrothermal atomization. Table 10.10, which compares theamount of analyte needed to achieve an absorbance of 0.20 when usingflame atomization and electrothermal atomization, is useful when selectingan atomization method. For example, flame atomization is the method ofchoice if our samples contain 1–10 mg Zn 2+ /L, but electrothermal atomizationis the best choice for samples containing 1–10 mg Zn 2+ /L.Selecting the Wavelength and Slit Width. The source for atomic absorptionis a hollow cathode lamp consisting of a cathode and anode enclosedwithin a glass tube filled with a low pressure of Ne or Ar (Figure 10.46). Applyinga potential across the electrodes ionizes the filler gas. The positivelycharged gas ions collide with the negatively charged cathode, sputteringatoms from the cathode’s surface. Some of the sputtered atoms are in theexcited state and emit radiation characteristic of the metal(s) from whichthe cathode was manufactured. By fashioning the cathode from the metallicanalyte, a hollow cathode lamp provides emission lines that correspondto the analyte’s absorption spectrum.