Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods603350–1200 o C. At these temperatures any organic material in the sample isconverted to CO 2 and H 2 O, and volatile inorganic materials are vaporized.These gases are removed by the inert gas flow. In the final stage the sample isatomized by rapidly increasing the temperature to between 2000–3000 o C.The result is a transient absorbance peak whose height or area is proportionalto the absolute amount of analyte injected into the graphite tube.Together, the three stages take approximately 45–90 s, with most of thistime used for drying and ashing the sample.Electrothermal atomization provides a significant improvement in sensitivityby trapping the gaseous analyte in the small volume within thegraphite tube. The analyte’s concentration in the resulting vapor phase maybe as much as 1000 greater than in a flame atomization. 15 This improvementin sensitivity—and the resulting improvement in detection limits—is offset by a significant decrease in precision. Atomization efficiency isstrongly influenced by the sample’s contact with the graphite tube, whichis difficult to control reproducibly.Mi s ce l l a n e o u s At o m i z at i o n Me t h o d sA few elements may be atomized by a chemical reaction that produces avolatile product. Elements such as As, Se, Sb, Bi, Ge, Sn, Te, and Pb, forexample, form volatile hydrides when reacted with NaBH 4 in acid. Aninert gas carries the volatile hydrides to either a flame or to a heated quartzobservation tube situated in the optical path. Mercury is determined by thecold-vapor method in which it is reduced to elemental mercury with SnCl 2 .The volatile Hg is carried by an inert gas to an unheated observation tubesituated in the instrument’s optical path.10D.2 Quantitative ApplicationsAtomic absorption is widely used for the analysis of trace metals in a varietyof sample matrices. Using Zn as an example, atomic absorption methodshave been developed for its determination in samples as diverse as waterand wastewater, air, blood, urine, muscle tissue, hair, milk, breakfast cereals,shampoos, alloys, industrial plating baths, gasoline, oil, sediments, androcks.Developing a quantitative atomic absorption method requires severalconsiderations, including choosing a method of atomization, selecting thewavelength and slit width, preparing the sample for analysis, minimizingspectral and chemical interferences, and selecting a method of standardization.Each of these topics is considered in this section.De v e l o p i n g a Qu a n t i t a t i ve Me t h o dFlame or Electrothermal Atomization? The most important factor inchoosing a method of atomization is the analyte’s concentration. Because15 Parsons, M. L.; Major, S.; Forster, A. R. Appl. Spectrosc. 1983, 37, 411–418.