Analytical Chem istry - DePauw University

630 Analytical Chemistry 2.0Plasmas also are subject to fewer spectral and chemical interferences. Forthese reasons a plasma emission source is usually the better choice.Se l e c t i n g t h e Wave l e n g t h a n d Sl i t Wi d t hThe choice of wavelength is dictated by the need for sensitivity and the needto avoid interferences from the emission lines of other constituents in thesample. Because an analyte’s atomic emission spectrum has an abundanceof emission lines—particularly when using a high temperature plasmasource—it is inevitable that there will be some overlap between emissionlines. For example, an analysis for Ni using the atomic emission line at349.30 nm is complicated by the atomic emission line for Fe at 349.06 nm.Narrower slit widths provide better resolution, but at the cost of less radiationreaching the detector. The easiest approach to selecting a wavelength isto record the sample’s emission spectrum and look for an emission line thatprovides an intense signal and is resolved from other emission lines.Pr e p a r i n g t h e Sa m p l eFlame and plasma sources are best suited for samples in solution and liquidform. Although a solid sample can be analyzed by directly inserting it intothe flame or plasma, they usually are first brought into solution by digestionor extraction.Minimizing Sp e c t r a l In t e r fe r e n ce semission intensityFigure 10.60 Method for correctingan analyte’s emission for the flame’sbackground emission.I ewavelengthThe most important spectral interference is broad, background emissionfrom the flame or plasma and emission bands from molecular species. Thisbackground emission is particularly severe for flames because the temperatureis insufficient to break down refractory compounds, such as oxidesand hydroxides. Background corrections for flame emission are made byscanning over the emission line and drawing a baseline (Figure 10.60).Because a plasma’s temperature is much higher, a background interferencedue to molecular emission is less of a problem. Although emission from theplasma’s core is strong, it is insignificant at a height of 10–30 mm above thecore where measurements normally are made.Minimizing Ch e m ic a l In t e r fe r e n ce sFlame emission is subject to the same types of chemical interferences asatomic absorption. These interferences are minimized by adjusting theflame’s composition and adding protecting agents, releasing agents, or ionizationsuppressors. An additional chemical interference results from selfabsorption.Because the flame’s temperature is greatest at its center, theconcentration of analyte atoms in an excited state is greater at the flame’scenter than at its outer edges. If an excited state atom in the flame’s centeremits a photon while returning to its ground state, then a ground state atomin the cooler, outer regions of the flame may absorb the photon, decreasing