Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods607atomization temperature. For example, by switching to a higher temperatureflame it may be possible to prevent the formation of interfering oxidesand hydroxides.If the identity of the matrix interference is unknown, or if it is not possibleto adjust the flame or furnace conditions to eliminate the interference,then we must find another method to compensate for the background interference.Several methods have been developed to compensate for matrixinterferences, and most atomic absorption spectrophotometers include oneor more of these methods.One of the most common methods for background correction is touse a continuum source, such as a D 2 lamp. Because a D 2 lamp is a continuumsource, absorbance of its radiation by the analyte’s narrow absorptionline is negligible. Only the background, therefore, absorbs radiation fromthe D 2 lamp. Both the analyte and the background, on the other hand, absorbthe hollow cathode’s radiation. Subtracting the absorbance for the D 2lamp from that for the hollow cathode lamp gives a corrected absorbancethat compensates for the background interference. Although this methodof background correction may be quite effective, it does assume that thebackground absorbance is constant over the range of wavelengths passedby the monochromator. If this is not true, subtracting the two absorbancesmay underestimate or overestimate the background.Minimizing Chemical Interferences. The quantitative analysis of someelements is complicated by chemical interferences occurring during atomization.The two most common chemical interferences are the formationof nonvolatile compounds containing the analyte and ionization of theanalyte.One example of the formation of nonvolatile compounds is the effect ofPO 4 3– or Al 3+ on the flame atomic absorption analysis of Ca 2+ . In one study,for example, adding 100 ppm Al 3+ to a solution of 5 ppm Ca 2+ decreasedthe calcium ion’s absorbance from 0.50 to 0.14, while adding 500 ppmPO 4 3– to a similar solution of Ca 2+ decreased the absorbance from 0.50 to0.38. These interferences were attributed to the formation of nonvolatileparticles of Ca 3 (PO 4 ) 2 and an Al–Ca–O oxide. 16When using flame atomization, we can minimize the formation ofnonvolatile compounds by increasing the flame’s temperature, either bychanging the fuel-to-oxidant ratio or by switching to a different combinationof fuel and oxidant. Another approach is to add a releasing agentor a protecting agent to the samples. A releasing agent is a species thatreacts with the interferent, releasing the analyte during atomization. AddingSr 2+ or La 3+ to solutions of Ca 2+ , for example, minimizes the effect ofPO 43–and Al 3+ by reacting in place of the analyte. Thus, adding 2000 ppmSrCl 2 to the Ca 2+ /PO 4 3– and Ca 2+ /Al 3+ mixtures described in the previousparagraph increased the absorbance to 0.48. A protecting agent reactswith the analyte to form a stable volatile complex. Adding 1% w/w EDTA16 Hosking, J. W.; Snell, N. B.; Sturman, B. T. J. Chem. Educ. 1977, 54, 128–130.Other methods of background correctionhave been developed, including Zeemaneffect background correction and Smith–Hieftje background correction, both ofwhich are included in some commerciallyavailable atomic absorption spectrophotometers.Consult the chapter’s additionalresources for additional information.