Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods609tissue weight (FFDT). The sample is digested at 68 o C for 20–24 h using3 mL of 0.75 M HNO 3 . After centrifuging at 2500 rpm for 10 minutes,the supernatant is transferred to a 5-mL volumetric flask. The digestionis repeated two more times, for 2–4 hours each, using 0.9-mL aliquotsof 0.75 M HNO 3 . These supernatants are added to the 5-mL volumetricflask, which is diluted to volume with 0.75 M HNO 3 . The concentrationsof Cu and Zn in the diluted supernatant are determined by flame atomicabsorption spectroscopy using an air-acetylene flame and external standards.Copper is analyzed at a wavelength of 324.8 nm with a slit widthof 0.5 nm, and zinc is analyzed at 213.9 nm with a slit width of 1.0 nm.Background correction using a D 2 lamp is necessary for zinc. Results arereported as mg of Cu or Zn per gram of FFDT.Qu e s t i o n s1. Describe the appropriate matrix for the external standards and for theblank?The matrix for the standards and the blank should match the matrixof the samples; thus, an appropriate matrix is 0.75 M HNO 3 . Anyinterferences from other components of the sample matrix are minimizedby background correction.2. Why is a background correction necessary for the analysis of Zn, butnot for the analysis of Cu?Background correction compensates for background absorption andscattering due to interferents in the sample. Such interferences aremost severe when using a wavelength less than 300 nm. This is thecase for Zn, but not for Cu.3. A Cu hollow cathode lamp has several emission lines. Explain whythis method uses the line at 324.8 nm.wavelength(nm)slit width(nm)mg Cu/L forA = 0.20 P 0 (relative)217.9 0.2 15 3218.2 0.2 15 3222.6 0.2 60 5244.2 0.2 400 15249.2 0.5 200 24324.8 0.5 1.5 100327.4 0.5 3 87With 1.5 mg Cu/L giving an absorbance of 0.20, the emissionline at 324.8 nm has the best sensitivity. In addition, it is themost intense emission line, which decreases the uncertainty inthe measured absorbance.