Analytical Chem istry - DePauw University

606 Analytical Chemistry 2.0See Chapter 7 to review different methodsfor preparing samples for analysis.are a source of stray radiation that leads to an instrumental deviation fromBeer’s law. The monochromator’s slit width is set as wide as possible, improvingthe throughput of radiation, while, at the same time, being narrowenough to eliminate the stray radiation.Preparing the Sample. Flame and electrothermal atomization require thatthe sample be in solution. Solid samples are brought into solution by dissolvingin an appropriate solvent. If the sample is not soluble it may bedigested, either on a hot-plate or by microwave, using HNO 3 , H 2 SO 4 , orHClO 4 . Alternatively, we can extract the analyte using a Soxhlet extractor.Liquid samples may be analyzed directly or extracted if the matrix is incompatiblewith the method of atomization. A serum sample, for instance, isdifficult to aspirate when using flame atomization and may produce an unacceptablyhigh background absorbance when using electrothermal atomization.A liquid–liquid extraction using an organic solvent and a chelatingagent is frequently used to concentrate analytes. Dilute solutions of Cd 2+ ,Co 2+ , Cu 2+ , Fe 3+ , Pb 2+ , Ni 2+ , and Zn 2+ , for example, can be concentratedby extracting with a solution of ammonium pyrrolidine dithiocarbamatein methyl isobutyl ketone.Minimizing Spectral Interference. A spectral interference occurs whenan analyte’s absorption line overlaps with an interferent’s absorption lineor band. Because they are so narrow, the overlap of two atomic absorptionlines is seldom a problem. On the other hand, a molecule’s broad absorptionband or the scattering of source radiation is a potentially serious spectralinterference.An important consideration when using a flame as an atomizationsource is its effect on the measured absorbance. Among the products ofcombustion are molecular species that exhibit broad absorption bands andparticulates that scatter radiation from the source. If we fail to compensatefor these spectral interference, then the intensity of transmitted radiationdecreases. The result is an apparent increase in the sample’s absorbance. Fortunately,absorption and scattering of radiation by the flame are correctedby analyzing a blank.Spectral interferences also occur when components of the sample’s matrixother than the analyte react to form molecular species, such as oxidesand hydroxides. The resulting absorption and scattering constitutes thesample’s background and may present a significant problem, particularly atwavelengths below 300 nm where the scattering of radiation becomes moreimportant. If we know the composition of the sample’s matrix, then we canprepare our samples using an identical matrix. In this case the backgroundabsorption is the same for both the samples and standards. Alternatively,if the background is due to a known matrix component, then we can addthat component in excess to all samples and standards so that the contributionof the naturally occurring interferent is insignificant. Finally, manyinterferences due to the sample’s matrix can be eliminated by increasing the