Analytical Chem istry - DePauw University

574 Analytical Chemistry 2.0fromsourceATRcrystalsampleglass substratetodetectorpressuretower andcompression tipsampleplatformsampleslotFigure 10.33 FT-IR spectrometer equipped with a diamond ATR sample cell. The inserts show a close-upphoto of the sample platform, a sketch of the ATR’s sample slot, and a schematic showing how the source’sradiation interacts with the sample. The pressure tower is used to ensure the contact of solid samples with theATR crystal.Further details about these, and othermethods for preparing solids for infraredanalysis can be found in this chapter’s additionalresources.the sample absorbs. ATR spectra are similar, but not identical, to thoseobtained by measuring the transmission of radiation.Solid samples also can be analyzed using an ATR sample cell. Afterplacing the solid in the sample slot, a compression tip ensures that it is incontact with the ATR crystal. Examples of solids that have been analyzedby ATR include polymers, fibers, fabrics, powders, and biological tissuesamples. Another reflectance method is diffuse reflectance, in which radiationis reflected from a rough surface, such as a powder. Powdered samplesare mixed with a non-absorbing material, such as powdered KBr, and thereflected light is collected and analyzed. As with ATR, the resulting spectrumis similar to that obtained by conventional transmission methods.10C.2 Quantitative ApplicationssampleslotFigure 10.18 shows the visible spectrumfor Fe(phen) 3 2+ .The determination of an analyte’s concentration based on its absorption ofultraviolet or visible radiation is one of the most frequently encounteredquantitative analytical methods. One reason for its popularity is that manyorganic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum. In addition, if an analyte doesnot absorb UV/Vis radiation—or if its absorbance is too weak—we oftencan react it with another species that is strongly absorbing. For example, adilute solution of Fe 2+ does not absorb visible light. Reacting Fe 2+ with o-phenanthroline, however, forms an orange–red complex of Fe(phen) 3 2+ thathas a strong, broad absorbance band near 500 nm. An additional advantageto UV/Vis absorption is that in most cases it is relatively easy to adjust experimentaland instrumental conditions so that Beer’s law is obeyed.