Beers Law in Chemical Analysis

For a compound to be analyzed by spectrophotometry, it must absorb light, and this absorption should be distinguishable from that due to other substances in the sample. Because most compounds absorb ultraviolet radiation, measurements in this region of the spectrum tend to be inconclusive, and analysis is usually restricted to the visible spectrum. If there are no interfering species, however, ultraviolet absorbance is satisfactory. Proteins are normally assayed in the ultraviolet region at 280 nm because the aromatic groups present in virtually every protein have an absorbance maximum at 280 nm. 

Solution Using Beer s law with the molar absorptivity from part (a), we find A 0.070 

Iron for biosynthesis is transported through the bloodstream by the protein transferrin. The following procedure measures the Fe content of transferrin 10 This analysis requires only about 1 p,g for an accuracy of 2-5%. Human blood usually contains about 45 vol% cells and 55 vol% plasma (liquid). If blood is collected without an anticoagulant, the blood clots, and the liquid that remains is called serum. Serum normally contains about 1 pg of Fe/mL attached to transferrin. 

This example illustrates the measurement of molar absorptivity from a single solution. It is better to measure several concentrations to obtain a more reliable absorptivity and to demonstrate that Beer s law is obeyed. 

Supernote the liquid layer above the solid that collects at the bottom of a tube during centrifugation. 

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