Aromatic amino acids tyrosine and tryptophan give proteins the characteristic ultraviolet (UV) absorption spectrum at 280 nm, which is routinely used to estimate protein concentration. Phenylalanine and disulfide bonds also contribute the absorption at that wavelength, albeit slightly. This method is simple, and requires an extremely small sample volume since new spectrophotometers employ a sample retention system during the measurement. However, the protein sample must be pure and does not contain any non-protein components with the same absorption spectrum, such as the contamination of nucleic acids [1]. This method is quickest, but error-prone.