UV–Vis spectra of different proteins and chemical solutions. (a) Ribonuclease A (at 30 mg ml−1) is non-absorbing at 295 nm. (b) Thermolysin (at 2.5 mg ml−1) absorbs at 295 nm. (c) Haematin (at 0.21 mM) absorbs at both 295 and 320–350 nm. (d) FeCl3 absorbs at both 295 and 320–350 nm. (e) Sodium nitrate absorbs at 295 and 320–330 nm. For all graphs, the vertical line is at 295 nm and the vertical band is between 320 and 350 nm.