Volatile thiols are easily and almost unerringly detected by their distinctive odor. Sulfur-specific analyzers for gas chromatographs are useful. Spectroscopic indicators are the DO-exchangeable SH signal in the H NMR spectrum (S is NMR-active but signals for divalent sulfur are very broad and of little utility). The ν band appears near 2400 cm in the IR spectrum. In the nitroprusside reaction, free thiol groups react with sodium nitroprusside and ammonium hydroxide to give a red colour.