The present invention is a device and system for the measurement of substances in exhaled breath. Its basic components include (i) an optional inlet unit (10) to provide controlled air to the subject, (ii) an outlet unit (20) for capturing exhaled breath from the subject, (iii) a sample chamber (30), (iv) a sensing element (40), such as a bioactive compound in a sol-gel matrix, (v) a light source (50), vi a detector (60), (vii) control circuitry (70), (viii) a signal processor (80), (ix) a display (90), and (x) optional storage means (100).
