We have investigated the application of narrow-band intracavity spectroscopy for high throughput characterization and detection of reaction products in-situ. An external cavity diode laser (ECDL), wavelength tunable from 810-890 nm, was used to measure varying concentrations of a range of compounds in solution. A linear calibration curve for parts per billion (ppb) concentrations corresponding to 17nM has been recorded. By replacing the grating with a high reflective mirror, parts per trillion (ppt) concentration detection was also achieved corresponding to 340pM. We believe that this technique provides a method for fast and safe measurements of reactions in real time and has potential applications in the pharmaceutical and chemical industry as well as for integrated bio-diagnostics lab-on-a-chip devices.