An analytical system and method for periodically monitoring an injection water distribution pipeline for the presence and concentration of formaldehyde or other aldehyde-functional biocide includes pumps, one of which provides a predetermined volume of injection water drawn from the pipeline at a sampling point and the other a predetermined volume of a reagent, preferably a buffered solution of dimedone, from a reagent storage vessel which are mixed and then heated in a chamber to a predetermined temperature to promote formation of any reaction products. The heated reaction mixture is passed to a detection cell and exposed to light of predetermined wavelength which, in accordance with the Hantzsch reaction, molecules having an aldehyde functional group that reacted with dimedone produce a fluorescence-emitting reaction product, the intensity of which is measured and compared to data previously obtained from standard aldehyde-containing solutions.