CONTINUOUS MONITORING OF GASEOUS FORMALDEHYDE USING AN IMPROVED FLUORESCENCE APPROACH

A continuous monitor for gaseous formaldehyde (HCHO) has been developed which avoids key limitations of previous monitors. The Hantzsch reaction, the cyclization of a beta-diketone, an amine, and formaldehyde, is used to produce a fluorescent derivative from HCHO. Use of 254 nm light for excitation provides fluorescence sensitivity nearly four times that obtained previously with 406 nm excitation. This improvement in fluorescence sensitivity leads to a gaseous detection limit of 0.2 ppbv with a glass coil scrubber as the collection device for gaseous HCHO. The Hantzsch reaction provides high selectivity, high reagent stability, and low reagent cost, while the glass coil provides high efficiency, stability of behavior, and simplicity of design. The monitor consequently provides sensitive and specific measurement of gaseous HCHO in a reliable and simple device. The rise and fall times of the instrument response are both 80 seconds, with a lag time of 120 seconds. Repetitive calibrations during extended field operations indicate a reproducibility of about +/- 7 percent (relative standard deviation). Ambient HCHO data from surface sites show strong diurnal variations, and measurements aboard an aircraft show a sharp vertical gradient in HCHO.