DEVELOPMENT OF AN AUTOMATED MEASUREMENT SYSTEM DIFFUSION SCRUBBER ANDHIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY FOR THE MONITORING OF FORMALDEHYDE AND ACETALDEHYDE IN AUTOMOTIVE EXHAUST-GAS

An automated measurement system for monitoring formaldehyde (HCHO) and acetaldehyde (CH3CHO) in automotive exhaust gas by using a diffusion scrubber in combination with high-performance liquid chromatography (H PLC) was developed. HCHO and CH3CHO are effectively collected by the d iffusion scrubber, which consists of a hydrophobic porous PTFE tube di sposed concentrically within a Pyrex-glass tube and a scrubbing soluti on. 2,4-Dinitrophenylhydrazine is used as the scrubbing solution for t rapping HCHO and CH3CHO, which are derivatized to formaldehyde 2,4-din itrophenylhydrazone (DNPH-HCHO) and acetaldehyde 2,4-dinitrophenylhydr azone (DNPH-CH3CHO), respectively, with phosphoric acid as an acid cat alyst. After the collection of the gas sample, the sample solution in the diffusion scrubber is injected into the HPLC system and DNPH-HCHO and DNPH-CH3CHO are separated and determined. All measurement operatio ns are sequenced by a programmable controller and an automated continu ous measurement can be performed at 10 min intervals. The collection e fficiencies of HCHO and CH3CHO were higher than 97% at a gas flow rate of 0.2 1 min(-1). The detection limit (3 sigma of the blank value) wa s 0.001 ppm v/v for HCHO and CH3CHO for a 1.6 1 gas sample volume. No interference of co-existing nitrogen dioxide (NO2) in the collection o f HCHO and CH3CHO was observed. The average concentration of HCHO in t he exhaust gas from methanol-fueled vehicles was 77.3 ppm v/v (n = 5) in the cold-phase mode when engines were first started. In the hot-pha se mode, the average concentration of HCHO was 3.3 ppm v/v (n = 15). T he concentrations of HCHO measured by this automated measurement syste m were in good agreement with those obtained using the impinger-HPLC m ethod.