Development of an automated measurement system using a diffusion scrubber and high-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 (HPLC) was developed. HCHO and CH₃CHO are effectively collected by the diffusion scrubber, which consists of a hydrophobic porous PTFE tube disposed concentrically within a Pyrex-glass tube and a scrubbing solution. 2,4-Dinitrophenylhydrazine is used as the scrubbing solution for trapping HCHO and CH₃CHO, which are derivatized to formaldehyde 2,4-dinitrophenylhydrazone (DNPH-HCHO) and acetaldehyde 2,4-dinitrophenylhydrazone (DNPH-CH₃CHO), respectively, with phosphoric acid as an acid catalyst. 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 operations are sequenced by a programmable controller and an automated continuous measurement can be performed at 10 min intervals. The collection efficiencies of HCHO and CH₃CHO were higher than 97% at a gas flow rate of 0.21 min^-1. The detection limit (3σ of the blank value) was 0.001 ppm v/v for HCHO and CH₃CHO for a 1.61 gas sample volume. No interference of co-existing nitrogen dioxide (NO₂) in the collection of HCHO and CH₃CHO was observed. The average concentration of HCHO in the 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-phase mode, the average concentration of HCHO was 3.3 ppm v/v (n = 15). The concentrations of HCHO measured by this automated measurement system were in good agreement with those obtained using the impinger-HPLC method.