Evaluation of the permeability of formaldehyde and water through a permeation tube for the preparation of an accurate formaldehyde reference gas mixture.

A system for continuous generation and analysis of formaldehyde (HCHO) in a nitrogen gas mixture prepared using a permeation method was fabricated in order to evaluate the permeability of HCHO and water (H2O) through a permeation tube. Specifically, the mass balance of HCHO and H2O through a permeation tube was evaluated using the system. The results indicated that the mass loss in the permeation tube accounted for the amount of HCHO and H2O measured using a spectrometer. The permeability of HCHO was calculated by subtracting the mass loss of H2O from the permeation tube per unit of time as determined from the mass balance results. The calculated permeability of HCHO was 75.7 ± 3.4 mg min(-1) (k = 2) for the HCHO gas mixture prepared by the permeation method using a permeation tube filled with paraformaldehyde that was vacuum-dried at 95 °C. The calculated permeability agreed with the permeability obtained using the dinitrophenylhydrazine-derivatization method (72.7 ± 4.4 mg min(-1) (k = 2)) within the level of uncertainty. This technique, in which the mass loss of H2O from the permeation tube is subtracted, can therefore provide a reference gas mixture with an accurate HCHO concentration using the permeation method.