A laboratory evaluation of two methods for measuring low levels of formaldehyde in indoor air

Two methods for measuring formaldehyde at ppb levels--the modified pararosaniline (PRA) and the modified chromotropic acid (CTA)--were evaluated in a laboratory study. A dynamic double dilution system was used to generate controlled test atmospheres of formaldehyde by the catalytic depolymerization of trioxane. Impinger samples were collected from the sampling manifold and analyzed accordingly. Both methods demonstrated good precision (3.5% for the PRA and 3.4% for the CTA) but differed in accuracy and collection efficiency. Accuracy was 87.7 +/- 7.5% for the PRA and 92.5 +/- 4.2% for the CTA, while collection efficiency was 91.9 +/- 6.9% and 98.7 +/- 4.7% respectively. These differences were mainly due to the use of 1% NaHSO3 as the absorbing solution in the CTA method. Additionally, the NaHSO3 solution has the advantage of long sample preservation, making the modified CTA the method of choice in most non-industrial indoor air sampling situations.