Evaluation of the performance of DNPH-coated silica gel and C18 cartridges in the measurement of formaldehyde in the presence and absence of ozone

Formaldehyde (HCHO) is an important photochemical product generated during the oxidation of volatile organic compounds in the troposphere. Accurate HCHO concentrations are essential for developing a quantitative basis for ozone formation and represent a primary monitoring objective for the Photochemical Assessment and Monitoring Stations (PAMS). In the PAMS network, measurements of HCHO are conducted by a derivatization method using cartridges coated with 2,4-dinitrophenylhydrazine (DNPH). In this study, a manifold system has been designed to evaluate the performance, accuracy, and importance of interferences of DNPH-coated silica gel and C-18 cartridges using gasphase calibration standards. Compared to the independent gas-phase calibration values, HCHO values from silica gel cartridges were typically 5% lower than the reference values over the range of 2-25 ppbv; C-18 cartridge values were systematically lower by 20%. These results were found to be independent of the level of humidity in the system. The HCHO measurements of the two cartridge systems were highly correlated and yielded the regression equation, [HCHO]C-18 = 0.84[HCHO]S-18 Gel. Measurement of HCHO in the presence of ozone by silica gel cartridges led to a negative interference, which has been widely reported. The magnitude of the interference was greater than 50% under conditions representative of urban environments (HCHO = 5 ppb; O-3 = 120 ppb). The interference was routinely removed through the use of a potassium iodide (KI) scrubber or denuder. However, these devices were found to require modest water concentrations (>4000 ppmv; RH > 10% at 25 degrees C) to remove ozone effectively. With the C-18 cartridges, the presence of ozone in the airstream produced a positive interference for the HCHO-hydrazone using standard analysis techniques. These results suggest that under ambient conditions offsetting errors are possible with the use of C-18 cartridges, if ozone removal devices are not used. However, with proper precautions, either of these cartridges can be a highly effective means of measuring formaldehyde and other carbonyl compounds in urban atmospheres.