EVALUATION OF THE PERFORMANCE OF DNPH-COATED SILICA-GEL AND C-18 CARTRIDGES IN THE MEASUREMENT OF FORMALDEHYDE IN THE PRESENCE AND ABSENCE OF OZONE

Formaldehyde (HCHO) is an important photochemical product generated du ring the oxidation of volatile organic compounds in the troposphere. A ccurate HCHO concentrations are essential for developing a quantitativ e basis for ozone formation and represent a primary monitoring objecti ve for the Photochemical Assessment and Monitoring Stations (PAMS). In the PAMS network, measurements of HCHO are conducted by a derivatizat ion method using cartridges coated with 2,4-dinitrophenylhydrazine (DN PH). In this study, a manifold system has been designed to evaluate th e performance, accuracy, and importance of interferences of DNPH-coate d 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 system atically lower by 20%. These results were found to be independent of t he level of humidity in the system. The HCHO measurements of the two c artridge systems were highly correlated and yielded the regression equ ation, [HCHO]C-18 = 0.84[HCHO]S-18 Gel. Measurement of HCHO in the pre sence of ozone by silica gel cartridges led to a negative interference , which has been widely reported. The magnitude of the interference wa s greater than 50% under conditions representative of urban environmen ts (HCHO = 5 ppb; O-3 = 120 ppb). The interference was routinely remov ed through the use of a potassium iodide (KI) scrubber or denuder. How ever, these devices were found to require modest water concentrations (>4000 ppmv; RH > 10% at 25 degrees C) to remove ozone effectively. Wi th the C-18 cartridges, the presence of ozone in the airstream produce d a positive interference for the HCHO-hydrazone using standard analys is techniques. These results suggest that under ambient conditions off setting 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 form aldehyde and other carbonyl compounds in urban atmospheres.