Study of inlet materials for sampling atmospheric nitric acid

The adsorption of nitric acid (HNO3) from a flowing gas stream is studied for a variety of wall materials to determine their suitability for use in atmospheric sampling instruments. Parts per billion level mixtures of HNO3 in synthetic air flow through tubes of different materials such that >80% of the molecules interact with the walls. A chemical ionization mass spectrometer with a fast time response and high sensitivity detects HNO3 that is not adsorbed on the tube walls. Less than 5% of available HNO3 is adsorbed on Teflon fluoropolymer tubing after 1 min of HNO3 exposure, whereas >70% is lost on walls made of stainless steel, glass, fused silica, aluminum, nylon, silica-steel, and silane-coated glass. Glass tubes exposed to HNO3 on the order of hours passivate with HNO3 adsorption dropping to zero. The adsorption of HNO3 on PFA Teflon tubing (PFA) is nearly temperature-independent from 10 to 80 degrees C, but below -10 degrees C nearly all HNO3 that interacts with PFA is reversibly adsorbed. In ambient and synthetic air, humidity increases HNO3 adsorption. The results suggest that Teflon at temperatures above 10 degrees C is an optimal choice for inlet surfaces used for in situ measurements of HNO3 in the ambient atmosphere.