Development of membrane based devices and ion chromatography systems and their applications in atmospheric measurements
Ion chromatography (IC) is presently the preeminent technique for ionic analysis in atmospheric measurements. High temporal resolution measurement of atmospheric gases and constituents of particulate matter (PM) is important to understand the chemistry and sources of atmospheric pollution. The application of hollow and filament-filled single asymmetric membrane fibers, consisting of a thin silicone layer on a tubular microporous support for removing CO2 in suppressed ion chromatography effluents is described. With appropriate choice of the removal device and operating conditions, CO2 can be essentially quantitatively (99+%) removed. This device further reduces the background conductivity from carbonate-based eluents after suppression. In hydroxide eluent chromatography, the device largely removes the response from CO2 present in the samples; this greatly aids atmospheric trace gas analysis by IC. A low-pressure (400 psi), fully automated, semi-continuous Gas Particle Ion Chromatography (GPIC) system has been developed. Soluble gas collection is accomplished with a parallel plate wet denuder (PPWD). A hydrophobic filter based continuous particle collector (PC) collects and continuously extracts the soluble component in atmospheric particulate matter. Two conductivity detectors are used for anion signals and ammonium, respectively. This instrument measures three most important constituents, ammonium, sulfate and nitrate in fine particulate matter (PM2.5) as well as soluble acidic and basic gases like HNO3, SO2 and NH3. A second Gas Particle Ion Chromatograph (GPIC) was developed. This compact instrument also measures soluble ionogenic gases and ionic constituents in PM2.5. A closed liquid flow path membrane denuder collects soluble gases and a continuously wetted hydrophilic filter-based particle collector collects the particles. A single, multi-port, syringe pump handles all the liquid aspiration and dispensation. Preceded by the sampling devices, the sample processing part interfaced with an ion chromatograph permits automated collection and measurements of collected samples. A single conductivity detector is used to measure not only anions but also ammonium. The flow configuration results in an ammonium peak before anion peaks in the chromatogram. The system measures ammonia, organic acids (such as acetic, formic and oxalic acids), HCl, HONO, SO2, HNO3, and the corresponding ions in the aerosol phase.