Demonstration of Extractive Cryocooled Inert Preconcentration with FTIR spectroscopy instrumentation and methodology for autonomous measurements of atmospheric organics.

摘要

At present researchers exclusively use gas chromatography (GC) systems to monitor multiple volatile organic compounds (VOCs) in either real- or near-real-time. We have designed, developed, and constructed an experimental atmospheric air-quality monitoring system capable of measuring low-concentration VOCs using advanced optical techniques. This system uses a commercial Fourier Transform Infrared spectrometer (FTIR), a commercial long-path gas cell, a commercial acoustic Stirling cryocooler, and a custom cryogen-free cryotrap to autonomously monitor a multi-pollutant suite of VOCs with on-board quality assurance/quality control (QA/QC) calibration. Every four hours, the system records a five minute co-added FTIR interferogram using preconcentrated batch samples which are thermally desorbed from the cryotrap into the gas cell. From this interferogram, the spectral processing algorithm calculates a corresponding absorption spectrum and derives trace gas concentrations using a peak fitting technique to achieve compound-specific detection limits of 6-60 parts per trillion volume (pptv). During the calibration cycle, the system acquires QA/QC measurements made in a similar fashion using high-purity calibration gas bottles.;The presented laboratory results show the system is capable of measuring single- and multi-component calibration gas mixtures within the manufacturer's accuracy specifications. In situ canister samples analyzed using gas chromatography with electron capture and flame ionization detection (GC/ECD/FID) presents a narrow background for possible VOC concentrations at the National Space Science and Technology Center (NSSTC) in Huntsville, AL. In situ observations by the FTIR-based system showcase the capabilities of the system to run fully autonomously and analyze a complex atmospheric mixture with a high degree of fidelity. Complex error analysis highlights the shortfalls of this methodology and presents quantitative correction factors for a number of systematic error sources. The results demonstrate the utility of this technology for a wide range of atmospheric gas-phase organics research and monitoring applications in a number of environments.