A new approach for measuring the carbon and oxygen content of atmospherically relevant compounds and mixtures
Abstract. Due to its complexity, gas- and particle-phase organic
carbon in the atmosphere is often classified by its bulk physicochemical
properties. However, there is a dearth of robust, moderate-cost approaches
to measure the bulk chemical composition of organic carbon in the atmosphere.
This is particularly true for the degree of oxygenation, which critically
affects the properties and impacts of organic carbon but for which routine
measurement approaches are lacking. This gap has limited the understanding of a
wide range of atmospheric components, including particulate matter, the mass
of which is monitored worldwide due to its health and environmental effects but the chemical characterization of which requires relatively high capital
costs and complex operation by highly trained technical personnel. In this
work, we demonstrate a new approach to estimate the mass of carbon and
oxygen in analytes and mixtures that relies only on robust, moderate-cost
detectors designed for use with gas chromatography. Organic compounds
entering a flame ionization detector were found to be converted with
approximately complete efficiency to CO2 , which was analyzed downstream
using an infrared detector to measure the mass of carbon analyzed. The ratio
of the flame
ionization detector (FID) signal generated to CO2 formed ( FID∕CO2 ) was shown to be
strongly correlated ( R2=0.89 ) to the oxygen-to-carbon ratio ( O∕C )
of the analyte. Furthermore, simple mixtures of analytes behaved as the
weighted average of their components, indicating that this correlation
extends to mixtures. These properties were also observed to correlate well
with the sensitivity of the FID estimated by structure activity
relationships (quantified as the relative effective carbon number). The
relationships between measured FID∕CO2 , analyte O∕C , and FID
sensitivity allow the estimation of one property from another with  % error for mixtures and  % error for most individual
analytes. The approach opens the possibility of field-deployable, autonomous
measurement of the carbon and oxygen content of particulate matter using
time-tested, low-maintenance detectors, though such an application would
require some additional testing on complex mixtures. With some instrumental
modifications, similar measurements on gas-phase species may be feasible.
Moreover, the potential expansion to additional gas chromatography detectors may
provide concurrent measurement of other elements (e.g., sulfur, nitrogen).