Diversity of Carbonyl Compounds in Biogas and Natural Gas Revealed Using High-Resolution Mass Spectrometry and Nontarget Analysis

Airborne carbonyl compounds such
as formaldehyde, acrolein, and
methyl ethyl ketone have long been chemicals-of-concern in the environment
due to their reactivity and their potential for negative health effects.
Standard methods for determining carbonyls in air, which focus on
a set of 15 or fewer compounds, involve derivatization to form nonvolatile
hydrazones, which can readily be analyzed via liquid chromatography
(LC) with ultraviolet detectors. Here, we apply a new LC-high-resolution
mass spectrometry (HRMS) method to natural gas and a variety of upgraded
biofuels to better assess their total carbonyl profile using the inherent
selectivity of the standard sampling methodology and the selectivity
and sensitivity of HRMS. The standard method accounted for only 64%
of the total carbonyl content in natural gas and between 26 and 45%
of the total carbonyl content in biogas sources, with the balance
detected by the new LC/HRMS method. An additional 540 compounds with
molecular formulas consistent with carbonyl compounds were detected
compared to only 14 target compounds using the standard method. These
results demonstrate that the established method dramatically under-reports
both the total carbonyl load and the diversity of carbonyl species
in natural gas and biogas samples.