Identifying potential antioxidant compounds in NaOH extracts of UK soils and vegetation by untargeted mass spectrometric screening

Abstract

NaOH extracts of soil and vegetation have been shown to have antioxidant capacity (AOC). However, the identification of specific antioxidant compounds within extracts has proved to be difficult. In earlier work we demonstrated that the presence of some lignin-derived phenolic compounds, which were antioxidants, could only account for a very small percentage of the overall antioxidant capacity of soil extracts. The aims of the work reported here were to measure AOC of both vegetation and soil extracts by the Trolox equivalent antioxidant assay and photochemiluminescence and to analyse them using pyrolysis-field ionisation mass spectrometry (Py-FIMS), which allows a tentative assignment of specific compounds and an amalgamation to broad groups of compounds (e.g. carbohydrates) for the soil extracts. With this information we could find which compounds were closely related to the AOCs of the extracts and of those which might potentially be antioxidants. We extracted six soil samples, which had a range of antioxidant capacities, and the vegetation associated with those soils. The AOC results by the two methods were well correlated, and comparing soil and vegetation AOCs suggested that some antioxidants in vegetation were lost during humification, and that some others were soil-derived. For the soil samples we found that, because most of the compound groups quantified by Py-FIMS were correlated with the antioxidant capacities, a clear assignment of antioxidant properties to groups was restricted. However, individual compounds could be linked to the AOC using multivariate statistics, grouping the samples by their antioxidant capacity. Some compounds were present in both soil and vegetation, and presumably survived humification, while others were only found in the soil, in which they had probably been formed. Further investigation of these tentatively identified compounds provides a way forward in explaining the AOC of soil and vegetation extracts.