Evaluation of sampling and sample preparation procedures for the determination of aromatic acids and their distribution in a podzol soil using liquid chromatography–tandem mass spectrometry

Abstract

In this work the distribution of free and weekly adsorbed aromatic acids (phthalic acid and ten phenolic acids; gallic, p-hydroxybenzoic, salicylic, vanillic, protocatechuic, p-coumaric, syringic, sinapic, ferulic and caffeic acid), which could participate in weathering and soil formation processes, were studied for O, E and B horizons in a podzol soil in central Sweden. For the analysis a simple and rapid quantitative and qualitative liquid chromatography–tandem mass spectrometry method (using gradient elution) was developed with LODs ranging from 5 to 25nM. Different soil solution sampling techniques (tension-lysimeter and soil centrifugation) and soil extraction with either 10mM phosphate buffer (pH7.2) or 50:50 (v/v) 10mM phosphate buffer:MeOH were compared. All eleven acids were found in detectable or trace concentrations. The most abundant aromatic acids were vanillic and phthalic acid with concentrations around 1μM for O and E horizon respectively. Lysimeter samples resulted in the lowest concentrations followed by centrifugation samples. In general, buffer:MeOH extraction resulted in the highest concentrations for the O horizon, likely due to MeOH's ability to compete for hydrophobic sites on soil organic matter (SOM). Then again, pure buffer with its higher ion strength, interfering with the acids electrostatic interactions with clay particles, leads to higher extracted concentrations for the E and B horizons. Since the efficiency of the extraction solutions, to a large extent, depends on the sample properties, a general approach is hard to appoint. However, the extraction of substituted cinnamic acids is in general facilitated by adding MeOH to the extraction solution. The use of statistical methods for the evaluation of the results showed a large and significant difference in aromatic acid concentrations received using different sampling techniques and sample preparations. In fact, sampling methods resulted in higher variations in aromatic acid concentrations than sampled horizon.