Addressing analytical uncertainties in the determination of trichloroacetic acid in soil

Abstract

Soil is an important compartment in the environmental cycling of trichloroacetic acid (TCA), but soil TCA concentration is a methodologically defined quantity; analytical methods either quantify TCA in an aqueous extract of the soil, or thermally decarboxylate TCA to chloroform in the whole soil sample. The former may underestimate the total soil TCA, whereas the latter may overestimate TCA if other soil components (e.g. humic material) liberate chloroform under the decarboxylation conditions. The aim of this work was to show that extraction and decarboxylation methods yield different TCA concentrations because the decarboxylation method can also determine "bound" TCA. Experiments with commercial humic acid solutions showed there was no additional chloroform formation under decarboxylation conditions, and that all TCA in a TCA-humic acid mixture could be quantitatively determined (108 +/- 13%). Anion exchange resin was used as a provider of solid-phase TCA binding; only 5 +/- 1% of a TCA solution mixed with the resin was present in the aqueous extract subsequently separated from the resin, yet the decarboxylation method yielded mass balance (123 +/- 22%) with TCA remaining in the resin. In aqueous extraction of a range of soil samples (with or without added TCA spike), the decarboxylation method was able to satisfactorily account for TCA in the extractant + residue post-extraction, compared with whole-soil TCA (+ spike) pre-extraction: e.g. mass balances for unspiked soil from Sikta spruce and larch forest were 99 +/- 8% and 93 +/- 6%, respectively, and for TCA-spiked forest and agricultural soils were 114 +/- 13% and 102 +/- 2%. In each case recovery of TCA in the extractant was substantially less than 100%(<20% for unspiked soils, <55% for spiked soils). Extraction efficiencies were generally lower in more organic soils. The results suggest that analytical methods which utilise aqueous extraction may underestimate whole-soil TCA concentrations. Application of both methodologies together may enhance insight into TCA behaviour in soil.

Methodological problems in determining TCAA in soils—the discovery of novel natural trichloroacetyl containing compounds and their interference with a common method for determining TCAA in soil and vegetation

“…a mass spectrometer (MS) or for low detection limits to an electron capture detector (ECD). 13,15,16 To investigate the presence and formation of TCAA in the forest environment, we decided to use thermal decarboxylation followed by GC/ECDanalysis of the chloroform formed in the decarboxylation process. The yield of chloroform upon heating has been shown by several authors to be close to 100%, when TCAA is heated for 90 min at 90 C or more at a pH between 3 and 8.…”

The production and degradation of trichloroacetic acid in soil: Results from in situ soil column experiments

“…This is the case irrespective of the different intrinsic TCA content of the two types of soil used. Furthermore, the soil TCA concentrations determined in this work are "whole soil" rather than extractable only (Dickey et al, 2005), which indicates non-recovered TCA must undergo chemical transformation rather than irretrievable binding within the soil matrix.…”

“…Details of the methodology as applied here to soil and aqueous samples have been extensively reported elsewhere (Heal et al, 2003b;Heal et al, 2003a;Dickey et al, 2004;Stidson et al, 2004a;Stidson et al, 2004b;Dickey et al, 2005). In brief, water or sieved soil samples were sealed in 20 mL headspace vials and heated at 100 C for 1.5 h to effect decarboxylation, and re-equilibrated at 60 C before CHCl 3 determination on a DB5 column held at 50 C. Any background CHCl 3 was accounted for by determining CHCl 3 in a parallel vial of sample equilibrated to 60 C only.…”

“…To investigate within-soil production of TCA, soil from the O horizons (5-20 cm) of the afforested and moorland areas of the Ballochbeatties catchment were made TCA-free by drying at 60 C for 8 day and 100 C for 2 h to decarboxylate TCA already present (Dickey et al, 2005), then rewetted with either deionised water or 7.6 mg L -1 sodium chloride solution, stored in an unheated greenhouse and soil TCA concentrations re-measured after 1, 28 and 54 days.…”

“…Heal et al, 2003a;Matucha et al, 2003a;Dickey et al, 2004;Matucha et al, 2007). The uncertainty in understanding soil-TCA relationships is exacerbated by the intrinsic heterogeneity of soil and associated difficulties in controlling soil parameters (Laturnus et al, 2005), and the use of different analytical methods which quantify different fractions of TCA within the soil (Dickey et al, 2005). Resolving the uncertainty is important because controlled experiments confirm TCA"s toxicity to trees and aquatic macrophytes at, or close to, environmental concentrations (Schröder et al, 1997;Cape et al, 2003;Dickey et al, 2004;Hanson and Solomon, 2004;Lewis et al, 2004), and consequently TCA is subject to formal risk assessment (Peters, 2003).…”