Sources indicate the composition of soil ideal for growing plants to be 25% air, 25% water, 45% minerals, and 5% organic matter. That does not seem like a daunting makeup, but the reality is that the relative proportion of the constituents can vary dramatically.
I have heard talks by Professor Milan Hutta about the complexity of humic substances (a major portion of the organic matter) and how soil is a model system for demonstrating the need for multiple and multidimensional separation approaches. Organic substances are present in a wide range of relative abundances and molecular weights; usually, we are interested in measuring some trace contaminant present in that matrix. Within the mineral portion are defined characteristics of the soil based on particle size (sand, 0.05–2 mm; silt, 0.002–0.05 mm; clay, <0.002 mm). These particles, which also can be mixed in any possible proportion, have different sorption characteristics. While sand and silt are largely inert pieces of rock, clay is largely made up from phyllosilicates, which are formed from the breakdown of other minerals. The amount of air and water in the soil will obviously vary with season and weather, and can even change daily. Spatially, in an area desired to be sampled, the composition of soil can even vary dramatically over short distances. These variations paint a picture of a highly variable matrix, which can contain virtually anything that has interacted with the environment.
We began our interest in soil contamination as part of the work we have been doing at the Collaborative Laboratories for Environmental Analysis and Remediation (CLEAR) at the University of Texas at Arlington. In an area of active unconventional oil and gas extraction in the Eagle Ford shale, we used a mobile mass spectrometer to measure benzene, toluene, ethyl benzene, and xylene (BTEX) released from gas flares on well-pad sites (1). In that work, we were able to pinpoint some inefficiencies in some of the well-pad infrastructure designed to scrub BTEX from the waste stream.
This work also prompted us to measure BTEX in the surrounding soil (2). We found large amounts of BTEX, which had clearly been released from the gas flares and was deposited in the surrounding soil area. We used EPA Method 5021A (3) to carry out the soil analysis. This method uses headspace gas chromatography to liberate volatile organic compounds for analysis. However, to make an accurate analysis, you have to determine and match the soil composition of the real sample to build a calibration curve. This is an arduous task, where, before the headspace analysis of samples can commence, some of the soil sample is mixed with a soapy solution and allowed to settle into different layers, which can be used to estimate the amount of sand, silt, and clay present. The matrix can then be effectively mimicked using commercially available clean soils. Water is added to the soil mixture in the headspace vial to help normalize water content and liberate BTEX (or other volatile organic compounds) from the matrix. The biggest problem is that each soil composition can yield slightly different sensitivity (that is, the slope of the calibration curve can change drastically from soil to soil). Thus, without an appropriate surrogate matrix, it is very difficult to obtain reliable data.
Given recent development of the use of ionic liquids (ILs) as headspace cosolvents by Professor Armstrong and coworkers for the determination of water in various matrices (4,5), we decided to try ionic liquids as headspace cosolvents for BTEX determination. ILs are molecular molten salts, which can be tuned to exhibit a variety of physicochemical properties. They exhibit nominal vapor pressure and thus do not contribute to the analytes liberated into the headspace. For water analysis, it was important to choose highly hydrophobic ionic liquids, especially characterized by negligible water content. For BTEX, we chose to use highly hydrophilic ionic liquids—the idea is to homogenize the matrix and provide an environment that facilitates release of the BTEX into the headspace upon heating.
We recently published this alternate method (6). Importantly, using hydrophilic IL cosolvents, such as 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][ESO4]), 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]), and tris(2-hydroxyethyl) methylammonium methylsulfate ([MTEOA][MeOSO3]), it was possible to normalize matrix effects for BTEX response from different soils. Additionally, because of the good thermal stability of the ILs, the temperature of the extraction could be increased and equilibration time could be reduced (from 50 min to 30 min), relative to the EPA method. With such a method, it is no longer necessary to matrix-match blank soil samples for calibration. One calibration curve will suffice for analysis of BTEX from different soils.