MA EPH GC x GC experimental design

by Robert G. Brown Principal Chemist/Group Leader Extractable Petroleum Hydrocarbons Approximately ten years have passed since the first generation of risk-based petroleum methods were developed and put into production in the environmental laboratory. The standard Extractable Petroleum Hydrocarbon (EPH) fractionation method still used in most laboratories follows the Massachusetts state protocol. The MA EPH method involves separating (“fractionating”) a total petroleum hydrocarbon (TPH) extract into its aromatic and aliphatic components. This fractionation is achieved by exploiting polarity differences between the two species and the solvents used to fractionate them. Once fractionated, the aromatic and aliphatic extracts are analyzed separately by capillary GC, typically using flame ionization detection (FID). However, the precise amounts of the several different solvents needed, in addition to variables affecting the fractionation media often result in “breakthrough” of target compounds into the wrong fraction(s) and/or contamination of the final extract(s). Advances in gas chromatographic and flow control technologies can now be used to replace the tedious sample preparation techniques previously required to obtain the separate sample extracts (“fractions”) used for site characterization/assessment. Experimental GC x GC Design The fractionation procedure of the MA EPH method should be able to be eliminated entirely by using a two dimensional GC approach (2D-GC; GC x GC). Capillary GC columns of opposing polarities should be able to replace the fractionation procedure (described below). Soil and wastewater samples are extracted with methylene chloride and concentrated to a final volume of 1.0ml. No solvent exchange takes place. Concentrated extracts are treated with ~ 0.3g loose silica gel to remove polar nonpetroleum related compounds. The (single) TPH extract is then analyzed on a two-dimensional GC system using a polar/nonpolar column set in order to chromatographically separate the aromatic/aliphatic compounds. Sample Preparation/GC Analysis Under the current (MA) protocol, soil and wastewater samples are extracted using methylene chloride (soils by sonication and waters by separatory funnel). The extract is dried over sodium sulfate and concentrated on a steam bath. During the final stage of the initial concentration, the extract is exchanged from methylene chloride into hexane, and the resulting hexane extract is concentrated to a final volume of 1.0ml. The hexane extract is then transferred to a 15ml, 3g disposable silica gel column for fractionation. The aliphatic components are first eluted off the silica gel by the addition of 1-2 ml aliquots of pentane. Each aliquot is allowed to elute by gravity – not vacuum - through the column until a final volume of 10ml of pentane has been collected. This is the aliphatic fraction. The elution procedure is repeated using 1-2 ml aliquots of methylene chloride until 10ml has been collected. This is the aromatic fraction. Each fraction is then concentrated to a final volume of 1.0ml for GC analysis. With the GC x GC approach, samples are extracted using methylene chloride. Extracts are dried with sodium sulfate, concentrated and treated with silica gel to remove polar, non-petroleum related compounds. The single, final extract is then analyzed using a two-dimensional gas chromatograph (2-D GC; GC x GC) designed to separate the aliphatic and aromatic species present in the extract still using FID. This new approach meets the original intent of the Massachusetts state and TPH Working Group methods to measure and quantitate collective aliphatic and aromatic hydrocarbon concentrations, as well as target polynuclear aromatic hydrocarbons (PAHs) with the following benefits: More accurate data – fractionation variability eliminated; separation done chromatographically. Shorter analysis time – essentially cut in half. Cost reduction – significant reduction in reagents due to elimination of fractionation prep procedure as well as reduction in labor to prep samples. Robert Brown has published an article in the Journal of Chromatographic Science on this topic. For more information, please contact Robert Brown at 717-656-2300.