Reliable Separation and Efficient Group-type Quantitation of Total Petroleum Hydrocarbons (TPHs)

Petroleum contamination from leaking underground storage tanks is a significant concern for both the environment and human health. Thorough characterization of the contamination is required to form appropriate risk assessments and remediation strategies, but until now, the determination of total petroleum hydrocarbons (TPHs) in soil has typically involved a convoluted and labour-intensive process. In this article, the analysis of TPH in environmental media is simplified using flow-modulated GC×GC–FID with quantitation based on pre-defined compound groupings. This approach overcomes the drawbacks of conventional solvent fractionation approaches, by eliminating the need for sample fractionation and automating data processing workflows.

Several risk-based methods have been developed for the analysis of total petroleum hydrocarbons (TPH) in environmental media. This includes those from the TPH Criteria Working group (1) as well as the UK Environment Agency (2) and the Massachusetts Department of Environmental Protection (3). The guidelines set out by the TPH Criteria Working Group (TPH-CWG) (1) state that both volatile petroleum hydrocarbons (VPH) and extractable petroleum hydrocarbons (EPH) must be characterized, with the wide-ranging EPH fraction (typically from C₈– C₄₀₊) being the most challenging.

Compliance with these methods requires quantitation of both the aliphatic and aromatic contaminants. Existing analytical approaches achieve this by solid-phase extraction (SPE) into hexane and dichloromethane respectively. However, in addition to requiring two separate GC–FID analyses for each sample, this approach suffers from a slow, labour-intensive sample fractionation process with high consumable costs, including SPE cartridges, solvents, etc.

Recently, two-dimensional gas chromatography (GC×GC) has been applied to address these challenges. GC×GC is an enhanced separation technique where two columns containing different stationary phases are used to separate complex samples based on two different chemical properties, for example, by volatility and then polarity. This approach avoids the drawbacks of conventional methods, eliminating the need for off-line sample fractionation and providing reliable group-type separation and quantitation of the aromatics and aliphatics in a single run.

In this study, dual-channel GC×GC–FID with flow modulation and automated data analysis is used to improve productivity and accuracy in EPH analysis.

Experimental

Samples: EPH calibration samples comprised diesel spiked into hexane at concentrations of 50, 100, 500, 2500, and 5000 ppm. Three soil extracts in hexane–acetone (1:1 v/v) and a hexane control blank were also analyzed.

GC×GC: Dual-channel configuration incorporating two INSIGHT-Flow modulators (SepSolve Analytical) installed in a single GC oven, with dual injection and detection.

Column set: Metal column set for optimal method robustness, with a typical lifetime of 3500–4000 analytical runs.

Software: ChromSpaceGC×GC software (SepSolve Analytical) for full instrument control (of the autosampler, GC, modulator, and FID) and data processing.

Results and Discussion

The analytical equipment and software described here comprises GC×GC hardware, software, method statements and consumables. Key aspects of this system for improving EPH analysis are described below.

Sample Preparation: When using GC×GC, the aromatics and aliphatics are separated by the chromatographic system (Figure 1) meaning that off-line sample fractionation is not required, significantly reducing consumable costs associated with EPH analysis. In addition, the elimination of dichloromethane means that waste disposal requirements and risk to laboratory staff are reduced. The use of GC×GC–FID analysis of a single sample with minimal sample handling also reduces the chance for human error, by simplifying
sample preparation.

Method Optimization: The purpose of the modulator in GC×GC is to sample the first-column effluent and re-inject it onto the second column in narrow bands. Flow modulators are consumable-free using a simple, valve-based approach to fill and flush a sample loop. Importantly, the flow modulator used in this study has an adjustable sample loop (in the range of 25-250 µL) for greater flexibility in method development. Here, a high-throughput EPH method has been optimized using a robust metal column set and a larger loop volume, enabling a fast run time of 19.3 min to be achieved (Figure 2). This would not be possible using a small fixed-volume loop. In addition, the system delivers excellent method robustness—specifically the repeatability of retention times (4)—through precise flow control by a dedicated EPC for each column, making it well-suited to routine GC×GC analyses. The repeatability of peak areas from 15 replicate injections of a banding standard over five days resulted in RSD values of less than 5%, confirming method reliability. Additionally, early adopters of GC×GC for TPH analyses have demonstrated system suitability through UKAS and MCERTS accreditation.

Data Processing: A key part of GC×GC for EPH analysis is a software platform that can enable fast, simple data processing and full LIMS compatibility. The availability of full instrument control and data processing in a single software platform results in streamlined workflows and simplified training requirements for analysts. An additional feature is the use of stencils defined by a banding standard, allowing real-world samples to be quickly integrated and quantified.

The workflow for EPH data analysis comprises four main steps: (1) Creation of stencil regions; (2) Quantitative method set-up; (3) Batch processing; (4) Review and reporting of quantitative results.

Creation of Stencil Regions

The structured ordering of GC×GC chromatograms ensures that chemical classes elute together in well-defined bands, simplifying group-type analysis. At the same time, interferences such as column bleed elute well away from the analytes of interest.

Bands for group-type EPH analysis are defined in terms of equivalent carbon numbers, for example>C₁₀–C₁₂, >C₁₂–C₁₄). The first step is to define these regions by analyzing a banding standard, which consists of a range of n-alkanes and aromatics.

Contiguous meshes are drawn and annotated using the stencil function and then applied to real samples, providing the foundations for the quantitative method. Figure 2 shows a completed EPH stencil applied to a real soil extract. Stencil regions are flexible to suit user preferences and can easily be assigned to hierarchical groups, for example, all aromatic regions can be assigned to an aromatics group, allowing quantitative results to be reported for the entire class as well as the individual regions.

Quantitative Method Set-up

To apply a calibration using the stencil, quantitative parameters must also be stored within the method. The region names are automatically transferred from the stencil to allow integration and calibration parameters to be added quickly.

Batch-processing

Once a method is saved, it can be applied in a batch-processing sequence. Sequences can be stored under the history tab and easily archived to a back-up system.

Review and Reporting of Quantitative Results

Full quantitative results can be launched from the results browser. The excellent linearity (R² >0.995) is achieved across the 50–5000 ppm calibration range. Note that this range can be extended depending on the sample matrix, with the approach also applicable to water samples. The browser displays all the quantitative information including region list, sample list, calibration curves and interactive colour plot. The latter shows the integrated stencil regions and includes a table detailing all the peaks found within each region. Results can be exported as a custom report, a simple .csv file or directly to a laboratory information management system (LIMS) interface, enabling alignment with existing laboratory protocols.

Dual Injection

By configuring two modulators within the same GC oven (Figure 4), dual injection can be used to double the productivity of the analysis. Combining this with the optimised separation using a metal capillary column set means that two analyses can be completed within 20 min, meeting the needs of this high throughput application.

Conclusions

This versatile and powerful approach for group-type analysis of extractable petrochemical hydrocarbons in environmental samples enables the need for off-line fractionation and multiple GC runs for each sample to be eliminated, saving time and reducing costs. It provides structured GC×GC chromatograms, increasing confidence in results, while flow modulation provides repeatable and affordable GC×GC, with metal columns increasing method robustness.

GC×GC software allows for automated group-type data processing for fast reporting of quantitative results, an additional advantage for contract laboratories routinely running large numbers of samples. System suitability has already been demonstrated at such sites through UKAS and MCERTS accreditation. Finally, the optional dual-channel configuration doubles productivity, for the analysis of two samples within 20 min.

References

(1) Analysis of Petroleum Hydrocarbons in Environmental Media, Total Petroleum Hydrocarbon Criteria Working Group Series (Volume 1), 1998.

(2) Performance Standard for Laboratories Undertaking Chemical Testing of Soil (version 4), Environment Agency, March 2012, www.gov.uk/government/publications/mcerts-performance-standard-for-laboratories-undertaking- chemical-testing-of-soil.

(3) Method for the Determination of Extractable Petroleum Hydrocarbons (MADEP-EPH-04, revision 1.1), Massachusetts Department of Environmental Protection, May 2004, www.mass.gov/eea/docs/dep/cleanup/laws/ eph0504.pdf.

(4) See SepSolve White Paper 006, www.sepsolve.com/petrochemicals/.

Laura McGregor completed a Ph.D. in Environmental Forensics at the University of Strathclyde in 2012. Her research interests included the chemical fingerprinting of coal tar contamination using GC×GC–TOF MS. In her current role at SepSolve Analytical, she oversees marketing activities for the full product portfolio and specializes in applying GC×GC and chemometrics to challenging samples.

James Ogden spent nine years working within an environmental analytical laboratory, where he was responsible for method testing, development, and implementation. In his current role at SepSolve Analytical, James supports customers through the development, demonstration, and handover of analytical methods across the company’s portfolio of instruments and software.

Anthony Buchanan received a master’s degree in Analytical Toxicology at King’s College London in 2012. After completing his degree, Anthony worked at several research contract labs using GC–MS. He joined SepSolve in 2018 as an Application Engineer and then Service Engineer, before becoming the Portfolio Product Manager for the GCxGC and TOF mass spectrometry technologies in 2022.

E-mail: lmcgregor@sepsolve.com

Website: www.sepsolve.com