What are common techniques for performing EPH fractionation and how do they compare?

Petroleum-based products play a pivotal role in our daily lives, serving as fuels, lubricants, and raw materials for countless industries. However, with their widespread use comes the potential for environmental contamination. Understanding and characterizing the hydrocarbon content of these products is crucial for effective management and remediation efforts. One key analytical approach for characterizing extractable petroleum hydrocarbons (EPH) is known as EPH fractionation for separation of aromatic and aliphatic hydrocarbons within a sample matrix. In this blog, we'll discuss three common techniques used for EPH fractionation and explore how they compare.

EPH fractionation technique #1: GCxGC-FID

Two-Dimensional Gas Chromatography with Flame Ionization Detection (GCxGC-FID) fractionation is a technique that separates aliphatic and aromatic compounds all in one sample that is injected into two different GC columns. The first column, or dimension, is more of a conventional non-polar GC column whereas the second dimension is a shorter, faster and polar GC type column. After the extracted sample is injected into the first GC column, the first dimension will separate compounds based on volatility. Next the effluent from the first dimension enters the modulator. In this phase, the effluent from the first dimension is collected in small fractions and quickly transferred to the second dimension in short bursts to maintain the separation from the first dimension. The second dimension will separate the compounds by polarity. Once the samples leave the GCxGC, they enter the flame ionization detector (FID), and signals are finally captured regarding the amount of carbons present. The final product is a 2-dimensional chromatogram that plots the first-dimension volatility results against the second-dimension polarity results. Aliphatic and aromatic compounds are categorized in their respective zones on the chromatogram to allow for individual quantification.

EPH fractionation technique #2: SPE & GC-FID

Solid Phase Extraction (SPE) fractionation is a process that separates extracted petroleum samples into separate aliphatic and aromatic fractions. The soil and/or water samples are extracted into an organic solvent and passed through a polymer based SPE cartridge after it has been rinsed and conditioned to remove unwanted impurities. Traditionally, silica SPE sorbents are an optimal choice because they retain the more polar aromatic compounds while allowing aliphatic hydrocarbons to flow through the sorbent more freely to help prevent co-elution. ISOLUTE® EPH columns consist of a proprietary blend of silica and alumina that has been optimized to achieve clean, robust fractionation of the aliphatic/aromatic EPH components. In addition, the Biotage® Extrahera™ provides and automated and reliable, high throughput fractionation with substantial reductions in solvent consumption and costs. Aliphatic compounds are typically eluted first using a strong organic solvent such as hexane. Following aliphatic elution, the column is rinsed with a second, more polar organic solvent, such as methylene chloride, to remove the aromatics. The resulting eluants are concentrated separately down to a specified volume before they are individually injected onto the GC-FID for analysis.

EPH fractionation technique #3: LC fractionation & GC-FID

Liquid Chromatography (LC) fractionation is a process that can also be used to separate aliphatic and aromatic compounds, these systems are typically referred to as “LC-fractionators”. Once the initial sample is extracted, it is placed on the LC fractionator and is subsequently injected into a silica LC column. The pump on the instrument controls the flow of the mobile phase and aliphatic compounds are eluted initially with hexane into their respective collection tubes. As seen with the SPE fractionation technique, aromatics are then eluted using methylene chloride and collected into a separate collection tube. Again, the tubes are separately concentrated down to a specified volume before they are individually injected into the GC-FID for analysis.

Comparing EPH fractionation techniques: Which is right for your lab??

As you can see, you have options for your EPH Fractionation. But which option is best for you depends on your specific laboratory needs and limitations. GCxGC-FID is pretty much a one-stop shop for your fractionation and analysis, but it can be costly. A typical system setup can be upwards of $200,000 USD depending on the brand, setup and add-ons. Additionally, some systems require cryo-cooling units so that is an added cost. Run times for analysis typically are ≥30 minutes, meaning that one system can process roughly 24 samples in about 12 hours.

LC Fractionation &GC-FID gives you automation using an LC approach. A new system can run upwards of around $40,000 USD. Many models have a large benchtop footprint so space can be a limiting factor. Run time on a Gilson is approximately 20 minutes/sample meaning a 24-sample batch will take 8 hours to fully fractionate. Additionally, since it employs liquid chromatography to perform fractionation, solvent consumption can be an issue. Current established methods detail using upwards of 220 mL of total solvent per sample to fractionate aliphatic and aromatic compounds. This can be a significant cost for smaller or budget-conscious labs. All that solvent will have to be concentrated prior to GC-FID detection which is more added time. It’s easy to consider this method to be a 2-day process from start to finish.

SPE-FID uses minimal solvent to perform fractionation in comparison to Gilson Fractionation. It can be performed manually or automated and uses less than 12mL of total solvent per sample from conditioning and rinsing to fractionation. Either system has a relatively small footprint to make them convenient for setups where bench space is a commodity. A manual SPE system can run around $5000 USD roughly and an automated system will be around $60,000 USD. Processing time for fractionation is roughly 1.5 hours to process 48 samples allowing you to have the capability of fractioning many samples daily. Elution volumes for each fraction will be 2-3mL so concentration will not take a significant amount of time, and you can begin analyzing much quicker. A standard GC-FID analysis usually takes around 15 minutes/sample for analysis.