Scientific Analysis Laboratories Ltd An Introduction to Aliphatic and Aromatic TPH analysis by two dimensional GC / FID David Smith, Technical Director Scientific Analysis Laboratories Ltd dsmith@salltd.co.uk
Scientific Analysis Laboratories Ltd Aliphatic and Aromatic TPH – A New Dimension ! ● A requirement of environmental fate and risk analysis is the separation of aliphatic from the aromatic hydrocarbons ● Further breakdown to carbon banding chain lengths ● The volatile fraction (C 5 -C 10 ) is analysed by GC/MS (headspace), where the aromatics can be identified and quantified individually and hence separated from the aliphatics using Mass Spectrometry and selected ions ● For years, the separation of aliphatic and aromatic fractions in the extractable fraction (C 10 -C 40 ) has involved the use of a physical bench top separation technique such as Solid Phase Extraction (SPE)
TPH CWG ● Measuring the total concentration of petroleum hydrocarbons (TPH) in soil does not give a useful basis for the evaluation of the potential risks to man and the environment ● Hydrocarbon fractions divided into aliphatic and aromatic fractions and supplemented by analysis of single compounds. This will be a much more useful basis for carrying out risk assessments
Criteria Working Group The "Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG)" was formed in the USA in 1993 with the goal to develop scientifically defensible information for establishing soil clean up levels that are protective of human health at hydrocarbon contaminated sites. The group had more than 400 participants from the oil industry, consultant, several state governments and the US EPA. The group has published 5 reports encompassing their findings and their recommendations: Vol-1- Analysis of Petroleum hydrocarbons in Environmental media Vol-2- Composition of Petroleum mixtures Vol-3- Selection of TPH fractions Vol-4- Fraction specific classes and reference concentrations Vol-5- Risk evaluation
Risk based Approach Two sites may have TPH measurements of 500 ppm but constituents at one site may include carcinogenic compounds while these compounds may be absent at the other site The risk at a specific site will change with time as contaminants evaporate, dissolve, biodegrade, and become sequestered A valid correlation between TPH and risk would have to be site- and time-specific, related to a single spill, and, even then, the correlation might not be the same around the periphery of a plume where the rate of compositional change accelerates
CWG Fractions The American Petroleum Institute (API) that found it necessary to modify the method slightly to incorporate evaluation of heavier petroleum fractions. This resulted in a suggestion to prolong the highest fraction (from EC35 to EC44 ), where aromatics and aliphatics are assessed separately, and add an even heavier fraction (EC44+ ) that includes both aliphatics and aromatics Aliphatic fractions Aromatic fractions >5 to 6 >5 to 7 >6 to 8 >7 to 8 >8 to 10 >8 to 10 >10 to 12 >10 to 12 >12 to 16 >12 to 16 >16 to 21 >16 to 21 >21 to 35 >21 to 35 >35 to 44 >35 to 44
CWG Fractions
Equivalent Carbon Numbers The Equivalent Carbon Number, EC, is related to the boiling point of a chemical . normalized to the boiling point of the n-alkanes or its retention time in a boiling point gas chromatographic (GC) column Hexane contains six carbons and has a boiling point of 69 o C. Its equivalent carbon number is six. Benzene, also containing six carbons, has a boiling point of 80 o C. Based on benzene’s boiling point and its retention time in a boiling point GC column, benzene’s equivalent carbon number is 6.5 This approach is consistent with methods routinely used in the petroleum industry for separating complex mixtures and is standard for CWG analysis Note that for molecules with higher relative carbon number indices, the disparity between aliphatic and aromatic hydrocarbons is great (see next two slides)
Equivalent Carbon Numbers
Equivalent Carbon Numbers
Hydrocarbons get into the environment !
Biodegradation Biodegradation processes can be very complex: The extent of biodegradation is dependent on many factors including: ● The type of microorganisms present ● Environmental conditions (temperature, oxygen levels, moisture etc.) ● Predominant hydrocarbon types ● Bioavailability of hydrocarbon contaminants n-Alkanes biodegrade rapidly with branched alkanes and single saturated ring compounds degrading more slowly
Environmental Fate Soil - weathering Petroleum products released into the environment undergo weathering processes with time. These processes include: ● Evaporation ● Leaching (transfer to the aqueous phase) through solution and entrainment (physical transport along with the aqueous phase) ● Chemical oxidation ● Microbial degradation (Christensen and Larsen, 1993) The rate of weathering is highly dependent on environmental conditions. For example: gasoline, a volatile product, will evaporate readily in a surface spill, while gasoline released below 10 feet of clay topped with asphalt will tend to evaporate slowly Evaporative processes are very important in the weathering of volatile petroleum products, and may be the dominant weathering process for gasoline
Environmental Fate .
Environmental Fate – Waters - Leaching Leaching processes introduce hydrocarbon into the water phase by solubility and entrainment. Aromatics, and especially BTEX, tend to be the most water soluble fraction of petroleum Petroleum contaminated groundwater tends to be enriched in aromatics relative to other petroleum constituents Relatively insoluble hydrocarbons may be entrained in water through adsorption into kaolinate particles suspended in the water or as an agglomeration of oil droplets (micro- emulsion) In cases where groundwater contains only dissolved hydrocarbons, it may NOT be possible to identify the original petroleum product because only a portion of the free product will be present in the dissolved phase. As whole product floats on groundwater, the free product will gradually lose the water-soluble compounds. Whole products have highly distinctive GC fingerprints relative to water-soluble fractions. Groundwater containing entrained product will have a GC fingerprint that is a combination of the free product chromatogram plus enhanced amounts of the soluble aromatics
Associated Quality in the Analysis Validation 11x2 by 3 matrices + CRM's Quality Contol charts – Spiked LRM's System Suitability Controls Ongoing Calibration checks every 19 Blanks (reagent blanks method blanks, Trip blanks) Multipoint Calibrations Proficiency Testing Contest / Aquacheck / RTC MCERTS UKAS
Scientific Analysis Laboratories Ltd Historical Aliphatic / Aromatic separation technique. Solid phase extraction Sample extracted in organic solvent Cartridge conditioned Aliphatic fraction collected Aromatic fraction collected Concentration Analysis Drawbacks Breakthrough More stages for errors to occur Reproducibility Interferences Fraction losses
Scientific Analysis Laboratories Ltd What is two dimensional GC and how it works
What is two dimensional GC and how it works ● Separation of complex compounds without the need of bench top techniques such as SPE ● Two columns working in series ● First column separating by boiling point ● Second column separating by polarity ● Controlled by a Modulator ● Ideal for use with an FID Advantages over conventional GC ● No physical split, no losses occur and a 100% split is achieved ● No 'breakthrough' can occur and variances in SPE cartridges and their conditioning are a thing of the past, ensuring that data is more consistent, reliable and reproducible ● Multi stage, error prone preparation steps are negated
What The following slides will show...... ● Sets of standards showing how 2 dimensions are better than one ● Real life samples what they look like performed this way ● Comparison of new chromatograms against 'old' ● Examples of interferences encountered in the conventional method and how GCxGC counteracts these
443743-017 total TPH chromatogram
443743-017 Cartridge split method
443743-017
Aliphatic only
Ali / Aro Standard
Aliphatic and Aromatic compounds
Kerosene
Diesel
Lube Oil
493659-009
493659-009
493659-009
493659-009
493659-009
493659-009
493659-009
Positioning Standard
493659-009
438161-001
432682-060
Humic acids
445758-001 Humic Acids
All Humic Acids (tail view)
Mineral oil humic acid mix (tail view)
Phenolic compounds (tail view)
Aliphatic and Aromatic TPH analysis by two dimensional GC / FID ● 100% recovery ● Guaranteed split ● More accurate and reliable results ● Able to deal with interferences better ● Far better chromatographic resolution Thank you
Recommend
More recommend
