Gas chromatography (GC) is a technique used to separate the individual components of a complex mixture.
A very small quantity of sample is needed (less than 1/1000th of a gram). The sample is injected onto one end of a chromatography column, down which gas (the carrier gas) is passing (usually helium or hydrogen). The column is situated in an oven. When the oven is cool (room temperature) only volatile compounds that boil at low temperature will evaporate from the sample and travel down the column to be detected at the far end by a detector. As the oven is heated, components of the mixture that boil at a higher temperature evaporate and are transported down the column by the flowing gas. In this way the components in the mixture are separated according to the temperatures at which they boil.
There are two basic types of chromatography column. These are packed columns and capillary columns.
Capillary column GC first became commonly used in the late 1970’s. The greatly increased separation efficiency of capillary columns compared with packed columns has resulted in their widespread use for oil and environmental analysis to the extent that few people now use packed columns. In parallel with the development of more efficient column, improvements in electronics and data capture and process by PC reduced the relative cost of chromatography as an analytical technique. Concurrent with these technical developments, increasing political awareness of green issues fueled a growth in the development of chromatographic methods for environmental analysis, which in turn were passed into legislation. The US Environmental Protection Agency developed many of these methods which are used worldwide.
Early Gas Chromatography instruments utilised either a thermal conductivity detector, suitable for analysing to a wide range of compounds or the more selective but higher sensitivity flame ionisation detector (FID) with is particularly suited to the analysis of hydrocarbons. Standard analyses developed around these instruments for example BTEX, (benzene toluene, ethyl benzene and xylene) analysis for water or soil samples contaminated by input of gasoline, diesel or fuel oil has been used for many years along side TPH (total petroleum hydrocarbons) both of which are still in widespread use today. Analysis by gas chromatography is based on a compound’s retention on the column as it passes along its length.
However, the possibility of co-elution of the compound of interest with other components of similar boiling point/polarity cannot provide unambiguous identification/quantification of the compounds present, particularly in a complex hydrocarbon mixture. Replacing the FID detector with a mass spectrometer (MS) provides a much greater degree of specificity. With the instrument operating in scanning mode, mass spectral information derived from fragmentation of the molecule under electron impact (EI) ionisation conditions can be compared with standard reference library spectra to assist in identification of the component.
Reference standards can be examined under the same analytical conditions to provide unequivocal confirmation of the identity of the compound and to calibrate the instrument for quantitative purposes. If required, a substantial increase in sensitivity (typically a factor of 10 or more) can be obtained by operating the detector in selected ion monitoring (SIM) mode, monitoring a restricted number of ions for a longer period. The use of the mass spectrometer as a detector has enabled analysts to perform the sophisticated analyses we expect today.
Aromatic hydrocarbons, in general, exhibit greater acute toxicity than aliphatic hydrocarbons. It is generally considered, therefore, that they have a more deleterious effect on the environment in the event of a pollution incident. For this reason, in many environmental surveys, special emphasis is placed on monitoring polycyclic aromatic hydrocarbons (PAHs).
The patterns obtained by GC analysis of PAHs from petrogenic sources are generally complex and are made even more difficult to interpret by biodegradation and weathering. In addition, due to their potential toxicity, it is often required to detect PAHs at very low concentrations in samples. This cannot be achieved using GC with a flame ionisation detector (FID); a more specific detector is necessary where, in addition to quantification, the identity of the component can be confirmed. This can be achieved using GC-MS.
Fingerprinting oil spills
One of the more demanding applications for chromatography is the fingerprinting of oil products associated with oil spills at sea. Whether due to deliberate emptying of bilges or accidental discharges, once the product has entered the sea the more volatile components are lost almost immediately and weathering and biodegradation begin to take place. These processes can affect the composition of the spilled oils significantly, and make correlation of the spilled and suspect source oils by gas chromatography with flame ionisation detection (GC-FID) problematic. The level of specificity afforded by GC-FID alone is also relatively limited, so unless the reference and spilled oils differ markedly in composition, further analysis by gas chromatography- mass spectrometry to compare the ‘biomarker’ fingerprints of the oil samples is often necessary.
A ‘biomarker’ is any organic compound detected in the geosphere whose basic skeleton suggests an unambiguous link with a known contemporary natural product. After organisms die the natural products they contain are incorporated into sediments where various processes take place to alter the structure of the ‘natural’ molecules. The reactions that take place depend on the temperature and type of the sediment, in which the organisms are buried. Increasing burial depth and the associated rise in temperature and pressure, give rise to further alterations of the molecular structure resulting in a complex range of ‘biomarkers’. It is during this latter stage that petroleum, containing the ‘biomarkers’, can be formed and be trapped in a reservoir.
The assemblage of ‘biomarkers’ in a crude oil not only reflects the environment of deposition of the original sediment, but also the geothermal history of the source rock. Thus their distribution is normally characteristic of a particular oil field and hence crude oil. Because of their specificity and resistance to biodegradation, petroleum biomarkers can be extremely useful in correlating suspected source oils (both crude oils and products refined from them) in environmental contamination incidents. Using gas chromatography-mass spectrometry (GC-MS) with selected ion monitoring (SIM), ions characteristic of particular biological marker and polyaromatic hydrocarbon (PAH) classes can be monitored to obtain the mass chromatograms used for oil ‘fingerprinting’ (e.g m/z 191 (triterpanes) and m/z 217 and m/z 218 (steranes)). Correlation of diagnostic ratios derived from these compound classes can then be used to give a quantitative assessment of the degree of similarity/dissimilarity between spilled and suspect source oils.
Ageing input into brownfield sites
Thankfully major pollution incidents associated with supertankers are rare. More common are small but continuous leaks of diesel or fuel oil from buried tanks on redundant brown field sites and the contamination passing in groundwater from the site into nearby streams and rivers. The weathering of diesel in soil is much better understood and therefore easier to quantify than in the oceans. The ageing of diesel input into soil can be estimated as long as care is taken to ensure that the sample is collected from above the water table (to prevent the input of fresh material in the water phase) and certain other prerequisite conditions are met.
The technique uses variations in the speed of biodegradation between selected normal-alkanes and more resistant branched hydrocarbons (isoprenoids), the ratio of the heights of the n-C17 and pristane peaks being used in the ageing calculations. In many cases the GC-MS ‘fingerprinting’ approach detailed above can also be used to provide a greater degree of correlation, using a range of lower-boiling biomarkers (bicyclic sesquiterpanes), which have been identified as being of utility in comparing and distinguishing middle distillate fuel sources (including kerosenes) in environmental samples.
Further correlation of the likely age of diesel inputs can also be obtained by determination of the sulphur content, as the sulphur content of automotive diesels this has been progressively reduced over the past 15 years, from 2000ppm to the <10ppm found in the ultra-low sulphur diesels of today.
Once the ownership of the site at the time of the input has been decided, the responsibility for the remediation can be established.
Brownfield sites that are undergoing remediation can contain drums of unknown chemicals. Mass spectrometry can be used to identify these materials so that they can be disposed of in an appropriate manner. The material in the drum is dissolved in a suitable organic solvent and then analysed using the GC-MS technique.
The mass spectra obtained from the components separated on the column can be compared with commercial databases of known compounds to determine their identity. Where there are no matches with compounds in the database, the mass spectrum has to be interpreted from first principles, often with the assistance of data obtained from other mass spectrometric and analytical methods.
Identification of unknown pollutants on brownfield sites
Brownfield sites can also contain unknown polymeric materials which require identification so that they can be disposed of safely. Two mass spectrometric techniques can be used for such analyses. In the first, Evolved Gas Analysis Mass Spectrometry (EGA-MS), the unknown material is heated from ambient temperature to ca. 650°C and the resulting volatile, semi-volatile and pyrolysis products passed to a mass spectrometer. The pyrolysis products produced are often characteristic of the original polymer present and can be used to identify the polymer.
When EGA-MS analysis does not provide unequivocal identification of the polymer, then a technique known as pyrolysis gas chromatography mass spectrometry (PY GC-MS) can be used. In this technique, the polymer is heated at a fixed temperature (e.g. 700°C) and the pyrolysis products analysed by GC-MS. Combining the separation power of a chromatographic column with the specificity of a mass spectrometer can identify polymers that cannot be distinguished by EGA-MS alone.
Making the polluter pay
Proof of ownership is key to determining responsibility for pollution events arising from leaks in petrol station tanks. The analysis of petrol to identify whether it is leaded, LRP or unleaded can provide insight into the age of a leak but more specific information can be obtained by branding the petrol residues.
Pump petrol is produced to an EU-wide standard (EN228). It is relatively straightforward to detect the presence of petrol in environmental samples, utilising target compounds such as monoaromatic hydrocarbons and the common oxygenate methyl tertiary butyl ether (MTBE). The relatively high degree of water-solubility (and its consequent mobility in the sub-surface environment) make MTBE useful as a ‘pathfinder’ component for gasoline in groundwaters. The close degree of compositional similarity of modern UK petrols does not readily permit specific identification of the source of a gasoline spill, even in cases where the spill is relatively recent and reference samples are available for comparison, which is frequently not the case.
However, each of the major suppliers has a unique additive package which is added to their fuel at the terminal. Chromatography based techniques can be used to identify some of these additives and thereby determine the brand (or brands) of the contaminating fuel. Petrol branding is particularly useful where a petrol station has been owned by a number of different suppliers over the years.
Not all soil and fresh water contamination is due to petrochemical products. Brownfield sites can contain a wide variety of contaminants (eg detergents, dioxins and PCBs), reflecting changes in use of the site over a century or more. There are however products that have only been used for a decade or so but the understanding of which has led to them being withdrawn from production and treated as a potentially harmful pollutant. Perfluoroctane sulphonate (PFOS) is one of these products. PFOS was previously used in the furniture industry for stain protection, in the garment industry for waterproofing rainware and in firefighting foams.
Most commonly found near fire training grounds at larger industrial sites and airports it leaches into soil and hence into groundwater. PFOS is now a proposed ‘Persistent Organic Pollutant (POP)’ so groundwaters from sites adjacent to drinking water supplies are sampled and analysed regularly. Even at low levels PFOS and its degradation products can be toxic and so highly sensitive methods based on liquid chromatography with either single or tandem mass spectrometry detection have been developed for its identification and quantification.
Liquid chromatography with mass spectrometry detection (LC-MS) is also used for the detection of another carcinogenic contaminant found in stream and river water. Pterosin B is a naturally occurring compound produced by ferns. Its carcinogenic effects are known to affect cattle that have eaten young fern fronds but it can also be washed by rainfall into streams and rivers. Where potable water is collected from upland springs and streams there is the possibility of low level contamination by Pterosin. This contamination is once again in the parts per billion range but can be quantified by LC-MS.
Air sampling for VOCs and odours
High sensitivity and sub part per billion detection limits are also required for the identification and quantification of odours and non odourous, volatile organic compounds in air. Although sorption tubes are easy to use and offer instantaneous results they tend to be useful for looking for specific, pre-selected compounds only.
Chromatography, particularly when linked to a mass spectrometer as a detector, while requiring the sample to be returned to the laboratory for analysis, offers improved detection limits (down to part per trillion levels) and identifies most of the volatile organic compounds present. The technique generally selected for the analysis of odours and other volatile organic compounds in air is Thermal Desorption Gas Chromatography with Mass Spectrometry Detection. To use this technique the sample is initially collected by being drawn onto a tube packed with the porous polymer Tenax using a small hand held pump. Tenax is selected as it is a good general adsorbent suitable for use with a wide range of compounds, though other materials are also available and can be used singly or as double packings. Having collected a known volume of VOCs the tubes are sealed and transferred to the laboratory for analysis.
The process of releasing the VOCs from the Tenax involves heating the tube while an inert carrier gas is passed over the porous polymer. The VOCs once liberated are collected on a cryotrap at -30oC prior to being flash vapourised into a GC-MS for analysis. When analysing odours, the concentration of each compound detected can be compared with known odour threshold levels; these are the concentration of the compound at which its odour can be detected by 50% of the population. In this way the compounds contributing to the odour can be separated from all the other volatile organic compounds present in the sample.
Thermal Desorption GC-MS is suitable for the analysis of both indoor and outdoor air and has also been used for the analysis of volatiles in vehicles and aircraft. There has recently been particular concern in the press about the presence of tricresyl phosphate vapours in aircraft cabins which can be analysed by this technique. The main areas of use however remain the boundary fence survey for volatiles emitted from industrial sites, which can be performed together with an analysis of pre- and post-scrubber samples, monitoring the efficiency of the site’s odour suppression equipment.
Not all applications of odour suppression monitoring are industrial. Restaurants can produce nuisance odours, particularly if you are vegetarian or do not like ethnic cuisine. Active measure to limit odours can in fact also benefit the restaurant, as selecting the best extraction/odour suppression systems can also reduce the hazardous build up of highly flammable grease deposits in ducting and pipe work carrying volatile laden extracted air from cooking areas.
For the future
Chromatography and allied techniques can identify a wide range of compounds. The identification of unusual compounds in the environment is the first step towards identification of their source and hence the polluter. The sensitivity and specificity of modern techniques particularly GC-MS and LC-MS enable even more detailed characterisation of compounds than was previously available with chromatography alone. New analytical methods, developed in response to our improved understanding of the toxicity of the compounds around us, are enabling our legislators to further reduce the acceptable levels of harmful compounds in our environment.
Published: 10th Mar 2009 in AWE International