Chromatography plays an important role in monitoring pollutants in the environment for identification and quantification. Capillary column gas chromatography greatly increased separation efficiency compared to the traditional packed columns. This led to widespread use and acceptance for oil analysis and other contaminants in the environment. Gas chromatography is often used for the analysis of environmental pollutants.
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Gas chromatography for environmental monitoring
Gas chromatography requires a very little sample. The methodology involves the sample being injected onto the end of a chromatography column containing a carrier gas, often helium or hydrogen. The column is situated in an oven and at room temperature, the volatile compounds boil at low temperatures and thus evaporate down the column.
Once the oven is heated, the other components of the sample mixture that boil at higher temperature evaporate and are then transported down the column by the flowing gas. The components in the mixture are separated according to their differing boiling points.
Additionally, gas chromatography instruments are often coupled with a high-sensitivity flame ionization detector (FID), which is especially suited to the analysis of hydrocarbons. Standard method development using GC-FID has been achieved for BTEX, (benzene toluene, ethylbenzene, and xylene) compounds and analysis for water or soil samples contaminated by diesel range organic (DRO) compounds or total petroleum hydrocarbon (TPH) compounds.
Analysis by gas chromatography is based on a compound’s retention on the column as it passes along its length and thus its affinity for the mobile or stationary phase.
One drawback of GC-FID is the possibility of getting co-elution of compounds in a sample mixture, which makes identification and quantification difficult to achieve, especially in complex heterogeneous mixtures. However, replacing the FID detector with a mass spectrometer (MS) can overcome this problem. Components of the mixture can then be compared with standard reference library spectra to help identify species.
Another contaminant of concern when monitoring the environment is aromatic hydrocarbons. These hydrocarbons are more harmful to the environment than aliphatic hydrocarbons and are often of greater interest for detection and remediation.
Additionally, GC-FID is used for fingerprinting oil spills at sea. The volatile components are usually lost immediately and weathering of the oil starts to take effect. The weathered oil can change composition, which makes it difficult to trace the spilled oil to the source product.
Furthermore, analysis of the GC chromatograph is quite difficult to interpret because of biodegradation, weathering, and low concentrations in samples. These compounds are better suited to GC-MS rather than GC-FID for quantification.
Gas chromatography-mass spectrometry (GC-MS) for environmental monitoring
Chromatography coupled with mass spectrometry is a more sensitive technique for environmental monitoring because it gives improved detection limits and resolves for co-elution of components, especially for volatile organic compounds.
Another technique used for the detection of volatile organic compounds is Thermal Desorption Gas Chromatography coupled with Mass Spectrometry. High sensitivity and detection limits of down to parts per billion are required for the identification and quantification of volatile organic compounds in the air.
Although many of the environmental monitoring using chromatographic techniques are for petroleum contamination in soil and water, there are a wide variety of other contaminants in the environment of concern. Some environmental pollutants have only been introduced relatively recently, but have been withdrawn from the production due to their harm to the environment.
Perfluorooctanesulfonic (PFOS) acid is commonly used in firefighting foam and is extremely toxic to the environment. Contamination of soil and groundwater is especially prevalent near firefighting training grounds, airports, and industrial sites. Low concentrations of PFOS can have significant effects on the environment, mainly soil and groundwater quality, thus very sensitive chromatographic methods coupled with mass spectrometry are used to detect and quantify levels.
Interferences in environmental monitoring
Interferences in environmental monitoring are a major concern when trying to isolate contaminants and quantify levels in soil and groundwater. There are many interferences present within soil samples due to the complexity of the sample matrix and the heterogeneity of the sample.
Soil type and particle size can have a significant effect when trying to separate components. Hence, the clean-up process during the sample preparation step in chromatography is a very important aspect in the method development stage.
The separation of components can be optimized by changing the separation phases. For example, changing the stationary phase to silica gel can separate components that are highly polar in contaminated sites. Another interference is the mixing of contaminants and the ability to accurately isolate them.
One example of this is the mixing of fats, oils, and greases (FOG) with DRO and PAH compounds in soil samples. Hence, method development is required to optimize the separation and speciate compounds. Additionally, coupling LC or GC techniques with MS has addressed many of the concerns around interferences and co-elution errors, thus improving environmental monitoring in general.
Sources:
- Santos, F.J & Galceran, M.T (2002) The application of gas chromatography to environmental analysis TrAC Trends in Analytical Chemistry 21:9-10 pp. 672-685.
- Paul AG, Jones KC, Sweetman AJ (January 2009). "A first global production, emission, and environmental inventory for perfluorooctane sulfonate". Environ. Sci. Technol. 43 (2): 386–92.
- Gosetti, F.; Mazzucco, E.; Gennaro, M.C.; et al. (2016). "Contaminants in water: Non-target UHPLC/MS analysis". Environmental Chemistry Letters. 14: 51–65.
Further Reading