Safe soil – identifying PAHs

Suli Zhao and Andy Zhai take us through some of the analytical techniques used for identifying polycyclic aromatic hydrocarbons in soil samples

Soil contamination is a growing environmental problem that involves the introduction of unwanted chemicals into the ground¹. Polycyclic aromatic hydrocarbons (PAHs) are chemical compounds that consist of fused aromatic rings and do not carry substituents. They exist naturally in the environment but can also be manmade, produced as the by-product of incomplete combustion². PAHs are contained in oil, coal, and tar deposits, but can also occur through natural combustion processes such as volcanic eruptions and forest fires³.

PAHs are lipophilic; therefore they mix easily with oil and not water⁴. This property of PAHs means that they are found primarily in soil, sediment and oily substances. Exposure to PAHs occurs primarily through inhalation, but it can also occur in small amounts through consumption of food grown in contaminated soil⁴.

PAHs are pollutants and are of concern because chronic exposure to some compounds has been shown to be mutagenic and carcinogenic in animals^3,5. The harmful effects of PAHs were brought to light following the 2010 Gulf of Mexico oil spill: there are fears that PAHs may have long-term sub lethal effects on marine organisms that were at the peak of their spawning season when the spill began. Should the PAHs stunt growth, this could affect ecosystems as smaller organisms are consumed by predators at a young age – depleting reserves for larger and rarer fish⁶.

Chemical pollution can cause environmental, economic, and health problems and therefore regular and reliable monitoring for PAHs is of utmost importance⁷. In 2008 in the UK alone, the PAH monitoring network consisted of 31 PAH monitoring sites (ranging in location from rural to industrial), with an additional six sites providing samples for additional PAH analysis on a quarterly basis⁸.

The US Environmental Protection Agency (EPA) has designated 16 PAH compounds as priority pollutants that are often targeted for measurement in environmental samples, these are: naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene, benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene.

The tightening of regulations by environmental agencies such as the EPA has caused an increase in the demand for rapid and accurate transportable instruments for the identification of chemical pollutants present in the field. In addition, as a result of advancements in this field, the analytical methods for PAHs are growing. A number of techniques have been developed for the analysis of PAHs in soil samples, such as gas chromatography-mass spectroscopy (GC-MS), high performance liquid chromatography (HPLC), and biological methods such as enzyme immunoassays (EIA)⁹. As well as the analytical instrument, pre-treatment of samples is important when analysing pollutants in the soil. In summary, the type of pre-treatment of your sample and the choice of analytical technique are important when detecting pollutants in environmental samples.

The coupling of an enzyme immunoassay with a superheated water extraction is an appealing method due to the reduced use of hazardous solvents and therefore their antagonistic effect on the environment¹⁰. Pre-treatment using water extraction will significantly reduce the extraction time as there is no need to dry the sample, the sample can be extracted almost as it was taken. This is an advantage to this method as field samples are often wet, but common extraction methods such as supercritical fluid extraction (SFE) require dry samples¹⁰. The enzyme immunoassay, unlike GC methods which can be resource intensive, is rapid and cheap. Current advancements in the biological field have led to the development of a colour encoded microbead-based flow cytometric immunoassay (FCIA) for PAHs in food¹¹. The main advantage of the FCIA over the EIA is increased sensitivity; it is likely in the future that more sensitive biological assays will be produced for screening pollutants in soil or food samples.

HPLC with, for example, fluorimetric detection, is a suitable method for the detection of PAHs in soil and food and may have shorter retention times than methods such as GC-MS. However, for maximum sensitivity, specific excitation and emission wavelengths must be set for each PAH to be anaylsed^12,13. High sensitivity is required for detection of extremely trace PAHs and their derivatives but is difficult to obtain with many techniques. Insufficient sensitivity and impurities or matrix interference are main limitations of modern chromatographic techniques¹³.

GC techniques may be coupled with various detection methods and a number have been described for analysis of PAHs. GC combined with a mass spectrometer (MS) has been shown to increase sensitivity and resolution, and allow simpler identification of PAHs in the sample¹³.

Recently Agilent have developed the 5975T LTM GC/MSD with a sample pre-treatment procedure, SampliQ QuEChERS, and a retention time locking (RTL) database. The SampliQ extraction QuEChERS method for the pre-treatment of soil samples provides cleaner chromatograms, overcoming the previous problem of matrix interference, especially for late eluters. In general, the later-eluted PAHs often show poor sensitivity, therefore requiring a cleaner matrix. The cleanliness of the soil background allows accurate determination of the high molecular weight PAHs. Additional benefits of the SampliQ extraction QuEChERS method include faster extraction time, simplicity, and reduced chemical reagent costs. Therefore this method is extremely complementary to the transportable 59575T GC/MSD. Lastly the RTL function provides excellent software for rapid peak identification and is amenable to field analysis when urgent determination of compounds is a necessity.

When natural disasters or suspicion of PAH contamination arise, it is essential that fast and reliable detection methods are available for the protection of workforces, the general public and the environment. Modern techniques such as GC-MS, with the right pre-treatment procedure, offer reliable results through clearer chromatograms with minimal matrix inference.