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The analysis of environmental contaminants is a challenging and complex science.
Soil and water samples are formidable matrices. They frequently contain compounds or elements which present interference with analytical determinations. The laboratory techniques used have evolved from simple ‘bucket’ chemistry and are being replaced with sensitive analytical instrumentation with greater power to resolve analytes from interferent species.
With new analytical techniques, new and emerging pollutants are being detected where previously none were found. For example, polychlorinated biphenyls (PCBs) were first detected as an interference in the analysis of organochlorine pesticides by Gas Chromatography with Electron Capture Detection (GC-ECD); it was only with the evolution of Mass Spectroscopy (MS) coupled with GC separation that the PCB compounds were resolved and identified.
This advancement of technology enabled more information to be gleaned regarding chemical species, both in terms of toxicology and environmental detection. A classic example of this is mercury, a metal that was once widely used industrially and domestically, now known to be toxic. Of more use is the ability to detect mercury in various states and to understand its differing toxicity, methyl mercury being more toxic than inorganic mercury.
Organic compounds are now presenting the most pertinent of analytical challenges for anyone wishing to analyse environmental samples. The original ‘dirty dozen’ list of persistent organic pollutants as defined by the Stockholm Convention in 2001, and enforced since 2004, was a platform from which a long and wide ranging list of chemicals of interest were identified. A further nine compounds have since been added to this list, while more than 180 emerging contaminants have been detected in groundwater samples across Europe and remain open for incorporation.
What are Persistent Organic Pollutants (POPs)?
POPs are carbon based compounds which possess a particularly potent combination of physical and chemical properties. They remain intact for very long periods of time and become widely distributed throughout the environment, for example. They are toxic, accumulate in the fatty tissue of living organisms and bio-accumulate through the food chain.
Organo Cholorine Pesticides
Of the ‘dirty dozen’, nine are organo chlorine pesticides.
Aldrin was a widely used insecticide which has now been universally withdrawn from use.
Chlordane is a broad spectrum insecticide used in crop treatment. It has a relatively short half-life in soil of one year and the primary exposure route for humans is through the air. Chlordane is known to affect the human immune system and is a possible human carcinogen.
Another organo-chlorine pesticide is DDT. This was extensively used during World War II for protection from malaria, typhus and other diseases spread by insects. It then saw continued use in disease control for crop protection, especially in cotton production. DDT is stable in soil, with a half-life in the region of 10 – 15 years. With this in mind, it’s hardly surprising that it is a widespread POP, being found as far as the Arctic. Adverse effects on health are seen with chronic rather than acute exposure. Despite being banned in many countries, there are some places where DDT is still in use against mosquitoes.
Dieldrin was widely used to control termites, insects and insect diseases in agricultural soils. In addition to direct use, it is also converted from Aldrin and is therefore found at higher levels in soil than would otherwise be expected of other organochlorine pesticides. It has a half-life in soil of approximately five years and enters the water course readily, which is particularly problematic as it is highly toxic to aquatic species.
Another ‘drin’ pesticide is Endrin. As well as widespread use as an insecticide, Endrin has also been used as a rodenticide. It is unusual in that it can be metabolised by the body and is therefore less prone to bioaccumulation. The half-life in soil is relatively long at 12 years. As with Dieldrin, the toxicity effect is most prevalent in the aquatic environment. Heptachlor is an insecticide used for crop protection, which is particularly harmful to wildlife, being fatal to animals ingesting heptachlor treated seeds. It is also a possible human carcinogen.
Toxaphene has seen wide use as a crop protection insecticide. It has been used to protect cotton, grain, fruit, nut and vegetable plants. It has also been used in tick and mite control. This compound also has a relatively long half-life of 12 years and is a possible human carcinogen.
Hexachlorobenzene is a by-product of the manufacture of organochlorine pesticides and has found specific use as a fungicide.
Mirex is somewhat unusual in that, in addition to being an insecticide, it has also been used as a flame retardant in plastics, rubber and electrical goods. It is a possible human carcinogen and is the most persistent and stable of the organochlorine pesticides in the dirty dozen.
Organochlorine pesticides have been measured in environmental matrices routinely for a number of years, but as our understanding of the compounds increases, the analytical levels of interest decrease to lower and lower concentrations.
PCBs
There are 209 chlorinated biphenyl compounds, with the same general structure:
PCBs were widely produced and used from the 1930s through to the 1980s, but their use has since been phased out. Despite this they are still routinely detected in soils, animal materials and the aquatic ecosystem. They are detected and measured as either ‘aroclor’ compound groups or discrete congeners.
There are strict regulations regarding concentrations in force across the USA and the EU, but these prescribed limits may be analytically challenging. As each congener is so chemically similar, the discrete detection and measurement of individual species may not be possible with certain mixtures without the use of high end analytical techniques.
The evolution of analytical mass spectroscopy, now with high resolution or time of flight capability, means that these can now be resolved. It is, however, worth noting that routine techniques often have limits of detection higher than the typical background concentrations of 0.1 – 40 µg/kg.
PCDD and PCDFs
PCDDs and PCDFs (Dioxins and Furans) are poly chlorinated aromatic compounds, chemically similar to PCBs, with the general structure as below:
Of a theoretical 210 compounds 17 have been identified by the UK Health Protection Agency (HPA) as compounds of the highest interest, due to their similarity to the most toxic of dioxins, 2,3,7,8-tetrachloro-p-dibenzodioxin.
PCDDs and PCDFs enter the environment through similar mechanisms such as incomplete combustion – particularly of municipal, hospital and hazardous waste – and pesticide and chlorinated chemicals production. In addition, PCDFs are generated as a by-product in PCB production.
The compounds are structurally similar enough to yield similar toxic effects – they are also classified as possible carcinogens. Being lipophilic compounds, they bio-accumulate and are not readily metabolised, yielding long residence times both in the environment and animals.
The Stockholm Convention has since added a further six groups of compounds to the original list. • α-, β-, γ-Hexachlorocyclohexane (γ-hexachlorocyclohexane commonly known as Lindane) • Chlordecone • Brominated compounds including Poly Brominated Diphenyl Ethers (PBDEs) • Pentachlorobenzene • Pefluorinated compounds • Endosulphan and isomers
Hexachlorocyclohexanes
Hexachlorocyclohexanes are all insecticides, the α- and β- forms being by-products from the formulation and production of Lindane. Lindane is a broad spectrum insecticide used to treat both soils and seeds. It bioconcentrates rapidly and is prone to long range transport.
These compounds are particularly persistent in colder regions of the world, bioaccumulating and biomagnifiying in Arctic food webs. They are all classified as potential human carcinogens. Routine chromatographic methods exist for the detection of the isomers of hexachlorocyclohexanes, although powerful modern deconvolution software aids detection when such chemically similar compounds are present in mixtures.
Chlordecone
Chemically related to Mirex, chlordecone is an organochlorine pesticide used widely in agriculture. It is highly prevalent, prone to bioaccumulation, biomagnification and long range transport. It is very toxic in the aquatic environment and is a possible human carcinogen. It can be measured in the same way as Mirex, lending itself to Gas Chromatographic analysis techniques.
Brominated compounds
Poly brominated compounds have been widely used in flame retardants. Like other POPs they are highly persistent and bioaccumulate. They are also susceptible to long range transport and have been found in the tissues of animals living in the Arctic and Antarctic circles.
Similar to PCBs, these brominated compounds can be detected using standard GC-MS techniques, although often the mixture of compounds and challenging matrices mean advanced mass spectroscopy techniques are often necessary.
Pentachlorobenzene
Pentachlorobenzene is often found in PCB products (transformer oil or electronics, for example), and is also used in dyestuffs, as a chemical intermediate and as a flame retardant. It has a relatively high solubility compared to PCBs and is therefore of significant risk to the aquatic environment, for which it is a particularly potent toxin. Pentachlorobenzene lends itself well to GC-MS detection and determination, and has long been on the USEPA list of semi-volatile compounds for standard methods of analysis.
Perfluorinated compounds
Perfluorooctanoic acid (PFOA) and perfluorooctane sulphonate (PFOS) are the most analytically prevalent of the perfluorinatedcompound group highlighted by the Stockholm convention. They have seen widespread use in electronics, fire-fighting foam, photo-imaging, hydraulic fluids and textiles.
PFOA and PFOS are different to most other POPs as they bioaccumulate via a different pathway. Most compounds are lipophilic and bioaccumulate in fatty tissue and cell membranes, whereas the perfluorinated compounds bind to proteins in the blood and liver.
They are still in use in developing countries, but their use is restricted.
Analysis of these compounds, while relatively simple analytically, is problematic due to the ubiquitous background of these compounds in the environment. The analysis is therefore also prone to contamination and great care should be taken in all stages of sample collection and analysis.
Endosulphan
Endosulphan is a mixture of two isomers – alpha and beta endosulphan, or endosulphan I and endosulphan II. They are broad spectrum insecticides which have also been used as wood preservatives. It is persistent and found in soils, sediments, water, and the atmosphere.
From the atmospheric pollution of endosulphan, long range transport is possible and it has been found in living organisms as far as the Arctic. It is toxic and has been directly linked to congenital physical disorders. Despite this, endosulphan is still in production and use.
Endosulphan analysis is predominantly by chromatographic means – typically GC.
Looking forward
We should be careful not to overlook non ‘POP’ contaminants. A great deal of research has concentrated on arsenic and its species. Arsenic and its compounds are known to have adverse health and environmental effects, and the toxicity is heavily dependent on the chemical form. The speciation of arsenic is of particular interest in ground waters.
Arsenic may be found as inorganic arsenic (arsenite – AsIII and arsenate – AsV ) and organic arsenic (typically mono methyl arsonic acid – MMA and dimethyl arsonic acid – DMA). Inorganic arsenites have a much higher acute toxicity than organic arsenates. In fact, there are abundant arsenic species in the marine environment which are completely non toxic, such as arsenobetaine, arsenocholine and arsenosugars.
Environmental sources of arsenic may be both natural and anthropogenic. Historically, arsenic has been used in medicine as treatments for syphilis and psoriasis, for example. It has also seen use as a chemical warfare agent, pesticide and wood preservative. Inorganic arsenic may undergo biomethylation to less toxic forms such as arsenobetaine. It is therefore important to understand the species of arsenic.
Arsenic is particularly susceptible to entry to groundwater in areas of geothermal activity. Not only does the mineralogy tend to be high in levels of arsenic, but also dissolution, weathering, sorption/desorption and leaching processes are enhanced by geothermal heating. It is therefore critical to monitor groundwater arsenic with a view to both total concentrations and species present.
Recent technological advancements have given a number of viable options for arsenic speciation analysis. Chromatographic separation techniques have been coupled to either inductively coupled plasma – mass spectroscopy (ICP-MS) – or hydride generation atomic fluorescence instruments (HG AFS).
Perhaps of more interest, however, is the use of solid phase extraction cartridges. These enable sampling at source, fixing the species in their relevant states and then analysis at a laboratory. This is in fact the recommended approach from the US Geological Survey’s ‘Techniques of Water-Resources Investigation’ report. As these techniques become more standardised and readily available, our understanding of environmental arsenic and the associated risks will grow.
What’s next?
New compounds are constantly being added to ‘watch lists’. These compounds may be either newly detectable due to advancements in analytical techniques, or only recently categorised. Often new and emerging pollutants have insufficient data to derive threshold values due to insufficient data relating to occurrence, toxicity, impact or environmental behaviour.
Surveys of environmental data from the EU have indicated detectable and measurable concentrations of DEET, caffeine, PFOS, carbamazepine and atrazine in groundwaters. Within the UK Bisphenol-A and Triclosan have also been detected. These compounds are rapidly making their way on to target compound lists, stimulating the laboratories to develop robust, sensitive methods for analysis. Often the challenging nature of this analysis yields further discoveries of ‘new’ compounds. One only has to consider the initial detection of PCBs to realise the potential impact of new and more sensitive techniques.
Published: 27th Feb 2014 in AWE International
