Polycyclic aromatic hydrocarbons

Toys, barbecued food, cosmetics – polycyclic aromatic hydrocarbons occur in many everyday products and are almost impossible to curb. What makes them so, how are they formed and what risks do they pose to health and the environment?

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Polycyclic aromatic hydrocarbons in the model
© Сергей Шиманович – adobe.stock.com

What are polycyclic aromatic hydrocarbons?

Polycyclic aromatic hydrocarbons (abbreviated to PAHs) are a substance group consisting of several hundred individual organic compounds. Benzo[a]pyrene (BaP), naphthalene and phenanthrene are the most well-known and most widely analysed representatives of this substance group due to their frequent occurrence in the environment and their toxicity.

The chemical structure of PAHs

Polycyclic aromatic hydrocarbons are complex molecules that consist of several benzene rings. A benzene ring is a simple aromatic ring consisting of six carbon atoms arranged in a planar, hexagonal structure. Each carbon atom in the ring is bonded to a hydrogen atom. This structure gives the benzene ring its aromaticity – a special form of chemical stability present over the entire ring due to delocalised electrons.

In polycyclic aromatic hydrocarbons, two or more of these benzene rings are bonded together – as indicated by the term polycyclic. This bond can be at either common edges or corners of the benzene rings. The exact way in which the rings are bonded together as well as the number of rings involved determine the specific characteristics and the reactivity of the related PAH.

Analysis for PAHs at Quality Analysis

Analysis for PAHs using gas chromatography with coupled mass spectrometry

At Quality Analysis, we rely on advanced, analytical methods for the precise identification of polycyclic aromatic hydrocarbons (PAHs) in accordance with AfPS GS 2019:01 PAK. Our laboratory is equipped with the latest equipment that enables us to identify and quantify reliably even the smallest traces of PAHs in numerous sample matrices.

Our expertise ranges from the analysis of environmental samples, foodstuffs, cosmetics and many other materials. Here we utilise established methods such as  gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC),to ensure precise, informative results. The measured data are evaluated using advanced software solutions that permit detailed analysis and interpretation of the results.

The characteristics of polycyclic aromatic hydrocarbons

The characteristics of the individual PAHs depend on the number of carbon rings present and their combination. In general, it can be stated that polycyclic aromatic hydrocarbons are lipophilic. This means that they are difficult to dissolve in water, but are easy to dissolve in lipids and oils. The more rings a PAH has, the greater its lipophilicity. This property is due to the increasing hydrophobicity and reducing polarity with an increasing number of rings.

PAHs are also predominantly neutral, non-polar substances that are absorbed well by dust and soil particles, irrespective of the molecular size of the PAHs. These characteristics affect their distribution in the environment and their tendency to accumulate in biological systems.

Lighter and heavier polycyclic aromatic hydrocarbons

The substance group is divided into lighter (low molecular weight) and heavier (high molecular weight) polycyclic aromatic hydrocarbons. Lighter PAHs consist of two to three rings and are volatile, that means they change to a gaseous state. Heavy PAHs consist of four to seven rings. With each ring their volatility reduces and their molecular weight increases. As such heavier PAHs tend to be more persistent, more bioaccumulative and more toxic.

The characteristics of pahs that are harmful for humans and the environment

Numerous PAHs are also demonstrably carcinogenic (that is they cause cancer), mutagenic as well as teratogenic. This statement applies particularly to heavy polycyclic aromatic hydrocarbons. After absorption, the PAHs are metabolised in the body; during this process they are converted into reactive compounds with the potential to interact with DNA. These interactions can result in genetic mutations that promote the development of cancer. Benzo[a]pyrene, a heavy PAH, is particularly well-known for its carcinogenic characteristics. The risks related to exposure to PAHs depend on the concentration of the PAHs, the duration of the exposure and the nature of the exposure. Extended or more intensive exposure to carcinogenic PAHs increases the risk of the development of cancer.

Formation and occurrence

Polycyclic aromatic hydrocarbons are formed during the incomplete combustion of organic materials such as coal, oil or wood. Although they can be formed by natural processes such as forest fires or volcanic eruptions, a large portion of the PAHs to be found today in nature can be traced back to human activities such as industrial processes related to refining oil or producing metal, generating energy or coal chemistry. Via waste gases, dust particles and ash, the PAHs enter the atmosphere and are distributed in the air. They also enter bodies of water and the ground by means of precipitation.

How are PAHs analysed?

The analysis of polycyclic aromatic hydrocarbons (PAHs) in different settings such as environmental samples, foodstuffs and materials requires specialised analytical methods to determine their presence and concentration reliably. An important step in this process is taking and preparing the sample; this procedure is adapted depending on the nature of the matrix – whether soil, water, air or biological material.

Analytical methods for the detection of PAKs

Samples from various sources are taken and correspondingly prepared for the analysis of PAHs; this activity often involves the extraction of the PAHs from the related matrix. Here various techniques such as solid phase extraction (SPE) or liquid-liquid extraction are used. One of the methods most frequently applied to separate and identify PAHs is gas chromatography, often in combination with  mass spectrometry (GC-MS). This combination makes it possible to break down complex mixtures into their constituents and to identify and quantify precisely the PAHs compounds contained in the mixtures, as is necessary above all during plastics analytics.

Along with gas chromatography, high-performance liquid chromatography (HPLC) is used, often in combination with a UV detector or fluorescence detector. This method is particularly useful for PAHs with a greater molecular mass that are not suitable for GC methods. Spectroscopic methods such as fluorescence spectroscopy can also be used for quick screening analyses. This method is helpful for the identification of PAHs in mixed samples because many PAHs have characteristic fluorescence spectra.

Quality control and standards

Quality control methods such as the use of internal standards and the implementation of replicate analyses are implemented to ensure the results of the analysis are dependable and accurate. Methods must also be validated to determine their accuracy, precision and verification limits. The analysis of PAHs is therefore a complex process that requires specialist knowledge and equipment. Using these methods, researchers and environmental experts can monitor and evaluate the presence and concentration of PAHs in different environments so that health and environment-related risks can be better understood and managed.

What can contain polycyclic aromatic hydrocarbons?

PAHs can occur in numerous products in everyday life, often as the result of specific manufacturing processes, or in the form of unintentional contamination. A classic example are foodstuffs such as barbecued meat and smoked foods. These often contain PAHs that are formed in the embers and smoke at high temperatures and can penetrate the food.

Combustion engines and rubber boots

A further ubiquitous source of PAHs is the exhaust gases from combustion engines. These PAHs can be deposited on surfaces close to busy roads and in this way indirectly enter our environment and our bodies. Plastic and rubber products such as car tyres, rubber boots or toys often contain PAHs for reasons of elasticity, durability or resilience; these PAHs are intentionally embedded in the material during the manufacturing process

Where durability and resilience are required

Products manufactured from tar or bitumen, such as roofing felt and road surfaces, can also contain PAHs. They are also sometimes used in wood preservatives and paints to increase durability and resilience to external effects. And they can even occur in cosmetics, for example in products that contain coal tar such as shampoos or skin creams.

Statutory regulations for limiting PAHs

The European Union has specified limits for PAHs in plastic and rubber products to curb the environmental and health effects emanating from polycyclic aromatic hydrocarbons. Since 27 December 2015 it is forbidden to place on the market everyday products that contain more than 1 mg/kg of one of eight PAHs classified as carcinogenic. This rule is anchored in the REACH regulation (no. 1907/2006).

Accordingly, manufacturers and importers had to modify their production and importation processes, so they continue to comply with the requirements. This transition has made a significant contribution to the protection of consumers and the reduction of PAH contamination in the environment.

Summary: polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a group of several hundred organic compounds known for their presence in numerous everyday products and their potential health and environmental risks due to their toxicity. They are formed primarily by the incomplete combustion of organic materials and they are persistent in the environment due to their lipophilic characteristics.

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