Polychlorinated biphenyls (PCBs)
Over the past 30 years, increased attention has been paid to the analysis of the group of persistent organic pollutants (POPs), which affect the environment at an extremely low level (the lower limit of detection is 10 -8 -10 -13%). Many of them were known for a long time and were widely used in industry and agriculture of most countries. These compounds belong to the class of organochlorine compounds and have a number of specific characteristics:
- bioconcentration (or bioaccumulation) – due to the fact that the solubility in water is low and high in fats and lipids;
- global prevalence due to the ability to be transported over long distances;
- extreme resistance to physical, chemical and biological changes;
- the ability to have a toxic effect on organisms in extremely small doses.
Currently, the UNEP (United Nations Environmental Project) highlights the group of 12 compounds and groups of compounds that should be given priority attention in environmental studies. This so-called “dirty dozen” includes the following substances: polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF), aldrin, dieldrin, dichloro-diphenyl-trichloroethane (DDT), di-dofenzofurans, daldrin, dichloromethane, dichloro-diphenyl trichloromethane (DDT), aldrin, dieldrin, dichloromethane chlordane, hexachlorobenzene (HCB), mirex, toxaphene, and heptachlor . This list was compiled as a result of a large number of international consultations and forums. The main result of this work was the adoption and signing on May 23, 2002 in Stockholm of the Global International Convention on the Prohibition of POPs, to which Russia also joined.
The Stockholm Convention contains a number of proposals and measures to study the effects of POPs on human, animal, plant health, to study the ways of dissemination of these substances, and to prohibit their production and use. Part of this program is the concept of environmental-analytical control, which involves conducting research to identify and assess pollution sources, determine the levels of pollution of natural and food POPs as a result of anthropogenic impact (direct, indirect or catastrophic) on the environment and humans.
Among POPs, PCBs are among the most common. They have been mass produced and used since 1929. From then until the end of their industrial production in 1986, about 2 million tons of PCB were produced worldwide.
PCBs belong to the class of aromatic compounds consisting of two benzene rings connected through the C – C internuclear bond and substituted from one to ten chlorine atoms in the ortho, meta or para positions (
According to their physico-chemical properties, PCB congeners are close to dioxins. The size of the molecule (I) is in the range of 9-10.5Å in length and about 3Å in width. PCBs have a number of unique physical and chemical properties: exceptional thermal and electrical insulation characteristics, heat resistance, inertness to acids and alkalis, fire resistance, good solubility in fats, oils and organic solvents, high compatibility with resins, excellent adhesion . This caused their widest use as dielectrics in transformers and capacitors, hydraulic fluids, coolants and coolants, lubricating oils, paint components, varnishes and adhesives, plasticizers and fillers in plastics and elastomers, flame retardants, solvents [1,2,3].
The industrial production of PCB is based on the replacement chlorination of biphenyl in the presence of an electrophilic substitution catalyst (usually Fe) .
The degree of chlorination depends on the duration of the reaction, which ranges from 12 to 36 hours. The reaction of electrophilic substitution takes place nonspecificly, therefore the product contains a mixture of a large number of individual PCBs – from 30 to 100 compounds. Most of them contain from 3 to 8 chlorine atoms, although there are small amounts of both more and less chlorinated PCBs. These mixtures are known under various brand names – Arochlor (USA), Kanehlor (Japan), Chlorphen (Germany), Delors (Slovakia), Fenochlor (France), Fenkhlor (Italy) Sovol and Sovtol (USSR, Russia), the composition of which is regulated ( Table 1).
Approximate composition of technical mixtures of Arochlorus (in%)
The number of chlorine atoms
Approximate molecular weight
The nature and dynamics of the distribution of PCBs in the environment are largely determined by their physical properties, such as chemical inertness, high vapor density, and the ability to sorb on particles. Despite the gradual reduction in the use of PCBs in economic activity, they continue to pollute the environment, and now these toxic products that have spread throughout the globe are present in the body of each of us [1,3,4]. As PCBs are incorporated into biological food chains, progressive loss of low-chlorinated components occurs due to their selective biotransformation. Therefore, the most dangerous highly chlorinated PCBs accumulate in humans and animals .
According to the World Health Organization , the main ways of release of PCB into the environment are as follows:
excretion during the incineration of household and industrial waste, as well as the ignition of transformers, capacitors and other industrial equipment that uses PCBs;
leaks with other industrial waste; removal of PCBs to landfills and aeration fields;
other uncontrollable ways.
Environmental pollution occurs mainly through the first three channels.
In Russia, the MPC values apply only to industrial PCB mixtures. Arochloride 1254  was taken as the standard mixture for which the MPC was calculated. MPC for PCB have the following meanings :
atmospheric air – 1 µg / m 3;
working area air – 1 mg / m 3;
water (water bodies for household, cultural and community water use) – 1 µg / l;
food products (in terms of fat):
In addition to these standard values, there are MACs in drinking water for monochlorbiphenyls – 1 µg / l; dichlorobiphenyls – 1 µg / l; trichlorobiphenyls – 1 µg / l; and pentachlorobiphenyls – 1 µg / l . Similarly, for soils, the MPCs (in mg / kg) are as follows: for TrHB-0.03, for TCB-0.06, for PeCB-0.1. However, the possibility of applying these standards is not justified either by the instrumentation and methodological base, nor by the fact that these groups of PCBs are not found separately, but only in the form of mixtures.
There are large differences in toxicity, bioaccumulation and bio-transformation properties for different PCB congeners. Congeners that do not contain chlorine atoms in the molecular ortho positions (ortho-unsubstituted PCBs) can take on a planar configuration that is energetically most beneficial  (
Congeners with one chlorine atom in the ortho position (mono-ortho-substituted PCBs) show a deviation from the planar configuration. Their dioxin-like toxicity is lower than that of ortho-unsubstituted. Congeners with two or more chlorine atoms in the ortho positions of the molecule take a globular configuration and have a very low toxicity of dioxin type [].
To compare the biological activity of various congeners of PCBs in 1987, the concept of toxic equivalents TEF – Toxic Equivalent Factor was proposed. According to this approach, the toxicity or biological activity of a particular congener is expressed relative to the activity of 2,3,7,8-TCDD. The so-called equivalent toxic concentrations – TEQ Toxic Equivalent Concentrations – are calculated by multiplying the concentrations of the individual PCB congeners by the TEF value corresponding to this congener. In tab. 2 shows the TEF coefficients for the most toxic PCBs.
International toxicity ratios.
PCB Congener Group
Structure by YUPAK
Not having a substitute in the ortho-position
The mechanism of action of ortho-unsubstituted and mono-ortho-substituted PCBs is similar to that of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD).
Recent studies of the pathogenesis of the effects of dioxin-like compounds on the human body have led to the creation of the concept of “dioxin disease”, which most fully describes the observed processes and changes .
Modern methods and approaches used by analysts in analyzing environmental objects for PCB content allow all congeners to be determined, despite the fact that not a single chromatographic column can currently separate all 209 PCB components. The dominant methods are:
1) gas-liquid chromatography (GLC) using an electron-capture detector selective for chlorine-containing compounds (ECD) [3, 11];
2) a combination of low resolution GC-MS [12, 13]; and to identify planar PCBs (No. 77, 81, 126, 169) a combination of GC with high resolution MS .
These methods allow you to solve any problems associated with the detection of PCBs, regardless of the nature of the natural matrix.
The choice of method of determination depends on the ultimate goal of the analysis. If you need to know the total or group content of PCBs, then use fairly simple methods. If the task is to determine the toxic properties of a particular sample, then more complex and expensive methods of congener-specific determination of the most toxic PCBs are used.
Definitions of total PCB content.
In this case, the quantitative determination of PCB is carried out by comparing the chromatographic profile of the analyzed sample with the profile of standard technical mixtures. Such analysis is carried out, as a rule, by means of GC ECD (EPA 600 and EPA 8082 A) and CMS.
On the chromatographic column, PCBs are not divided into individual isomers, they elute as clusters, rarely reaching the baseline. For calibration, various commercial mixtures are used, usually Arochlor chlorine mixtures, in order to find the one that is best suited for the sample being analyzed. That is, the identification is carried out solely by the retention time.
In cases where a sample contains only one technical mixture, this technique gives satisfactory results, but if the sample is contaminated with two or more technical mixtures in an unknown ratio, then none of the technical mixtures "Arohlor" can not be taken as a standard, and the results are not accurate enough. In addition, along with PCB, other pollutants can also elute, such as polychlorterphenyls, sometimes butyl monochlorodiphenyl ethers, isopropyl chlorobiphenyls, chlorobenzenes, chlorohydrocarbons (pesticides) that are not identified, but are included in the total “sum of PCBs”. Despite the inaccuracy of the method is simple and convenient.
For this kind of analysis, you can also use a mass spectrometric detector (MS), which significantly increases the selectivity. It possesses no less high sensitivity than the EZD, and allows identification on the basis of structural information on molecular and fragmentation ions in the mass spectrum. High sensitivity in GLC-MCHP is achieved by using selective detection of selected ions (MID). The characteristic picture of the isotope distribution of chlorine-containing ions provides additional opportunities for identification.
The concept of “PCB amounts” has a number of drawbacks both in terms of their definition in the environment and in technical mixtures, and in terms of toxicology. Knowledge of the “sum of PCBs” without their detailed composition does little for a toxicological assessment of pollution.
Congener-specific definition of PCB.
Up to 100 individual congeners are usually found in environmental objects in the air, fish, sediments, soil, plant materials. Since the physicochemical and toxicological properties of different congeners are very different, a congener-specific definition of PCB is necessary. For example, a change in the congener profile can provide information about the relationship between the structure of PCBs and their distribution in nature or changes in a living organism. In addition, if you define a limited number of congeners, this can lead to an underestimation of the total amount of PCBs.
The inclusion of dioxin-like PCBs along with PCDD and PCDF in the calculation of dioxin load and risk has made a significant modification to the PCB determination methods. The “dioxin-like” includes planar PCB congeners that do not contain chlorine atoms in the ortho position and some mono-ortho-substituted congeners. Since their concentrations are usually low, and reliable identification and precise determination of their content is required, they use GLC-MCH, including GLC-MSVR, as well as to determine PCDD and PCDF .