Methods of POPs analysis
To monitor the concentration of most POPs in the environment new methods of analysis shall be developed, which, however, presents a very challenging task.
First of all, the task appears difficult, because due to POPs’ high toxicity the analysis method shall be able to detect amounts estimated in picograms (10-12 g) and sometimes in femtograms (10-15g). The other difficulty is the structural variety of the chemicals and of related compounds whose potential number might be very large. For example, the group of PCDD/PCDF includes 210 compounds – isomers and homologues (congeners), the PCB group – 209 congeners, the group of PBDE – 209 congeners; toxaphene (commercial name of pesticides), which is produced by chlorination of pinene or camphene, is a mixture of some 2000 enantiomers with different chlorination degree.
The toxicity of the compounds of different groups is also different. Identification and estimation of the amount of the most toxic compounds poses a significant analytical challenge.
Isomer-specific analysis and identification of POPs in the surrounding objects, in biological objects and other matrices normally use capillary gas chromatography in combination with mass spectrometry (GC/MS). To identify trace amounts of POPs, especially of dioxins and furans, PBDE and toxaphenes, high resolution MS or chemical ionisation are applied.
Preparation of samples for analysis implies extracting them from the matrix by means of various extraction methods and separating them from impurities (sometimes up to 1000 compounds) by column chromatography in multilayer columns. To achieve complete extraction of the analyte at different stages of analysis and to control the quality of analysis, a number of internal standards, usually isotopically labelled compounds and imitators added to the sample matrix, are used. The reliability of compounds identification is ensured by mass spectra and the coincidence of chromatographic retention times between isotopically labelled analogues and the analyte compound.
Reliability and comparability of analysis results are ensured by running a program of internal quality control, which involves an assessment of replicability of results, a reliability evaluation of conducted measurements, control of matrix effects, determining the causes of deviation and their elimination. For each sample set, an analysis of appropriate certified reference standards, of quality control samples, or of blank samples is conducted; alternatively, a reanalysis of one of the sample sets is done.
One of the most important elements of quality control is a regular participation in international interlaboratory intercalibration exercises and demonstration of positive results.
According to the UNEP , the global monitoring system for POPs shall involve laboratories of three levels depending on their capacity and equipment. The lowest third level includes laboratories equipped with high performance gas chromatography with capillary column and electron capture detector. These laboratories are thought to be able to analyse basic chlororganic pesticides (COP) and PCBs, if quality control standards are satisfied. Laboratories of the second level shall have low resolution GC/MS and be able to conduct analysis of all PCBs and all COPs including toxaphenes (when chemical ionisation MS is available). Laboratories of the first level shall be equipped with high resolution GC/MS or equipment ensuring a similar degree of assay sensitivity, and be able to analyse all types of POPs, also in trace amounts, including PCDD/F, in all matrices, including human blood and breast milk.
UNEP has developed and currently runs a database of monitoring laboratories for POPs . At the moment, the database covers more than 200 laboratories from the countries-Parties to the Stockholm Convention on POPs. Moreover, currently UNEP is implementing the project “Assessment of existing capacity and capacity-building needs to analyse POPs in developing countries” with financial support from the GEF, as well as from the governments of Canada, Germany, and Japan.
A new document “Renewed general technical guidelines for the environmentally sound management of wastes consisting of, containing or contaminated with persistent organic pollutants ”, developed by UNEP, lists selected methods to analyse POPs (Annex III).