Empirical relationship between precision and ultra-trace concentrations of PCDD/Fs and dioxin-like PCBs in biological matrices

Dioxin analysis in food and feed can be characterized as an analytical application where very high accuracy is required at very low levels of contamination. Gas chromatography (GC) in combination with 13C-label isotope dilution (ID) high resolution mass spectrometry (HRMS) is the reference congener-specific technique characterized by pronounced selectivity, precision and trueness at parts-per-trillion (ppt) and sub-parts-per-trillion (sub-ppt) levels. The quality of the analytical data produced routinely by a laboratory should be adequate for its intended purpose, i.e., one will seek a compromise between the cost and time needed and the consequences of incorrect decisions due to erroneous results. The requirements for reproducibility are usually dependent on the analyte concentrations and have been expressed in various empirical functions. While Horwitz or modified functions are widely useful for many purposes, it would be difficult to expect these functions to cover every analytical problem. This study reports on precision characteristics achieved by the GC-ID-HRMS reference method for polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and dioxin-like polychlorinated biphenyls (DL-PCBs) in food and feed in two interlaboratory method-performance studies among expert laboratories with long-standing experience in this field. Striking linear functions in log scale between reproducibility standard deviation and congener's level over a concentration range of 10(-8)-10(-14)g per g fresh weight are observed. The data fit very well to a Horwitz-type function of the form sR=0.153c0.904, where sR and c are dimensionless mass ratios expressed in pg g(-1) on fresh weight, regardless of the nature of the toxic congeners, food and feed matrices, or sample preparation methods. We demonstrate that the proposed function is suitable for use as a fitness-for-purpose criterion for proficiency assessment in interlaboratory comparisons on dioxins and related compounds in food.     

Reduced analytical availability of polychlorinated biphenyls (PCB's) in colored surface water

It has been generally accepted, during the last few decades, that the dissolved natural organic matter in water [DNOM] appearing as yellow brownish color, has become more and more "polluted" by inorganic and organic micro-pollutants. Due to the complexing properties of NOM, lipophilic organic micro-pollutants, such as, PCBs will be mobilized into water together with the DNOM. A mixture of eight PCBs, with Cl-content from tri-Cl to hepta-Cl, was added to solutions of ten different DNOMs. The DNOMs were aqua's solutions of RO-(reverse osmosis)-isolated material, having approximately the same concentration of carbon. After a contact time of three days, standard analytical PCB-method was used to determine the recovery of the added PCBs. The results show that the analytical availability of the added PCB was significantly reduced in the presence DNOM, compared to distilled water. The percentage loss in recovery of PCB increased with the content of Cl, in mean, from 3%/mg C for tri-Cl to 9%/mg C for hepta-Cl. The results also suggest that the analytical recovery of PCB was affected by the quality and the nature of the organic matter. For example the longer the DNOMs had been in the aquatic phase, the less efficient they are attached to the PCBs.     

(−)-Acetylcarnitin und Succinatoxydation

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