MeO-PCBs

OH-PCB compounds are much more labile than the MeS02-PCBs. OH-PCBs may undergo oxidation if exposed to air for a prolonged period of time. Most oxidation products are as yet unidentified, although catechols ((OH)2-PCB) and quinone-type products are known to be formed [116]. A number of OH-PCBs, analyzed as their corresponding MeO-PCB derivatives, have so far been identified by comparison to the authentic standards listed in Table 2. 

El mass spectra can be used for the structural determination of MeO-PCBs. Similar to the MeS02-PCBs, the sensitivity of EI-MS towards MeO-PCBs have been shown to be one to two orders of magnitude lower than for ECNI-MS [47], MeO-PCBs with the methoxy group in orffio-position imparts a characteristic, abundant [M-CH3C1]+ ion in the El mass spectra corresponding to the formation of a dibenzofuran-type structure [47,112,113]. Meta- and para-substituted MeO-PCBs are differentiated by i) the abundance of [M-CH3]ii) the relative intensity ratio of [M-15]/[M-43] of approximately 1.0 or greater for 4-MeO-PCBs, and iii) a [M-15]/[M-43] ratio less than 0.2 for 3-MeO-PCBs (calculated from [47]).The [M-43] ion corresponds to [M-(CH3+CO)]. Themolecular ion is generally the base peak in both El and ECNI mass spectra [47]. 

GC separation of the isolated phenolic compounds occurs prior to identification and quantification using ECD- and MS-coupled detectors (Sect. 4.3). However, phenolic compounds are polar, and derivatization gives better GC peak shapes in the most commonly used types of GC columns. Hitherto the standard method of derivatization of OH-PCBs retained in blood has been methylation by diazomethane to form the corresponding methoxy-PCB (MeO-PCB) [39,43, 135]. OH-PCB methylation by ion-pair alkylation with methyl iodide is an alternative to the diazomethane technique [43,140]. Acetylation has been shown to give comparable recoveries of OH-PCB derivatization as the methyl iodide and diazomethane approaches to OH-PCB methylation [139]. Silylation reactions have also been applied for the derivatization of various OH-PCBs [137]. 

GC separations have mainly been performed on columns such as 5%-phenylmethyl-derivatized column (e.g., DB-5 or XTI-5) [39,42]. However, coelution of MeO-PCBs occurs, which may require separation on two columns of different polarity. A cyano-derivatized column (e.g., SP-2331) has been used for this reason to facilitate full identification/separation of the congeners present in the OH-PCB envelope of elution [39]. GC(ECD) and GC/MS(EI) [39,42], and GC/MS(ECNI) [43] have been used to detect and quantify the MeO-PCBs. The OH-PCB congeners so far identified in wildlife and human blood are listed in Table 2. 

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