Biphenyl compounds, resolution

R. M. Kinghorti, P. J. Marriott and M. Cumbers, Multidimensional capillary gas chr O-matography of polychlorinated biphenyl marker compounds , 7. High Resolut. Chromatogr. 19 622-626 (1996). 

In general, capillary gas chromatography provides enough resolution for most determinations in environmental analysis. Multidimensional gas chromatography has been applied to environmental analysis mainly to solve separation problems for complex groups of compounds. Important applications of GC-GC can therefore be found in the analysis of organic micropollutants, where compounds such as polychlorinated dibenzodioxins (PCDDs) (10), polychlorinated dibenzofurans (PCDFs) (10) and polychlorinated biphenyls (PCBs) (11-15), on account of their similar properties, present serious separation problems. MDGC has also been used to analyse other pollutants in environmental samples (10, 16, 17). 

Figure 13.1 Monitor (FID) (a) and analytical (ECD) (b) channel responses for PCB congeners in Aroclor 1254, showing selection of the six heart-cut events Frr-st columns, HT8 second columns, BPX5. Reprinted from Journal of High Resolution Chromatography, 19, R. M. Kinghorn et al., Multidimensional capillar-y gas chr omatography of polychlorinated biphenyl marker compounds , pp. 622-626, 1996, with per-mission from Wiley-VCH.

The simplex approach to SFC optimization has proven to be extremely helpful with real samples as well as synthetic mixtures. In recent work with Arochlor 1254 (Crow and Foley, manuscript in preparation), a mixture of polychlorinated biphenyls (PCBs), the number of resolved peaks increased from 19 to 28 in going from vertex 1 to vertex 21 (optimum). Resolution was improved sufficiently that baseline resolution of most if not all compounds could have been obtained by transferring the optimized method to a longer column as described earlier in the section on Minimizing the Time Required for Optimization. 

This compound, in common with other suitable substituted biphenyls, possesses a chiral axis (p. 6) and is isolated from the reaction as a racemate. Although several resolution procedures have been reported, the superior method to date4 is that in which the binaphthol is first converted by treatment with phosphorus oxychloride into the binaphthyl phosphoric acid (14). Resolution is then effected by formation of diastereoisomeric salts with (+ )-cinchonine, appropriate fractional crystallisation and recovery of the (S)-( + )-binaphthyl phosphoric acid. Suitable hydrolysis gives (S)-( — )-l,l -bi-2-naphthol (15). 

In a field closely related to biphenyl, hindered rotation around the pyridyl-amide bond allowed the resolution of the nicotinamide analogue 142 into enantiomers. Compound 142 was optically stable at 25°C in water at higher temperatures racemization occurs (e.g., t1/2 = 3.1 hr at 100°C) (82RTC191 ... 

A recent method of detection is electron capture negative ionisation (ECNI) as ionization technique in combination with GC-MS analysis. This method is advantageous because it offers a high sensitivity for compounds with four or more bromine atoms [36]. The sensitivity of ECNI for these compounds is approximately 10 times higher than with the use of an electron capture detector (ECD) [5]. In the analytical method which was developed to quantitate PCBs and PBBs in human serum, GC/ECD was used [30]. Because the response, and therefore the sensitivity, of the ECD depends on the position of the halogen on the biphenyl nucleus as well as the number of halogen atoms, it is necessary to run a standard for each compound to be determined [2], The use of narrow bore (0.15 mmi.d.) capillary columns is advised to obtain the required resolution [5]. 

Biphenyl-bridged bis-Cp titanocene dichloride and dimethyl complexes have been synthesized, and the kinetic resolution of the racemic final mixture of the products has been carried out. A mixture of diastereomers is obtained by treatment of the dimethyl compound with O-acetyl-mandelic acid, while enantiomerically pure products result in the reaction of the dichloro derivative with (i )-binaphthol and 1 equiv. of LiBun (Scheme 661).1050... 

Micellar electrokinetic chromatography (MEKC), a modified CE, enabled the separation of electrically neutral analytes. However, highly hydrophobic compounds including PAHs, corticosteroids, fat-soluble vitamins and polychlorinated biphenyl (PCB) congeners could not be separated by MEKC in electrophoretic medium of sodium dodecylsulfate (SDS) [59]. The use of CDs to the SDS solution can remarkably improve the resolution of analytes. For example, a mixture of water-soluble and fat-soluble vitamin or esterom can be successfully separated by MEKC by the addition of y-CD in the SDS electrophoretic medium [60]. 

CE has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various toxic pollutants such as phenols, polyaromatic hydrocarbons, and so on. Sarac, Chankvetadze, and Blaschke " resolved the enantiomers of 2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl)propanoic acid using CD as the BGE additive. The CDs used were native, neutral, and ionic in nature with phosphate buffer as BGE. Welseloh, Wolf, and Konig investigated the CE method for the separation of biphenyls, using a phosphate buffer as BGE with CD as the chiral additive. Miura et al., used CE for the chiral resolution of seven phenoxy acid herbicides using methylated CDs as the BGE additives. Furthermore, the same group resolved 2-(4-chlorophenoxy) propionic acid (MCPP), 2-(2,4-dichlorophenoxy) propionic acid (DCPP), (2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chlorophenoxy) propionic acid (2,4-CPPA), [(2,4,5-... 

A further example for the analysis of trace levels of pollutants is the determination of polychlorinated dibenzo-dioxins and polychlorinated dibenzofurans in milk. The method includes gel permeation chromatography, alumina cleanup, and porous graphitized carbon chromatography, followed by analysis by GC/high-resolution MS. Moreover, the potential of porous graphitic carbon as an HPLC adsorbent for the isolation of halogenated aromatic compounds has been demonstrated by the fractionation of polychlorinated biphenyls (PCBs), chlorinated pesticides, polychlorinated dibenzo-p-dioxins (PCDDs), and... 

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