The Mass Spectrometric Ortho Effect for Distinguishing the Coeluting Isomers of Polychlorinated Biphenyls and the Coeluting Isomers of Polybrominated Biphenyls: Qualitative and Quantitative Aspects

Abstract

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) are persistent organic pollutants still widespread in the environment and in the food chain. Both groups of these synthetic xenobiotics consist of 209 possible congeners depending on the number and position of halogens. PCBs with the same number of chlorine atoms and PBBs with the same number of bromine atoms are isomers: ten different degrees of halogenation are allowed, which results in a lot of existing isomers for both groups. The isomers have perfect correspondence in the number and type of atoms with differences only in positioning, so their mass spectra are expected to be identical with a consequent significant analytical problem in the event of coelution of the chromatographic peaks. This is not always the case, since the mass spectrometric ortho effect is capable of effectively discriminating many coeluting PCB or PBB isomers, although not all possible ones. The present paper investigates, for the first time, the reliability of qualitative and quantitative analysis by using the ortho effect: this was conducted through targeted experimental measurements on real samples of food by using different detectors. In this context, it is shown how to recognize the presence of a PCB that does not have the ortho effect when coeluting with an isomer that has. This is an important aspect that has never been studied until now. The ortho effect is extremely simple to operate once the ordinary GC-MS runs have been performed: the analyst only needs to recheck the mass spectrum for measuring the intensity of the first dehalogenation ion. The topic is of practical relevance since two different isomers can have different health hazards, and the presence of a very toxic isomer could be masked by a less toxic one. The same mass spectrometric ortho effect also deals with PXBs (i.e., mixed poly-brominated/chlorinated biphenyls), which are emerging contaminants.

Keywords: analytical tool; coelution of isomers; emerging contaminants; mass spectrometry; metrology; mixed poly-brominated/chlorinated biphenyls (PXBs); ortho effect; polybrominated biphenyls (PBBs); polychlorinated biphenyls (PCBs).

Figure 1

(A). Generic molecular structure of PBBs. (B). Generic molecular structure of PCBs. (C). Example of PCB congener: hexachlorobiphenyl, whose IUPAC name is 2,2′,4,4′,5,5′-hexachlorobiphenyl. The notation shown on the right (PCB 153) is the nomenclature introduced by Ballschmiter and Zell (numbers from 1 to 209), which is generally adopted [7,8]. A corresponding nomenclature is used for PBBs [1]. For example, 2,2′,4,4′,5,5′-hexabromobiphenyl is also referred to as PBB 153.

Figure 2

Mass spectra of PCB 74 and PCB 66, which are isomers (tetra-Cl). The mass spectra were obtained by injecting a standard mixture in the GC-MS and using the experimental conditions in Section 3.4.2. It can be noted in both spectra that the ion fragments differed from each other for the loss of one chlorine atom upon electron impact ionization (292 − 35 = 257, 257 − 37 = 220, 220 − 35 = 185, 185 − 35 = 150). The chlorine atom has a mass number of 35 or 37 (isotopic abundances: ³⁵Cl about 76%, ³⁷Cl about 24%). The two mass spectra were indistinguishable. In this case, the retention times were different.

Figure 3

Mass spectrum of PCB 44. This was obtained by injecting a standard mixture in the GC-MS and using the experimental conditions in Section 3.4.2.

Figure 4

Chloromium ion intermediate (B) starting from a 2,2′-substituted PCB (A). The chloromium intermediate (first dechlorination ion) benefits from stabilization due to the formation of a 5-membered ring. The actual process involved in the rate of formation of the first dechlorination ion may involve the displacement of one ortho chlorine by an ortho chlorine from the other ring of the biphenyl nucleus. Data from the ion kinetic energy spectra support this pathway [18].

Figure 5

GC-MS overlaid chromatograms of standard PCB 138 (200 µg L⁻¹) and PCB 163 (100 µg L⁻¹) injected separately. Chromatograms were obtained by displaying the molecular ion 360 m/z. Experimental conditions are the same as in Section 3.4.2.

Figure 6

Mass spectra of PCB 138 and PCB 163 relating to the peaks in Figure 5.

Figure 7

GC-MS chromatogram of Food Sample 1. (Top) Chromatogram obtained by displaying the molecular ion 360 m/z. (Bottom) Chromatogram obtained by displaying the ion 325 m/z. Sample injected using the same experimental conditions as in Section 3.4.2.

Figure 8

GC-MS chromatogram of Food Sample 2. Indicated by the arrow: PCB 138 + 163. Experimental conditions the same as in Section 3.4.3.

Figure 9

Mass spectrum of the peak indicated by the arrow in Figure 8.

Figure 10

4′-Br-2,3,3′,4,5-Cl-biphenyl. This molecule was investigated in a 2011 research study by Fernandes et al. [33]: it is an analog to PCB 156 and PBB 156. Depending on the position of the bromine atom, there are six possible PXB 156. Due to the absence of 2,2′-substitution, it is to be expected that the ortho effect is absent for the six possible PXB 156.