Differentiation of (Mixed) Halogenated Dibenzo-p-Dioxins by Negative Ion Atmospheric Pressure Chemical Ionization

Abstract

Brominated and mixed halogenated dibenzo-p-dioxins (PBDDs and PXDDs) may well be as toxic as 2,3,7,8-tetrachloro-dibenzo-p-dioxin (2378-TCDD), a compound reputed as one of the most toxic chemicals known to exist. However, studies on the occurrence of PXDDs have been hampered by a lack of authentic standards as well as separation techniques capable of resolving the enormous number of potential isomers. Electron ionization (EI) mass spectrometry based methods are of limited value due to the lack of isomer specific fragmentation. Negative ion atmospheric pressure chemical ionization (APCI(-)) of 2378-TCDD was described in this journal over 30 years ago. Under these conditions, the reaction between O2(-•) and 2378-TCDD results in structure diagnostic cleavages of the C-O bonds, which can distinguish TCDD isomers on the basis of Cl distribution between the two aromatic rings. In the present study, the analogous ether cleavages of PBDDs and PXDDs were studied using a gas chromatograph-quadrupole time-of-flight (GC-QTOF) mass spectrometer coupled using APCI. The results indicate comparable detection limits for the radical cations [M(•+)] and negative pseudomolecular ions [M-Cl+O](-): approximately 5 fg and 10 fg, respectively, for 2378-TCDD and 5-10 fg and 10-30 fg, respectively, for the 2,3,7,8-substituted PXDDs. Detection limits obtained by monitoring the ether cleavage products were somewhat higher (between 100 and 600 fg) but still acceptable for trace analysis of PXDDs. Such reactions may resolve coeluting isomers, which is crucial for the identification of PXDDs. The technique is demonstrated by differentiating PXDD isomer classes in a sample obtained from a major industrial fire that would not be feasible using EI or positive ion APCI(+).

Is Nontargeted Screening Reproducible?

“…Unfortunately, this is often difficult information to get, especially with electron ionization mass spectrometry. Alternative ionization approaches can be helpful; for example, isomeric dioxin congeners can be differentiated by reactions with dioxygen anions …”

Nontargeted Screening Using Gas Chromatography–Atmospheric Pressure Ionization Mass Spectrometry: Recent Trends and Emerging Potential

“…Therefore, separating toxic from non-toxic isomers is a major challenge that cannot be solved using GC alone, as witnessed by the coeluting isomers 2,3,7,8-TBDD (toxic) and 1,2,3,4-TBDD (less toxic) in Figure 5d. Fernando et al [103] showed that ion-molecule reactions with oxygen could be exploited to separate the coeluting isomers because 2,3,7,8-TBDD reacts with oxygen to produce the ether cleavage product C6H2BrO2 •− (m/z 265.840), whereas 1,2,3,4-TBDD cannot. When Structure-diagnostic reactions have also been observed in the positive ion mode 68 .…”

Automated Isotopic Profile Deconvolution for High Resolution Mass Spectrometric Data (APGC-QToF) from Biological Matrices

“…The current instrument configuration generates soft ionization fragments that differ from traditional electron impact (EI) ionization. Fernando et al 10 showed that polyhalogenated dibenzo-p-dioxins and furans (PXDDs and PXDFs) undergo an oxygen displacement reaction and produce pseudomolecular ions [M − X + O] − (where X = Cl, Br) using an APGC source in negative mode. A similar reaction product ion [M − Br + O] − was observed in PBDEs, although it was not always the most abundant fragment (Table S were not detected or had very low intensities in negative mode.…”

“…Softer ionization such as atmospheric pressure chemical ionization (APCI) methods generate fewer fragments and potentially produce a more intense molecular ion that may aid in the identification of an unknown through reconstruction of the molecular formula from the measured mass-to-charge (m/z) and observed isotopic distribution profile. 9,10 However, data processing for soft ionization high-resolution mass spectrometry (HRMS) data could be a challenge when confirmatory fragments are not available to support nontargeted analyses. The natural abundance of the two prominent isotopes of Br ( 79 Br, 81 Br) and Cl ( 35 Cl, 37 Cl) provide distinct M, M + 2, M + 4... isotopic signatures for compounds that contain these atoms.…”

“…A softer ionization technique could be used as an alternative means of identification. Softer ionization such as atmospheric pressure chemical ionization (APCI) methods generate fewer fragments and potentially produce a more intense molecular ion that may aid in the identification of an unknown through reconstruction of the molecular formula from the measured mass-to-charge ( m / z ) and observed isotopic distribution profile. , However, data processing for soft ionization high-resolution mass spectrometry (HRMS) data could be a challenge when confirmatory fragments are not available to support nontargeted analyses.…”