Preventing false negatives with high-resolution mass spectrometry: the benzophenone case

Benzophenone (BP) is one of the many contaminants reported as present in foodstuffs due to its migration from food packaging materials. Liquid chromatography/tandem mass spectrometry (LC/MS/MS) is acknowledged in the literature as the method of choice for this analysis. However, cases have been reported where the use of this methodology was insufficient to unambiguously confirm the presence of a contaminant. In previous work performed by the authors, the unequivocal identification of BP in packaged foods was not possible even when monitoring two m/z transitions (precursor ion – product ion), since ion ratio errors higher than 20% were obtained. In order to overcome this analytical problem a fast, sensitive and selective liquid chromatography/high‐resolution mass spectrometry (LC/HRMS) methodology has been developed and applied to the analysis of BP in packaged foods. A direct comparison between LC/HRMS and LC/MS/MS data indicated better selectivity when working with LC/HRMS at a resolving power of 50 000 FWHM (full width at half maximum) than when monitoring two m/z transitions by LC/MS/MS. The resolving power used enabled the detection and identification of Harman as the compound impeding the confirmation of BP by LC‐MS/MS. Similar quantitative results were obtained by an Orbitrap mass analyser (Exactive?) and a triple quadrupole mass analyser (TSQ Quantum Ultra AM?). Copyright © 2011 John Wiley & Sons, Ltd.     

Voltammetry of labile metal—complex systems with induced reactant adsorption. Theoretical analysis for any ligand-to-metal ratio

The validity of the hypothesis of ligand excess is discussed for the voltammetric reduction of a metal ion (M) in the presence of a ligand (L). The following basic assumptions are made: (i) formation of the electroinactive ML complex, (ii) equal mobility of species M, ML and L, (iii) reversible charge transfer, (iv) labile complex and (v) Langmuirian adsorption of the ligand and the complex. The model proposed is solved rigorously for normal pulse polarography (NPP) in the four possible cases assuming either ligand excess or ligand deficiency or either adsorption or non-adsorption. For the case without adsorption, the assumption of ligand excess affects only the increasing part of the NPP wave independent of the total ligand-to-metal ratio. Then (I_NNP)_lim has the same value for both ligand excess and ligand deficiency while E differs, thus preventing the use of the DeFord—Hume method to obtain the stability constant. An analytical expression for E is performed, which allows the evaluation of the stability constant at any total ligand-to-metal ratio and provides a quantitative estimation of the error made by applying the classical procedure assuming ligand excess. In the presence of adsorption, the assumption of ligand excess modifies the whole wave. The discrepancy between the currents obtained from the hypothesis of ligand excess with respect to that with ligand deficiency is even higher for the case with adsorption than for the case without adsorption.     

New carrier electrolytes for the separation of chlorophenols by capillary electrophoresis

Optimum conditions for the separation of positional isomers of chlorophenols by capillary zone electrophoresis (CZE) were established. The behavior of five volatile electrolytes (L-cysteic acid, 3-amino-1-propanesulfonic acid, aminomethanesulfonic acid, diethylmalonic acid, and ammonium acetate) was compared. The best performance based on low electrophoretic current and high separation efficiency was obtained for diethylmalonic acid as working electrolyte. The influence of pH on the separation, using both uncoated fused-silica capillaries and modified capillaries (NaAMPS from EKT) with anionic coating, was discussed. Moreover, the effect of electrolyte concentration and applied voltage using fused-silica capillaries was studied. The optimum CZE conditions that allowed the separation of 16 chlorophenols were 20 kV, 30 mM diethylmaIonic acid, pH 7.25, and uncoated fused-silica capillary. Figures of merit such as run-to-run and day-to-day precision, linearity, and limits of detection were calculated.     

Solid-phase microextraction liquid chromatography/tandem mass spectrometry for the analysis of chlorophenols in environmental samples

Liquid chromatography with atmospheric pressure chemical ionisation mass spectrometry (LC/APCI-MS), using negative ion detection in a triple quadrupole instrument, was used for the determination of chlorophenols (CPs) in environmental samples. In-source collision-induced dissociation (CID) was compared with MS/MS fragmentation. In general, less fragmentation was observed in MS/MS as compared with in-source CID, with the latter providing more intense fragment ions due to chemical ionisation. Under MS/MS conditions [M - H - HCl](-) was the main fragment ion observed for all compounds except for pentachlorophenol, which showed no fragmentation. For multiple reaction monitoring (MRM) acquisition mode, the transition from [M - H](-) to [M - H - HCl](-) was selected, leading to detection limits down to 0.3 ng injected. Direct and headspace-solid-phase microextraction (HS-SPME) were used as preconcentration procedures for the analysis of CPs in wood and in industrially contaminated soils. CPs were quantified by standard addition, which led to good reproducibility (RSD between 4 and 11%) in both SIM and MRM modes, and detection limits down to ng/g. The combination of MS/MS and in-source CID allowed confirmation of the presence of CPs in environmental samples.