Atmospheric pressure MS method gives hope on OPEs

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  • Published: Jan 1, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: Atmospheric pressure MS method gives hope on OPEs

Organophosphate esters are ubiquitous, but are they dangerous?

Although they have previously tended to keep a low profile, organophosphate esters are widespread chemicals, which have become a hot topic in the analytical world due to their potential harmful effects on health and the environment.

Organophosphate esters (OPEs) are used as flame retardants, plasticizers, solvents, lubricants and chemical intermediates. Their widespread use has led to them being very common environmental contaminants. Only a limited amount of work has been carried out on the their possible toxic effects, but animal studies suggest that long-term exposure may lead to endocrinal, reproductive and systemic effects, as well as promoting allergies, such as asthma. A possible link to ‘sick-building syndrome’ has also been noted. Some of the chlorinated organophosphate esters, used as flame retardants, have been shown to be neurotoxic and carcinogenic in animal studies.

Organophosphate pesticides are a special class of organophosphate ester. These insecticides are acetylcholinesterase inhibitors, and most have a much greater acute toxicity than the ‘ordinary’ organophosphate esters discussed in this article, which generally have little or no effect on acetylcholinesterase.

In order to determine the concentrations of OPEs in the environment, it is important to have accurate analytical techniques. On the whole, GC/MS is preferred to LC/MS due to the low solubility of many of these compounds in the usual aqueous solvent reverse phase systems, their long retention times in such HPLC systems and the poor UV responses of the aliphatic OPEs.

APCI, EI, PCI and NCI all tested for organophosphate ester detection

APCI is increasingly popular, both for LC/MS and GC/MS. The French/Lebanese team aimed to compare it with the traditional techniques of EI, PCI (positive chemical ionisation) and NCI (negative chemical ionisation) in the GC/MS of 18 OPEs.

The authors note that contamination with OPEs from the environment was a potential problem. Glassware, baked at 400 °C for 4 hours prior to use, was preferred to plastic apparatus wherever possible. An exception was plastic pipette tips, but these were shown not to give any signals for OPEs.

Although EI was a sensitive method, it showed fragmentation peaks rather than molecular ions. These fragmentation peaks were often not specific; for example, in the cases of alkyl OPEs, the base peak was at m/z = 99, due to protonated phosphoric acid. The PCI and NCI methods gave clear molecular ions, but gave less sensitivity than EI.

ACPI was first investigated under dry charge transfer conditions. Strong [M+30]+ ions were noted. These were attributed to the formation of NO adducts (presumably NO was formed from the nitrogen used in the system). Switching to proton transfer conditions (aqueous methanol put into the source) supressed the formation of the NO adducts and gave strong molecular ion [M+1]+ signals.

Optimisation was carried out for GC-MS/MS (tandem MS) using ACPI and selective reaction monitoring (SRM). Good results were found for the 18 OPEs (although two brominated OPEs needed a shorter GC column in order to reduce their long retention time). In addition, optimisation was also carried out for GC-MS/MS using EI. The optimised methods were applied to spiked fish samples. The samples were extracted by pressurised liquid extraction, and then treated by gel permeation chromatography prior to GC-MS/MS.

ACPI is a sensitive method for organophosphate ester detection

The authors of the paper showed that ACPI could be a sensitive method for OPE detection, being in general at least equal to EI, but showing much better molecular ion peaks. Both EI and ACPI were clearly more sensitive than PCI or NCI. The optimized conditions were successfully tested with spiked fish samples. The authors state that they intend to examine sample preparation techniques and to look at matrix effects for both the optimized ACPI and EI conditions.

Related Links

Journal of Mass Spectrometry, 2016, Early View paper. Halloum et al. APCI as an innovative ionization mode compared to EI and CI for the analysis of a large range of organophosphate esters using GC-MS/MS.

Environmental Science & Technology, 2016, 50, 12831-12839. Castro-Jiménez et al. Organophosphate Ester Flame Retardants and Plasticizers in the Global Oceanic Atmosphere.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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