Multiple mass spec methods measure dirty money

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  • Published: Sep 6, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: HPLC
thumbnail image: Multiple mass spec methods measure dirty money

Trace analysis of bisphenols is increasingly important

Bisphenols are used in various polymers, such as polycarbonates, as plasticizers. They are also used in the thermal paper used in till receipts; they can then be transferred to humans, either directly or by transfer via other artefacts, such as paper money. Bisphenol A (BPA), the most commonly used of the group, has been shown to be an endocrine disruptor, which may well also be the case with analogous bisphenol compounds. Bisphenol S (BPS) has been increasingly used in place of bisphenol A; it has a similar structure, with the CMe2 group in bisphenol A being replaced by a sulfone group, SO2. The use of bisphenol A in thermal paper is being phased out in the EU by 2020.

Analysis of trace amounts of bisphenols has been achieved using a wide variety of methods, including LC-MS. A systematic comparison of the various mass spectrometry ionisation methods was needed to find out which was the most effective. The Pennsylvania researchers compared the conventional ESI (electrospray ionisation) method to two other methods: atmospheric pressure photoionisation (APPI) and atmospheric pressure chemical ionisation (APCI). The former method, APPI, uses a nebuliser and a UV lamp for ionisation, while APCI uses a nebuliser in combination with a corona discharge electrode. Atmospheric pressure ionisation methods, such as these, have become increasingly popular in recent years for LC-MS.

Different LC-MS ionisation methods compared for bisphenols

Analytical standards of seven bisphenols, including BPA and BPS, were examined by LC-MS. HPLC was carried out using a Shimadzu Prominence system with an Agilent Eclipse XDB-C18 column. Methanol and water were used for gradient elution, the proportion of the former solvent being increased from 70 to 95%. Mass spectrometry employed a Sciex 3200 QTRAP instrument, in which the ionisation source can be readily replaced. The HPLC separated most of the compounds, although two peaks overlapped. These two compounds could be distinguished by the mass spectrometry results.

The sensitivity of the system using ESI, APPI and APCI sources was examined. All three techniques used negative ion mode. The multiple reaction monitoring (MRM) transitions with the greatest sensitivity were identified for each compound. Comparison of the sensitivity showed that APPI gave the highest sensitivity, being 37 to 112 times as sensitive as ESI for non-sulfone compounds. APCI was slightly less sensitive than APPI. For sulfone compounds, such as bisphenol S, the differences between the methods were much smaller. The optimised on-column limits of detection (LODs) using the APPI source varied from 21 to 51 pg for the seven bisphenols.

Extraction of till receipts was carried out with methanol, while high-pressure methane was used to extract currency notes. Bisphenol S was the most common compound in till receipts, although bisphenol A was also detected in some. On currency notes, bisphenol S was slightly more prevalent than bisphenol A, while traces of BPSIP (the mono-isopropyl derivative of BPS) were also found on a few notes.

APPI shown to be more sensitive than ESI for most bisphenols

The results show that for non-sulfone bisphenols, APPI was much more sensitive than ESI, while APCI was slightly less sensitive than APPI, but still clearly superior to ESI. Laboratories that automatically carry out LC-MS with ESI may well be generating inferior results, not just for bisphenols, but for many other groups of compounds. Further similar comparison studies for other types of analyte would be useful in determining when a particular ionisation method should be preferred. This work has confirmed the widespread presence of bisphenols in U.S. till receipts and currency notes; presumably the latter bisphenols are derived from transfer from receipts. The overall implications of bisphenol exposure to human health remain a subject of controversy.

Related Links

Rapid Communications in Mass Spectrometry, 2017, Early View article. Eichman Jr. et al. A comparison of ESI, APCI, and APPI ionization for the LC-MS-MS analysis of bisphenols. Application to bisphenols in thermal paper receipts and U.S. currency notes.

Mass Spectrometry Reviews, 2003, 22, 318-331. Raffaelli et al. Atmospheric pressure photoionization mass spectrometry.

Wikipedia, Atmospheric-pressure chemical ionization

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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