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PET is a general-purpose thermoplastic polymer. It is clear, strong and lightweight so is widely used for packaging foods
and beverages. It was previously thought that bottles made with PET were benign but new research indicates that PET
could be leaching into bottled drinks during storage. PET and other similar polymers have the potential to harm humans as
they can interfere with oestrogen and other reproductive hormones.(6)
First, the team made up samples from bottled beverages – two brands of still water, a carbonated water and a cola – by
filtering the liquids through 0.2 µm filter papers, before washing them to remove organic matter, leaving only microplastics
on their surfaces. The papers retain microplastics down to 0.2 µm in size, enabling nanoplastics to be analysed too
(particles less than 1 µm). The papers were dried in an oven for 30 minutes then folded and placed inside thermal
desorption tubes ready for thermal desorption followed by analysis using GC–MS (Figure 1).
Figure 1: Sample preparation and analysis stages for the bottled beverage study.
With Markes’ thermal desorption instruments, there are two stages (Figure 2). First, the sample tube is heated to enable
volatile organic compounds (VOCs) to be emitted. A carrier gas sweeps the VOCs to an electrically-cooled focusing
trap, where the compounds are concentrated – this maximises the sensitivity to enable the detection of low levels of
target compounds.
At the second stage, the focusing trap is heated rapidly in a reverse flow of carrier gas – known as backflushing – to
transfer the VOCs to the GC column in a narrow band of vapour for separation and detection. Backflushing enables a
wide range of compounds to be analysed in a single run, which is essential for environmental samples in which the
compounds are unknown.
This technique is well established for direct desorption of materials and combines sample preparation, desorption,
preconcentration and GC injection into a single automated process.
Split flows can be applied at both stages of desorption to improve the desorption efficiency and handle wide
concentration ranges. Also, re-collection of a split flow onto a clean sorbent tube allows a portion of the sample to be
archived for re-analysis, saving time spent re-preparing the filter papers and avoiding the need to store large volumes
of liquid samples.
Figure 2: Two-stage thermal desorption and split flows.
The team ran analyses on a PET standard to find volatile marker compounds that they could use to identify and
measure concentrations of PET – tetrahydrofuran (THF) was used to confirm its presence and 2,4 di-tert-butylphenol
(2,4-DTBP) was used to calculate its concentration. This was done by calibrating the mass of PET against 2,4-DTBP
peak area (Figure 3).