QUALITATIVE LIQUID-CHROMATOGRAPHIC ATMOSPHERIC-PRESSURE CHEMICAL-IONIZATION MASS-SPECTROMETRIC ANALYSIS OF POLYETHYLENE TEREPHTHALATE OLIGOMERS

The oligomeric fraction of polyethylene terephthalate (PET) has been studied as it has the potential to migrate to foods and beverages packaged in virgin and recovered PET plastics. We have applied positive ion atmospheric pressure chemical ionisation (APCI) to extracts of food-grade PET resin and beverage bottles. A reversed-phase HPLC system was connected directly to a VG Platform mass spectrometer. An acetonitrile-water-acetic acid gradient elution was performed. Low APCI probe temperatures (as appropriate for a polyethylene glycol calibrant) produced no significant ions from a PET cyclic trimer standard. A high probe temperature of 500-600 degrees C gave a strong protonated molecular ion. Characteristic spectra of the cyclic oligomers from the trimer to the heptamer were obtained. A second homologous series of substances 44 mass units higher than each PET oligomer eluted prior to each PET oligomer. The mass spectra indicated these to be oligomers with one monoethylene glycol unit replaced by a diethylene glycol unit. To our knowledge this is the first time that cyclic oligomers above the tetramer have been confirmed by LC-MS. The technique was considerably more sensitive than published thermospray methods and gave good spectra with sub-microgram quantities injected. This work demonstrates the advantages of APCI over thermospray as an MS technique for substances of this type.