Direct analysis of an oligomeric hindered amine light stabilizer in polypropylene materials by MALDI-MS using a solid sampling technique to study its photostabilizing action.

A novel method for the direct analysis of small amounts of an oligomeric hindered amine light stabilizer (HALS) occluded in polypropylene (PP) material was developed to study its photostabilizing action on the basis of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using a solid sampling technique while avoiding troublesome solvent extraction. In this sampling protocol, the powdered mixture of PP composite sample containing trace amounts of an oligomeric HALS, Adekastab LA-68LD (MW = 1900), and the matrix reagent (dithranol) was spotted on the sample plate, then ion exchanged water was deposited onto the mixture to make a suspension, and finally, the dried mixture adhered on the plate was subjected to MALDI-MS measurement. On the mass spectrum thus obtained by the solid sampling MALDI, the molecular ions of the HALS desorbed from the PP composite were clearly observed as three major series of the HALS components in the range up to about m/z 7000 with little interference by the PP substrate and the other additives. Moreover, in the MALDI-MS spectra for the UV-exposed sample, the satellite peaks around the major HALS components proved to enhance significantly, reflecting the oxidized HALS species at the tetramethylpiperidine units to cause the photostabilizing action. In addition, hydrolyzed HALS species were also observed for the irradiated sample. These results suggest that not only the oxidation reaction but also the hydrolysis or decomposition of the oligomeric HALS components competitively proceed in the PP composites during UV exposure.