Quantitative analysis of an oligomeric hindered amine light stabilizer in polypropylene by matrix-assisted laser...

A small amount of an oligomeric hindered amine light stabilizer (HALS) (Adekastab LA-68LD) in polypropylene (PP) materials was directly determined by solid sampling matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using an internal standard method. First the matrix reagent (dithranol), 20 mg, and the empirically selected internal standard, angiotensin I (MW = 1296.5), 5 microg, were premixed in the solid state. The matrix mixture was then co-ground with the PP sample containing the HALS in liquid nitrogen using a freezer mill. The powdered sample mixture was spotted on the sample plate, suspended in ion-exchanged water, dried to adhere on the plate, and subjected to MALDI-MS. Three series of the HALS components accompanied by the oxidized species were clearly observed as their molecular ions (M*+)) along with that of the internal standard in the mass spectra. A fairly good linear relationship (R2 = 0.9991) with a relative standard deviation of ca. 11% was observed between the relative peak intensities of the HALS components and the HALS contents ranging from 0.1-2.5 wt%, which could be used as the calibration line to determine the HALS content in PP composites directly by MALDI-MS. The UV-exposed PP composite samples were evaluated by this method to interpret the photostabilizing action of HALS in the PP materials based on the observed change in the relative abundances of the original and oxidized HALS components as a function of UV-exposure time.