ToF-SIMS quantification of polystyrene spectra based on principal component analysis (PCA)

Monodisperses polystyrenes (PS) with molecular weight (M-n) ranging from 850 to 20 000 g mol(-1) and with different butyl end-groups have been analysed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Only the fingerprint part of the spectra (m/z < 200) has been considered. This part comes from the fragmentation of the macromolecules and this is the only contribution in the secondary ion spectra of bulk polymers. In a former study, it has been shown that M-n has a significant influence on the PS secondary molecular ion intensities, It was found that not only the end-group characteristic ion intensities are modified for M-n < 50 000 g mol(-1), but also those related to main chain fragments. In this study, the possibility to use this effect to quantify the molecular weight of the macrochains at the surface is investigated. Because the quantification method should preferably include all the information contained in the spectra (over 150 peaks), statistical methods are needed, Principal component analysis is applied to these ToF-SIMS data. Principal component analysis builds the best linear combination of the variables, namely the principal components that describe the greatest amount of variation of the sample set, It is found that only one principal component is required to account for the molecular weight variations and a second one allows the samples to be discriminated depending on their butyl end-groups. Whatever the PS end-groups (linear, secondary or tertiary butyl), the correlation found between the first principal component and the sample molecular weight leads to the definition of a universal curve that allows the molecular weight determination for an unknown monodisperse PS sample. (C) 1997 by John Wiley & Sons, Ltd.