Complementarity of Solvent-Free MALDI TOF and Solid-State NMR Spectroscopy in Spectral Analysis of Polylactides

We report systematic studies of solvent-free modification of matrix-assisted laser desorption/ionization time-of-flight (SF MALDI-TOF) mass spectrometry in analysis of synthetic polymers employing solid-state NMR spectroscopy as a supporting technique. In the present work oligomeric (M_n = 4000 g mol⁻¹) poly(l-lactide) (PLLA) was employed as a reference sample. The analyte was embedded into four matrixes commonly used in MALDI-TOF analysis of polymers: 1,8-dihydroxy-9-anthracenone (DT), 2,5-dihydroxybenzoic acid (DHB), 2-(4 hydroxyphenylazo)-benzoic acid (HABA), and trans-3-indoleacrylic acid (IAA). Solid-state NMR measurements clearly showed that the initial crystallinity of PLLA had no influence on quality of SF MALDI-TOF spectra since the crystalline structure of the analyte was not preserved during analyte/matrix grinding. Interestingly, the matrix remained crystalline during the sample’s preparation. It was also found that, on the contrary to the dried droplet (DD) method, the SF approach leads to highly resolved mass spectra for a large variety of matrixes. Finally, problems of polymorphism and mechanochemical processes that can occur during the analyte/matrix grinding are briefly discussed.