the reflectron mode using a Voyager-DE PRO (AB Sciex, Framingham,
MA) equipped with a nitrogen laser emitting at 337 nm. Ions were accelerated
to a 20 kV final potential. Positive ions were detected, and mass
spectra were a sum of 300 shots. External mass calibration was provided
by a mixture of peptides (Sequazyme standards kit, AB Sciex). Samples
were prepared by dissolving polymers in THF at a 10 g L–1concentration. The matrix, 3-indoleacrylic acid (IAA; Sigma-Aldrich,
used without further purification), was dissolved in THF at 0.25 M.
Matrix and polymer solutions were mixed at a 9:1 (v/v) ratio, and
then 1 μL of the mixture was deposited and dried onto the MALDI
sample plate.