“Fraction base” KMD plots for a high molecular weight poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolyester following its on-plate alkaline degradation and SpiralTOFTM analysis
High molecular weight polymers are often MS-silent due to their inherent high dispersity (ĐM) or detected in the high mass range with low resolving power. High-resolution mass spectrometry (HRMS) is indeed limited to the low mass range (< 3000 Da) for an unambiguous evaluation of the nature of repeating units and/or end-groups or the isolation of isobaric compounds. An “on-plate” alkaline degradation has thus been developed as a sample pre-treatment on the MALDI target with tenths of ng of polymer to cut long industrial polyester chains into short oligomers amenable to MALDI-HRMS . The complexity of the associated mass spectra can be greatly reduced with the appropriate resolution-enhanced Kendrick mass defect (KMD) analysis using the “fraction base” option of msRepeatFinder to produce compositional maps.
Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV, 12 mol% 3HV, Mn=1.4×105 g mol-1, ĐM=2.5) was from Sigma-Aldrich (St. Louis, MO, USA) while all other chemicals and solvents were from Wako pure chemicals (Osaka, Japan) unless otherwise mentioned. 1 μL of PHBV (1 mg mL−1 in THF) was drop-casted on a disposable MALDI target plate (Hudson Surface Technology, NJ, USA). 1 μL of sodium hydroxide in methanol-d4 at 10 mg mL−1 was further deposited and left to air-dry. The excess alkaline reagent was washed with distilled water. 1 μL of 2,4,6-trihydroxyaceto-phenone (Protea Biosciences, WV, USA) in THF at 20 mg mL−1 was applied on the sample spot. Mass spectra were recorded with a JMS-S3000 SpiralTOF™ mass spectrometer. Plots were computed using msRepeatFinder 3.0 from the peak list exported using MSTornado.