“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
Introduction: mathematical background
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. 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 complicated mass spectrum of P3HB oligomers was analyzed by fraction base Kendrick mass defect (KMD) plot . Fraction base KMD analysis has been developed from the regular KMD analysis to modify the aspect and separation capabilities of the KMD plots depending on a divisor (noted X) in addition to the base unit (noted R) via a fraction base R/X .
Poly(3-hydroxybutyrate) (P3HB, Mn = 2.6 × 105 g mol−1, ĐM=2.7) was from Sigma Aldrich (St. Louis, MO,USA). All other chemicals and solvents were from Wako pure chemicals (Osaka, Japan) unless otherwise mentioned. 1 μL of P3HB in THF at 1 mg mL−1 was drop-casted on a disposable MALDI target plate (Hudson Surface Technology, New Jersey, USA) to form a polymer thin layer. 1 μL of sodium hydroxide in methanol (10 mg mL−1) was deposited on the sample layer and left to air-dry for 5 min. The excess amount of alkaline reagent was washed with distilled water. After drying, 1 μL of 2,4,6-trihydroxyacetophenone (THAP, Protea Biosciences, West Virginia, USA) in THF (20 mg mL−1) was applied on the sample spot as matrix. Mass spectra were recorded with a JMS-S3000 SpiralTOFTM mass spectrometer. Plots were computed using msRepeatFinder 3.0 from the peak list exported using msTornado.