Maldi target plate

Elemental analysis was performed on freeze-dried samples of HS (5 mg) by using Vario EL cube (Elementar Analysensysteme GmbH, Germany). The solid-state 13 C-NMR was performed on a Bruker AVANCE III 400WB spectrometer. MALDI had the advantage of forming singly charged ions. Combined with TOFMS, it could detect the absolute MW of HS rather than the apparent MW detected in high performance size exclusion chromatography [39] . In details, the HS samples were dissolved in 50% acetonitrile (ACN) and 0.1% trifluoroacetic acid (TFA), then mixed with same amount of matrix solution containing 10 mg/mL of α-cyano-4-hydroxycinnamic acid (CHCA) in 0.1%TFA-50%ACN, and spotted onto a 384-well stainless-steel MALDI target plate (Applied Biosystems, USA). An ABI 4800 MALDI TOF/TOF mass spectrometer (Applied Biosystems, USA) was used to analyze the samples on the MALDI target plates. For MS analyses, typically 1600 shots were accumulated for each sample. The MALDI spectra of HS in current study were consistently found to contain a number of common peaks. It was difficult to assign the origins to peaks in the low m/z range, as they could be attributed to matrices, contaminants or HS fragments during ionization. Thus only the largest m/z peak with signal-to-noise ratio over 3 in each spectrum, which was more likely associated with the ionized HS molecule, was reported in this work.