Improved matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry of carboxymethyl cellulose

A refined sample preparation procedure for matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS) was developed for the evaluation of the degree of substitution (DS) in partially depolymerised carboxymethyl cellulose (CMC). By adding ammonium sulphate to the sample mixture prior to the analysis, good quality mass spectra could be acquired. The usual time-consuming search for 'sweet-spots' at the crystalline rim of the MALDI target spot was also avoided. This quality improvement made it possible to investigate whether various positions on the target spot generated mass spectra in which the measured DS varied. The accuracy and reproducibility of the sample preparation procedure were tested by applying it on three commercial CMCs. The study shows that the DS values that were calculated from the spectra acquired from the centre region of the MALDI target spot were in better agreement with the DS provided by the supplier than were the values obtained from the large crystals at the target spot rim. This observation could be one reasonable explanation for the higher DS values reported in other publications. By applying our refined MALDI sample preparation procedure DS values that were in good agreement with the values provided by the manufacturer could be obtained. This indicates that MALDI-TOFMS of partially depolymerised CMCs can be used for an estimation of the DS as a complement to the more established methods, e.g. NMR, titrimetry, and chromatographic techniques.