Characterization of the quantitative relationship between signal-to-noise (S/N) ratio and sample amount on-target by MALDI-TOF MS: Determination of chondroitin sulfate subsequent to enzymatic digestion

Chondroitin sulfate (CS) is an important glycosaminoglycan of the extracellular matrix and its quantitative detection is of interest in different pathologies. Although there are already methods of quantitative CS determination, many of them are laborious, require time-consuming sample workup and/or suffer from low sensitivity. It will be shown here that the CS content of biological samples can be easily assessed in the picomole range subsequent to enzymatic digestion. MALDI-TOF MS (matrix-assisted laser desorption and ionization time-of-flight mass spectrometry) was used to determine the concentrations of the unsaturated disaccharide of CS obtained by enzymatic digestion of native CS with chondroitin ABC lyase. The signal-to-noise (S/N) ratio can be used as a quantitative measure: amounts of CS (measured as the disaccharide) down to at least 500 fmol could be detected and there is a direct correlation between the S/N ratio and the amount of CS between about 2 and 200 pmol although the curve per se is sigmoidal. The influence of different parameters such as the used matrix, the applied laser intensity and different methods of data analysis were also tested. Advantages and drawbacks of this approach are critically discussed. Finally, the method was validated by the determination of the CS content in samples of known concentration as well as in enzymatically digested bovine nasal cartilage and compared with two further established methods of CS determination (Carbazole and Alcian Blue method).