Differentiation of Ganglioside Isomers by MALDI Tandem Mass Spectrometry with Alkali Earth Metal Additives

Gangliosides, acidic glycosphingolipids with one or more sialic acid residues, are challenging to analyze with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) because loss of sialic acid residues is significant during ionization process, especially in the positive-ion mode. Recently, we showed that 5-methoxysalicylic acid (MSA) could be a matrix of choice for ganglioside analysis in the positive-ion mode. In this study, we investigated whether GD1a and GD1b, structural isomers with two sialic acid residues, are distinguishable by MALDI MS or tandem MS (MS/MS) with MSA and a proper salt additive. Our results showed that characteristic fragment ions of GD1a and GD1b were successfully observed by the positive-ion mode MALDI MS/MS and these diagnostic ions were more reliably detected with increased intensities when an alkali earth metal additive such as CaCl2 was employed instead of an alkali metal additive such as NaCl.