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Linear oligogalacturonic acids (1,4-linked alpha-D-galacturonic acid oligomers), obtained by partial acid hydrolysis of orange polygalacturonides, were studied by negative-ion electrospray ionization mass spectrometry, without prior sample derivatization. After preparative separation using high-resolution anion-exchange chromatography, some fractions enriched in uronic acids were desalted, transferred into a methanol+water solution adjusted to pH 10, and directly submitted to electrospray ionization mass spectrometry in the negative-ion recording mode. Clear molecular mass assignments of the oligomers, covering a degree of polymerization between 4 and 7, were obtained.