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Low molecular weight heparins (LMWHs), derived from unfractionated heparin (UFH) by chemical or enzymatic degradation, have lower side effects than that of heparin. Enoxaparin, nadroparin, and dalteparin are the most widely used LMWHs. Nadroparin and dalteparin are prepared through nitrous acid degradation followed by a subsequent reduction process, in which specific residue, 2,5-anhydro- d -mannitol (An-Man), is formed at the reducing terminals of generated sugar chains. However, few practicable analytical method was available to analyze An-Man qualitatively and quantitatively. In this work, a HPAEC-PAD-MS method was developed to analyze monosaccharides in heparin and LMWHs, especially An-Man. An ion suppressor is set up between HPAEC and MS to remove the nonvolatile salts from HPAEC and make the elute compatible to MS. Various monosaccharides were separated well with HPAEC. Online MS and MS/MS confirmed all sugar residues in the hydrolysates of heparin samples. The specific residue, An-Man, was only observed in the hydrolysates of LMWHs prepared with nitrous acid degradation and reduction. In addition, major glucosamine and minor arabinose/galactose were observed in all heparin samples. The amounts of these sugar residues were quantitated with PAD, simultaneously. The ratio of glucosamine to An-Man could be used to calculate the molecular weight of LMWHs. The calculated values are comparable to the value measured with size-exclusion chromatography-multiple angle laser scattering detection. Higher the ratio of glucosamine to An-Man higher the molecular weight. The HPAEC-PAD-MS platform is an accurate, precise and efficient way to identify LMWHs by determination of An-Man. It is also an alternative method to detect the MWs of LMWHs having An-Man for quality control purposes. [ABSTRACT FROM AUTHOR]