A simple, sensitive, and rapid quantitative LC-MS/MS assay was designed for the simultaneous quantification of free and glycoprotein bound monosaccharides using a multiple reaction monitoring (MRM) approach. This study represents the first example of using LC-MS/MS methods to simultaneously quantify all common glycoprotein monosaccharides, including neutral and acidic monosaccharides. Sialic acids and reduced forms of neutral monosaccharides are efficiently separated using a porous graphitized carbon column. Neutral monosaccharide molecules are detected as their alditol acetate anion adducts [M ϩ CH 3 CO 2 ] Ϫ using electrospray ionization in negative ion MRM mode, while sialic acids are detected as deprotonated ions [M Ϫ H] Ϫ . The new method exhibits very high sensitivity to carbohydrates with limits of detection as low as 1 pg for glucose, galactose, and mannose, and below 10 pg for other monosaccharides. The linearity of the described approach spans over three orders of magnitudes (pg to ng). The method effectively quantified monosaccharides originating from as little as 1 g of fetuin, ribonuclease B, peroxidase, and ␣ 1 -acid glycoprotein human (AGP) with results consistent with literature values and with independent CE-LIF measurements. The method is robust, rapid, and highly sensitive. arbohydrates play vital roles in the control of many key biological processes by acting as reciprocating compounds with proteins in molecular recognition events [1,2]. Associative interactions between oligosaccharides on glycoproteins and lectins on the surfaces of binding partners can be involved in the initiation steps of many diseases such as influenza, cholera, stomach cancer, and cancer metastasis [3]. An understanding of these initiating binding processes at the molecular level requires the structural determination of oligosaccharides and the quantification of the glycan monosaccharide constituents, which would be essential towards the development of possible cures for many diseases [4,5].Monosaccharides compositional analysis is commonly performed using gas-chromatography [6 -10], liquid-chromatography [11-13], or capillary electrophoresis [14 -21]. GC/MS chromatography with an electron-impact ion source has been the traditional method of choice for the quantification of neutral monosaccharides. GC analysis of the trimethylsilyl ether derivatives of sugars is a well-known procedure [9], as is the GC separation of sugars as alditol acetates [7]. However, sample preparation for GC is extensive, involving the derivatization of sugars with specialty reagents to render them more volatile [10]. Also, the sensitive chromatographic operating parameters for GC are not well suited for routine analysis. Liquid chromatography analysis of monosaccharides is complicated by the fact that these compounds do not possess a UV chromophore and consequently, laborious derivatization with chromophores or fluorophores is required before UV or fluorescence detection [22][23][24][25][26]. Neutral sugars have also been analyzed by HPLC using...