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Improved ESI-MS Sensitivity via an Imidazolium Tag (DAPMI-ITag) for Precise Sialic Acid Detection in Human Serum and CMAH-Null Mouse Tissues Sialic acids (Sias), consisting primarily of N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), play crucial roles in many biological processes. The detection and quantification of Sias are essential for understanding their roles in health and disease progression. Although numerous techniques have been developed to enhance the specificity and sensitivity of Sias analysis, traditional methods such as derivatization with fluorescent tags coupled with HPLC-MS analysis often suffer from low limits of detection, limiting the quantification of Sias in trace samples. Here, we introduce DAPMI, a novel imidazolium-based ITag for sensitive Sia detection. We demonstrate its utility in the detection and quantification of Sia composition in human serum, and in different tissues from CMAH (cytidine monophosphate-N-acetylneuraminic acid hydroxylase) knockout mice, using ESI-MS analysis and with a limit of detection (LOD) down to the low fmol range. The results showed that both Neu5Ac and Neu5Gc were present in varying proportions in wild-type mice and CMAH heterogeneous mice. Trace amounts of Neu5Gc were also detected in the tissues of CMAH null homogeneous mice (CMAH–/−) and in human blood serum using ESI-ToF-MS, suggesting its presence may be linked to dietary intake of Neu5Gc-containing foods, as Neu5Gc cannot be synthesized endogenously in CMAH–/– mice, and in humans. The DAPMI-ITag and the labeling technology developed in this study significantly improve the sensitivity of Sias detection compared to conventional tags such as o-phenylenediamine (OPD), and provide a new chemical tool for the exploration of Sias’ biological roles and their use as biomarkers in different human conditions. https://pubs.acs.org/doi/10.1021/acs.analchem.5c00752