An inventive substrate is provided which includes a substrate compound of formula A - B1 - B2 - B3: wherein A is a sugar moiety; B1 is a linker moiety allowing the conjugation of moiety A and the remaining structure of the substrate; B2 contains a permanently charged element such as a quaternary ammonium group so as to increase proton affinities and ionization efficiencies for mass spectrometry analysis; and B3 of various carbon length conferring specificities to targeted enzymes. Also provided is a process to detect lysosomal diseases by contacting a sample with the inventive substrate along with an internal standard which is isotope-labeled analog of the product cleaved by a targeted enzyme upon the substrate.
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