Abstract: Methods, compositions, kits, and apparatuses for MS-based quantification of a target analyte (e.g., a peptide, a hormone) in a sample are provided for increasing the productivity and/or throughput of samples by reducing the time and/or cost associated with conventional techniques for external instrument calibration that typically utilize a series of calibrators that are sequentially run through the same analytical process as a batch of samples. In various aspects, calibration mixtures are provided containing a known quantity of a plurality of calibrants for a target analyte of interest, with each calibrant being distinguishable in the calibration mixture by mass spectrometry such that a complete calibration curve can be generated from a single external calibration run.

Abstract: Provided are molecules and methods for detecting enzymatic activity of various lysosomal storage enzymes. The molecules may be used as internal standards that may be combined with substrates that have improved solubility.

Abstract: Substrates are provided that include compounds suitable for detecting the activity of an enzyme such as a lysosomal storage enzyme where the substrates include: a sugar moiety; a linker moiety allowing the conjugation of sugar moiety with the remaining structure of the substrate; and two or more fatty acid chains or derivatives thereof at least one of which is sufficiently structured to provide improved solubility in aqueous or organic solvent systems. Also provided are methods for using substrates for detecting enzymatic activity using the inventive substrates.