• Internal Standard (IS) is a compound (preferably a deuterated or labeled analog of the reference standard) that is added at a constant level to the standards, zero-level standards and samples. The uniform addition of an internal standard reduces variability during batch acquisition that can be attributed to outside factors (e.g., temperature, drift, autosampler errors, etc.) by using the area ratio or ratio of analyte response to internal standard response to perform calculations (i.e., calculated concentrations).