What is an internal standard and why is it used in LC-MS/MS analyses?

• A. An internal standard is the analyte of interest.
• B. An internal standard changes the sample pH.
• C. An internal standard increases instrument noise.
• D. An internal standard is a compound similar to the analyte added in a constant amount to correct for variability in extraction, injection, and ionization.

Answer: (D)

In LC-MS/MS analyses, variability from sample preparation, extraction efficiency, injection volume, and ionization (matrix effects) can cause the measured signal to wander from run to run. An internal standard is a compound that is chemically similar to the analyte of interest, added to every sample (and to calibration standards and quality controls) in a constant amount. Because it behaves similarly to the analyte during extraction, cleanup, and ionization, any fluctuations affect both compounds in a related way. By comparing the signal of the analyte to the signal of the internal standard (often using a ratio), you normalize these variations, leading to more accurate and precise quantification across samples and runs. The internal standard is typically not the analyte itself and is chosen to be distinguishable by the instrument, often a stable isotope-labeled version or a closely related compound, and it’s added in a fixed amount to ensure the ratio truly reflects changes in the analyte concentration.