Why might isotopically labeled internal standards be preferred over structural analogs for LC-MS quantitation?

Why might isotopically labeled internal standards be preferred over structural analogs for LC-MS quantitation?

• A. They co-elute and behave almost identically in LC-MS but have a distinguishable mass, providing accurate correction for matrix effects, ionization efficiency, and recovery.
• B. They always have higher ionization efficiency, increasing signal compared with analytes.
• C. They are cheaper to use than unlabeled analytes and require no validation.
• D. They do not interact with the matrix and therefore eliminate the need for calibration.

Answer: (A)