Why use isotopically labeled internal standards in LC-MS quantitation?

Why use isotopically labeled internal standards in LC-MS quantitation?

• A. They co-elute and behave identically in LC-MS but have a distinguishable mass, providing accurate correction for matrix effects.
• B. They are cheaper than unlabeled standards.
• C. They ionize better than unlabeled standards.
• D. They always simplify the chromatography.

Answer: (A)