Which isotopic internal standard is commonly used for cannabinoids LC-MS/MS analysis?

• A. THC-d5
• B. THC-d3
• C. THC-13C
• D. CBD-d3

Answer: (B)

Using a stable isotope–labeled internal standard is essential in LC-MS/MS to correct for differences in extraction, matrix effects, and instrument response. The internal standard should closely resemble the target analyte so it behaves almost identically during sample prep, chromatography, and ionization, while still being distinguishable by mass.

THC-d3 is a deuterated version of THC that mirrors the chemical behavior of THC closely. It co-elutes with cannabinoids, undergoes similar extraction efficiency, and ionizes and fragments in a way very similar to THC, but it can be distinguished by a three-mass-unit shift in the mass spectrometer. This makes THC-d3 an effective internal standard for quantifying cannabinoids across LC-MS/MS analyses.

THC-d5 could serve as an internal standard as well, but THC-d3 is more commonly used in routine methods due to availability and established performance for cannabinoid panels. A carbon-13 labeled standard (THC-13C) would be excellent in principle but is typically more expensive and less commonly adopted for routine screening. Using CBD-d3 as the internal standard would not track THC behavior as accurately, since it is a different compound with different retention and ionization characteristics.