Determination of 11-Nor- 9-Tetrahydrocannabinol-9-Carboxylic Acid in a Urine-Based Standard Reference Material by Isotope-Dilution Liquid Chromatography-Mass Spectrometry with Electrospray Ionization

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Abstract
A new method is presented for the determination of 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid (THC-9-COOH) in urine-based standard reference materials (SRMs) for marijuana metabolite. This method is based on isotope-dilution liquid chromatography-mass spectrometry (LC-MS) using electrospray for ionization (ESI). An isotopically labeled compound, 5′,11-nor-Δ9-THC-9-carboxylic acid-d3, was used as an internal standard. Solid-phase extraction (SPE) was used to prepare samples for LC-MS analyses. LC was performed on a C18 column with an isocratic mobile phase consisting of 0.05M of ammonium acetate in methanol/water (75:25). Electrospray ionization in the negative ion mode was used to monitor the [M-H]-ions at m/z 343 and 346 for THC-9-COOH and its labeled internal standard, respectively. Samples of SRM 1507b (THC-9-COOH in freeze-dried urine) were prepared and measured on three separate sets. The within-set coefficients of variation ranged from 0.32% to 2.77%. The correlation coefficients of the regression lines were 0.999 to 1.000. The detection limit for THC-9-COOH with this method is estimated to be 5 pg/mL. This method compared well with the gas chromatography-mass spectrometry method used for certification, and results were within the certified values of THC-9-COOH for SRM 1507b.

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