MSACL 2024 Abstract

Title: Failed proficiency result for 11-nor-9-carboxy-THC
1. Problem
Laboratory failed a proficiency sample result for THC-COOH. Investigation was launched to determine root cause of failure.
2. Method Information
• Waters LC-MS system
o Acquity Binary Solvent Manager
o Acquity Sample Manager
o Acquity Column Manager
o Xevo TQ-S micro
• Mobile Phase A: 0.005% Formic Acid in Water
• Mobile Phase B: 0.01% Formic Acid in Acetonitrile
• 7 minute gradient method, 0.4 mL/min
• Column: 2.1 x 100 mm, 1.7 µm C18
• Column Temp: 50°C
• Injection volume: 7.5 µL
• Quantitative MRM acquisition
• UTAK drug free urine spiked with KF
• Cerilliant Certified Reference Material
o Calibrator analyte: (±)-11-nor-9-Carboxy-Δ9-THC (Item # T-006)
o Internal Standard: (±)-11-nor-9-Carboxy-Δ9-THC-d3 (Item # T-004)

3. Troubleshooting Steps
THC-COOH slope drift was observed in calibration as the 5-day work week passed. Protocol at the time was to pull new calibrator set from -80°C freezer on Monday, use throughout the week (interim storage at 4°C refrigerated temperature), and discard on Friday. Slope drift appeared to coincide with days of the week. A downward trend in slope value was determined. Stability of THC-COOH in calibrator solution was questioned. Outlier sample was recalculated using calibration slope from different days of the week. The recalculated results would pass using calibrations from earlier days of the week.

4. Outcome
Calibrator set protocol was changed so that sets would be discarded after use on Wednesday, and a new set would be pulled on Thursday. This set would then be discarded on Friday. Ideally, a new set would be pulled every day, but the current composition of the calibrator set would make it too laborious a task to make daily aliquots. The slope of the calibration curve is monitored daily, and any variation exceeding the cutoff is promptly addressed.