Rapid, sensitive, and reliable quantitation of nicotine and its main metabolites cotinine and trans-3'-hydroxycotinine by LC-MS/MS: Method development and validation for human plasma

Abstract

New nicotine delivery products are gaining market share. For evaluation of their characteristics, toxicokinetic investigations are in current research focus. For reliable determination of blood plasma levels of nicotine and its main metabolites cotinine and trans-3'-hydroxycotinine, a quantitation method based on LC-ESI-MS/MS was developed and validated. Addition of isotope labeled internal standards prior to rapid sample preparation using protein precipitation with methanol was chosen for sample preparation. Different stationary phases were tested and phenyl-hexyl separation was found to be superior to HILIC, C18, and C8 stationary phases. Ion suppression effects caused by hydrophilic early eluting matrix were eliminated by the adjustment of an adequate retention utilizing a phenyl-hexyl separation stationary phase. Exchange of acetonitrile as organic mobile phase by methanol and elevation of pH value of aqueous mobile phase containing 5 mM NH₄Ac to 4.50 improved the chromatographic resolution. The limits of quantitation for nicotine, cotinine, and hydroxycotinine were 0.15, 0.30, and 0.40 ng/mL, respectively. Linearity was proven by matrix matched calibration for the whole working range from 0.50 ng/mL to 35.0 ng/mL for nicotine and from 6.00 to 420 ng/mL for cotinine and hydroxycotinine (Mandel's fitting test with R² > 0.995). Quality control samples at four different levels (0.50, 1.50, 17.5, 28.0 ng/mL for nicotine and 6.00, 18.0, 210, 336 ng/mL for cotinine and hydroxycotinine) in plasma were analyzed six times on three days. Mean accuracies ranged from 87.7% to 105.8% for nicotine, from 90.3% to 102.9% for cotinine, and from 99.9% to 109.9% for hydroxycotinine. Intra- and inter-day precisions (RSD %) were below 15% for all analytes (<20% for LLOQ). As proof of concept, the method was successfully applied to a real plasma sample from a cigarette smoking volunteer.

Keywords: Cotinine; Hydroxycotinine; Nicotine; Phenyl-hexyl column; Protein precipitation.