Determination of cotinine and 3-hydroxynicotinine in human serum by liquid chromatography-tandem mass spectrometry and its application.

In this study, we developed a method for determining cotinine and 3-hydroxycotinine in human serum and established a methodology for an in-depth study of tobacco exposure and health. After the proteins in the human serum samples were precipitated with acetonitrile, they were separated on a ZORBAX SB-Phenyl column with a mobile phase of methanol encompassing 0.3% formic acid-water encompassing 0.15% formic acid. The measurement was performed on an API5500 triple quadrupole mass spectrometer in the multiple reaction monitoring mode. Cotinine, 3-hydroxycotinine, and cotinine-d3 isotope internal standards were held for 2.56 minutes, 1.58 minutes, and 2.56 minutes, respectively. In serum, the linear range was 0.05 to 500 ng·mL-1 for cotinine and 0.50 to 1250 ng·mL-1 for 3-hydroxycotinine. The lower limit of quantification (LLOQ) was 0.05 ng·mL-1 and 0.5 ng·mL-1 for cotinine and 3-hydroxycotinine, respectively. The intra-day and inter-day relative standard deviations were <11%, and the relative errors were within ± 7%. Moreover, the mean extraction recoveries of cotinine and 3-hydroxycotinine were 98.54% and 100.24%, respectively. This method is suitable for the rapid determination of cotinine and 3-hydroxycotinine in human serum because of its rapidity, sensitivity, strong specificity, and high reproducibility. The detection of cotinine levels in human serum allows for the identification of the cutoff value, providing a basis for differentiation between smoking and nonsmoking populations.