During the first visit, a registered nurse collected a blood sample from an antecubital vein to a 7 mL vacutainer tube with sodium heparinate. All samples were iced immediately and centrifuged for five minutes at 3000 rpm. The plasma was then removed and frozen at −20°C until time of assay. Measurements of plasma nicotine, and its metabolites cotinine and 3-hydroxycotinine, were performed by Labstat Inc. (Kitchner, Ontario, Canada). Nicotine and cotinine were measured by highresolution capillary-column gas chromatography [22, 23]. Plasma 3-hydroxycotinine was measured using capillary gas chromatography-mass spectrometry (GC-MS) [24].
