Research cigarettes 3R4F and 1R5F (University of Kentucky, Lexington, KY) were used as quality control (QC) samples and were included in each smoking machine run. These QC cigarettes were smoked with 3 unlit clearing puffs collected after smoking was completed. Material blanks for the vapor phase were gas sampling bags filled with approximately 350 mL of laboratory air, equivalent to 10 × 35 mL puffs, and spiked with ISTD. Material blanks for the particulate phase were new filter pads inserted into a clean headspace vial and spiked with ISTD. Matrix blanks for both phases were collected as normal QC samples on the smoking machine, with empty, but filtered, cigarette holders. Matrix blanks accounted for carryover in the smoking machine and sidestream smoke contamination from cigarettes on neighboring ports. Cigarette and QC samples were accepted based on modified Westgard rules.22 (link) Nitrobenzene and 2-nitropropane deliveries were below the lowest calibrator in both research cigarettes.