In order to evaluate the possibility of using external
calibration method, calibration curves obtained by standard addition
method using 1%m/v lipstick emulsion and external calibration of aqueous
standardwere compared. The slopes obtained by external calibration
method and standard addition method were 1,890,000 counts ng−1 mL
and 2,270,000 counts ng−1 mL, respectively. As can be seen, the sensitivities
of Pb increased significantly in the emulsion matrices. It could be
due to the presence of organic acid added in the lipsticks which caused
difference in the vapor generation efficiency between emulsion matrix
and aqueous solution. Therefore, external calibration method could not
be used for the quantification of Pb in these samples. Hence, isotope dilution
and standard addition methods were used for the determination
of Pb in selected lipstick samples. Aliquots of 100 μL of the emulsified
lipstick were injected for the determination of Pb using the FI vapor
generation system. Typical mass-selective flow injection signal (ICP-MS
detection) for a solution containing Lipstick 3 emulsion (1% m/v) is
shown in Fig. S2 (Supplementary data).