Development and Validation of a Method for the Analysis of Zinc Oxide in Cosmetic Matrices by Flame Atomic Absorption Spectroscopy

Table 1

Difference values for the variables studied under the Youden–Steiner model.

Parameter	Difference
	Raw material	Lipstick	Foundation	Emulsion
Ethanol (mL)	0.56	0.07	0.07	0.21
Ultrasound bath temperature (°C)	0.37	0.07	0.15	0.02
First sonication step (min)	0.09	0.01	0.04	0.17
Second sonication step (min)	0.37	0.00	0.04	0.03
Final concentration (ppm)	0.29	0.13	0.24	0.24
Critical value	0.974	0.093	0.128	0.159

Bold values correspond to critical variables.