3.4. Analytical figures of merit of the proposed method
To evaluate the performances of the proposed method, calibra- tion linearity range, correlation coefficients (r), limits of detection (LODs) and quantification (LOQs), and precision were investigated. The results were shown in Table 3. The calibration curves, which related the concentrations to the peak areas, were tested over a wide range of 0.3–50 mg L−1 (including six concentration levels). The correlation coefficients of five analytes using a least squares linear regression fit ranged from 0.9995 to 0.9998. LODs and LOQs were calculated as 3 and 10 times the standard deviation (SD) values that acquired from analyzing 11 blank skin cream extracts fortified with 0.3 mg L−1 for each analyte.
Potential matrix effects were studied by comparison of the slopes of the calibration curves obtained with the standard addition method and external calibration method. The calculated calibration parameters and their values were listed in Table S1. Percentage deviations between slope values were in the range of 0–4.9% in all cases. Hence, matrix components were not expected to interfere with the determination of analytes, and the external calibration with aqueous standards could be done.
The repeatability of the instrument (expressed as RSD) was measured by carrying out seven independent determinations for each multi-component standard solution at low and high concen- trations. Also, the reproducibility of the method was evaluated by analyzing two group of skin cream extracts (seven independent samples for each group) spiked at two levels of all the analytes within a day (intra-day variation) and in seven non-consecutive days (inter-day variation). The precision (RSD values) varied from 0.70% to 3.6% for instrument, from 1.4% to 5.1% for the intra-day and from 1.8% to 6.1% for the inter-day variations (Table 3).