2.6. Limit of detection and limit of quantitation
The limit of detection (LOD) and the limit of quantitation (LOQ)
were estimated according to the three times of signal-to-noise ratio
(LOD = 3S/N) and ten times of signal-to-noise ratio (LOQ = 10S/N)
using the matrix-matched standard calibration curve method
[30]. Six standard solutions with concentration (including zero
concentration) close to the LOD for each NSAID were prepared as
described in Sections 2.1 and 2.3. A plot is constructed by plotting
the peak areas of standard solution against their corresponding
concentrations. Then, linear least-square regression method was
applied to find the best fit linear equation (y = ax + b) of the best
linear standard calibration curve. The overall standard deviation of
the standard calibration curve (sy/x) in the y-direction is calculated
from the experimental signals (peak areas) and the theoretical signals
of the standard calibration curve. The LOD is found by dividing
three times of the sy/x value with the slope of the calibration curve,
i.e. 3sy/x/a, and the LOQ is calculated by dividing ten times the sy/x
value with the slope of the calibration curve, i.e. 10sy/x/a.