The matrix effect was tested by comparing the slopes of thematrix-free calibration curves to the matrix-matched calibrationcurves. Samples were first extracted according to the procedureas described the Experimental section. Matrix effect was investi-gated by calculating the percentage (C%) of signal enhancement orsuppression, according to Eq. (1).
C%
=
1
− ss
sm
×
100 (1)where ssis the slope of calibration plot with matrix-matchedcalibration solutions and smis the slope of calibration plot withcalibration solutions in solvent.