3.6. Matrix Effects

The post-extraction addition method was employed for matrix effect evaluation. Briefly, ten samples were extracted according to the SPE procedures. After extraction, five samples were spiked with defined amounts of standard stock solutions, while the remaining five samples were kept as blank samples. Simultaneously, identical amounts of standard stock solutions and internal standards were spiked into five clean vials. All samples were then evaporated and dissolved in the mobile phase according to sample preparation and extraction, respectively. The matrix effect was calculated by the ratio of the measured peak area in the matrix spiked after the extraction (minus the peak area in the blank matrix sample) to the peak area in the standard solution.