*2.6. Matrix E*ff*ect*

Matrix effect should be considered in the detection process, which includes intrinsic organic or inorganic compounds after extraction and cleanup, and extrinsic inorganic ions, organic acids, detergent, etc. These interfere material can comprehensively enhance or suppress the response of the target compounds. In our research, the matrix effect (*ME*%) was calculated based on the following equation [30,31]:

$$ME\% = (\frac{A}{B} - 1) \times 100\%$$

where *A* is the integration area in matrix-matched standard solution and *B* is the integration area in a standard solution with identical concentration for each compound. In general, the matrix effect within ±20% can be regarded as acceptable and the calibration can be performed without considering matrix effect. Otherwise, it should be considered during quantification [16,32].

The fishery drugs were dissolved and diluted up to 1000 times, which would significantly decrease the matrix effect. Standards diluted with more than 90% blank matrix solution were used to test for the matrix effect. The result showed less than 20% matrix effect for all the compounds of interest at the concentrations of 100 and 500 ng/mL for compounds with SDL above 100 ng/mL. Because of the acceptable matrix effect, it is feasible to use a methanol–water (1:1, *v*/*v*) solution to dilute a series of standard solutions, for quantification of positive compounds.
