Preparing a sample for analyzing a residual agricultural chemical in a caffeine-containing food such as tea involves: a step for extracting the residual agricultural chemical from the food by using an organic water-soluble extraction solvent to obtain an extract; a step for obtaining a mixture of a treatment agent, which contains powdery ion exchange group-modified carbon material-containing silica gel and powdery carbon material-containing silica gel, the extract, and an organic purification solvent capable of dissolving the residual agricultural chemical; and a step for fractionating a part of a liquid component from the mixture. The ion exchange group of the powdery ion exchange group-modified carbon material-containing silica gel is an amine-based functional group having anion exchange ability, for example, a 3-(2-aminoethylamino)propyl group or a 3-aminopropyl group.

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