To provide a method for preanalytical treatments of nitropolycyclic aromatic hydrocarbons present in emission particles etc. of diesel engines.

In this method, a test liquid 160, in which a sample containing polycyclic aromatic hydrocarbons, is dissolved in a solvent; a plurality of eluents 110, 120, and 130 different from the solvent and having different polarities to be a mobile phase of the test liquid; and a silica gel column 170 are used; and after the test liquid is supplied for the column through the use of the test liquid and the eluent having the lowest polarity, the polar solvents are sequentially made to flow from lower polarity, to preparatively isolate nitropolycyclic aromatic hydrocarbons and other polycyclic aromatic hydrocarbons.