The present invention provides a method for mass spectrometry of a negatively charged organic synthetic compound, and a matrix that can be used in said method. A method for mass spectrometry of a negatively charged organic synthetic compound, the method comprising using, as a liquid matrix, an ionic liquid comprising an amine ion and an acidic group-containing organic substance ion. For example, the amine is 3-aminoquinoline (3-AQ), and the acidic group-containing organic substance is p-coumaric acid (p-CA). The negatively charged organic synthetic compound to be analyzed is, for example, an organic synthetic polymer compound or a complex compound.