To obtain a mass spectrometry whose detecting sensitivity, analytical capability and analytical accuracy are increased by ionizing a platinum complex by an atmospheric pressure ionization method and mass-spectrometrically analyzing the platinum complex by using a mass spectrometer.

As a platinum complex, a quadridentate ligand platinum (II) complex in Formula I, a hexadentate complex platinum (IV) complex in Formula II and the like are enumerated. In the formulae, L, M, Q and R resresent anionic ligands, and X and Y represent neutral ligands. First, the platinum complex is dissolved in a protonic solvent or the like, and a sample solution at a sample concentration at which the platinum complex becomes about 0.1% (in terms of a weight ratio) is prepared. The prepared platinum complex solution is introduced into a mass spectrometer equipped with an ion source by an atmospheric pressure ionization method by means of high-performance liquid chromatography. As the atmospheric pressure ionization method, e.g. an atmospheric pressure chemical ionization method, an ion spray ionization method and the like are enumerated. The platinum complex is introduced into the mass spectrometer, and parent ions which are generated by the atmospheric pressure ionization method are mass-spectrometrically analyzed. Thereby, the molecular weight of the platinum complex can be measured.