A number of copper salts, Cu(OOCCH₃)₂, Cu(ClO₄)₂, Cu(NO₃)₂, CuCl₂ and CuSO₄ have been tested for their ability to form binuclear copper–caffeine complexes. The electrospray ionization (ESI) mass spectra of methanol solution containing caffeine and CuCl₂ or CuSO₄ show signals of two copper atom containing ions, so the signals correspond to binuclear complexes: [2Caf + Cu₂SO₄]⁺, [2Caf + Cu₂]⁺, [2Caf + Cu₂Cl]⁺, [2Caf + Cu₂Cl₂]⁺ and [2Caf + Cu₂Cl₃]⁺. Sulfate and chloride anion are characterized by charge densities higher than those of the carboxylate, nitrate and perchlorate anion. Thus, due to the electrostatic forces, the binuclear complexes containing SO₄^2– or Cl^– can survive the transfer from solution to the gas phase and then can successfully be observed on ESI mass spectra. The ion [2Caf + Cu₂Cl₃]⁺ is present in solution and could be detected when using methanol/chloroform as solvent. The ions [2Caf + Cu₂]⁺, [2Caf + Cu₂Cl]⁺ and [2Caf + Cu₂Cl₂]⁺ are formed from the [2Caf + Cu₂Cl₃]⁺ ion (by subsequent loss of Cl atoms) on transfer from the solution to the gas phase or in the gas phase. The ion [2Caf + Cu₂]⁺ does not contain a bridging agent, thus it is reasonable to assume that it contains a Cu–Cu bond.

Keywords: caffeine, copper, binuclear complexes, electrospray ionization mass spectrometry