Identification of hexahydroimidazo[1,2-a]pyrazine-3,6-diones and hexahydroimidazo[1,2-a]imidazo[1,2-d]pyrazine-3,8-diones, unusual products of silica-catalyzed amino acid thermal condensation and products of their thermal decomposition using coupled high-performance liquid chromatography–particle beam mass spectrometry and gas chromatography–Fourier transform infrared spectroscopy–mass spectrometry