A new high photoluminescent metal-organic complex is proposed as optical marker for the indentification of gunshot residues (GSR) produced from firearms. The marker containing Eu3 + ion was synthetized producing the [Eu(PIC)3(NMK)3] complex, where PIC is picric acid and NMK is n-methyl caprolactam. It was chemically characterized using emission spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis, and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR MS) and then added to conventional ammunition as function of its concentration (from 2 to 50 mg by ammunition). Marked conventional GSR was visually and chemically detected by UV irradiation (λ = 395 nm) and ESI-FT-ICR MS. The tests indicated that marked GSR is easily visible in proportions above of 25 mg on the target, bullets and firearms regions and on the arm of shooter. It was also detected at lower concentrations using ESI-FT-ICR MS. A final cost of the process is evaluated be lower than US$ 1.50/by ammunition, where one gram of marker suffices to tag approximately 40 .40 bullets.