Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation

Abstract The fast and nearly non-destructive criteria of laser induced breakdown spectroscopy (LIBS) technique has been exploited for forensic purposes, specifically, document investigation. The dependence of the optical emission spectra of different black gel ink samples on the excitation laser wavelength, namely the visible wavelength at λ = 532 nm and the IR wavelength at λ = 1064 nm, was studied. The inks of thirty black gel-ink pens comprising ten brands were analyzed to determine the variation of the chemical composition of ink and to discriminate among them with minimum mass removal and minimum damage to the document's paper. Under the adopted experimental conditions, the ability of the visible LIBS to differentiate among the different ink samples was successful compared to IR LIBS at the same laser pulse energy (~ 25 mJ/pulse, laser fluence is ~ 1400 J·cm - 2 for visible laser and ~ 1100 J·cm − 2 for IR laser) which could be attributed to the IR absorption effects by the black ink. However, the visible LIBS produces deeper crater with respect to that produced by IR LIBS. Applying IR LIBS with higher pulse energy of ~ 87 mJ (laser fluence is ~ 4100 J·cm − 2 ), identification and differentiation of the adopted samples was performed with producing a larger-diameter but superficial crater. The plasma parameters are discussed at the adopted experimental conditions. The results support the potential of LIBS technique using both the visible and IR lasers to be commercially developed for forensic document examination.