Femtosecond Laser Vaporization Detection of Dinitrotoluene Using Ion Mobility Spectrometry

Femtosecond laser vaporization at 10¹⁴ W/cm² is investigated as a method to increase the concentration of the explosive dinitrotoluene (DNT) and trinitrotoluene (TNT) in the gas phase for ion mobility spectrometry (IMS) detection. While the identification specificity of IMS is limited by the fact that single ion mobility value can correspond to multiple ions, the formation of NO₃^- provides further confirmation that the feature originates from DNT or TNT. Quantitative analysis of DNT is demonstrated with a linear response range from 0.1 to 5 μg. The limit of detection (LoD) of DNT with the femtosecond laser vaporization method is 49 ng deposited on a metal coupon with noncontact detection using a Venturi pump sampling system. The LoD for laser vaporization IMS can be improved to 5 ng by direct detection of laser-vaporized DNT without the Venturi sampling system. The LoD of DNT using the femtosecond laser vaporization is 15 times lower than that using thermal desorption at 250 °C. Optical and spectroscopic investigations of the laser vaporization process as a function of position on the metal substrate reveal that a quench of laser-induced plasma occurs upon interaction with the deposited DNT sample. IMS measurements reveal the formation of NO₃^- when both plasma generation and DNT vaporization occur simultaneously. NO₃^- formation is due to the decomposition of DNT by the laser-induced plasma from the metal substrate.