Detection of explosive-related compounds by laser photoionization time-of-flight mass spectrometry

We report studies using laser photoionization and time-of-flight mass spectrometry to detect several explosive-related compounds (ERCs). The ultimate goal of this work is the detection of buried land mines through their chemical signatures. Resonantly enhanced multiphoton ionization using jet expansion cooling, with a nanosecond pulse laser, results in complete photofragmentation of the parent ERC and appearance only of the NO+ ion, which forms all of the detected signal. This will also occur for compounds naturally occurring in the environment, such as NO2 or peroxyacetylnitrate, rendering too many false alarms for this approach to be viable. Therefore, two other techniques were evaluated. Single-photon ionization with nanosecond pulses in the vacuum ultraviolet is shown to produce only the parent ion, and is probably the most suitable choice. For 2,4-dinitrobenzene we find a limit of detection of about 40 ppb, for a signal to noise ratio of 3. This may be sufficient for land-mine detection, but improvement is likely with future work. Nonresonant multiphoton ionization in the ultraviolet with a femtosecond laser produces fragmentation but retains some parent ERC ion signal. The limit of detection is similar to that of single-photon ionization but it is harder to implement with lasers now commercially available. Future directions are outlined.