Laser induced multiphoton ionization (MPI) has been used to produce positive and negative ions at atmospheric pressure in an ion mobility spectrometer. This paper deals particularly with the application of this technique to the ultra‐trace detection of a number of energetic materials such as RDX and PETN.In conventional ion mobility spectrometry (IMS), a radioactive 63Ni β‐foil is used as the ionization source. The laser IMS spectra will be compared with the 63Ni spectra and it will be shown that the analyte ions produced by laser ionization have a wavelength dependent fingerprint which provides an additional degree of selectivity. It is felt that this new approach to IMS will develop considerably the scope and applicability of this powerful technique.

Resonance-enhanced multiphoton ionization spectrometry (REMPI), using time-of-flight mass spectrometers and tuned lasers, has proved an important ultra-sensitive analytical technique, Nevertheless, conventional nanosecond REMPI suffers from a number of shortcomings: most importantly, REMPI often fails through rapidly (pre)dissociating states, In the case of thermally labile molecules, which include the nitro-molecules, either no or very small parent or high/mass fragment ion peaks exist, making the interpretation of the mass spectra ambiguous at best and often impossible, Femtosecond laser mass spectrometry (FLMS) can often 'defeat' these dissociative states, resulting in large parent or high-mass fragment ion peaks which make the interpretation less ambiguous. In the present paper, nanosecond and femtosecond multiphoton ionization and fragmentation are compared using time-of-flight mass spectrometry for NO2 gas and a number of different nitro-molecules: nitromethane, nitrobenzene, m-nitrotoluene, dinitrotoluene and trinitrotoluene.

Resonance-enhanced multiphoton ionization has been used to detect the presence of vapour-phase explosive-type samples in a linear time-of-flight (TOF) mass spectrometer. In particular, nitrobenzene, 2-nitrotoluene, 2,4-dinitrotoluene, 2,4,6-trinitrotoluene, ethylene glycol dinitrate, pentaerythritol tetranitrate (PETN), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and SEMTEX (PETN plus RDX plus plasticizer) have been analysed at a laser wavelength of 226.3 nm corresponding to a strong resonant transition of the neutral NO molecule. TOF mass spectra have been recorded and the intensity of the NO+ ion monitored as a function of temperature for comparison with the temperature dependence of the various vapour pressures.

The photofragmentation of the nitrotoluene isomers in the gas phase is studied in the wavelength region 210-270 nm using a pulsed UV laser in conjunction with a time-of-flight mass spectrometer. Laser-induced mass spectra are analysed and compared with those produced by the electron impact technique. The generation of the observed fragment ions is explained by invoking different fragmentation pathways followed by these molecules. Observed differences in the mass spectra of the ortho- meta-, and para-nitrotoluene isomers and in the wavelength dependence of the NO fragment released from these molecules are discussed as a possible way of providing a laser-based method for their identification.

Nitrobenzene and o-nitrotoluene were detected in trace concentrations in gas mixtures at atmospheric pressure in a simple unity-gain ionization chamber. The detection procedure relies on tunable ultraviolet laser radiation that identifies the characteristic wavelength dependence of the NO+ ion fragment. Sensitivity levels are 0.25 +/- 0.05 ppm for nitrobenzene and 0.75 +/- 0.1 ppm for o-nitrotoluene, with the current limitations being background ionization, laser power and small sampling volume. Additionally, nominally 1 ppm NO and NO2 gas samples were analysed and the saturation fluence (via a resonant two-photon process at 226.3 nm) for NO+ production from NO gas was determined. Sensitivity levels of 120 and 50 ppb were estimated from the data on NO2 and NO, respectively. It is shown that an increase in sensitivity of an order of magnitude is possible in all instances if the experimental parameters are optimized.

Multiphoton processes in open laboratory air have been studied in the spectral range 224-230 nm using a simple 'open-to-the-atmosphere' ionization detector. A number of oxygen molecular (O2) and atomic (O) transitions have been identified in this region. It is pointed out that this information is essential if multiphoton laser analytical techniques are applied in an open air regime as well as to predominantly air samples containing trace quantities of environmentally sensitive compounds.