A simple fiber-optic probe suitable for remote analysis with the use of resonance-enhanced multiphoton ionization has been developed and demonstrated by measuring vapor-phase toluene and gasoline above aqueous solutions. An optical fiber transmits a high-power laser pulse (266 nm or 532 nm) to the sample, ionizing it, and the subsequent ions are collected with a platinum electrode mounted at the fiber tip. Measurements take approximately 100 s to perform, require no sample preparation, and have been demonstrated over a distance of five meters. The limit of detection for toluene in water with the use of this probe is 1.47 ± 0.02 ppb (wt/wt). In addition to two-photon excitation at 266 nm, the feasibility of a 2 + 2 excitation scheme using 532 nm has also been shown.
Brian M. Cullum, Slade K. Shealy, and S. Michael Angel, "Fiber-Optic Resonance-Enhanced Multiphoton Ionization Probe for in Situ Detection of Aromatic Contamination," Appl. Spectrosc. 53, 1646-1650 (1999)