Atmospheric pressure laser ionization. An analytical technique for highly selective detection of ultralow concentrations in the gas phase.

In this contribution a new analytical technique is presented for the direct mass spectrometric (MS) detection of gas-phase trace species at atmospheric pressure. Employing resonance-enhanced multiphoton ionization (REMPI) close to the inlet nozzle orifice, i.e., at high molecule densities, the sensitivity of the instrument has been increased by up to 3 orders of magnitude as compared to conventional REMPI-MS with ionization in a differentially coupled ion source. Furthermore, adiabatic cooling, resonant ionization, and mass-selective detection establish a highly selective analytical technique. Several atmospherically relevant compounds were investigated. The current detection limit for NO is 0.9 pptv, for acetaldehyde 1.7 pptv, for CO 15 pptv, and for 2,5-dichlorotoluene 12 pptv. We discuss APLI with regard to applications in medical and environmental research.