Signal enhancement by crossing the sample flow at a small angle against the laser beam in multiphoton ionization mass spectrometry.

To enhance sensitivity in multiphoton ionization/time-of-flight mass spectrometry, the sample, in the form of an effusive molecular flow, was introduced at a small angle relative to the laser beam. This approach enlarged the volume of interaction between the sample and the laser beam, thus increasing the number of ions formed by multiphoton ionization. The sensitivity obtained using this technique was compared with that obtained using a previous method based on the off-axis sample introduction technique with detection limits for organic hydrocarbons at the subfemtogram level. The detection limit was examined for dioxins and an improvement of 2.5 ± 0.4 fold was found. The technique developed herein is applicable for practical ultratrace analysis of persistent organic pollutants.