In this paper we describe the first use of laser desorptionin conjunction with membrane introduction mass spectrometry (MIMS). In this technique, a low-poweredcarbon dioxide laser is used to irradiate the low-pressure(vacuum) side of a silicone membrane during a typicalMIMS analysis of an aqueous solution. The absorptionof laser energy results in direct membrane heating andrapid desorption of permeate molecules. This improvesboth the sensitivity and response times of MIMS whenanalyzing compounds having high molecular weight andlow volatility. Two simple interfaces are described forperforming laser desorption inside and outside the vacuummanifold of a GCQ ion trap mass spectrometer. Togetherwith flow injection (FI) sample introduction, we demonstrate direct on-line monitoring of aqueous solutions ofhigh boiling point (200-530
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C) polynuclear aromatichydrocarbons such as naphthalene, anthracene, pyrene,benzo[
b]fluoranthene, and indeno[123-
cd]pyrene.