Trace Detection of Non-Uniformly Distributed Analytes on Surfaces Using Mass Transfer and Large-Area Desorption Electrospray Ionization (DESI) Mass Spectrometry

Ambient ionization methods such as desorption electrospray ionization (DESI) allow the analysis of chemicals adsorbed at surfaces without the need for sample (or surface) pretreatment. A limitation of current implementations of these ionization sources is the small size of the area that can be sampled. This makes examination of surfaces of large areas time-consuming because of the need to raster across the surface. This paper describes a DESI source that produces a spray plume with an effective desorption/ionization area of 3.6 cm2, some 200 times larger than given by conventional DESI sources. Rhodamine 6G and several drugs of abuse (codeine, heroinand diazepam) were used to demonstrate the ability to use large-area DESI MS to perform rapid (a few seconds) representative sampling of areas of the order of several square centimetres without scanning the probe across the surface. The large area ion source displayed high sensitivity (limits of detection in the high nanogram range) and high reproducibility (∼20 to 35% relative standard deviation). The rapid analysis of even larger surfaces (hundreds of cm2) for traces of explosives is possible using a sorbent surface wipe followed by large-area DESI interrogation performed directly on the wipe material. The performance of this mass transfer dry wipe method was examined by determination of the limits of detection of several explosives. Surfaces with different topographies and compositions were also tested. Using this method, absolute limits of detection observed for HMX and RDX from plastic surfaces and skin were found to be as low as 10 ng cm−2. The concentration of residue from large surface areas in this technique allowed the detection of 100 ng of explosives from surfaces with areas ranging from 1.00 × 103 cm2 to 1.40 × 104 cm2