EXPRESS: Trace Detection of Spatially Confined Pure Contraband using a Handheld Raman System

While handheld Raman spectroscopy is typically used for bulk material identification, it is rarely viewed as a tool for trace chemical analysis (nanogram to microgram) of powders due to the inherent challenges associated with the low interaction volume of the laser with dispersed particulate samples. Our study challenges this convention by demonstrating that with appropriate spatial confinement, commercial handheld systems can achieve detection limits in the single-nanogram range. We experimented with two approaches for sample confinement. One was to use inkjet-printed deposits on aluminum substrates. This produced a hemispherical deposit with micrometer dimensions, to simulate trace contraband particles, where we were able to successfully identify pure explosives and drugs with mass loadings as low as 2 ng to 17 ng, depending on the compound. Another was to physically confine the sample by preparing a novel testbed of laser-drilled silicon microwells with depths varying from 25 µm to 500 µm for packing pure contraband powders. In addition to achieving tens of nanograms in sensitivity, this allowed us to determine the effective sampling depth of the Raman probe at approximately 100 µm, beyond which any additional sample will not increase the signal. Also, the plot suggested that the minimum bulk film thickness for detection was in the order of tens of micrometers. These observations suggest that trace chemical screening for pure substances such as fine powders on a surface is feasible given appropriate sample collection and preconcentration methods to maximize the analytical thickness of the trace sample.