The importance of sample preparation method in testing air jet removal efficiencies of trace explosive particles relevant to non-contact sampling

In particular, we study the influence of sample preparation methods on the removal efficiency of cyclotrimethylenetrinitramine (RDX) particles from clean glass slides <i>via</i> air jet impingement. Five different sample preparation methods are investigated covering a range of potential contamination scenarios. These include (1) dry sieve, (2) artificial fingerprint without sebum, (3) artificial fingerprint with sebum, (4) dry transfer and (5) direct pipetting of an aqueous suspension. Results show clear differences in RDX particle removal efficiency with sample preparation method, with a range from undetectable to almost 90%. A custom aerodynamic sampling testbed was developed for the testing that tightly controls air jet pressure, standoff distance and impingement angle while minimizing particle losses to the lab environment. Differential interference contrast (DIC) light microscopy was used to image RDX particles ranging in diameter from 5 μm to over 100 μm before and after jet impingement, with semi-automated post-processing used to determine total particle removal efficiency. The results of this study suggest that testing and evaluation procedures used for trace contraband screening systems that utilize non-contact aerodynamic sampling should consider the role of test sample preparation when designing and defining testing methodologies.