## 2 Results and Discussion
described a low recovery efficiency of a ricin-like protein, it was unexpected to the authors that the recovery efficiency of several of the swab materials on multiple surface types would be below detectable ECL limits when assaying an aerosol deposited ricin considering the sensitivity levels of the ECL assay [1]. Only a single material, Dacron was capable of recovering a relatively significant amount of aerosolized material compared to that of liquid deposited protein. The recovery efficiencies of the 10 \(\upmu\)g of recombinant ricin seeded onto the various coupon types of liquid deposited material for all four swab types was between 47% and 76%, depending on the surface material. recovery efficiencies of aerosolized material, recovering 13% to 39% of the 10 ng of aerosolized agent deposited onto the coupons, depending on the surface material, while the other three swab types only recovered up to 10%. The inability to sample the small molecule agent at levels comparable to that of _B. atrophaeus_ spores can be attributed to differences in agent and swab interactions, the amount of material deposited onto the coupons, and the deposition method. Unfortunately, little effort has been placed on understanding the physical and molecular interactions between agent and recovery material and the forces and factors responsible for these interactions are still largely unknown. An argument can be made that the low levels of aerosolized agent could play a role in the low level detection of the ricin. While concentration very likely has an impact on recovery efficiency, it is important to note that the control samples and liquid deposition samples were diluted to the picogram concentrations while the experimental aerosol samples were not diluted due to lower levels of agent recovery. The authors also point out that the recovery efficiency of the Dacron swab was rather significant with the respect to the lower concentration of material deposited onto the coupons as an aerosol. Although there is 1000X less aerosolized material deposited onto the coupons than the liquid deposition method, the recovery efficiencies of the aerosolized material using the Dacron swabs was only 2X-5X better than the recovery efficiencies of the liquid deposited material. It is likely that differences in recovery efficiencies due to the concentration of starting material are insignificant in comparison to the deposition method. Another potential impact on our aerosol deposition recovery efficiencies is the deposition method itself. With the liquid deposition method, the agent is applied to the coupons in few and relatively concentrated locations. As the liquid evaporates, presumably, the agent will concentrate into small droplets until the suspension dries completely into smaller areas than initially deposited as observed with SEM of biological material [12]. This will produce few but significantly higher concentrated overall deposits of agent which is likely to be easier to recover with sampling technologies. However, with an aerosol deposition, an evenly distributed coating of dry agent likely is applied to the entirety of the coupon, as observed with SEM of biological material, and no mechanism is present to concentrate the toxin [12]. This evenly distributed coating of material is not as easily recovered as the localized and more concentrated evaporated liquid depositions. To truly understand the reasoning between differences in recovery efficiencies among deposition methods, sampling materials, and surface substrates, thorough measurements of critical components of agent substrate interactions, such as adhesion forces and concentration of deposited material need to be thoroughly investigated. Not only will this insight better explain the observations of this work and others which have previously been performed, but it could also be used in developing technologies which are more consistent in agent recovery and hazard assessment amongst many different possible classes of surface substrates.