Use of force-sensing array films to improve surface wipe sampling

Abstract

Surface wipe sampling, as performed by human operators, is widely used in environmental monitoring and currently subject to a high degree of variability. Two factors relating to sampling efficiency, applied force and area coverage, have not previously been amenable to measurement during wipe sampling. A force-sensing resistor (FSR) array film is used for the first time in this study to measure these factors and provide feedback for operator training, with the goal of reducing inter-operator variability. A volunteer population of 20 performed wipe sampling within a 10 cm by 10 cm templated area under an instruction to use "firm" force. The applied force for the population ranged from 3 Newtons (N) (306 g-force) to 14 N, with intra-operator variability within each experiment of 20% RSD on average. A statistically significant difference in force among operators was determined by a single-factor analysis of variance (ANOVA) (p-value = 1.6 × 10 (-7)). An average force of 7.4 N (SD = 2.9 N) was determined for the population, and an operator who fell outside the mean was trained with feedback from the FSR, achieving an average force of 6.8 N (SD = 1.8 N). Retention of the training was demonstrated by testing after 6 days resulting in an average force of 6.8 N (SD = 0.8 N). Direct measurements of the sampling path can be made from the FSR using center-of-force (COF) data. Operators were evaluated against a path designed to fill the 10 cm by 10 cm area with 98% coverage. With training, operators achieved coverage of up to 96% using only guidelines marked on the paper template to define the sampling path. Fluorescent polymer microspheres were used to evaluate collection efficiency when performing dry wipe sampling under controlled conditions of force and area coverage. Over the range of forces measured earlier for the population, the collection efficiency varied from 10% to 50%, with an approximate increase of 4% in collection efficiency with each additional 1 N of force. The locations of particles remaining on the surfaces after sampling were compared with the COFs. A high percentage of particles (up to 45%) were redeposited to the surface after collection, indicating that sampling patterns should be designed to minimize the length of the sampling path while still providing maximum area coverage.

Design of free‐standing microstructured conducting polymer films for enhanced particle removal from non‐uniform surfaces

“…Unfortunately, existing commercial swabs used for trace explosive detection [e.g., muslin cloth and poly(tetrafluoro ethylene)‐coated (PTFE‐coated) fiberglass] have been shown to provide vastly different particle collection efficiencies . Additionally, in the present state‐of‐the‐art the force and surface area of sampling have not been controlled actively . As such, there exists a critical need to improve the sample collection of these devices and to rationally design novel swabs with improved ability to interrogate surfaces and increased adhesion to explosive particles.…”

“…8 Additionally, in the present state-of-the-art the force and surface area of sampling have not been controlled actively. 9,10 As such, there exists a critical need to improve the sample collection of these devices and to rationally design novel swabs with improved ability to interrogate surfaces and increased adhesion to explosive particles. Here, we address this opportunity in a direct manner through the introduction of a microscale, template-assisted electropolymerization methodology, and we demonstrate that the swabbing polymers that are resultant from this process have structural, mechanical, and physical properties that allow these next-generation swabs to have superior collection properties relative to current commercial swabs.…”

Compressive Behavior of Idealized Granules for the Simulation of Composition C‐4

“…Composition C-4 is comprised of 91 %c yclotrimethylenetrinitramine (RDX) with 9% polymeric binder. Current military specifications for RDX list ranges of particle diameters from < 44 mmt o< 2000 mm [ 1,2].D ue in part to this wide range of particle diameters, variability existsr egarding the size of particles encounteredi nC -4 during swipe sampling [3][4][5][6][7].N otably,t he size of particles deposited by at humbprinto nasurface of interest has not been fully evaluated, and the ability to recreate as tandard print is also lacking [3][4][5][6][7].…”

“…Accordingly,akey aspect of the development of improved swabs and swabbing protocols for explosives detection involves understanding the mechanisms of adhesive/cohesive failure within the energetic residue as it is removed from as urface.Composition C-4 is comprised of 91 %c yclotrimethylenetrinitramine (RDX) with 9% polymeric binder. Current military specifications for RDX list ranges of particle diameters from < 44 mmt o< 2000 mm [ 1,2].D ue in part to this wide range of particle diameters, variability existsr egarding the size of particles encounteredi nC -4 during swipe sampling [3][4][5][6][7].N otably,t he size of particles deposited by at humbprinto nasurface of interest has not been fully evaluated, and the ability to recreate as tandard print is also lacking [3][4][5][6][7].Robust trace explosives samplingt echniques also have yet to be firmly established [3].F or IMS, certain standards apply:as wab must effectively removes olid particulates from as urface, withstandt emperatures up to 300 8Ca se mployed by the IMS, and be affordable [3,4].M ost current studies consider eitherc loth or Te flon-coated fiberglass swabs [3][4][5][6][7].Anumber of parameters have not been successfullyc ontrolled during the development of an optimal wiping technique [6].V ariability existsi nt he applied force during swiping, the surface area covered, the swab material, the roughness of the swab and substrate materials, the swipe velocity,a nd the number of times as wab may be reused before it is discarded [3,6,7].S tudies by Verkouteren et al claim that the critical parameters in determining removale fficiency are applied load and the translational force requiredt oo vercome the frictional resistance at the swipe-substrate interface to maintain ac onstants wiping velocity [3]. They also note ad irect linear correlation between increaseda pplied forceo fs wiping and particle removal efficiency [3,5,6].W hile Verkouteren et al indicate as wipe speedo f0 .7 cm s À1 in their studies, the Environ-mentalP rotection Agency indicates swipe sampling speeds [a] M.Abstract:I nt he most common approacht od etect trace explosives at security checkpoints, any i...…”

“…Robust trace explosives samplingt echniques also have yet to be firmly established [3].F or IMS, certain standards apply:as wab must effectively removes olid particulates from as urface, withstandt emperatures up to 300 8Ca se mployed by the IMS, and be affordable [3,4].M ost current studies consider eitherc loth or Te flon-coated fiberglass swabs [3][4][5][6][7].Anumber of parameters have not been successfullyc ontrolled during the development of an optimal wiping technique [6].V ariability existsi nt he applied force during swiping, the surface area covered, the swab material, the roughness of the swab and substrate materials, the swipe velocity,a nd the number of times as wab may be reused before it is discarded [3,6,7].S tudies by Verkouteren et al claim that the critical parameters in determining removale fficiency are applied load and the translational force requiredt oo vercome the frictional resistance at the swipe-substrate interface to maintain ac onstants wiping velocity [3]. They also note ad irect linear correlation between increaseda pplied forceo fs wiping and particle removal efficiency [3,5,6] Abstract:I nt he most common approacht od etect trace explosives at security checkpoints, any illicit residues must first be removed from the surface of interest (contact sampling) beforet hey are delivered to ad etector,s uch as an ion mobility spectrometer.C ontact sampling involves applying ac ompressive shearing load to dislodge the residue from as urface. Optimizing this step requires an understanding of the properties of the residues andt heire ffect on residue behavior.T his study seeks to evaluate the previously unstudiede ffect of the binder mechanical properties and particle size distributionso nt he behavior of Composi-tion C-4.…”