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.