Abstract
Sample collection for Ion Mobility Spectrometry (IMS) analysis is typically completed by swiping a collection wipe over a suspect surface to collect trace residues. The work presented here addresses the need for a method to measure the collection efficiency performance of surface wipe materials as a function of the number of times a wipe is used to interrogate a surface. The primary purpose of this study is to investigate the effect of wipe reuse, i.e., the number of times a wipe is swiped across a surface, on the overall particle collection and IMS response. Two types of collection wipes (Teflon coated fiberglass and Nomex) were examined by swiping multiple times, ranging from 0 to 1000, over representative surfaces that are common to security screening environments. Particle collection efficiencies were determined by fluorescence microscopy and particle counting techniques, and were shown to improve dramatically with increased number of swiping cycles. Ion mobility spectrometry was used to evaluate the chemical response of known masses of explosives (deposited after reusing wipes) as a function of the wipe reuse number. Results show that chemical response can be negatively affected, and greatly depends upon the conditions of the surface in which the wipe is interrogating. For most parameters tested, the PCE increased after the wipe was reused several times. Swiping a dusty cardboard surface multiple times also caused an increase in particle collection efficiency but a decrease in IMS response. Scanning electron microscopy images revealed significant surface degradation of the wipes on dusty cardboard at the micrometer spatial scale level for Teflon coated wipes. Additionally, several samples were evaluated by including a seven second thermal desorption cycle at 235°C into each swipe sampling interval in order to represent the IMS heating cycle. Results were similar to studies conducted without this heating cycle, suggesting that the primary mechanism for wipe deterioration is mechanical rather than thermal.
Highlights
Swipe sampling is a common method for collecting particles for environmental sampling and forensic applications
The x-axis is the wipe reuse number in logarithmic format, the left axis is the Particle collection efficiencies (PCE) from 0 % to 100 %, and the right axis is the ion mobility spectrometry (IMS) response
Similar to the results of Teflon1 -coated fiberglass (TCFG) repeatedly swiped on canvas, Nomex PCE increased significantly with reuse compared to wipes that were new (t-test resulting in a P value of 0.0007, 95 % confidence interval)
Summary
Swipe sampling is a common method for collecting particles for environmental sampling and forensic applications. This physical swiping of surfaces is used at airports and other security settings to screen people and their belongings for explosive materials that could suggest the presence of potential terrorist threats [4, 24]. The screening technique that commonly utilizes dry wipe sampling in these security settings is ion mobility spectrometry (IMS), which is often chosen due to its sensitivity, rapid analysis time and ease of use. While biological and environmental screening often uses wet wipes for collection and extraction for an analytical procedure [3, 14, 15], the wipes for IMS screening must be dry to avoid chemical interferences with the IMS technique [5]. IMS is used to screen for illicit narcotics, this technology is used for screening visitors and mail at prisons [2], detecting potential drug smuggling activities [6, 17], and for medical and biological purposes [1, 18]
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More From: International Journal for Ion Mobility Spectrometry
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