Evaluating the Suppression Tolerance of a Novel Walk-up Desktop Trace Detector

Natalie Dunn^a, Lance Hiley^a, Charles Liddell^a, Peter Luke^a, David Douce^b, Ashley Sage^b with Scott Campbell^cand John Moncur^c

a: Mass Spec Analytical Ltd, Future Space, UWE North Gate, Filton Road, Bristol, BS34 8RB, UK. b: Waters, Stamford Avenue, Altrincham Road, Wilmslow, Cheshire SK9 4AX UK c: Spectralworks, The Heath, Weston, Cheshire, WA7 4QX.

First presented as a poster at BMSS Ambient Ionisation SIG Meeting February 2024

Introduction

Traditional forensic practices utilising Mass Spectrometry Trace Detection (MS-TD) boast high-throughput and assured analysis of trace surface samples. But this powerful technique is no longer confined to forensic labs. We present a prototype system from Mass Spec Analytical Ltd. (MSA) that redefines desktop, portable trace detection. This “Next Generation Desktop Threat Detector” demonstrates a novel approach capable of identifying a diverse range of threats (explosives, pesticides, drugs, chemical warfare simulants) from various compound groups, using swab sampling.

Direct thermal analysis reveals impressive detection limits (low nanogram range), while swabbed liquid samples dried on glass slides and collected exhibit sensitivities as low as 100-200 ng. However, common environmental interferents like cleaning products, cosmetics, and oils pose a significant challenge to trace detection, generating false positives/negatives, excessive background noise, or even complete inhibition.

Our poster tackles this critical issue by evaluating the prototype’s suppression tolerance towards these everyday interferences. We explore how the system handles these challenges, paving the way for future real-world deployments in accessible, walk-up systems.

Methods and Materials

This study leverages a prototype system designed for high-throughput, high-assurance trace analysis of diverse threats. The core of the system lies in the synergistic integration of:

Mass Spec Analytical Ltd. (Bristol, UK) Thermal Desorber (TD) Ion Source: Seamlessly integrated with the mass spec instrument, the TD source offers precise control and optimized performance. Novel materials within the source enhance sensitivity and overall analytical capabilities.
Waters Instruments™ (Wilmslow, UK) RADIAN™ Single-Quad Mass Spectrometer (MS): This instrument provides robust mass analysis. Its variable cone voltage capability facilitates tailoring in-source fragmentation to specific analytes, significantly increasing performance compared to traditional Ion Mobility Spectrometers while maintaining cost-effectiveness. Each swab was analysed using Waters MassLynx™ software.

For this study, three structurally diverse explosive compounds – PETN, RDX, and TNT – were chosen, reflecting a customer project running in parallel. These compounds were simultaneously detectable using negative Atmospheric Pressure Chemical Ionization (APCI) mode.

Trace detection faces the constant challenge of emerging challenges. Our prototype system readily accommodates the addition of new compounds, making it virtually future proof. The adaptability of the system, coupled with its robust analytical capabilities, paves the way for its widespread adoption in commercial applications.

Results

Environmental Tests

To assess the prototype’s ability to handle background contaminants, over 500 environmental swabs were collected from a diverse range of locations, including homes, offices, vehicles, and public spaces.

Each swab was analysed following a rigorous protocol:

·Over 447 analyses were conducted over a 7-hour period, with check swabs interspersed throughout. The results were encouraging:
·Minimal performance degradation: Check swabs consistently maintained acceptable detection levels, demonstrating the prototype’s resilience to environmental contaminants.
Gradual decline: The slow decrease in sensitivity over time allows for convenient monitoring and proactive intervention.

Extrinsic Residue Suppression

This section examines the prototype’s resilience to common environmental interferents known to hinder some detection techniques. Eight readily encountered compounds were chosen, spanning colognes, cleaners, moisturizers, and hand sanitizers.

Figure 3 shows that the selected explosive signals significantly exceed the background noise induced by any residue (lower graphs). The graphs for PETN and TNT were very similar.

No noticeable performance decline was observed for any explosive check swabs throughout the investigation, despite analysing 24 residue samples per experiment.

Intrinsic Residue Suppression

This section explores the prototype’s susceptibility to “intrinsic suppression,” where interferents directly suppress the target explosive by competing for ions or raising background noise.

Figure 4 confirms intrinsic suppression, but the effect is not debilitating. Results for PETN and TNT are similar. TNT is most affected, likely due to its interaction with the chlorine used to analyze RDX and PETN. Moisturizer emerges as the most potent suppressor across all explosives due to its thick composition. Notably, the isolated explosive results suggest minimal extrinsic suppression from the residue sequence, implying the major impact comes from the combined swab.

Real World Application

As a real-world application, consider the integration of this detector with MSA’s Scentinel™ II Walk-up software for explosives detection, coupled with Spectralworks’ RemoteAnalyzer™ for data analysis. This combination streamlines complex mass spectrometry data into intuitive Yes/No results, empowering non-expert users like security personnel. A user-friendly touch-screen interface provides easy instrument operation, while instant feedback allows for immediate action.

Collect sample.
Start analysis: Use touchscreen or mouse to select sample type and method, then follow on-screen/audio prompts for swab insertion.
View results: Analysis takes 8-10 seconds. Screen displays compounds tested; positive results indicated in red.
Review data: Remote expert review via data archiving ensures trend analysis and future research capability.

This approach can be extended to other analytes of interest and has been demonstrated with drugs of abuse and pesticides.

Conclusions

Trials confirmed the prototype’s exceptional environmental tolerance for reliable detection in harsh settings. Rapid analysis and proactive monitoring bolster field-ready potential. Minimal intrinsic suppression poses negligible real-world impact. This high-performance system, with its walk-up software, excels in diverse trace detection, especially under demanding conditions.