Ammonium Nitrate/Fuel Oil Vapour Detection with Conducting Polymer Percolation Network Sensors
Authors: Lefferts Merel J. , Humphreys Lisa H. , Mai Nathalie , Murugappan Krishnan , Armitage Ben I. , Pons Jean-François , Castell Martin R. 
Published: July 2021

Ammonium nitrate mixed with fuel oil (ANFO) is one of the most commonly used materials for improvised explosive devices (IEDs). ANFO detection is challenging because ANFO itself is not volatile. However, ammonium nitrate (AN) slowly decomposes into NH3 and HNO3.1 Additionally, fuel oils such as diesel are omnipresent and can cause a high false positive rate. Existing detection methods often require large pieces of equipment and are therefore impractical for many safety, security, and humanitarian applications. Conductive polymer (CP) based chemiresitive vapour sensors offer a potential solution because they can be used for stand-off detection, are small scale, relatively cheap, and operate at room temperature.2 Here we use percolation networks of conducting polymers to detect ANFO, instead of the more traditional thin films, to increase the sensitivity.3-5

Using a custom made sensor testing chamber, sensors based on a percolation network of polypyrrole (PPy) were exposed to a series of vapours emitted by 7 different fertilisers and ANFOs made with those fertilisers (Figure 1). The sensor responses to the fertiliser and ANFO samples were rapid and reversible. It is likely that the sensor responses measured are responses to the NH3 emitted by the fertiliser in the sample. For this reason a calibration curve based on known NH3 concentrations can be used to determine the NH3 concentration detected as a result of an unknown fertiliser or ANFO sample. Ammonia vapour concentrations emitted by the fertiliser and ANFO samples of 13-180 ppb were detected.
Exposing the sensors to all 7 fertilisers and their ANFOs results in different sensor responses for the different fertilisers (Figure 1). Although one might expect that the sensor response is directly related to the nitrogen content of the fertiliser, we found that this is not the case. It is possible that other components such as additives or internal desiccants play a role. Even in small quantities around 0.1% of common additives such as magnesium nitrate play an important role in the stability of the fertiliser.6 Additionally, the porosity of the fertiliser granules or prills might affect the measured NH3 concentration because the pores may store NH3 and thus influence the amount of available NH3. The relationship between sensor response and granule porosity could provide valuable information for sensing applications because porosity is known to be one of the most important factors determining explosives properties such as detonation velocity.7

Furthermore, it was shown that our sensors do not respond to diesel alone, and that there is no significant difference between the sensor responses to the fertiliser and ANFO made with the same fertiliser. This eliminates false positives due to the presence of diesel vapours in the environment.
At the end of each series of exposures the sensor was exposed to the first fertiliser from the series for a second time. This control experiment demonstrates that the sensor has good stability and reproducibility, and doesn’t degrade over time or after repeated exposures.
Our sensor responses were validated by comparison to gas chromatography/mass spectroscopy (GCMS) measurements conducted for the same fertilisers and ANFOs. Although our sensors detected much smaller NH3 concentrations, the pattern of different NH3 concentrations detected for the different fertilisers was similar for the two methods. Furthermore, in both methods the fuel oil component did not affect the detection of the fertiliser or ANFO.
Figure 1: Top: Schematic of custom made sensor testing chamber. The testing chamber has 2 gas inlets, allowing for gases directly from a cylinder as well as vapours emitted by solid or liquid samples carried into the chamber by an N2 flow. Bottom: Vapour concentrations measured by our conducting polymer percolation network sensors for 7 different fertilisers and ANFOs made with those fertilisers. Ammonia vapour concentrations emitted by the fertiliser and ANFO samples of 13-180 ppb were detected.
References:
1 Steinkamp, F. L., Giordano, B., Collins, G. & Rose‐Pehrsson, S. Volatile emissions of ammonium nitrate under flowing conditions. Propellants, Explosives, Pyrotechnics
40, 682-687 (2015).
2 Lefferts, M. J. & Castell, M. R. Vapour sensing of explosive materials. Analytical Methods
7, 9005-9017 (2015).
3 Armitage, B. I., Murugappan, K., Lefferts, M. J., Cowsik, A. & Castell, M. R. Conducting polymer percolation gas sensor on a flexible substrate. Journal of Materials Chemistry C
8, 12669-12676 (2020).
4 Murugappan, K. & Castell, M. R. Bridging electrode gaps with conducting polymers around the electrical percolation threshold. Electrochem. Commun.
87, 40-43 (2018).
5 Lefferts, M. J. et al. ANFO vapour detection with conducting polymer percolation network sensors and GCMS. Submitted (2020).
6 Kiiski, H. Properties of ammonium nitrate based fertilisers. (2009).
7 Zygmunt, B. & Buczkowski, D. Influence of ammonium nitrate prills' properties on detonation velocity of ANFO. Propellants, Explosives, Pyrotechnics: An International Journal Dealing with Scientific and Technological Aspects of Energetic Materials
32, 411-414 (2007).

Figure 1