In the wide range of possibilities offered by the available working principles for chemical detection, 2 different technologies (PID and IMS) have been explored for this sub-system. By coupling those 2 working principles, it is possible to perform a qualitative and a quantitative assessment of the detected release jointly. Indeed, the first one put in place a working principle chosen for its reliability in assess concentration in the open air, its sensitivity, low weight, and low cost. The second one has been signposted for its capability to perform identification, with a given certainty at least for the class of chemical substances.
Other instruments, based on different working principles, were not investigated because they have been deemed too heavy or voluminous to be integrated on a mini category drone. Other ones were not considered because too expensive or not compatible with possible releases of combustible or explosive substances, for which the detector could become an ignition point.
Photo-Ionization Detectors (PID)
The first working principle chosen for this application is photo-ionization detection (PID). Detectors using PID are effective in detecting and monitoring numerous hazardous substances and ware commonly low cost. Moreover, they provide a fast response [44
]. Compared to several other methods of detecting dangerous gases available on the market, PIDs enclose a combination of response rates, ease of use and maintenance, compact size, and ability to detect low concentrations, including most volatile organic compounds (VOCs). PIDs used the working principle of CAs ionization. When the sample gas absorbs energy by a PID lamp, the gas gets excited, and its molecular content is altered. The compound loses an electron (e−
) and becomes a positive ion [45
]. Once this process occurs, the substance is considered ionized (Figure 5
Most substances can be ionized, some more easily than others. The aptitude of a substance to be ionized is measured as ionization potential (IP) with an energy scale in electron-volt (Ev). This scale generally ranges from a value of 7 to a value of about 16, the higher the relevant value of IP, the more difficult it will be to ionize the substance.
To ionize the compound object of the monitoring, PIDs use an ultraviolet (UV) lamp. The lamp, which is often the size of a common flashlight bulb, emits at a specific voltage, for instance, 10.6 Ev lamp emits enough UV energy to ionize any compound with an IP value less than 10.6 Ev. By measuring the current produced from the ionized compounds is possible to obtain its concentration as parts-per-million (ppm).
PID detectors can measure most of the organic compounds and some inorganic compounds, such as ammonia and sulfuric acid. The best way to calibrate the instrument is to expose it at a specific concentration of a certain compound, typically isobutylene, selected because it is located at a midpoint of ionization value for most of VOCs, moreover, it is not flammable or toxic at the concentration used for calibration. Once the instrument is calibrated, to conduct the measurement and obtain the concentration for a certain substance is necessary to convert the obtained result with a response factor (RF), that is compound specific. The RF is the ratio between the sensitivity of PID to the calibration gas compared to one of the gases that the user wants to measure. By the application of RF, it is possible to determine the concentration of a large number of compounds with a single gas calibration. Nowadays, databases of RF for many different chemical substances are freely available [46
PID detectors are widely used in many different scenarios, such as leakage from industrial equipment, perimetral monitoring of industrial building, or storages whether the nature of spill is known, delimitation of contaminated areas after an emission, technical investigation after a fire. Therefore, due to their sensitivity and fast response, those instruments are extremely useful both for industrial applications for first responder’s usage [48
Therefore, PID detectors are able to measure the concentration of a known gas dispersed in the atmosphere, but they do not have identification capability. Indeed, they can be used to give a significative quantitative notice, but have to be coupled with an instrument with a working principle for the substance identification (qualitative evaluation).
Few applications have been previously conducted with the integration of those sensors with UAS, most of them devoted to combustion products [11
] or particulate matter [12
] investigation. For what regards industrial pollutants, evidence has been found of studies using electrochemical sensors [13
], less sensitive than PID to VOC.
Regarding the future integration with a UAV suitable for this application, it has also been explored the need in terms of hardware to interface and collect sensor data. Among the PID sensors commercially available, Alphasense designs and manufactures sensors, such as optical monitors for aerosol and particulate matter (PM 10/PM 2.5), inorganic gases and VOCs with a limit of detection (LOD) of ppb, with a complete low-cost and lightweight microcontroller solution to program the sensor, process the acquired data and communicate it to the control station [50
This solution presents a low power demand, both from the sensor than for the microcontroller, reducing further the overall payload of the drone. Furthermore, it allows the use in parallel an array of different detectors and features at the same time, controlled by the same hardware.
Among the several families of microcontrollers available on the market, one of the most popular is the Arduino series. It is mainly based on the Atmel AVR processor and provides many inputs and outputs in only one self-piece of hardware. Therefore, it offers flexible solutions for different needs, with a high-level language, cross-platform toolchain, and the capability of interface embedded devices. Hence, this family of microcontrollers will be the desirable choice to prototype the hardware and software interface with the sensors chosen for this application.
Ion-Mobility Spectrometry (IMS)
The second working principle selected for this application is Ion-Mobility Spectrometry (IMS). This is a well-established technology developed during the last century and widely applied in many types of detection instruments used as portable analyzers.
IMS detectors are selective equipment due to the different effects of the ionization process on different substances. Thus, they make it possible to discriminate different components in an air sample. The IMS working principle foresees 2 main stages. The first one is the sample reception and its ionization in the ionic reactor, with the creation of ions containing analyte molecules or their fragments. The second stage consists of the ion’s separation, which occurs in the transducer part of the detector (Figure 6
). The separation phase of ions distinguish IMS from other simpler ionization methods, such as flame ionization detectors (FID) and electron capture detectors (ECD) [51
]. The output signal is generated by the transfer of the ions produced in the reactor towards the collector. This ion movement take place in a flowing stream of the carrier gas (advection) and in an electric field (drift).
The process can generate positive or negative products on the gaseous sample volume withdrawn from the detector pump, employing different methods of ionization: Isotopic source emitting radiation, by a corona discharge (CD) [52
] or UV [53
In case of integration with a UAV, radioactive sources such as ionizing agents are to be avoided, due to the regulation connected with aerial transport of dangerous goods [54
Within the commercially available portable IMS detectors that employs corona discharge as an ionization method, the SMITH Detection instrument LCD3.3 could be a suitable solution. In a previous study, the same technology had been applied to evaluate an airborne methyl salicylate spread [14
]. This instrument was AA battery-powered and performed continuous sampling and air analysis [55
]. Selective and sensitive, the IMS, combined with the quantitative response given by the PID, can give a qualitative answer about the nature of the release.
Miniaturized Sampling Systems for UAV Application
Technical specifications already spotted for the other sub-systems such as lightweight, plug-and-play function, low cost, and low energy consumption are still valid guidelines for the effective sampling system choice. In the field of environmental sampling, many different technological solutions are commercially available for the collection of significant and representative specimens. The main aim is to bring back a withdrawal to a laboratory from the location noticed by the Lidar network as the source of release and recognized from the other instruments integrated into UAS for the characterization and confirmation of the evidence. Assuming that the task will be absolved by the design of new equipment, it is proposed to realize a multi-sampling system working in parallel consisting of a device simultaneously CAs or unwanted TICs and particulate matter, preserving the characteristics of the sample until the time of analysis. The system will foresee specific sectors to selectively retain different substances, including sintered cartridge sampling systems [56
], single-use or reusable after desorption, with the capability to intake samples from different environmental matrices, gaseous matrix, and aerosol samples.
Commercially available solutions with the indicated characteristics are air-sampler pumps. These devices are commonly used as personal equipment for environmental monitoring and providing low flows from 20 to 500 mL/min for gas and vapor sampling, with the possibility of Bluetooth interface to program the sampling at a distance [57
]. This equipment, with its restrained weight, commonly less than 250 g, can employ different sampling media, such as impinger for powder trap or sintered metal cartridges able to immobilize VOC components and pollutants. The latest configuration gives the possibility to use a low flow tube holder to have many samples in parallel and have the possibility to replicate the analysis once the withdrawal is collected from the laboratory.
Regarding the integration of the sampling system to the UAS, the same interface used for PID detectors, an Arduino microcontroller, can be implemented to control in real-time duration, nature, and flowrate of the withdrawal.