Miniaturized high-performance drift tube ion mobility spectrometer
Developing powerful hand-held drift tube ion mobility spectrometers (IMS) requires small, lightweight drift tubes with high analytical performance. In this work, we present an easy-to-manufacture, miniaturized drift tube ion mobility spectrometer, which is manufactured from polyether ether ketone, stainless steel foils and printed circuit boards. It is possible to operate the drift tube IMS with a radioactive 3H ionization source or a non-radioactive X-ray ionization source with 3 kV acceleration voltage. The drift tube design provides high resolving power of Rp = 63 at a drift length of just 40 mm, 15 mm × 15 mm in cross-section (outer dimensions) and a drift voltage of 2.5 kV. The limits of detection for less than one second of averaging are 40 pptv for dimethyl-methylphosphonate and 30 pptv for methyl salicylate. For demonstration, the miniaturized drift tube IMS is integrated into a stand-alone battery-powered mobile device, including a closed gas-loop, high performance driver electronics and wireless data transmission. In a proof-of-concept study, this device was tested in an international field evaluation exercise to detect the release of a volatile, hazardous substance inside a large entry hall.
KeywordsMiniaturized drift tube ion mobility spectrometer Miniaturized drift tube IMS Non-radioactive ionization mini-IMS
During the past decades, ion mobility spectrometers (IMS) have been established in various kinds of applications [1, 2, 3, 4], while safety and security application still make up the largest share. Some IMS are used as stationary detectors for monitoring purpose; others are embedded into mobile or hand-held devices [5, 6, 7]. Especially in the latter case, compact, robust, lightweight and low cost drift tubes are required to provide easy to operate, hand-held devices. Thus, in recent years, several groups worked on the development of miniaturized drift tubes IMS. Some designs are based on low temperature co-fired ceramic (LTCC) . Other designs use stacked electrode-insulator-pairs [6, 9] or printed circuit boards [10, 11]. However, the key challenge of miniaturization is to maintain the analytical performance of the IMS, which strongly depends on the geometrical dimensions of the drift tube [10, 12].
In this work, we show a drift tube design, which is suitable for low cost and portable applications while maintaining the resolving power of laboratory-grade systems. The design is based on stainless steel straps, which are bended over a printed circuit board (PCB) and form the drift electrodes. Since the drift electrodes are connected to a dual-in-line package (DIP) footprint, the drift tube is easy to integrate simply being an electrical part of the driver electronics of the system.
Miniaturized drift tube
The miniaturized drift tube IMS is completed by an ionization chamber and a Faraday detector. The ionization chamber is also manufactured from PEEK and allows the adaption of two different ion sources to the drift tube. Ion generation is initiated either by electron emission from a radioactive 3H source (130 MBq) or by radiation from a custom non-radioactive X-ray source (Model XRT-50-2-Rh-0.6-125 by Newton Scientific Inc., Cambridge, Massachusetts, USA). It has been shown, that X-rays can be considered as non-radioactive substitute for the 3H ionization source, but the X-ray source should be placed orthogonally to the axis of the drift tube to avoid any offset current on the detector [14, 15]. Ions are injected into the drift region using a field switching shutter as described in . To keep production cost low, the Faraday detector is manufactured from standard PCBs. Ionization chamber, X-ray tube respectively, and Faraday detector are coupled to the drift tube using polytetrafluoroethylene (PTFE) O-rings and water jet cut foils.
For drift and sample gas inlet, ionization chamber and detector are equipped with bores fitting a PEEK capillary (1/16″ outer diameter, 1 mm inner diameter). The sample gas is directly introduced into the ionization region, while the drift gas enters the drift tube at the detector through channels milled in the PCB.
Battery-powered mobile demonstrator with closed gas-loop
To achieve high analytical power, a homogeneous drift field is crucial. Therefore, the drift tube is connected to a resistive voltage divider via the DIP socket, as described above. For all experiments, drift voltages in the range of 2.5 kV are used. In addition, another high voltage of 3 kV is required as acceleration voltage for the X-ray source in the respective setup. The filament of the X-ray source is driven with a constant current of 640 mA. The field-switching shutter is operated at injection voltages in the range of 500 V.
A defined sample gas flow of 100 mls/min (at 20 °C and 1013.25 hPa) is periodically pumped from the sample inlet (Fig. 3 - orange, continuous line) through the 250 μl sample loop. During measurements, the sample loop pump is turned off to avoid any interferences caused by mechanical vibrations, while the closed gas-loop pump is continuously running. When a measurement is started, the gas within the sample loop is injected via 6-port-valve into the IMS by a carrier gas flow of 10 mls/min (Fig. 3 - red, dotted line). The 6-port-valve also allows easy integration of a gas chromatographic column for pre-separation (GC-IMS). An ambient pressure reference (Fig. 3 - grey, continuous line) is added before the filters to avoid unwanted pressure variations inside the closed gas-loop e.g. during valve switching.
Operating parameters of the IMS
X-ray tube filament current
X-ray tube acceleration voltage
Drift gas flow
120 mls /min
Sample gas flow
100 mls /min
Analyte gas flow
Sample loop volume
Inner IMS pressure
Results and discussion
To analyze the performance of the presented IMS, we measured its resolving power and detection limits. In a first step, the miniaturized drift tube is characterized without being integrated in the stand-alone demonstrator. Afterwards, the stand-alone demonstrator is investigated.
Analytical performance of the miniaturized drift tube
In order to determine the detection limit and linear dynamic response range of the drift tube IMS, dimethyl-methylphosphonate (DMMP) has been used as model substance in the positive ion mode and methyl salicylate (MS) has been used as model substance in the negative ion mode. In this work, all limits of detection are given for measurement times less than one second and are based on the 3σ- definition.
Analytical performance of the stand-alone demonstrator
As described in the previous section, the miniaturized drift tube IMS with X-ray ionization has been integrated into a mobile demonstrator. Using the closed gas-loop with 6-port-valve sample injection results in a detection limit of 90 pptV for MS in purified, dry air. Compared to the detection limit using the drift tube IMS without closed gas-loop and 6-port-valve sample injection, this value is higher by a factor of three. This is due to dilution of the sample when mixing with the carrier gas.
In this work, an easy-to-manufacture, miniaturized drift tube ion mobility spectrometer is presented. The design is based on stainless steel straps, which are bended over a printed circuit board (PCB) and form the drift electrodes. Since the drift electrodes are connected to a dual-in-line package (DIP) footprint, the drift tube is easy to connect to the driver electronics (plug and play).
The dimensions of the drift tube are only 15 mm × 15 mm in cross-section (outer dimensions) and 56 mm in length (outer dimension). Nevertheless, the design provides a high resolving power of Rp = 63 at a drift length of just 40 mm and a drift voltage of 2.5 kV. The limits of detection for one second of averaging are 40 pptv for dimethyl-methylphosphonate monomer, 500 pptV for the dimer and 30 pptv for methyl salicylate using a 3H ionization source. Using X-ray ionization, the limits of detection are 60 pptv for dimethyl-methylphosphonate monomer, 800 pptV for the dimer and 60 pptv for methyl salicylate. It is important to note, that an increasing X-ray filament current leads to significantly better limits of detection. However, here, the operating parameters of the used X-ray ionization source were set to generate RIP intensities comparable to 3H ionization. The system has been successfully tested in an international field evaluation exercise to detect the release of volatile, hazardous material inside a large entry hall.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 653409.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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