Ion mobility spectrometer with orthogonal X-Ray source for increased sensitivity

Abstract

Ion mobility spectrometers (IMS) are compact devices for extremely sensitive detection of proton and electron affine volatile compounds down to low ppt concentrations within less than a second. The measuring principle requires ionization of the target analyte. Most IMS employ radioactive electron sources, such as Ni orH. These radioactive materials suffer from legal restrictions limiting the fields of application. Furthermore, the electron emission has a predetermined intensity and cannot be controlled or disabled. In a previous work, we replaced the axially mounted H source of our ion mobility spectrometer with a commercially available X-ray source operated at low acceleration voltage of 4.5 kV to be applicable in most application without legal restrictions. However, the high penetration depth of the radiation together with the statistical behavior of the X-ray ionization process led to an increase of Fano noise and thus a limited signal-to-noise ratio. Therefore, the X-ray source is now mounted orthogonal to the drift tube in order to avoid Fano noise. Here, we compare the analytical performance of this orthogonal setup with the axially mounted X-ray source. The noise level is significantly reduced. This improves the signal-to-noise ratio from 700 with the axially placed source to more than 3000 with the orthogonally placed source, while the resolving power still remains at R = 100. Furthermore, typical limits of detection for some model substances in the low ppt range in positive and negative ion mode are given.

Miniaturized high-performance drift tube ion mobility spectrometer

“…Ion generation is initiated either by electron emission from a radioactive 3 H source (130 MBq) or by radiation from a custom nonradioactive 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 nonradioactive substitute for the 3 H ionization source, but the Xray 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 [16].…”

Compact and Sensitive Dual Drift Tube Ion Mobility Spectrometer with a New Dual Field Switching Ion Shutter for Simultaneous Detection of Both Ion Polarities

“…Ion mobility spectrometers (IMS) combining atmospheric pressure chemical ionization (APCI) with field switching ion shutters are extremely sensitive trace gas detectors, reaching limits of detection (LoD) in the single digit ppt v range within measurement times of a second. − However, depending on the target molecule properties, positive and/or negative ions may form during the ionization process. − Thus, analyzing of both ion polarities is required in many applications.…”

Ion density of positive and negative ions at ambient pressure in air at 12–136 mm from 4.9 kV soft x-ray source

“…Soft x rays between 2.25 and 5 keV were demonstrated with an alternate ion mobility design, the differential ion mobility spectrometer, by Kuklya et al, 9 and ion production was found identical to the widely distributed ion source for IMS, a beta emitter 63 Ni. Other applications with x-ray sources were given by Bunert et al, 10 who used a 4.9 keV x-ray source with an orthogonal installation to avoid secondary consequences of x-ray penetration into the inner volume of the drift tube, and Ahrens et al, 11 who used 3 keV x rays as an alternative for tritium in a miniature IMS drift tube. A design with two 4.9 keV x-ray sources in a tandem arrangement permitted fast switching between multiple reagents in atmospheric pressure chemical ionization mass spectrometry by Rissanen et al 12 Riebe et al 13 used 2.8 keV soft x rays as the APCI ionization source for research of photoionization of explosives and dopants.…”