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

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 or H. 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.