Due to the core assumptions of kinetic theory and the drive toward realizing reproducible gas-phase measurements, ion mobility experiments are commonly conducted in the presence of an inert, neat buffer gas, usually nitrogen or helium. Mixing drift gases in defined, static ratios can provide useful information not only for optimizing the separation of analytes but also for defining the interaction between the ion and neutral particle. In a foundational effort, we seek to validate the role of the drift gas polarizability on the observed mobility of the ions by systematically mixing drift gases to discretely access a range of bulk gas polarizabilities not given by pure drift gases. Compared to historical efforts to probe the role of polarizability on the ion−neutral collisional cross section where a linear relationship was assumed, the data collected in the present effort clearly illustrate a quadratic dependency of the ion−neutral particle collision cross section and polarizability (R 2 > 0.999). When translating these data into the mobility dimension, we illustrate that the gas-phase mobility of polyatomic ions conforms to Blanc's law. These observations combined with considerations related to Langevin's polarization limit provide an experimental mechanism to estimate to what degree an ion−neutral interaction conforms to either the hard-sphere or induced-dipole model. To support these observations, additional comparisons are made with the respective reduced masses, polarizabilities, and mobilities of ions in mixtures where different degrees of hard-sphere interactions are present.
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.
For time dispersive ion mobility experiments detail control over the mechanism of ion beam modulation is necessary to establish optimum performance as this parameter greatly influences the temporal width of the ion beam arriving at the detector.
Abstract. Split-ring resonators are electrical circuits, which enable highly sensitive readout of split capacity changes via a measurement of the shift in the resonance frequency. Thus, functionalization of the split allows the development of biosensors, where selective molecular binding causes a change in permittivity and therefore a change in split capacity. In this work, we present a novel approach using transmission line theory to describe the dependency between permittivity of the sample and resonance frequency. This theory allows the identification of all relevant parameters of a split-ring resonator and thus a target-oriented optimization process. Hereby all setup optimizations are verified with measurements. Subsequently, the split of a resonator is functionalized with aptamers and the sensor response is investigated. This preliminary experiment shows that introducing the target protein results in a shift in the resonance frequency caused by a permittivity change due to aptamer-mediated protein binding, which allows selective detection of the target protein.
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