Optimization of the design and operation of FAIMS analyzers

Field asymmetric waveform ion mobility spectrometry (FAIMS) holds significant promise for post-ionization separations in conjunction with mass-spectrometric analyses. However, a limited understanding of fundamentals of FAIMS analyzers has made their design and operation largely an empirical exercise. Recently, we developed an a priori simulation of FAIMS that accounts for both ion diffusion (including anisotropic components) and Coulomb repulsion, and validated it by extensive comparisons with FAIMS/MS data. Here it is corroborated further by FAIMS-only measurements, and applied to explore how key instrumental parameters (analytical gap width and length, waveform frequency and profile, the identity and flow speed of buffer gas) affect FAIMS response. We find that the trade-off between resolution and sensitivity can be managed by varying gap width, RF frequency, and (in certain cases) buffer gas, with equivalent outcome. In particular, the resolving power can be approximately doubled compared to "typical" conditions. Throughput may be increased by either accelerating the gas flow (preferable) or shortening the device, but below certain minimum residence times performance deteriorates. Bisinusoidal and clipped-sinusoidal waveforms have comparable merit, but switching to rectangular waveforms would improve resolution and/or sensitivity. For any waveform profile, the ratio of two between voltages in high and low portions of the cycle produces the best performance.     

Optical bistability produced by resonant Joule's heating of semiconductors

The thermal hysteresis of Joule's absorption of infrared radiation in then-type semiconductor magnetoplasma is analysed. Such bistability is connected with the temperature dependence of the relaxation time of free electrons, their density being constant. The resonant Joule's absorption bistability of the pumping infrared wave near a Langmuir resonance of free electrons is discussed as well as the hysteresis cross-modulation of an infrared probing wave by means of a pumping microwave. The possibilities of smooth detuning of the hysteresis regime due to a magnetic-field control and low energy thresholds of these regimes are illustrated for InSb thin films in relatively weak magnetic fieldsH1–2 kG.     

Aspect-ratio dependent oscillatory thermocapillary convection in the floating half zone

The dependency of the critical Marangoni number on the geometrical aspect ratio of the floating half zone is essential to predict the onset of oscillatory thermocapillary convection.The experimental studies in the microgravity conditions on floating half zones of several centimeters in diameter have predicted that the critical Marangoni number increases with the increasing aspect ratio,and the terrestrial experimental studies have predicted the contradictory conclusion for floating half zones of several mil...     

Non-Gravitational Effects with Density-Matching in Evaluating the Influence of Sedimentation on Colloidal Coagulation

The method of density matching between the solid and liquid phases is often adopted to effectively eliminate the effect of sedimentation of suspensions in studies on dynamic behaviour of a colloidal system. However, the associated changes in the solvent composition may bring side effects to the properties investigated and therefore might lead to a faulty conclusion if the relevant correction is not made. To illustrate the importance of this side effect, we present an example of the sedimentation influence on the coagulation rate of suspensions of 2μm （diameter） polystyrene. The liquid mixtures, in the proper proportions of water （H2O）, deuterium oxide （D2O） and methanol （MeOH） as the liquid phase, density-matched and unmatched experiments are performed. Besides the influence of viscosity, the presence of methanol in solvent media, used to enhance the sedimentation effect, causes significant changes （reduction） in rapid coagulation rates compared to that in pure water. Without the relevant corrections for those non-gravitational factors it seems that gravitational sedimentation would retard the coagulation. The magnitude of the contribution from the non-gravitational factor is quantitatively determined, making the relevant correction possible. After necessary the influence of the sedimentation on coagulation rates at corrections for all factors, our experiments show that the initial stage of the coagulation is not observable.     

Heat exchange between a selectively emitting liquid and a laminar gas flow in the presence of an external source of radiation

An investigation is conducted in the solution of a number of practical problems of the radiative and combined heat exchange in nonuniform systems having widely different physical properties. The processes of thermal interaction between the ocean and the atmosphere have been treated in the paper [1], the effect of thermal radiation on the melting and solidification of semitransparent crystals has been investigated in [2], the flow of a selectively emitting gas around the lateral surface of an object evaporating under the action of radiative heating has been discussed in [3], and heat transfer from a jet to the molten mass of glass in a glassmaking furnace tank has been investigated in [4]. The radiative and combined heat exchange between a selectively emitting liquid and a transparent heat-conducting laminar gas flow in the case of a specified external thermal radiation field is discussed in this paper. The energy conservation equations are set up taking into account the heat transfer by radiation, convection, and molecular thermal conduction. A differential approximation is used in calculating the values of the radiation fluxes. A system of fundamental computational equations is solved by the method of finite differences and iterations and by the Runge-Kutta method. The results of the calculations are presented in the form of graphs.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 116–122, May–June, 1976.