Sodium affinity of caffeine and adenine: the effect of microsolvation and electrostatic field of solvent on the sodium affinity

<p>The sodium affinity (SA) of caffeine (CAF), adenine (AD) and their microsolvated clusters containing one X molecule (CAF-X and AD-X; X = H₂O, NH₃, H₂S and HF) has been calculated in the gas phase and water, separately. The density functional theory (DFT) employing CAM-B3LYP functional has been used for all of the calculations in this work. The solvent was modelled by the polarised continuum model (PCM) which considers the electrostatic field of solvent on solute. The calculated SA of [CAF-X] and [AD-X] was higher than that of CAF and AD in the gas phase, respectively, which showed that the microsolvation of molecules in the gas phase could be used for changing the tendency of molecules for binding to Na⁺. Also, it was observed that the electrostatic effect of solvent decreases the SA of the species compared to the gas phase, considerably. The symmetry adapted perturbation theory (SAPT) calculations were also used to interpret the change in the SA of CAF and AD due to the clustering with one X molecule in the gas phase. In addition, there is a detailed study on the position of Na⁺ relative to AD and CAF structures in different conditions including gas phase, microsolvation and electrostatic field of solvent in this work.</p

Ion Mobility Spectrometry of Heavy Metals

A simple, fast, and inexpensive method was developed for detecting heavy metals via the ion mobility spectrometry (IMS) in the negative mode. In this method, Cl⁻ ion produced by the thermal ionization of NaCl is employed as the dopant or the ionizing reagent to ionize heavy metals. In practice, a solution of mixed heavy metals and NaCl salts was directly deposited on a Nichrome filament and electrically heated to vaporize the salts. This produced the IMS spectra of several heavy-metal salts, including CdCl₂, ZnSO₄, NiCl₂, HgSO₄, HgCl₂, PbI₂, and Pb­(Ac)₂. For each heavy metal (M), one or two major peaks were observed, which were attributed to M·Cl^– or [M·NaCl]­Cl^–complexes. The method proved to be useful for the analysis of mixed heavy metals. The absolute detection limits measured for ZnSO₄ and HgSO₄ were 0.1 and 0.05 μg, respectively

Effect of Hydration on the Kinetics of Proton-Bound Dimer Formation: Experimental and Theoretical Study

A kinetic study was performed on the proton-bound dimer formation of cyclopentanone, cyclohexanone, and cycloheptanone at atmospheric pressure with ion mobility spectrometry (IMS) at the temperature range of 30 to 70 °C. Measured rate constants were in the range of 9.5 × 10^–11 to 4.5 × 10^–10 cm³ s^–1. Rate constants were also calculated using average dipole orientation (ADO) theory employing density functional theory (DFT). Calculated rate constants were in the range of 1.0–5.5 × 10^–9 cm³ s^–1. The difference between experimental and calculated rate constants was interpreted based on the hydration of the protonated monomers so that water molecules were replaced with a neutral monomer molecule in the process of dimer formation. This process requires activation energy for the formation of dimer and consequently reduces the rate constants. To verify our hypothesis, an effective rate constant (<i>k</i>_eff) was introduced, which accounted for the energetically activated water–monomer replacement in the dimer formation reactions. A good agreement was observed between the experimental rate constants and calculated <i>k</i>_eff, confirming the validity of the proposed model in explaining the kinetics of dimer formation in atmospheric pressure

Field Switching Combined with Bradbury–Nielsen Gate for Ion Mobility Spectrometry

Bradbury–Nielsen gate (BNG) is commonly used in ion mobility spectrometers. It, however, transmits only a small fraction of the ions into the drift region, typically 1%. In contrast, all ions in the ionization chamber could be efficiently compressed into the drift region by the field switching gate (FSG). We report in this paper on the simultaneous use of BNG and field switching (FS) to enhance ion utilization of the BNG. In this technique, the FS collects the ions existing in the region between the FS electrode and the BNG and drives them quickly, going through the BNG in the period of gate opening. The BNG acts as the retarding field in the reported FSG to stop ions from diffusing into the drift region in the period of gate closing. Using this technique, an increase of at least 10-fold in the ion peak height without any loss of resolution is achieved for acetone compared with the BNG-only approach at a gate pulse width of 150 μs, and an even larger improvement factor of 21 is achieved for heavier DMMP dimer ions. This technique can be adapted to the current BNG-based ion mobility instruments to significantly enhance their sensitivity without any modification of the drift tube hardware