Selective ion suppression as a pre-separation method in ion mobility spectrometry using a pulsed electron gun

Ion mobility spectrometry is a well-known method for fast trace gas detection. Detection limits in the very low ppb- and even ppt-range, fast response times down to a second and good separation power combined with a reasonable instrumental effort make ion mobility spectrometry more and more attractive. Aiming for higher separation power we investigate the ion specific lifetime of different ion species in a field free reaction region of a drift tube ion mobility spectrometer equipped with a pulsed non-radioactive electron gun. When turning off the electron gun ionization stops and the total ion concentration in the reaction region starts to decrease, while different ion species have different decay times. By varying the time delay between the end of the ionization and the injection pulse transferring all remaining ions of one polarity from the reaction region into the drift region the individual decay times can be measured. Our experimental data show that the lifetime of ion species in a field free reaction region mainly depends on ion-ion-recombination and charge transfer reactions leading to significant lifetime differences. Therefore, short-lived ions can be effectively suppressed in the reaction region by introducing a sufficient time delay between the end of the ionization and the injection pulse. This allows detecting even smallest concentrations of long-lived ions in a complex short-lived background. From our experimental data it can be also concluded that wall losses and the ion transport within the sample gas stream out of the reaction region just play a minor role in the ion loss.     

Nanodiamonds in sugar rings: an experimental and theoretical investigation of cyclodextrin-nanodiamond inclusion complexes

We report on the noncovalent interactions of nanodiamond carboxylic acids derived from adamantane, diamantane, and triamantane with β- and γ-cyclodextrins. The water solubility of the nanodiamonds was increased by attaching an aromatic dicarboxylic acid via peptide coupling. Isothermal titration calorimetry experiments were performed to determine the thermodynamic parameters (K(a), ΔH, ΔG and ΔS) for the host-guest inclusion. The stoichiometry of the complexes is invariably 1:1. It was found that K(a), ΔG and ΔH of inclusion increase for larger nanodiamonds. ΔS is generally positive, in particular for the largest nanodiamonds. β-Cyclodextrin binds all nanodiamonds, γ-cyclodextrin clearly prefers the most bulky nanodiamonds. The interaction of 9-triamantane carboxylic acid shows one of the strongest complexation constants towards γ-cyclodextrin ever reported, K(a) = 5.0 × 10(5) M(-1). In order to gain some insight into the possible structural basis of these inclusion complexes we performed density functional calculations at the B97-D3/def2-TZVPP level of theory.     

Awakening of a ferrous complex's electronic spin in an aqueous solution induced by a chemical stimulus

A low-spin, macrocyclic iron(II) complex in an aqueous solution responds to the addition of a chemical reactant (dithionite) by transformation into a high-spin complex, detectable by measurement of the longitudinal relaxation time (T(1)) of surrounding water hydrogen nuclear spins. The initial compound does not modify T(1) of pure water at concentrations as high as 4 mM. The response is pH-dependent, and the complex is robust at a variety of conditions.     

Rapid synthesis of radioactive transition-metal carbonyl complexes at ambient conditions

Carbonyl complexes of radioactive transition metals can be easily synthesized with high yields by stopping nuclear fission or fusion products in a gas volume containing CO. Here, we focus on Mo, W, and Os complexes. The reaction takes place at pressures of around 1 bar at room temperature, i.e., at conditions that are easy to accommodate. The formed complexes are highly volatile. They can thus be transported within a gas stream without major losses to setups for their further investigation or direct use. The rapid synthesis holds promise for radiochemical purposes and will be useful for studying, e.g., chemical properties of superheavy elements.     

Control and femtosecond time-resolved imaging of torsion in a chiral molecule

We study how the combination of long and short laser pulses can be used to induce torsion in an axially chiral biphenyl derivative (3,5-difluoro-3',5'-dibromo-4'-cyanobiphenyl). A long, with respect to the molecular rotational periods, elliptically polarized laser pulse produces 3D alignment of the molecules, and a linearly polarized short pulse initiates torsion about the stereogenic axis. The torsional motion is monitored in real-time by measuring the dihedral angle using femtosecond time-resolved Coulomb explosion imaging. Within the first 4 picoseconds (ps), torsion occurs with a period of 1.25 ps and an amplitude of 3° in excellent agreement with theoretical calculations. At larger times, the quantum states of the molecules describing the torsional motion dephase and an almost isotropic distribution of the dihedral angle is measured. We demonstrate an original application of covariance analysis of two-dimensional ion images to reveal strong correlations between specific ejected ionic fragments from Coulomb explosion. This technique strengthens our interpretation of the experimental data.     

Production of a beam of highly vibrationally excited CO using perturbations

An intense molecular beam of CO (X(1)Σ(+)) in high vibrational states (v = 17, 18) was produced by a new approach that we call PUMP - PUMP - PERTURB and DUMP. The basic idea is to access high vibrational states of CO e(3)Σ(-) via a two-photon doubly resonant transition that is perturbed by the A(1)Π state. DUMP -ing from this mixed (predominantly triplet) state allows access to high vibrational levels of CO (X(1)Σ(+)). The success of the approach, which avoids the use of vacuum UV radiation in any of the excitation steps, is proven by laser induced fluorescence and resonance enhanced multi-photon ionization spectroscopy.