Current status and need for standards in ion mobility spectrometry

Ion mobility spectrometry (IMS) is a well established technique for the detection of many compounds of interest based on the reduced mobility (K₀) values of their ions. While having the advantage of small size, weight, and power, IMS has been subject to low specificity and is subject to interferences that can cause false alarms in detectors used for security applications. The rate of false positive alarms is directly related to the detection window width required to maintain a high rate of true positive detections. These window widths are in turn a result of the historically available accuracy of reference measurements and the range of responses by multiple detectors. The windows cannot be arbitrarily reduced without risking an increase in the rate of false negative responses. Ongoing work has focused on high accuracy calibration as a means of decreasing the false alarm rates by reducing the variability between detectors which would allow for narrower detection windows. Central to the calibration procedure is the selection of an appropriate calibrant (or reference standard) that can be easily characterized and known with a high degree of certainty across a range of instrumental conditions. This review evaluates a number of previously proposed and potential calibrants against seven recommended criteria of suitability. We examine the sources of false positive alarms in IMS-based detectors and propose a calibration procedure based on high accuracy reference measurements. Initial results of applying this procedure in a post-processing manner are promising towards reducing detector variability and detection window width.     

Method validation parameters for drugs and explosives in ambient pressure ion mobility spectrometry

An approach using method validation (MV) parameters, otherwise known as analytical figures of merit was combined with electrospray ionization high performance ion mobility spectrometry (ESI-HPIMS) to describe an approach for evaluating drugs and explosives analysis in the field. MV parameters such as reduced mobility (K _o ), conditional reduced mobility (K _c ), resolving power (R _p ), theoretical plates (N), linearity, accuracy, precision, limit of detection (LOD), limit of quantitation (LOQ), repeatability, range, and reporting limit were investigated and developed for eleven drugs and six explosives. Our investigation estimated resolving power at 66 ± 0.64 for the ESI-HPIMS used. The LOD’s calculated ranged from 0.45–2.97 ng of material electrosprayed into the ESI-HPIMS. The LOQ’s calculated falls in the range 4.11–8.63 ng of material electrosprayed into the ESI-HPIMS. The key findings from this investigation were the following: K _c proves to be a measure of the identity of an explosive or drug ion; a parameter that may be applied to help aid IMS devices when detecting drugs and explosives. MV parameters, especially, K _c , introduced in this study is an effective parameter for establishing a unique identity of a drug or explosive. A control chart is an effective way to monitor the performance of an instrument and may be a useful tool for establishing reliability of confirmatory data in forensic investigations. MV parameters may be a reliable, accurate and unique identification marker for target drugs and explosives capable of differentiating these substances from false positive responses.     

Thermal deformation thermodynamics of a smectite mineral

A method has been purposed to calculate some of the thermodynamic quantities for the thermal deformation of a smectite without using any basic thermodynamic data. The Hanç?l? (Keskin, Ankara, Turkey) bentonite containing a smectite of 88% by volume was taken as material. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of the sample were obtained. Bentonite samples were heated at various temperatures between 25–900°C for the sufficient time (2 h) until to establish the thermal deformation equilibrium.Cation-exchange capacity (CEC) of heated samples was determined by using the methylene blue standard method. The CEC was used as a variable of the equilibrium. An arbitrary equilibrium constant (K _a) was defined similar to chemical equilibrium constant and calculated for each temperature by using the corresponding CEC-value. The arbitrary changes in Gibbs energy (ΔG _a ⁰ ) were calculated from K _a-values. The real change in enthalpy (ΔH ⁰) and entropy (ΔS ⁰) was calculated from the slopes of the lnK vs. 1/T and ΔG vs. T plots, respectively. The real changes in Gibbs energy (ΔG ⁰) and real equilibrium constant (K) were calculated by using the ΔH ⁰ and ΔS ⁰ values. The results at the two different temperature intervals are summarized as below: ΔG ₁ ⁰ =ΔH ₁ ⁰ ?ΔS ₁ ⁰ T=?RTlnK ₁=47000?53t, (200–450°C), and ΔG ₂ ⁰ =ΔH ₂ ⁰ -ΔS ₂ ⁰ T=?RTlnK ₂=132000?164T, (500–800°C).     

Absolute and relative p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> predictions via a DFT approach applied to the SAMPL6 blind challenge

In this work, quantum mechanical methods were used to predict the microscopic and macroscopic pK_a values for a set of 24 molecules as a part of the SAMPL6 blind challenge. The SMD solvation model was employed with M06-2X and different basis sets to evaluate three pK_a calculation schemes (direct, vertical, and adiabatic). The adiabatic scheme is the most accurate approach (RMSE?=?1.40 pK_a units) and has high correlation (R²?=?0.93), with respect to experiment. This approach can be improved by applying a linear correction to yield an RMSE of 0.73 pK_a units. Additionally, we consider including explicit solvent representation and multiple lower-energy conformations to improve the predictions for outliers. Adding three water molecules explicitly can reduce the error by 2–4 pK_a units, with respect to experiment, whereas including multiple local minima conformations does not necessarily improve the pK_a prediction.     

Evaluation of Acid Dissociation Constants in DMSO and DMF by Quantum-Chemical Methods

We have calculated total electronic energies (E) and Gibbs energies (G) of a large number of acids and their anions in water, dimethylsulfoxide, and dimethylformamide using the hybrid B3LYP functional DFT method in the 6-31++G(d,p) basis set, taking into account the solvent effect by the conductor-like polarizable continuum model method. A linear correlation has been found between the experimental values of acid dissociation constants (pK_a) of different nature and the difference between anion and acid E values, and between pK_a and the difference between anion and acid G values. The obtained correlations allowed us to evaluate the pK_a values of both inorganic and organic acids. Such an evaluation is of special importance for nonaqueous solvents as it is quite problematic to determine these dissociation constants.     

Volumetric,Viscometric and Speed of Sound Studies of Binary Mixtures of <Emphasis Type="Italic">Tert</Emphasis>-butyl Acetate with Fluorobenzene,Chlorobenzene and Bromobenzene at (298.15 and 308.15) K and at Atmospheric Pressure 0.087 MPa

Volumetric, viscometric and speed of sound studies of binary mixtures of tert-butyl acetate with fluorobenzene, chlorobenzene and bromobenzene have been made over the entire range of composition, at (298.15 and 308.15) K and at atmospheric pressure (p?=?0.089 MPa). From the experimental values of density, viscosity, and speed of sound, the excess molar volumes V^E, deviations in viscosity ?η and deviation in isentropic compressibility ΔK _S have been calculated. The V^E and ΔK _S values are negative while the ?η the values are positive over the entire composition range for the binary mixtures. The derived parameters have been fitted with the Redlich–Kister polynomial equation. The interaction parameters of McAllister model are used to correlate the experimental values of density, viscosity and speed of sound.     

SAMPL6: calculation of macroscopic p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values from ab initio quantum mechanical free energies

Macroscopic pK_a values were calculated for all compounds in the SAMPL6 blind prediction challenge, based on quantum chemical calculations with a continuum solvation model and a linear correction derived from a small training set. Microscopic pK_a values were derived from the gas-phase free energy difference between protonated and deprotonated forms together with the Conductor-like Polarizable Continuum Solvation Model and the experimental solvation free energy of the proton. pH-dependent microstate free energies were obtained from the microscopic pK_as with a maximum likelihood estimator and appropriately summed to yield macroscopic pK_a values or microstate populations as function of pH. We assessed the accuracy of three approaches to calculate the microscopic pK_as: direct use of the quantum mechanical free energy differences and correction of the direct values for short-comings in the QM solvation model with two different linear models that we independently derived from a small training set of 38 compounds with known pK_a. The predictions that were corrected with the linear models had much better accuracy [root-mean-square error (RMSE) 2.04 and 1.95 pK_a units] than the direct calculation (RMSE 3.74). Statistical measures indicate that some systematic errors remain, likely due to differences in the SAMPL6 data set and the small training set with respect to their interactions with water. Overall, the current approach provides a viable physics-based route to estimate macroscopic pK_a values for novel compounds with reasonable accuracy.     

Molecular orientation ordering in surface layers of polymer films

The spontaneous ordering of fragments of chain molecules near the surface in polymer films is described in terms of the multichain model that allows for local intra-and interchain orientational interactions of chain segments and for transverse fluctuation of their orientation in the approximation of strong planar-orientation order in the layers. Chain packing in the plane-ordered state is impossible unless the interchain interaction parameter has a value of b = 2K ₂/k _B T > b _c. The value of b _c decreases with a growth in chain rigidity (parameter a = 2K ₁/k _B T). The calculated dependence of the limiting values of the quadrupole orientation order parameter S ₀ on the length of the Kuhn statistical segment A = 2a reasonably well describes the experimental data obtained in a study of polymer-homolog (polysaccharide) films by means of the tilted polarized-beam technique at an interchain interaction parameter of b = 1.75. The monolayer thickness (d ? 10 mn) is films of some polysaccharides was calculated from the fit of the theoretical to the experimental dependences of the orientational order parameter and the birefringence on the number of layers.     

Solute–solvent interactions in 2,4-dihydroxyacetophenone isonicotinoylhydrazone solutions in N,N-dimethylformamide and dimethyl sulfoxide at 298–313 K on ultrasonic and viscometric data

The speed of sound (u), density (ρ), and viscosity (η) of 2,4-dihydroxyacetophenone isonicotinoylhydrazone (DHAIH) have been measured in N,N-dimethyl formamide and dimethyl sulfoxide at equidistance temperatures 298.15, 303.15, 308.15, and 313.15 K. These data were used to calculate some important ultrasonic and thermodynamic parameters such as apparent molar volume (V _{? s} ^st ), apparent molar compressibility (K _?), partial molar volume (V _? ⁰ ) and partial molar compressibility (K _? ⁰ ), were estimated by using the values of (V _? ⁰ ) and (K _?), at infinite dilution. Partial molar expansion at infinite dilution, (? _E ⁰ ) has also been calculated from temperature dependence of partial molar volume V _? ⁰ . The viscosity data have been analyzed using the Jones–Dole equation, and the viscosity, B coefficients are calculated. The activation free energy has been calculated from B coefficients and partial molar volume data. The results have been discussed in the term of solute–solvent interaction occurring in solutions and it was found that DHAIH acts as a structure maker in present systems.     

Investigation of Solution p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> and Thermodynamic Values of Lamivudine and Pefloxacin Drugs by Ab initio and DFT Methods

In this work we investigate the thermodynamic properties and pK_a value of lamivudine and pefloxacin drugs, in aqueous solutions, by ab initio and density functional theory (DFT) methods at different temperatures. Molecular structures and solute–solvent effects of the anions, cations, and neutral molecules of lamivudine and pefloxacin were studied by the polarizable continuum model (PCM). The calculation was done at the DFT-B3LYP/6-31+G(d) level of theory using Tomasi’s method to analyze the formation of intermolecular hydrogen bonds (IHB) in aqueous solution. The pK_a1 values of lamivudine and pK_a2 values of pefloxacin increase with temperature increase. In contrast, the pK_a1 values of pefloxacin decrease when the temperature increases. Further, the thermodynamic properties of the ionization processes (?H, ?S and ?G) of the drugs in aqueous solution were determined and discussed. The results of this work are in good agreement with the literature data at 298.15 K.     

Elastic properties of a nematic in a tetrapalladium organyl-pentadecane system

The temperature dependences of birefringence Δn, anisotropy of permittivity ?_a, and elastic constants K ₁₁ and K ₃₃ in the nematic phase of a tetrapalladium organyl-pentadecane system with a pentadecane content of 55 wt % have been investigated experimentally. It has been shown that, as temperature is elevated, ?_a, K ₁₁, and K ₃₃ values decrease and Δn remains unchanged. Elastic constants K ₁₁ and K ₃₃ have been established to vary from 3.4 × 10^?7 to 5.6 × 10^?6 dyn and from 1.3 × 10^?6 to 27.4 × 10^?5 dyn, respectively. The value of ?_a has been revealed to vary over the range 0.2–0.5. It has been found that, at temperatures above the N₂ → Cr phase transition by 6°C, an imposed electric field induces the growth of tetrapalladium organyl crystals.     

Ermittlung der Solvolysenkonstante von Nitrosylchlorid in Methanol

The equilibrium constantK of the methanolysis of nitrosyl chloride was determined. The activities of HCl and CH₃OH in methanolic solutions of hydrogen chloride have been already published. The equilibrium was established from the right side by adding known amounts of methyl nitrite to methanolic hydrochloric acid (0–18m). The equilibrium concentration of NOCl was measured by means of aPulfrich photometer. For 25°C it was foundK=14.1, for 15°CK=18.9, and for 0.5°CK=30.3. From these results the thermodynamic functions of the methanolysis of nitrosyl chloride ΔG°, ΔH° and ΔS° were calculated.     

Phase Ratio and Equilibrium Constant in RP-HPLC Obtained from Octanol/Water Partition Constant Through Solvophobic Theory

The experimentally known dependence in RP-HPLC of the retention factor k′ on octanol/water partition coefficient (K _ow) has been examined based on solvophobic theory. The result showed that the dependence provides a means for the evaluation of phase ratio (Φ) of RP-HPLC columns, and of the equilibrium constant for a given compound and mobile phase. Using this theory, the phase ratio was evaluated for a set of seven different C18 columns (five having fully porous particles and two core–shell particles), and the equilibrium constants were calculated for four homologous series of compounds in two mobile phase systems. One mobile phase was methanol/aqueous solution of 0.1% H₃PO₄, and the other was acetonitrile/aqueous solution of 0.1% H₃PO₄. Besides providing the values for Φ for the evaluated columns, the results of the study indicated that for a specific composition of the mobile phase and for a given compound displaying only hydrophobic interactions, the equilibrium constant K(X) for different C-18 columns is basically the same. The data were further used to provide guidance in the selection of a chromatographic column for a specific separation based on K _ow values and chemical structure of the analytes. The study indicated that the separation of compounds with identical polar groups (or no polar groups) and with very close values for the K _ow cannot be achieved based only on hydrophobic interactions that dominate the separation on RP-type columns. Only column that displays polar interactions may provide a solution to such separations. For hydrocarbons with close K _ow values, the separation cannot be achieved even on columns with some polarity. On the other hand, even compounds with equal K _ow values, but with different functionalities can be separated on RP-HPLC columns without involving polar interactions. The compounds with different K _ow values are expected to be easily separated on RP-HPLC columns.     

Constants of acid‒base equilibria in an aqueous amikacin aminoglycoside solution at 298 K

The acid dissociation constants of form pK₁ = 7.34 ± 0.01, pK₂ = 7.84 ± 0.01, pK₃ = 8.77 ± 0.01, pK₄ = 9.49 ± 0.01, and pK₅ = 10.70 ± 0.02 of cationic amikacin are determined by pH-metric titration at 25°C against the background of 0.1 mol/L KNO₃. K₁, K₂, K₃, and K₄ correspond to the dissociation of protons coordinated to amino groups, while K₅ characterizes the dissociation of the hydroxyl hydrogen atom, testifying to the amphoteric character of amikacin molecules. Applying density functional theory (DFT) with the B3LYP hybrid functional and the 6-311G**++ basis set, the partial charges on the atoms of an amikacin molecule are calculated. It is concluded that the dissociation of H(55)hydrogen atom occurs with a greatest partial charge of +0.53631.     

Simulation of electrical percolation in disperse systems at a small difference between specific conductivities of components

The possibility of development of electrical percolation upon variations in the contents of components in a disperse system has been studied in terms of Bruggeman’s ideas of an effective medium and the continual model of the electrical conductivity of composite materials. The case of a small difference between the specific conductivities of a dispersed phase K_m and a dispersion medium K_d has been considered. It has been shown that the dependence of electrical conductivity K of a disperse system on volume fraction p of a wellconducting component may exhibit an inflection in the vicinity of the p* value, which depends on the K_m/K_d ratio. Different methods for determining the p* value have been described. At low ratios between the conductivities of a dispersed phase and a dispersion medium, the inflection is very weakly pronounced. However, the representation of the K(p) dependences in semilog coordinated substantially facilitates its identification.     

Stability of 3<Emphasis Type="Italic">d</Emphasis>-element cyclotetraphosphates in water

The hydrolysis kinetics of the anion in 3d-element cyclotetraphosphates is considered. The thermodynamic functions of formation (Δ _f H ⁰, Δ _f G ⁰, and Δ _f ? _at ⁰ ) of the cyclotetraphosphates are calculated using the ion increment method. A linear correlation is established between and log K Δ _f ? _at ⁰ for these compounds.     

Electrical-percolation effects in micellar solutions of alkyltrimethylammonium bromides

Specific conductivity K of aqueous solutions of alkyltrimethylammonium bromides has been studied in a wide range of concentrations c of surfactants containing 10, 12, 14, and 16 carbon atoms in alkyl chains. In general, three break points have been observed in the K(с) dependences. The first point observed upon increasing overall solution concentration corresponds to critical micelle concentration CMC₁. The CMC₁ values of alkyltrimethylammonium bromides decrease with an increase in the alkyl chain length. They are in satisfactory agreement with the published data. It has been supposed that the second break point in the K(с) dependences corresponds to the formation micellar structures as clusters and the appearance of channels with a higher specific conductivity, which is provided by the contribution from the overlap of electrical double layers existing in the vicinities of micelles. Surfactant concentrations corresponding to these break points have been called “critical percolation concentrations” (CPCs). The position of a CPC in the concentration scale strongly depends on alkyl radical length. All K(с) curves exhibit a third break, which corresponds to second critical micelle concentration CMC₂, at which the properties of ionic-surfactant solutions may substantially change because of the appearance of supramicellar structures. The experimental data obtained have been used to evaluate the parameters of the model of electrical percolation for micellar solutions, i.e., effective conductivity \({\tilde K_m}\) and effective micelle radius r ₀.     

Synthesis and crystal structure of a cyanide-bridged one-dimensional nickel complex with <Emphasis Type="Italic">trans</Emphasis>-Cn-Ni(En)<Subscript>2</Subscript>-CN structure

The synthesis and crystal structure of the cyano-bridged complex {Ni(En)₂[Ni(CN)₄]} _n are reported. The complex was synthesized by the reaction of [Ni(En)₂](ClO₄)₂ and K₂[Ni(CN)₄] in an H-shaped tube. The crystals are monoclinic, space group P2₁/c with a = 7.0917(4), b = 10.6629(5), c = 9.4975(5) Å, β = 108.193°, M = 170.85, Z = 4, V = 682.27(6) ų. The title complex reveals the trans-CN-Ni(En)₂-NC structure.     

Dissociation Constants of Silanol Groups of Silic Acids: Quantum Chemical Estimations

Dissociation constants of silanol groups on the silica surface are calculated with the DFT quantum chemical method using B3LYP and M06 functionals. Structural features of silanol fragments and the presence of hydrogen-bonded water clusters are shown to have significant effects on pK_a values of silanol groups. In particular, pK_a values are shown to vary widely depending on the features of the system of hydrogen bonds.     

The frequency dispersion of dynamic transfer coefficients and elastic moduli of magnetic liquids

The analytic equations for viscosity coefficients and the corresponding elastic moduli obtained in [3] were used to calculate these values over a wide range of reduced frequency values (ω* ≈ 10^?7?10). The volume η _v (ω) and shear η _s (ω) viscosity coefficients decreased as the frequency increased. The dispersions η _v (ω) and K _r (ω) were only caused by the contribution of structural relaxation, and the dispersions η _s (ω) and μ(ω), by translational and structural relaxation. The shear elastic modulus μ(ω) and relaxation volume elastic modulus K _r (ω) increased as the frequency grew. The results obtained were in satisfactory agreement with the conclusions from general relaxation theory.