Aqueous and non-aqueous Li+/H+ ion exchange in Li0.44La0.52TiO3 perovskite

The topotactic Li⁺/H⁺ exchange in Li_0.44La_0.52TiO₃ has been studied in different aqueous and non-aqueous media with different acidity. With this purpose, aqueous hydrochloric and nitric acid solutions and benzoic acid/ethanol solution were employed. The pristine and exchanged samples have been characterized by ¹H and ⁷Li MAS-NMR, TGA and XRD techniques. Aqueous hydrochloric and nitric acid solutions produce the powders degradation and the subsequent formation of Li₂TiO₃ and La₂Ti₂O₇ phases. A detailed analysis of the ¹H MAS NMR spectra of exchanged samples indicate that Li⁺/H⁺ exchange reaction in pure water produced formation of LiOH at the particles surface, band at 0 ppm, that could subsequently carbonated after exposition in air. Furthermore the presence of other OH signals at 8, 6 and 2 ppm has been related to differences on octahedral Ti-O distances, produced by La/vacancy ordering in alternating planes of perovskites. In samples immersed into benzoic acid/ethanol solution results are similar, however the amount of LiOH species in particle surface is considerable lower. The exchange degree improved when increasing exchange temperature. The mechanical grinding of powders decreases the particle size improving exchange reactions; however, grinding treatments eliminated specific NMR bands of perovskite. In ground materials new ¹H NMR bands at 6 and 4 ppm were ascribed to an amorphous phase.     

Negative ion formation in laser ion mobility increment spectrometer

A new method is proposed and implemented, according to which small concentrations of organic compounds in the atmosphere are measured by combining the ion mobility increment spectrometer and laser ionization. The experimental setup makes use of the pulsed fourth-harmonic radiation of a YAG:Nd³⁺ laser (λ = 266 nm). At high intensities (q ∼ 10⁷ W/cm²) of laser radiation, ion spectra with reactant peaks are observed, which are identical to the spectra obtained with traditional sources of ionization by corona discharge and β radiation. The ion spectra obtained at low laser radiation intensities (q ∼ 10⁵ W/cm²) are free of background and reactant peaks, which ensures the high selectivity of the analysis. It is concluded that the mechanism of negative ion formation from the molecules of nitro compounds can change depending on the intensity of ionizing laser radiation.     

Peak-peak repulsion in ion mobility spectrometry

The space charge effect has an important role in instruments dealing with ion packets and charged particles in gas phase such as the mass spectrometer and ion mobility spectrometer (IMS). It has been shown that the space charge is partially responsible for peak broadening in IMS depending on the ion density. Here, we explore the effect of space charge on peak shifting in IMS. We show that the field created by a large peak influences the drift time of a neighboring small peak. An experimental method was introduced to accurately measure the effect of space charge between two peaks. In this method, a double pulse was applied to the shutter grid to create two closed ion packets with a given initial spacing. The final spacing was then measured at the collector through the separation of the two peaks. This study shows that space charge repulsion must be considered for accurate measurements of ion mobilities. The experiments were performed in both normal and inverse modes. A theoretical model was also proposed to describe the repulsion between two ion packets in IMS.     

A tandem ion trap/ion mobility spectrometer

A tandem quadrupole ion trap/ion mobility spectrometer (QIT/IMS) has been constructed for structural analysis based on the gas-phase mobilities of mass-selected ions. The instrument combines the ion accumulation, manipulation, and mass-selection capabilities of a modified ion trap mass spectrometer with gas-phase electrophoretic separation in a custom-built ion mobility drift cell. The quadrupole ion trap may be operated as a conventional mass spectrometer, with ion detection using an off-axis dynode/multiplier arrangement, or as an ion source for the IMS drift cell. In the latter case, pulses of ions are ejected from the trap and transferred to the drift cell where mobility in the presence of helium buffer gas is determined by the collision cross section of the ion. Ions traversing the drift cell are detected by an in-line electron multiplier and the data processed with a multichannel scaler. Preliminary data are presented on instrumental performance characteristics and the application of QIT/ IMS to structural and conformational studies of aromatic ions and protonated amine/crown ether noncovalent complexes generated via ion/molecule reactions in the ion trap.     

Hadamard transform ion mobility spectrometry

Traditionally, the spectrum acquired using ion mobility spectrometry (IMS) is an average of multiple experimental cycles. Each cycle is initiated by passing a short burst of ions into a drift tube containing a homogeneous electric field. Prior to starting the subsequent cycle, all ions in the system must arrive at the detector or spectral overlap may occur. To maximize resolution, the ion pulse admitted to the drift tube is small in relation to the total scan time with the unfortunate consequence of an inherently low duty cycle (approximately 1%). Offering an improved SNR through a 50% duty cycle, the Hadamard transform (HT) applied to ion mobility spectrometry represents a fresh alternative to signal-averaged data acquisition. Initial results from measurements of amphetamine and cytochrome c samples indicate a 2-10-fold increase in SNR for the HT-IMS technique with no reduction in resolution.     

Surface ionization ion mobility spectrometry

A surface ionization (SI) source was designed and constructed for ion mobility spectrometry (IMS). Compared with a conventional (63)Ni source, the surface ionization source is as simple and reliable, has an extended dynamic response range, is more selective in response, and does not have regulatory problems associated with radioactive ionization sources. The performance of this SI-IMS was evaluated with several different classes of compounds. Triethylamine was employed for studying the behavior of the ionization source under different source conditions and gaseous environments. Amines, tobacco alkaloids, and triazine herbicides were also investigated. Picogram level detection limits were achieved for target compounds with a response dynamic range of 5 orders of magnitude. Selective monitoring by IMS was also demonstrated. While the surface ionization source does not have the universality of response that is obtained with a (63)Ni ionization source, it is an excellent nonradioactive alternative for the ionization and ion mobility detection of those compounds to which it responds.     

Voltage sweep ion mobility spectrometry

Ion mobility spectrometry (IMS) is a rapid, gas-phase separation technique that exhibits excellent separation of ions as a standalone instrument. However, IMS cannot achieve optimal separation power with both small and large ions simultaneously. Similar to the general elution problem in chromatography, fast ions are well resolved using a low electric field (50-150 V/cm), whereas slow drifting molecules are best separated using a higher electric field (250-500 V/cm). While using a low electric field, IMS systems tend to suffer from low ion transmission and low signal-to-noise ratios. Through the use a novel voltage algorithm, some of these effects can be alleviated. The electric field was swept from low to high while monitoring a specific drift time, and the resulting data were processed to create a 'voltage-sweep' spectrum. If an optimal drift time is calculated for each voltage and scanned simultaneously, a spectrum may be obtained with optimal separation throughout the mobility range. This increased the resolving power up to the theoretical maximum for every peak in the spectrum and extended the peak capacity of the IMS system, while maintaining accurate drift time measurements. These advantages may be extended to any IMS, requiring only a change in software.     

Pulsed-ionization miniature ion mobility spectrometer

We have demonstrated a miniature ion mobility spectrometer (IMS) that employs single pulses of corona discharge ionization. IMS spectra of both positive and negative ions generated from ambient air were measured as a function of drift field under various ionization conditions. Ion mobility spectra were studied with various pulse widths for both positive and negative ions, giving insights into mechanisms and kinetics of corona discharge ionization used in the miniature IMS. A combination of a pulsed potential with a steady dc bias was used to generate ions in the miniature IMS. There was a threshold dc potential for ion generation for a given pulse height. The dc ionization threshold was found to decrease linearly with increasing pulse height.     

Hadamard transform ion mobility spectrometry

A detection scheme that makes use of the Hadamard transform has been employed with an atmospheric-pressure ion mobility spectrometer fitted with an electrospray ionization source. The Hadamard transform was implemented through the use of a linear-feedback shift register to produce a pseudorandom sequence of 1023 points. This pseudorandom sequence was applied to the ion gate of the spectrometer, and deconvolution of the ion signal was accomplished by the Hadamard transform to reconstruct the mobility spectrum. Ion mobility spectra were collected in both a conventional and Hadamard mode, with comparisons made between the two approaches. Initial results exhibited low spectral definition, so an oversampling technique was applied to increase the number of data points across each analyte spectral peak. The use of the Hadamard transform increases the duty cycle of the instrument to 50% and results in a roughly 5-fold enhancement of the signal-to-noise ratio with a negligible loss of instrument resolution. It is also shown that any potential multiplex disadvantage, which limits the attractiveness of some high-throughput techniques, is not a limiting factor in this new implementation.     

Nonlinear wavelet compression of ion mobility spectra from ion mobility spectrometers mounted in an unmanned aerial vehicle

Linear and nonlinear wavelet compression of ion mobility spectrometry (IMS) data are compared and evaluated. IMS provides low detection limits and rapid response for many compounds. Nonlinear wavelet compression of ion mobility spectra reduced the data to 4-5% of its original size, while eliminating artifacts in the reconstructed spectra that occur with linear compression, and the root-mean-square reconstruction error was 0.17-0.20% of the maximum intensity of the uncompressed spectra. Furthermore, nonlinear wavelet compression precisely preserves the peak location (i.e., drift time). Small variations in peak location may occur in the reconstructed spectra that were linearly compressed. A method was developed and evaluated for optimizing the compression. The compression method was evaluated with in-flight data recorded from ion mobility spectrometers mounted in an unmanned aerial vehicle (UAV). Plumes of dimethyl methylphosphonate were disseminated for interrogation by the UAV-mounted IMS system. The daublet 8 wavelet filter exhibited the best performance for these evaluations.     

Lithium ion conductivity and mobility in the Li0.12Na0.88Ta0.4Nb0.6O3 solid solution

Lithium ion mobility and the superionic phase transition in the Li_0.12Na_0.88Ta_0.4Nb_0.6O₃ solid solution have been studied using temperature-dependent ionic conductivity measurements and Raman spectroscopy. From the temperature dependences of the conductivity and the width of a Raman line corresponding to Li⁺ and Na⁺ vibrations in the AO _x (A = Na⁺, Li⁺) polyhedra, the average lifetime of the Li⁺ ion in its equilibrium position and the height of the barrier to hopping have been estimated at ?3.9 × 10^?13 s and ?16 kJ/mol, respectively.     

Mass spectrometry for on-line monitoring of perfluoro compounds using Li+ ion attachment techniques

Ion attachment mass spectrometry is being developed for continuous measurement of perfluoro compounds (PFCs) found in the atmosphere as a result of semiconductor manufacturing processes. Studies were made on 5 greenhouse gases, CF4, CHF3, C2F6, SF6, and c-C4F8, to develop improved methods for PFC analysis, particularly at levels found in the atmosphere (the parts-per-billion concentration range). The results demonstrate the feasibility of performing real-time measurements of the trace amounts of PFCs encountered in process facilities by generating adduct ions from Li+ ion attachment. The identification and detection of c-C4F8 is described as an example of the utility of this new method.     

Low-temperature plasma ionization ion mobility spectrometry

In this research work, the capability of low-temperature plasma (LTP) as an ionization source for ion mobility spectrometry (IMS) has been investigated for the first time. This new ionization source enhances the potential of IMS as a portable analytical tool and allows direct analysis of various chemical compounds without having to evaporate the analyte or seek a solvent or reagent whatsoever. The effects of parameters such as the flow rate of the discharge gas, plasma voltage, and positioning of the LTP on the IMS signal were investigated. The positive reactant ions generated by the LTP ionization source were similar to those created in a corona discharge ionization source, where the proton clusters ((H(2)O)(n)H(+)) are the most abundant reactant ion, and in the negative mode, in addition to a saturated electron peak, several negative reactant ions (e.g., NO(x)(-)) were observed too. These reactant ions subsequently ionized the gaseous samples directly and liquids or solids after evaporation by plasma desorption. The ion mobility spectra of a few selected compounds, including explosives, drugs, and amines, were obtained to evaluate the new ionization source in positive and negative modes, and the reduced mobility values (K(0)) of the originated ions were calculated. Furthermore, the method has also been applied to obtain the figures of merit for acetaminophen as a test compound. The results obtained are promising enough to ensure the use of LTP as a desorption/ionization source in IMS for analytical applications.     

Proton and hydroxide ion mobility in capillary electrophoresis

A capillary electrophoresis method for measuring the effective mobilities of proton and hydroxide ions was developed. Photodiode array detection coupled with pH-indicating dyes enabled the detection of proton and hydroxide ions using a conventional CE instrument. The effects of ionic strength and pH of the background electrolyte on the measurement of effective mobility of the proton were first investigated. Mobility measurements were found to be independent of applied voltage. Hydroxide ion mobility was also measured and found to be in agreement with literature values. The effects of pH indicator and proton source on the ion mobility were determined and are described herein.     

Theory of ion mobility in liquid3He

An ion interacting with quasiparticles in liquid³He is treated theoretically by summing most divergent terms in perturbation series of the self-energy of the ion and the vertex part. The ion Green's function is renormalized by a factorZ(T), and the vertex part byZ(T) ^?1, where \({\text{Z(T)}} = {\text{(T/T}}_F {\text{)}}^{{\text{2V}}_{{\text{0}}^{\rho ^{\text{2}} } }^{\text{2}} } \) , forT ₀?T?T _F. Here,T ₀=(m/M)T _F, withm the³He mass andM the ion mass, and \({\text{V}}_{0^\rho } \) is the strength of the interaction. The factor explains the weak temperature dependence of the mobility around the minimum atT ₀; we also discuss its effect on the behavior of the mobility in³He-B nearT _c.