Computer simulations of a dielectric barrier discharge used for analytical spectrometry

A model developed for a dielectric barrier discharge (DBD) in helium, used as a new spectroscopic source in analytical chemistry, is presented in this paper. The model is based on the fluid approach and describes the behavior of electrons, He⁺ and ions, He metastable atoms, He atoms in higher excited levels, and He₂ dimers. The He ground-state atoms are regarded as background gas. The characteristic effect of charging/discharging of the dielectrics which cover both electrodes is also simulated. Typical results of the model include the distribution of potential inside the plasma (and the potential drop across the dielectrics), the electric current and gap voltage as a function of time for a given applied potential profile, the spatial and temporal number-density profiles of the different plasma species, and the relative contributions of the mechanisms of their production and loss. Figure Schematic diagram of the dielectric barrier discharge (left) and typical temporal profiles of voltage and current, as obtained from the simulations (right)     

Cation identity dependence of crown ether photonic crystal Pb<Superscript>2+</Superscript> sensing

We quantitatively modeled the volume phase transition of a hydrogel containing a crystalline colloidal array with a crown ether ligand which binds Pb²⁺. The hydrogel volume response and the wavelength diffracted depend on the Pb²⁺ concentration and on both the ionic strength and the valence of the nonbinding ionic species. We successfully modeled the response of this hydrogel Pb²⁺ sensor to ionic strength and the cation valence of the added salts. Figure Cation identity dependence of crown ether photonic crystal Pb⁺ sensing     

Characterization of cellular chemical dynamics using combined microfluidic and Raman techniques

The integration of a range of technologies including microfluidics, surface-enhanced Raman scattering and confocal microspectroscopy has been successfully used to characterize in situ single living CHO (Chinese hamster ovary) cells with a high degree of spatial (in three dimensions) and temporal (1 s per spectrum) resolution. Following the introduction of a continuous flow of ionomycin, the real time spectral response from the cell was monitored during the agonist-evoked Ca²⁺ flux process. The methodology described has the potential to be used for the study of the cellular dynamics of a range of signalling processes. Figure Spectral mapping of a single CHO cell     

Nuclear magnetic resonance of mass-limited samples using small RF coils

Figure Schematic diagram of a typical arrangement used for hyphenating chemical microseparations (e.g. capillary HPLC, CE, or CEC) with microcoil NMR detection     

Analysis of glycans in glycoproteins by diffusion-ordered nuclear magnetic resonance spectroscopy

Enzymatically cleaved glycans from sub-milligram quantities of erythropoietin (EPO) and ovalbumin have been analyzed, without further purification, by two-dimensional diffusion-ordered nuclear magnetic resonance spectroscopy. At NMR sample concentrations below 50 μmol L⁻¹ the major components of the oligosaccharide fractions could be distinguished by their anomeric proton chemical shift and their size-dependent diffusion coefficients. Figure ¹H NMR diffusion decay curves of anomeric protons in the EPO glycan fraction     

Detection of molecular processes in the intact retina by ATR-FTIR spectromicroscopy

We used Fourier transform infrared spectromicroscopy in the attenuated total reflection configuration to study biochemical events associated with the response to light of an intact retina. We show that the technique is suitable for the detection in real time of molecular processes occurring in rod outer segments induced by light absorption. Two-dimensional correlation analysis was applied to the identification and interpretation of specific spectral changes associated to the evolution of the system. The technique allows us to observe an extensive protein translocation, which we interpret as arising from the release of transducin from the disk membrane and its redistribution from the outer segment towards the inner segment of rod cells. These results are in full agreement with our current understanding of retinal physiology and validate the technique as a useful tool for the study of complex molecular processes in intact tissue. Figure Spectral changes in the mid infrared region following exposure of an intact retina to light     

Fourier transform ion cyclotron resonance mass spectrometry of covalent adducts of proteins and 4-hydroxy-2-nonenal, a reactive end-product of lipid peroxidation

Covalent adduction of the model protein apomyoglobin by 4-hydroxy-2-nonenal, a reactive end-product of lipid peroxidation, was characterized by nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FTICR). The high mass resolving power and mass measurement accuracy of the instrument facilitated a detailed compositional analysis of the complex reaction product without the need for deconvolution and transformation to clearly show the pattern of adduction and component molecular weights. Our study has also demonstrated the value of electron capture dissociation over collision-induced dissociation for the tandem mass spectrometric determination of site modification for the 4-hydroxy-2-nonenal adduct of oxidized insulin B chain as an example. Figure FTICR allowed characterization of 4-hydroxy-2-nonenal (HNE)-modified apomyoglobin (an expanded spectrum of the +15 charge state is shown)     

Aptamers as molecular recognition elements for electrical nanobiosensors

Recent advances in nanotechnology have enabled the development of nanoscale sensors that outperform conventional biosensors. This review summarizes the nanoscale biosensors that use aptamers as molecular recognition elements. The advantages of aptamers over antibodies as sensors are highlighted. These advantages are especially apparent with electrical sensors such as electrochemical sensors or those using field-effect transistors. Figure Feeling proteins with aptamer-functionalized carbon nanotubes     

Instrumental modification intended to save time,and volumes of sample and reagent solutions,in the atomic fluorescence spectrometric determination of mercury

Use of small membrane pumps, instead of peristaltic pumps, to introduce sample and reagent solutions into the spectrometer has several advantages in atomic fluorescence spectrometric determination of mercury. This simple modification results in a substantial saving in the time required for the measurements and so 90% of reagent solution volumes and 95% of sample solution volumes are saved, with a consequent decrease in the volume of waste generated. The sampling frequency is almost tripled, with no deterioration in sensitivity, which is similar to that obtained by use of peristaltic pumps. The relative standard deviation for ten consecutive measurements of a 1 μg L⁻¹ mercury solution was approximately 2%. Figure Small membrane pumps for the atomic fluorescene spectro metric determination of mercury     

Novel gel-based rapid test for non-instrumental detection of ochratoxin A in beer

A rapid easy-to-use immunoassay was optimised for the non-instrumental detection of ochratoxin A (OTA) in beer. The analytical method involves preconcentration on the immunoaffinity layer inside a column followed by direct competitive ELISA detection in the same layer. The visual cut-off value, i.e. the lowest OTA concentration resulting in no colour development, was 0.2 μg L^-1. Assay validation was performed using samples spiked with OTA. Thirty-seven naturally contaminated samples were screened with the gel-based method developed and no false-negative results were obtained. The method described offers a simple, rapid and cost-effective screening tool, thus contributing to better health protection of consumers. Figure Gel-based immunoassay of spiked beer samples.     

Modifying argon glow discharges by hydrogen addition: effects on analytical characteristics of optical emission and mass spectrometry detection modes

An overview of the effects produced by the presence of hydrogen in a glow discharge (GD), generated either in argon or in neon, is given. Extensive work related to the addition of hydrogen to GDs, coupled with optical emission spectrometry (OES) and mass spectrometry (MS), has been published in the last few years in an attempt to explain the processes involved in the discharge of mixed gases. Although numerous experimental results have already been explained theoretically, a complete understanding of the effects brought about by mixing hydrogen with argon (or another discharge inert gas) has not been reported yet. The use of theoretical models implemented using a computer has allowed the importance of some collisional and radiative processes in the inert gas plasma when hydrogen is present to be evaluated. This review shows, however, that both experimental work and theoretical work are still needed. The influence of small quantities of hydrogen on discharge parameters, such as electrical current or dc bias voltage, on crater shapes and on sputtering rates is thoroughly reviewed along with the effect on the analytical signals measured by OES and MS. Also, hydrogen-effect corrections needed to carry out proper calibrations for direct solid quantitative analyses are discussed. Figure Hydrogen induced changes in the Ar glow discharge reactions.     

Sequential preconcentration by coupling of field amplified sample injection with pseudo isotachophoresis–acid stacking for analysis of alkaloids in capillary electrophoresis

A novel on-column sequential preconcentration method based on the combination of field-amplified sample injection induced by acetonitrile and pseudo isotachophoresis (ITP)–acid stacking is developed for simply but efficiently concentrating alkaloid cations in a high-salt sample matrix in capillary electrophoresis. Acetonitrile (70%) added to a sample solution with a high-salt sample matrix not only induces field-amplified sample stacking by decreasing conductivity but also acts as a termination reagent in the succeeding pseudo ITP. After sample injection had been completed, a plug of H⁺ was injected electrokinetically and a neutralization reaction between H⁺ and tartrate from the buffer solution produced a low conductivity zone, in which the injected analyte cations were further concentrated. With the sequential preconcentration method, a 3 orders of magnitude detection sensitivity (1,400-fold) increase could be observed compared with the conventional electrokinetic injection method, without compromising separation efficiency and peak shape, and detection limits of 0.1 ng/mL for myosmine and 0.3 ng/mL for anabasine with the conditions selected were achieved. The calibration curves demonstrated good linearity in the concentration ranges 1.3–600 ng/mL for myosmine and 4.9–900 ng/mL for anabasine, respectively. The proposed method has been used to analyze successfully trace alkaloids in cigarette samples. Figure Sequential preconcentration processes: a sample injection; b introduction of HCl; c capillary zone electrophoresis separation. A ⁻ tartrate, white circles acetonitrile, black circles Na⁺, sample zone, myosmine, anabasine     

Characterization of imazamox degradation by-products by using liquid chromatography mass spectrometry and high-resolution Fourier transform ion cyclotron resonance mass spectrometry

The photodecomposition of imazamox, a herbicide of the imidazolinone family, was investigated in pure water. The main photoproducts from the photolysis were followed over time by liquid chromatography mass spectrometry and structures were proposed from exact mass determinations obtained by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The method comprised exact mass determination with better than 0.2 ppm mass accuracy and a corresponding structural visualization taking care of respective isotopes with an adapted van Krevelen diagram that enabled a systematic approach to the characterisation of the elementary composition of each photoproduct. By taking advantage of the high resolving power of FT-ICR MS to make precise formula assignments, the derived 2D van Krevelen diagram (O/C; H/C; m/z) enabled one to structurally differentiate the formed photoproducts and to propose a degradation pathway for imazamox. Figure Overview of applied method to analyse the photolysis process of imazamox herbicide     

Assessment of metastable atom bombardment (MAB) ionization mass spectrometry for the fast determination of heterocyclic aromatic amines in cooked meat

An investigation of metastable atom bombardment (MAB) ionization mass spectrometry for the fast characterization of mutagenic/carcinogenic heterocyclic aromatic amines (HAAs) formed during heating processes of meats is presented. The aim of our study was to use the selective ionization of MAB to develop a detection method for HAAs in non-purified meat extracts, thus avoiding purification and concentration steps and reducing analysis time. Sample introduction into the MAB ion source was achieved by pyrolysis, allowing the direct and fast insertion of complex food extracts into the mass spectrometer. Analysis conditions were optimized on standard HAAs by using different ionization gases for the MAB process. Metastable nitrogen was selected as the best MAB gas for the analysis of HAAs. Ionization selectivity is shown by the detection of heterocyclic amines in non-purified chicken meat extracts spiked with HAAs. A quantitative approach is also presented by using pyrograms as chromatograms for quantification purposes. HAAs determination using Py-MAB-ToF was finally performed on cooked chicken breast extracts and compared to an LC-APCI-MS/MS method. Although Py-MAB-ToF sensitivity remains to be improved in the present state of development of our prototype device, only 2 h from the cooking were required to obtain quantitative results in good agreement with HAAs concentrations measured by LC-MS/MS in 36 h. Figure Experimental set-up for pyrolysis-MAB-ToF mass spectrometry experiments.     

Rapid response behavior,at room temperature,of a nanofiber-structured TiO<Subscript>2</Subscript> sensor to selected simulant chemical-warfare agents

A chemical prototype sensor was constructed based on nanofiber-structured TiO₂ and highly sensitive quartz resonators. The gas-sensing behavior of this new sensor to selected simulant warfare agents was investigated at room temperature. Results showed rapid response and good reversibility of this sensor when used with high-purity nitrogen. This provides a simple approach to preparation of materials needed as chemical sensors for selected organic volatiles or warfare agents. Figure Sensing behavior of TiO₂ nanofiber sensor to chemical vapors     

Determination of trace elements in suspensions and filtrates of drinking and surface water by wavelength-dispersive X-ray fluorescence spectrometry

An X-ray fluorescence method (XRF) is presented that allowed low detection limits (at the 0.1–23 ng mL⁻¹ level) to be obtained for Cr, Mn, Fe, Ni, Zn, Sr, Pb, Bi and Br in water. The samples were prepared using a thin layer method. Trace elements were determined via the calibration curve and standard addition. Absorption effects and inhomogenities in prepared samples were checked for using the emission–transmission method and internal standards, respectively. The results from the XRF method were compared with the results from the inductively coupled plasma atomic emission spectrometry method.      

Multidimensional electrochemical imaging in materials science

In the past 20 years the characterization of electroactive surfaces and electrode reactions by scanning probe techniques has advanced significantly, benefiting from instrumental and methodological developments in the field. Electrochemical and electrical analysis instruments are attractive tools for identifying regions of different electrochemical properties and chemical reactivity and contribute to the advancement of molecular electronics. Besides their function as a surface analytical device, they have proved to be unique tools for local synthesis of polymers, metal depots, clusters, etc. This review will focus primarily on progress made by use of scanning electrochemical microscopy (SECM), conductive AFM (C-AFM), electrochemical scanning tunneling microscopy (EC-STM), and surface potential measurements, for example Kelvin probe force microscopy (KFM), for multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers. Figure Electrochemical and electrical tools like scanning electrochemical microscopy, conductive atomic force microscopy, electrochemical scannig tunneling microscopy and Kelvin probe force microscopy (see figure) are powerful tools for the multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers.     

Determination of major, minor and trace elements in cobalt-substituted lithium nickelate ceramic powders by inductively coupled plasma optical emission spectrometry

An analytical method was developed for the determination of three major (Li, Ni and Co) and fourteen minor or trace elements (Al, Ba, Ca, Cu, Cr, Fe, K, Mg, Mn, Na, Si, Sr, Ti and V) in LiNi_1−x Co _x O₂ (x = 0.2–0.8) ceramic powders by inductively coupled plasma optical emission spectrometry. Sample dissolution was achieved by 25% nitric acid digestion in a microwave oven. For each element, an analytical line free from spectral interferences was selected. A detailed study of matrix effects over a wide interval of total excitation energy (TEE) lines (1.62–16.50 eV) was performed at near-robust plasma conditions. A remarkable enhancement in atomic lines with TEE <4 eV was noticed, whereas a significant reduction in atomic and ionic lines with TEE >4 eV was observed. The extrapolation to infinite dilution method was successfully used to overcome these nonspectroscopic interferences. Detection limits (3σ) varied from 0.21 mg kg⁻¹ for Sr to 49.7 mg kg⁻¹ for Na. The precision of determination (obtained as the relative standard deviation) was lower than 1% for the major elements Li, Ni and Co and between 0.69 and 10% for minor and trace elements. The accuracy of the method ranged from 91 to 101% for major elements, and from 90 to 110%, or close to this range, for most of the impurities in both of the samples studied.