Effect of collision gas pressure and collision energy on reactions between oxygen and the negative molecular ion of tetrachlorodibenzo-p-dioxins in the collision cell of a triple -quadrupole mass spectrometer

Low-energy reactive collisions between the negative molecular ion of a tetrachlorodibenzo-p-dioxin (TCDD) and oxygen inside the collision cell of a triple-stage quadrupole mass spectrometer produce a substitution ion [M ? Cl + O]^?, a phenoxide ion [C₆H_4-nO₂Cl_n]^?·, [M ? HCl]^?·, and Cl^? by which 1,2,3,4-, 1,2,3,6/1,2,3,7- and 2,3,7,8-TCDD isomers can be distinguished either directly or on the basis of intensity ratios. The collision conditions have an important effect on the relative abundances. Energy- and pressure-resolved curves show that the ions formed by a collisionally activated reaction (CAR) process, i.e. [M ? Cl + O]^? and [C₆H_4-n,O₂Cl_n]^?·, are favoured by a high pressure of oxygen (3-6 mTorr) (1 Torr = 133.3 Pa) and a low collision energy (0.1-7 eV), whereas the ions formed by a collisionally activated dissociation (CAD) process, i.e. [M ? HCl]^?· and Cl^?, are favoured by high pressure and high energy. By choosing a relatively low collision energy (5 eV) and high pressure (4 mTorr), the CAR and CAD ions can be clearly detected.     

A segmented radiofrequency-only quadrupole collision cell for measurements of ion collision cross section on a triple quadrupole mass spectrometer

The gas collision cell of a triple quadrupole mass spectrometer has been modified to consist of ten short quadrupole rod segments that allow an axial field to be applied to the cell in order to make measurements of ion mobility. The radiofrequency (rf)-quadrupole field provides effective radial confinement that greatly reduces diffusional losses at low pressure. The mobilities of mass-selected ions from an ionspray source have been measured at a pressure of 8 × 10^?3 torr at electric fields of 0. 1 to 3 V/cm, and used to calculate the collision cross sections of the ions. The measured cross sections compare well with those measured by other techniques.     

The effect of resting transmembrane voltage on cell electropermeabilization: a numerical analysis

The transmembrane voltage induced due to applied electric field superimposes to the resting transmembrane voltage of the cell. On the part of the cell membrane, where the transmembrane voltage exceeds the threshold transmembrane voltage, changes in the membrane occur, leading to increase in membrane permeability known as electropermeabilization. This part of the cell membrane represents the permeabilized area through which the transport of molecules occurs. In this paper we calculated numerically the permeabilized area for different electric field strength, resting transmembrane voltage, cell shape and cell orientation with respect to the applied electric field. Results show that when the transmembrane voltage is near the threshold transmembrane voltage, the permeabilized area of the cell is increased on the anodic side and decreased on the cathodic side due to the resting transmembrane voltage. In some cases, only anodic side of the cell is permeabilized. Therefore, by using bipolar pulses, the permeabilized area can be significantly increased and consequentially also the efficiency of electropermeabilization. However, when the induced transmembrane voltage is far above the threshold, the effect of the resting transmembrane voltage is negligible. These observations are valid for different cell shapes and orientations.     

Correlation between formation conditions and breakdown voltage of anodic oxide films on aluminum

Two series of samples were investigated: a) aluminum films with a sublayer of tantalum; b) industrial aluminum alloys AMg-2m; D16; AMc; AD-1n. The optimum composition of re-anodizing electrolyte was chosen on the basis of a solution of citric acid and ethylene glycol. The results of investigations of the sparking voltage at re-anodizing for various aluminum alloys and thicknesses of primary porous oxide are presented. The analytical dependence of breakdown voltage value on forming voltage value for alloy AD-1n was obtained. The original design of re-anodizing cell allowing increase of the sparking voltage was developed.     

Comparing the capability of collision/reaction cell quadrupole and sector field inductively coupled plasma mass spectrometers for interference removal from 90Sr, 137Cs,and 226Ra

Here we report a quantitative comparison of sector field inductively coupled plasma mass spectrometry (ICP-SFMS) and collision/reaction cell inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) for the detection of ⁹⁰Sr, ¹³⁷Cs, and ²²⁶Ra at ultra-trace levels. We observed that the identification and quantification of radioisotopes by ICP-MS were hampered by spectral (both isobaric and polyatomic ions) and non-spectral (matrix effect) interferences. ICP-QMS has been used to eliminate the isobaric ⁹⁰Sr/⁹⁰Zr interference through the addition of O₂ into the collision cell as a reactant gas. Zr⁺ ions were subsequently converted into ZrO⁺, whereas Sr⁺ ions were not reactive. In addition, the isobaric interference of ¹³⁷Ba on ¹³⁷Cs was eliminated by the addition of N₂O gas in the cell, which led to the formation of BaO⁺ and BaOH⁺ products, whereas Cs⁺ remained unreactive. Furthermore, He and H₂ were used in the collision/reaction cell to eliminate polyatomic ions formed at m/z 226. A comparison of the results obtained by ICP-SFMS after a chemical separation of Sr from Zr and Cs from Ba was performed. Finally, to validate the developed analytical procedures, measurements of the same samples were performed by γ-ray spectroscopy.     

The effect of charge-exchange collision gas on ‘resolved’ E/2 mass spectra

A survey of the mass spectroscopic technique producing resolved E/2 mass spectra is undertaken, primarily to determine its dependence on the charge-exchange collision gas used. The applicability of the resultant spectra to ion structure studies is discussed. Instrumental effects, influencing the spectral pattern, are explained and will aid those utilizing this technique for ion structure determination.     

The effect of local structural disruption on the yield of photochemical ligation between anthracene-oligonucleotide conjugates.

The techniques of chemical ligation have attracted great attention as an alternative to enzymatic joining of DNA ends. Here we introduce the photoligation of anthracene-modified ODN conjugates through anthracene cyclodimer formation. The effect of the positions and the kinds of single base mismatch on the template was evaluated using eight templates with one-base displacements. We found out that the yield of the ligation was affected by mispairing in a position-dependent manner. Such results would be attributed to the disruption of the local structure at the ligation site.     

The effect of mass transfer on electrochemical impedance of a solid oxide fuel cell anode

This paper presents electrochemical impedance simulation of a solid oxide fuel cell (SOFC) anode in order to investigate the effect of mass transport processes on the impedance spectra. The current model takes in to account the gas-phase transport processes both in the gas channel and within the porous electrode and couples the gas transport processes with the electrochemical kinetics. The impedance simulation is carried out in time domain, and the correlation between the anode harmonic responses to the sinusoidal excitation and the impedance spectra is analyzed. In order to solve the system of non-linear equations, an in-house code based on the finite difference method is developed and utilized. Results show a depressed semicircle in the Nyquist plot, which originates from gas transport processes in the gas channel, in addition to a Warburg diffusion impedance originates from gas transport in the thick porous anode. The influence of parameters such as electrode thickness, inlet gas composition, and temperature is also investigated and the results are discussed. The simulation results are in good agreement with published data.     

Dichromate dosimetry: The effect of acetic acid on the radiolytic reduction yield

The radiation chemical yield for the reduction of dichromate, Cr(VI) → Cr³⁺, in an acidic aqueous perchloric acid solution of potassium dichromate, may be increased from 0.04 to >0.2 μ mol J^-1 by adding acetic acid. The increased yield, G[-(Cr₂O₇)^2-] is about the same in N_2- and O₂-saturated solutions. The molar linear absorption coefficient at 350 nm also is the same in both solutions (ϵ_m = 2800 M^-1cm^-1) at pH 0.4. The proposed mechanism to explain the enhanced response in N₂-saturated solutions involves the efficient reaction of acetic acid with hydroxyl radicals by the abstraction of H from the methyl group; the resulting acid radicals react with relatively high yield to reduce Cr(VI). In O₂-saturated solution, the acetic acid radical apparently goes through an acetic acid peroxyl radical by a bimolecular reaction to the tetroxide intermediate of acetic acid, which releases H₂O₂ with relatively high yield by a Bennett-type reaction. This additional H₂O₂, as a reducing agent, reacts slowly with dichromate and boosts the value of G[-(Cr₂O₇)^2-]. The negative slope of the response (ΔA vs dose) continues to increase during the period immediately after irradiation of oxygenated solution, due to slow reaction of radiolytically-produced H₂O₂ with dichromate. There is also in both O₂- and N₂-saturated solution a long-term slow reaction involving oxidation of the organic substrate (in this case, acetic acid). Because of these instabilities, the solutions cannot readily be used for dosimetry without the presence of silver ions, which in the oxidized state, Ag²⁺, act to stabilize the solution after irradiation. The addition of silver dichromate at a concentration of 0.1 mM decreases the yield to G[-(Cr₂O₇)^2-] = 0.17 μmolJ^-1, but greatly improves the stability of the solution after irradiation. The absorbed dose range for the modified dichromate dosimeter when analyzed spectrophotometrically at 350 nm wavelength is approx. 2 × 10²-2 × 10³ Gy.     

Systematic determination of ion score cutoffs based on calculated false positive rates: application for identifying ubiquitinated proteins by tandem mass spectrometry

We report a simple approach for determining ion score cutoffs that permit the confident identification of ubiquitinated proteins by tandem mass spectrometry (MS/MS). Initial experiments involving the analysis of gel bands containing multi-Ubiquitin chains with quadrupole time-of-flight and quadrupole ion trap mass spectrometers revealed that standard ion score cutoffs used for database searching were not sufficiently stringent. We also found that false positive and false negative rates (FPR and FNR) varied significantly depending on the cutoff scores used and that appropriate cutoffs could only be determined following a systematic evaluation of false positive rates. When standard cutoff scores were used for the analysis of complex mixtures of ubiquitinated proteins, unacceptably high FPR were observed. Finally, we found that FPR for ubiquitinated proteins are affected by the size of the protein database that is searched. These observations may be applicable for the study of other post-translational modifications.     

Immobilized pH gradients: effect of salts, added carrier ampholytes and voltage gradients on protein patterns

Salts formed from strong acids and bases (e.g. NaCl, Na2SO4, Na2HPO4), present in a protein sample applied to an immobilized pH gradient (IPG) gel, induce protein modification (oxidation of iron moiety in hemoglobin) already at low levels (5 mM) and irreversible denaturation (precipitation) at higher levels (greater than 50 mM). This effect is due to production of strongly alkaline cationic and strongly acidic anionic boundaries formed by the splitting of the salt's ion constituents, as the protein zone is not and can not be buffered by the surrounding gel until it physically migrates into the gel matrix. Substitution of "strong" salts in the sample zone with salts formed by weak acids and bases, e.g.. Tris-acetate, Tris-glycinate, Good's buffers such as (N-[2-acetamido]-2-iminodiacetic acid (ADA), (2-[(2-amino-2-oxoethyl)-amino] ethanesulfonic acid (ACES), (3-[N-morpholino]propane sulfonic acid (MOPS), essentially abolishes both phenomena, oxidation and irreversible denaturation. Suppression of "strong" salt's effects is also achieved by adding, to the sample zone, carrier ampholytes in amounts proportional to the salt present (e.g. by maintaining a salt: carrier ampholytes molar ratio of at least 1:1). This suppression is due to the strong buffering power of the added carrier ampholytes, able to counteract drastic pH changes in the two moving boundaries. A reduction of these deleterious effects of strong salts is also achieved when the IPG run is performed at low voltage for a prolonged time (4 h at 500 V instead of only 1 h at 500 V, before switching to high-voltage settings). Guidelines are given for trouble-free IPG operations.     

Chemical synthesis inside the collision cell of a MS/MS system: 1—Formation of adduct ions between protonated esters and ammonia

An evaluation of the TAGA 6000 tandem quadrupole MS/MS capabilities for the study of the formation of adduct ions is presented. It is shown that under chemical ionization conditions the protonated molecular ions of esters react with neutral ammonia inside the collision cell of the instrument to yield adduct ions. Under the same reaction conditions the molecular ions obtained under electron impact conditions do not yield any adduct. Although it is not yet clear whether the adduct structure is of the cluster or covalently bonded type, the experimental results argue mainly in favor of the latter. The center-of-mass energy range usable for the formation of adduct ions is narrow and an order of magnitude lower than the energy required for collision-induced dissociation. However, despite the weak parent ion center-of-mass energies used, the spectra exhibit fragments characteristic of the adduct ion structures. Finally, by controlling the parent ion center-of-mass axial kinetic energy, the number of parent ions and the target thickness, it is possible to measure and control all the usual physicochemical parameters for adduct ion formation.     

Correlation between the effect of sonic irradiation on the microfibrils of chitin and cellulose and the polarity of their chains

The effect of sonication on the morphology of microfibrils of chitins and celluloses was correlated with the polarity of their constituent chains. For given conditions, fibrillation of the microfibrils takes place when the polarity of the chains is antiparallel but not when it is parallel. If this correlation may be extended to cases of unknown structure, it permits an inference about the parallelity or antiparallelity of the chains of samples of both polymers where polarity cannot be determined directly.     

Thin-layer chroamtography of pt-metals complexes with ketoanils: Analysis and effect of chelate ring contraction on RF values

Summary Thin-Layer chromatogrtaphy on silica gel was used for the separation of platinum group metal complexes using one-, two-, and three-component solvent systems. In octahedral and square planar, cationic complexes, containing two identical five-membered chelate rings formed by three different ketoanils, a linear dependence on chelate ring contraction (and also metal-ligand bond frequencies and ligand field parameters) of their R_F values has been established in one-component solvent systems.     

Kinetics of pyrolysis of sugarcane bagasse: effect of catalyst on activation energy and yield of pyrolysis products

Cellulose - In this work we have attempted to use biomass as energy source which is abundantly available throughout the world. The work is focused on pyrolysis of sugarcane bagasse in a...     

MALDI-TOF mass spectrometry of a combinatorial peptide library: effect of matrix composition on signal suppression

The effect of matrix composition on signal suppression caused by a dominant compound under MALDI ionization was studied using the combinatorial TQTXT pentapeptide library as a model system. The peptide library is composed of 19 components with all proteinogenic amino acids except cysteine in position X. From these compounds, only the Arg peptide (TQTRT) was detected with sufficient intensity in the MALDI-TOF mass spectrum under typical MALDI conditions (CCA matrix). The analysis of a set of compounds utilized as different matrix components, additives and a cationizing agent revealed that the composition of the matrix is a critical point in signal suppression. Highly improved ion yields were achieved by using a CCA/DHB mixture as a matrix. The addition of K(+) as a cationizing agent to the CCA matrix resulted in MALDI-TOF mass spectra with relative ion intensities very similar to those obtained by electrospray ionization.