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Gradient System (gradient + system)
Selected AbstractsCultivation of methanotrophic bacteria in opposing gradients of methane and oxygenFEMS MICROBIOLOGY ECOLOGY, Issue 3 2006Ingeborg Bussmann Abstract In sediments, methane-oxidizing bacteria live in opposing gradients of methane and oxygen. In such a gradient system, the fluxes of methane and oxygen are controlled by diffusion and consumption rates, and the rate-limiting substrate is maintained at a minimum concentration at the layer of consumption. Opposing gradients of methane and oxygen were mimicked in a specific cultivation set-up in which growth of methanotrophic bacteria occurred as a sharp band at either c. 5 or 20 mm below the air-exposed end. Two new strains of methanotrophic bacteria were isolated with this system. One isolate, strain LC 1, belonged to the Methylomonas genus (type I methantroph) and contained soluble methane mono-oxygenase. Another isolate, strain LC 2, was related to the Methylobacter group (type I methantroph), as determined by 16S rRNA gene and pmoA sequence similarities. However, the partial pmoA sequence was only 86% related to cultured Methylobacter species. This strain accumulated significant amounts of formaldehyde in conventional cultivation with methane and oxygen, which may explain why it is preferentially enriched in a gradient cultivation system. [source] Quantitative analysis of EO9 (apaziquone) and its metabolite EO5a in human plasma by high-performance liquid chromatography under basic conditions coupled to electrospray tandem mass spectrometryJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 10 2006Liia D. Vainchtein A sensitive and specific LC-MS/MS assay for the quantitative determination of EO9 and its metabolite EO5a is presented. A 200-µl human plasma aliquot was spiked with a mixture of deuterated internal standards EO9- d3 and EO5a- d4 and extracted with 1.25 ml ethyl acetate. Dried extracts were reconstituted in 0.1 M ammonium acetate,methanol (7 : 3, v/v) and 25 µl-volumes were injected into the HPLC system. Separation was achieved on a 150 × 2.1 mm C18 column using an alkaline eluent (1 mM ammonium hydroxide,methanol (gradient system)). Detection was performed by positive ion electrospray followed by tandem mass spectrometry. The assay quantifies a range from 5 to 2500 ng/ml for EO9 and from 10 to 2500 ng/ml EO5a using 200 µl of human plasma samples. Validation results demonstrate that EO9 and EO5a concentrations can be accurately and precisely quantified in human plasma. This assay will be used to support clinical pharmacologic studies with EO9. Copyright © 2006 John Wiley & Sons, Ltd. [source] A liquid chromatographic method optimization for the assessment of low and high molar mass carbonyl compounds in winesJOURNAL OF SEPARATION SCIENCE, JSS, Issue 20 2009Luciana C. de Azevedo Abstract Carbonyl compounds (CC) play an important role in beverage aroma since they may affect flavor of wines, brandies, and beers, among others. For this reason, it is necessary to identify and quantify CC through adequate analytical techniques. This study is a proposal of both developing and optimization of a new analytical methodology that allows investigate C1,C8 CC in wines simultaneously by quantifying even those ones that are predominantly present in the adduct form hydroxylalkylsulfonic acids (HASA). The HASA dissociation is undertaken by specific alkaline media (pH 11). The developed methodology employed the LC with UV/VIS detection (, = 365 nm) technique under gradient elution in the way to reach both free-CC and bound-CC quantification. Results showed that binary gradient system using eluent A (MeOH/ACN/H2O 74.5:0.5:25% v/v/v) and eluent B (MeOH) reached the best separation condition of both lower and higher molecular mass CC. This proposed method allowed simultaneous quantification of formaldehyde, acetaldehyde, propanone, furfuraldehyde, butyraldehyde, benzaldehyde, hexanaldehyde, 2-ethyl-hexanaldehyde, E-pent-2-en-1-al, and cyclohexanone , all of them were found in white wine (Moscato Canelli) and red wine (Shiraz) produced in the São Francisco Valley, in the Northeastern Region of Brazil , although this optimized method may probably be suitable for quantification of propionaldehyde, isobutyraldehyde, heptanaldehyde, octanaldehyde, benzaldehyde, and E-hex-2-en-1-al as well. We could not prove if this method is also able to determine the latter CC group since we have not found these substances present in detectable levels in our real samples considered in this study. [source] Experimental determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient systemMAGNETIC RESONANCE IN MEDICINE, Issue 3 2009Rebecca E. Feldman Abstract In MRI, strong, rapidly switched gradient fields are desirable because they can be used to reduce imaging time, obtain images with better resolution, or improve image signal-to-noise ratios. Improvements in gradient strength can be made by either increasing the gradient amplifier strength or by enhancing gradient efficiency. Unfortunately, many MRI pulse sequences, in combination with high-performance amplifiers and existing gradient hardware, can cause peripheral nerve stimulation (PNS). This makes improvements in gradient amplifiers ineffective at increasing safely usable gradient strength. Customized gradient coils are one way to achieve significant improvements in gradient performance. One specific gradient configuration, a planar gradient system, promises improved gradient strength and switching time for cardiac imaging. The PNS thresholds for planar gradients were characterized through human stimulation experiments on all three gradient axes. The specialized gradient was shown to have significantly higher stimulation thresholds than traditional cylindrical designs (y-axis SRmin = 210 ± 18 mT/m/ms and ,Gmin = 133 ± 13 mT/m; x-axis SRmin = 222 ± 24 mT/m/ms and ,Gmin = 147 ± 17 mT/m; z-axis SRmin = 252 ± 26 mT/m/ms and ,Gmin = 218 ± 26 mT/m). This system could be operated at gradient strengths 2 to 3 times higher than cylindrical configurations without causing stimulation. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source] Simple linear formulation for magnetostimulation specific to MRI gradient coilsMAGNETIC RESONANCE IN MEDICINE, Issue 5 2001Blaine A. Chronik Abstract A simple linear formulation for magnetostimulation thresholds specific to MRI gradient coils is derived based on established hyperbolic electrostimulation strength vs. duration relations. Thresholds are derived in terms of the gradient excursion required to cause stimulation, and it is demonstrated that the threshold curve is a linear function of the gradient switching time. A parameter , is introduced as being fundamental in the evaluation of gradient coil stimulation. , is a map of the induced electric field per unit gradient slew rate, and can be calculated directly from the gradient coil wire pattern. Consideration of , alone is sufficient to compare stimulation thresholds between different gradient coil designs, as well as to evaluate the expected dependency of stimulation threshold on position within the gradient coil. The linear gradient threshold curve is characterized by two parameters: SRmin and ,Gmin. SRmin is the slope of the threshold curve and represents the minimum slew rate required to cause stimulation in the limit of infinite gradient strength. ,Gmin is the vertical axis intercept of the curve and represents the minimum gradient excursion required to cause stimulation in the limit of infinite slew rate. Both SRmin and ,Gmin are functions of both , and the standard tissue parameters Er (rheobase) and ,c (chronaxie time). The ease with which both the gradient system performance and the stimulation thresholds can be plotted on the same axes is noted and is used to introduce the concept of a piece-wise linear operational limit curve for a gradient system. Magn Reson Med 45:916,919, 2001. © 2001 Wiley-Liss, Inc. [source] High-resolution blood flow velocity measurements in the human fingerMAGNETIC RESONANCE IN MEDICINE, Issue 4 2001M. Klarhöfer Abstract MR phase contrast blood flow velocity measurements in the human index finger were performed with triggered, nontriggered, and cine acquisition schemes. A strong (Gmax = 200 mT/m), small bore (inner diameter 12 cm) gradient system inserted in a whole body 3 Tesla MR scanner allowed high-resolution imaging at short echo times, which decreases partial volume effects and flow artifacts. Arterial blood flow velocities ranging from 4.9,19 cm/sec were measured, while venous blood flow was significantly slower at 1.5,7.1 cm/sec. Taking into account the corresponding vessel diameters ranging from 800 ,m to 1.8 mm, blood flow rates of 3.0,26 ml/min in arteries and 1.2,4.8 ml/min in veins are obtained. The results were compared to ultrasound measurements, resulting in comparable blood flow velocities in the same subjects. Magn Reson Med 45:716,719, 2001. © 2001 Wiley-Liss, Inc. [source] Highly sensitive and quantitative analysis of polyeptides using a new gradient system based on an abrupt change in adsorption of polypeptide to the reversed-phase column packingBIOMEDICAL CHROMATOGRAPHY, Issue 10 2007Ryoya Goda Abstract During a study of 100 µL aliquots of urocortin containing various acetonitrile contents, we hypothesized that a change in the acetonitrile content in the solution across a specific content of acetonitrile (critical threshold) causes an abrupt change in adsorption capacity to the column packing. Circular dichroism measurements suggest that the conformational change induced by acetonitrile in the solution causes the abrupt change in adsorption capacity, and this solvent-induced conformational change is reversible across the critical threshold. This hypothesis can apply to various polypeptides with molecular weights range from 1007 to 6789 and to other organic solvents. A new gradient system utilizing the instant recovery of the adsorption capacity across the critical threshold was designed, and applied to the analysis of a 100 µL aliquot of various polypeptide solutions. The results suggest that use of a solution containing organic solvents more than the critical threshold allows successful dilution of polypeptides up to picomolar concentration range without any loss due to its adsorption during handling procedures and loading onto the LC system, and that a new gradient system enables quantitative analysis of polypeptides at picomolar concentrations in such solutions. Copyright © 2007 John Wiley & Sons, Ltd. [source] High performance liquid chromatographic,mass spectrometric assay for the quantitation of BMS-204352 in dog K3EDTA plasmaBIOMEDICAL CHROMATOGRAPHY, Issue 3 2002Ming Yao A high performance liquid chromatographic-mass spectrometric (LC/MS) assay was developed and validated for the determination of BMS-204352 in dog K3EDTA plasma. A 0.5,mL aliquot of control plasma was spiked with BMS-204352 and internal standard (IS) and buffered with 1,mL of 5,mM ammonium acetate. The mixture was then extracted with 3,mL of toluene. After separation and evaporation of the organic phase to dryness using nitrogen at 40°C, the residue was reconstituted in the mobile phase and 25,µL of the sample were injected onto a Hypersil C18 column (2,×,50,mm; 3,µm) at a flow rate of 0.5,mL/min. The mobile phase was consisted of two solvent mixtures (A and B). Solvent A was composed of 5,mM ammonium acetate and 0.1% triethylamine in 75:25 v/v water:methanol, pH adjusted to 5.5 with glacial acetic acid, and solvent B was 5,mM ammonium acetate in methanol. A linear gradient system was used to elute the analytes. The mass spectrometer was programmed to admit the de-protonated molecules at m/z 352.7 (IS) and m/z 357.9 (BMS-204352). Standard curves of BMS-204352 were linear (r2,,,0.998) over the concentration range of 0.5,1000,ng/mL. The mean predicted quality control (QC) concentrations deviated less than 5.1% from the corresponding nominal values (ie 4, 80, 400 and 2000,ng/mL); the within- and between-assay precision of the assay were within 5.5% relative standard deviation. Stability of BMS-204352 was confirmed after at least three freeze/thaw cycles and BMS-204532 was stable in dog plasma when stored frozen at or below ,20°C for at least 16 weeks in spiked QC samples and for at least 4 1/2 weeks for in vivo study samples. BMS-204352 and IS were stable in the injection solvent at room temperature for at least 24,h. The assay was applied to delineate the pharmacokinetic disposition of BMS-204352 in dogs following a single intravenous dose administration. In conclusion, the assay is accurate, precise, specific, sensitive and reproducible for the pharmacokinetic analysis of BMS-204532 in dog plasma. Copyright © 2002 John Wiley & Sons, Ltd. [source] Shaping and timing gradient pulses to reduce MRI acoustic noise,MAGNETIC RESONANCE IN MEDICINE, Issue 2 2010Marcel Segbers MSc Abstract A method to reduce the acoustic noise generated by gradient systems in MRI has been recently proposed; such a method is based on the linear response theory. Since the physical cause of MRI acoustic noise is the time derivative of the gradient current, a common trapezoid current shape produces an acoustic gradient coil response mainly during the rising and falling edge. In the falling edge, the coil acoustic response presents a 180° phase difference compared to the rising edge. Therefore, by varying the width of the trapezoid and keeping the ramps constant, it is possible to suppress one selected frequency and its higher harmonics. This value is matched to one of the prominent resonance frequencies of the gradient coil system. The idea of cancelling a single frequency is extended to a second frequency, using two successive trapezoid-shaped pulses presented at a selected interval. Overall sound pressure level reduction of 6 and 10 dB is found for the two trapezoid shapes and a single pulse shape, respectively. The acoustically optimized pulse shape proposed is additionally tested in a simulated echo planar imaging readout train, obtaining a sound pressure level reduction of 12 dB for the best case. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] | ||||||||||