Absolute Concentration Measurements Research Articles

Three-dimensional simulation of a HFCVD reactor

Abstract A three-dimensional (3D) model of a hot-filament CVD reactor is developed to study the gas-phase and surface processes of diamond growth. The gas-phase and surface reaction mechanisms, the molecular diffusion and thermodiffusion, catalytic hydrogen dissociation at the filament and surface kinetics at the substrate are taken into account in transport equations that are numerically solved. The model has been checked using experimental measurements of absolute methyl and atomic hydrogen concentration. An importance of 3D effects for the species concentration and gas temperature distribution is revealed. The filament and substrate temperature dependence of species concentrations are calculated and compared with experimental data.

Measurement of absolute concentration and viability of CD34+ cells in cord blood and cord blood products using fluorescent beads and cyanine nucleic acid dyes

Conventional flow cytometric methods for CD34+ cell counting may be affected by the high number of nucleated red blood cells or nonviable cells in cord blood and its products. We developed a simple flow cytometric no-wash procedure that avoids these shortcomings because it provides absolute CD34+ cell counts and assesses cell viability. Samples were incubated with phycoerythrin (PE)-labeled anti-CD34 (Becton Dickinson Immunocytometry Systems [BD], San Jose, CA) and peridinin chlorophyll protein (PerCP)-labeled anti-CD45 (BD) in bead-containing TRUCOUNT tubes (BD). After red cell lysis with a fixative-free reagent, the impermeant nucleic acid dye YO-PRO-1 (Molecular Probes, Eugene, OR) was added and samples were analyzed on a single-laser FACSCalibur (BD). A comparison with the ProCOUNT progenitor cell assay (BD) in 57 samples revealed excellent correlation of results (r = 0.98, intercept -0.2 cells/microl, slope 1.01). Precision studies conveyed coefficients of variation of 6.4 and 8.9% at concentrations of 35 and 16 CD34+ cells/microl, respectively. In untreated and leukocyte-enriched cord blood 4.5+/-3.8% of CD34+ cells were stained by YO-PRO-1, representing apoptotic or necrotic cells. In post-thawing cryopreserved samples this number increased to 10.4+/-5.5%. Isotype controls showed very low blank values of viable cells (0.1+/-0.4 cells/microl, maximum 2.4) and seemed unnecessary. We found no washing-related alteration of results in 35 samples, indicating that the method may also be performed with cell washing. Replacing YO-PRO-1 with TO-PRO-3 facilitated four-color analysis of subpopulations of viable CD34+ cells on a FACSCalibur equipped with a second (diode) laser. We found the proposed method to be a rapid, efficient, and flexible procedure that improved validity of CD34+ cell counts.

Nitric oxide abundance in the mesosphere/lower thermosphere region: Roles of solar soft X rays, suprathermal N(4S) atoms, and vertical transport

This paper carefully examines the inability of photochemical models to account for the large nitric oxide densities of ∼108 cm−3 at ∼105 km obtained from IR, UV, and microwave measurements. A detailed and up‐to‐date photochemical model is constructed that incorporates measured YOHKOH soft X ray fluxes, hot N atom chemistry with an energy dependent thermalization cross section and seven reaction sources, and laboratory‐constrained N(2D) yields. The resulting model which has well‐constrained chemistry compared to past models fails to generate high enough NO densities in comparison with the most reliable measurements of absolute NO concentrations in the lower thermosphere. The sensitivity of the model results and the known uncertainties in the inputs are used to identify where future efforts should be focused. A deficit remains despite an increase in the vertical mixing rates in the lower thermosphere from the very low Kzz profile used in our calculations and/or an increase in the N(2D) yield from electron impact dissociation of N2 from its nominal value of 0.54 to 0.62. The sensitivity of NO profiles to the nascent energy distributions of the atmospheric sources of suprathermal N atoms is illustrated by including the thermalization of suprathermal N atoms with an updated thermalization cross section. The diurnally averaged NO concentration at 105 km is enhanced by factors of 1.2 and 2.6 when the energy distributions of the N atoms from electron impact dissociation of N2 are chosen with peaks near 0.6 eV or 3–4 eV, but deficits of factors of ∼7 and ∼3, respectively, remain. There is higher sensitivity to vertical transport than to variations of chemistry within known uncertainties.

Absolute concentration, temperature, and velocity measurements in a diamond depositing dc-arcjet reactor

Spatial distributions of the absolute concentration of H, CH, C 2 , and C 3 reactive intermediate species have been measured by laser-induced fluorescence (LIF) in the freestream plume of a dc-arcjet. Diamond film grows from this reactive mixture on a water-cooled molybdenum substrate in a 25 Torr reactor. The temperature of the gas jet is determined from LIF measurements of the rotational distribution of CH radicals in the plume. The directed velocity of the gas plume is derived from the Doppler shift of LIF. The fraction of the feedstock hydrogen dissociated is obtained from calorimetry. The atomic hydrogen concentration and gas temperature are used to constrain a model of the gas phase chemistry in the reactive plume. Concentration measurements of CH, C 2 , and C 3 provide a stringent test for the model of the diamond precursor chemistry.

Absolute radical concentration measurements and modeling of low-pressure CH 4/O 2/NO flames

An experimental and theoretical investigation of CH and CN radical formation and destruction in a low-pressure 13.3-hPa (10 Torr) premixed stoichiometric CH 4 /O 2 flame seeded with NO is presented. Relative concentration profiles of CH and CN are measured by linear unsaturated laser-induced fluorescence (LIF). An absolute calibration of the relative profiles is obtained by Rayleigh scattering. A computational study is performed to identify key uncertainties in the formation and destruction chemistry of the CH and CN radicals. It is shown that the reaction of the CH radical with molecular oxygen is of particular importance in the present flame. Prevailing uncertainties in the reactions of 3 CH 2 with hydrogen atoms and molecular oxygen are also discussed. The present quantitative measurements of the CN radical also indicate that further attention should be given to the formation and oxidation chemistry of HCN. Nevertheless, computational results are encouraging and reasonable agreement is obtained for both the CH and CN radicals. It is further shown that the effects on CH concentration levels of introducing NO dopants may be reproduced. Comparisons of absolute concentration profiles of CH 3 , OH, and CH radicals as well as NO are also made with computed results obtained using GRI Mech. 2.11 and a reaction mechanism developed by Warnatz. The computations highlight significant differences in reaction paths and rate selection. The major areas of uncertainty are outlined and tentative recommendations are made in relation to the key reaction paths.

Organic geochemical studies of soils from the Rothamsted Classical Experiments—II, Soils from the Hoosfield Spring Barley Experiment treated with different quantities of manure

Total lipid extracts (TLEs) were obtained from soil samples taken in the years 1882, 1913, 1946, 1965 and 1995 from three treatments of the Hoosfield Spring Barley Experiment at Rothamsted Experimental Station, Harpenden, Hertfordshire, U.K. The extracts were fractionated and molecular analyses performed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC/MS). In addition to the soil samples (contemporary and archived), the two primary organic inputs, barley and farmyard manure (FYM), were studied so that the composition and diagenetic behaviour of extractable lipids from the two inputs could be assessed. The major aliphatic soil lipids exhibited variable dominance with respect to the expression of barley and FYM derived lipids. Wax esters were of low abundance and too strongly affected by degradation and transesterification processes to identify a dominant input whilst the composition of soil n-alkanols was largely determined by FYM with a minor pedogenic input. n-Alkanoic acids increased in overall abundance in soils with a continual FYM input and showed appreciable degradation in soils receiving no manure. C 32 ββ hopanoic acid was detected in two plots and appeared to degrade at a rate similar to 5 β-stanols with the most likely source of this compound being the FYM. Measurements of absolute concentrations of 5 β-stanols, biomarkers characteristic of manuring, revealed that a manuring signal persisted for >120 years within the soil which had been intensively cultivated annually and had received no manure since 1871. The persistence of a manure signal in soils has important implications for archaeological studies of agricultural practices based on 5 β-stanols.

Elastic recoil detection analysis (ERDA) of thick films

Elastic Recoil Detection Analysis (ERDA) with primary ion beams having energies in the MeV/nucleon range allows the measurement of absolute concentrations and depth profiles especially of low Z elements in the surface layers of a solid. In the present work, we use simple mathematical relations for defining the optimal measurement geometry with respect to measuring large depths with high intensities. Results obtained by the straightforward “surface approximation” are shown and compared to calculations with a more complete model. A parameter discussion is given examining optimal geometries for different experimental set-ups. When using detectors sensitive to very low energies in order to be able to analyse large depths, the intensity loss due to the typically smaller solid angles of these detectors can be compensated by the higher intensity of the recoils in the corresponding geometry.

A new method for absolute quantitation of MRS metabolites.

A new method for absolute quantitation of MRS spectra is presented. This method is not based on a reference peak, derived from a real NMR signal, but rather on a synthesized NMR reference produced by an electronic device, transmitted by a broad-band antenna to avoid quality factor variations. This signal is therefore received at the same time as the sample signal. The reference line produced is stable in time (maximum variation lower than 2%) and allows precise and accurate measurement of absolute concentrations (mean error lower than 3%) in vitro and in vivo.

Absolute concentration measurements of CH radicals in a diamond-depositing dc-arcjet reactor

Laser-induced fluorescence in the CH (B - X) and CH (A - X) electronic transitions is used to measure absolute number density versus position for CH radicals in the plume of a 25-Torr hydrogen/argon/methane (0.8:1:0.005) dc arcjet during the chemical vapor deposition of diamond film. The laser-induced-fluorescence signal is calibrated with argon Rayleigh scattering, and the resultant concentration of the CH radical in the center of the arcjet plume is found to be (3.5 +/- 0.8) x 10(12) molecules/cm(3). The characterization of the plasma plume shows three different regions in the reacting gas: nozzle, plume, and boundary layer. We observe substantial differences in spatial distribution of gas temperature, collisional quenching, and CH number density among these regions.

Spatially resolved measurements of absolute CH3 concentration in a hot-filament reactor

Abstract Methyl radicals are generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (20 mm long) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 is measured during deposition using a line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements are carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross section has been shown, by others, to be independent of temperature over a large range. We observe a strong sensitivity of the methyl radical concentration throughout the reactor to the substrate temperature. At some operating conditions, we also observe the methyl radical concentration to peak at a location several millimeters from the filament surface. This behavior of CH 3 with distance from filament is in qualitative agreement with two-dimensional models of the deposition environment, and is attributed to the effect of Soret diffusion on the balance of the primary methyl production/destruction reaction.

Measurement of absolute cerebral haemoglobin concentration in adults and neonates.

The concentration and saturation of the main oxygen carrier of the blood—the haemoglobin—may be an important clinical factor to judge the oxygenation of tissue. The brain is very sensitive to under-oxygenation. So far, the measurement of absolute cerebral haemoglobin concentration by near infrared spectrophotometry (NIRS) has not taken into account the contribution of skull and skin, which causes an underestimation by a factor of 3 [Owen-Reece, 1996] in adults.

SLCP—The scanning diffusion limited current probe: A new method for spatially resolved analysis

A new analytical method of measuring absolute concentration profiles inside microstructures called SLCP is presented. The combination of chronoamperometry at microelectrodes of 15–20 μm with scanning and positioning techniques (precision in x, y, z: 1 μm) sets up this SXM/SXP † † Scanning X-Microscopy and Scanning X-Probe, where X denotes the measuring principle. -type method. The SLCP method is capable of ion selective and localized absolute concentration measurement. It is applicable to microstructures with high aspect ratios. The measured parameter is the diffusion limited current I lim which is directly proportional to concentration. It is recorded at a substance specific potential. The principle works with all metal ions and electroactive substances that can be deposited or consumed almost quantitatively and under diffusional control. By using potential/time programs with two deposition potentials, the simultaneous concentration measurement of two ion species is possible. The multiple pulse sequence is successfully applied to alloy deposition. Three experimental modes have been developed; in addition to local concentration measurement, local cyclic voltammograms can be measured as well. Local resolution is 10–20 μm. Very thin microdisk analysis electrodes of 15–20 μm tip diameter have been developed.

Absolute concentration measurements by pulsed laser-induced fluorescence in low-pressure gases: allowing for saturation effects

We present a simple method to account for the inevitable partial saturation of the excitation process in pulsed laser-induced fluorescence. The method allows the absolute concentration of a transitory species in a low-pressure gas to be determined by comparing the signal with that from a calibrating (stable) species of known concentration: the two species are excited with different laser powers, in the inverse ratio of their transition strenghts. The validity of the method is justified both theoretically and experimentally, and is applied to the measurement of CF radicals in a fluorocarbon discharge.

Fluorescence-optical measurements of chloride movements in cells using the membrane-permeable dye diH-MEQ.

Fluorescence-optical measurements of the intracellular chloride concentration facilitate identification of chloride movements across the cell membrane of living cells. The two main dyes used for this purpose are 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-quinolyl acetoethyl ester (MQAE). The use of both substances is impaired by their poor membrane permeability and therefore limited loading of the cells to be studied. Here we report the use of 6-methoxy-N-ethylquinolinium iodide (MEQ), a chloride-sensitive dye for which a membrane-permeable form is easily prepared. This makes the loading procedure as easy as with the acetoxymethyl (AM) forms of other dyes for sensing intracellular ions. In addition, the original method, which described absolute concentration measurements of chloride in the cytosol, was modified in so far as only relative measurements were made. This avoids the known limitations of single wavelength excitation and emission dyes with respect to exact concentration measurements. Moreover, to enhance the signal-to-noise ratio the driving force for chloride was considerably increased by changing the original direction of the anion flux in the cells under investigation. We verified the method by using fibroblasts and activating ICln, a putative chloride channel cloned from epithelial cells and of paramount importance in the regulatory volume decrease in these cells. In the presence of SCN- the MEQ quench measured in NIH 3T3 fibroblasts is dramatically enhanced in hypotonically challenged cells compared with cells under isotonic conditions. Antisense oligodeoxynucleotides sensing ICln considerably impeded the swelling-induced chloride current (ICl) in NIH 3T3 fibroblasts. Accordingly, the chloride movement measured by the SCN- quench of the MEQ signal was significantly reduced. Similar results can be obtained in the presence of 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) or 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), two known blockers of chloride transport in the plasma membrane of a variety of cells. In conclusion, fluroscence-optical measurements using MEQ as the chloride-sensitive dye provide a reliable and easy-to-use method for measuring changes of the chloride flux across the cell membrane of living cells.

Measurement of absolute CH3 concentration in a hot-filament reactor using cavity ring-down spectroscopy

Methyl radicals were generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH4 in H2. The UV absorbance of CH3 was measured during deposition using a new line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements were carried out at 213.9 nm, a wavelength at which the CH3 absorption cross-section has been shown by others to be independent of the temperature over a large range. We observed that the CH3 absolute concentration varied strongly as a function of the position between the substrate and the filament, and its value was strongly influenced by the substrate temperature.

Absolute radionuclide concentration measurement using maximum-likelihood expectation-maximization iterative reconstruction, attenuation, and scatter correction

The aim of this study was to evaluate the accuracy with which radionuclide concentration could be measured after implementation of the channel ratio (CR) scatter correction method and incorporation of transmission attenuation coefficients into a maximum-likelihood expectation-maximization iterative reconstruction algorithm regularize using a multinomial prior. A water-filled thorax phantom containing a liver insert and a variable spleen volume was used to simulate different clinical situations. An uncollimated Co-57 sheet source was used to obtain attenuation matrices. All emission data were acquired in two 10% energy windows straddling the photopeak. Planar and SPECT sensitivities were determined. After scatter correction was performed data were first reconstructed using the measured attenuation matrices, and, second, using the good geometry attenuation coefficient for water. Radionuclide concentration with the attenuation matrix using 64 projections varied between 48.9/spl plusmn/3.1% (49.6/spl plusmn/3.1%) and 76.5/spl plusmn/3.0% (76.5/spl plusmn/3.2%) when 25 and (50) iterations were used. Similar results were obtained using 128 projections, and no statistical difference could be found (p<0.05). The inaccuracy of the results obtained with the implementation of the attenuation matrix from the transmission tomogram is due to the effective attenuation coefficients used in conjunction with the scatter compensation method. Results obtained with the attenuation coefficient of water varied between 70.1/spl plusmn/3.1% (70.8/spl plusmn/3.0%) and 103.2/spl plusmn/3.5% (103.3/spl plusmn/3.4%). The influence of volume and concentration is clearly demonstrated. Edge detection plays an important role in the accuracy of concentration calculations.

The noninvasive measurement of absolute cerebral deoxyhemoglobin concentration and mean optical path length in the neonatal brain by second derivative near infrared spectroscopy.

We have used second differential near infrared spectroscopy of water to determine the mean optical path length of the neonatal brain. By obtaining the ratio of the second differential features of deoxyhemoglobin to those of water, the absolute cerebral concentration of deoxyhemoglobin can be monitored continuously and noninvasively. Nineteen neonates were studied; the gestational age at birth varied from 23 to 38 wk, and the postconceptual age, when the spectra were recorded, ranged from 35 to 48 wk. The calculated mean deoxyhemoglobin concentration was 14.6 +/- 4.0 microM; the differential path length factor (mean optical path length/optode separation) calculated from the water peak at 730 nm was 4.66 +/- 1.01, and that calculated at the 830-nm peak was 3.91 +/- 0.75. These values are consistent with path length measurements using laser time-of-flight spectroscopy on postmortem neonates and phase-resolved spectroscopy on live neonates. Induced arterial oxygen saturation decreases from 98 to 93% showed no significant change in the mean optical path length, despite significant cerebral desaturation. Changes in the deoxyhemoglobin concentration after this procedure were identical, whether measured by second differential analysis at 760 nm or by multilinear regression over the wavelength range 740-900 nm. When combined with existing methods of measuring total cerebral hemoglobin concentration, second differential near infrared spectroscopy can be used to derive the mean cerebral oxygen saturation. A preliminary experiment outlined the feasibility of this approach and yielded a saturation value of 63%, consistent with near infrared sampling of a predominantly venous pool in the brain.

Absolute concentration measurements inside a jet plume using video digitization

An experimental system based on digitized video image analysis is used to measure the local value of the concentration inside a plume. Experiments are carried out in a wind-tunnel for a smoke-seeded turbulent jet plume illuminated with a laser beam. Each test is filmed, subsequently video images are digitized and analysed in order to determine the smoke absolute concentration corresponding to each pixel gray level. This non-intrusive measurement technique is first calibrated and different laws connecting gray level to concentration are established. As a first application, concentration measurements are made inside a turbulent jet plume and compared with measurements conducted using a classic gas analysis method. We finally present and discuss the possibilities offered for the measurements of absolute concentration fluctuations.

Absolute concentration measurements of C2 in a diamond CVD reactor by laser-induced fluorescence

By use of Laser-Induced Fluorescence (LIF) the absolute concentration of the C2 radical in a microwave excited diamond chemical vapour deposition plasma has been measured for the first time. LIF spectra of thed3Πg–a3Πu (1,0) Swan band near 473 nm were recorded and synthesized theoretically allowing the plasma temperature of 2100 ± 200 K to be inferred. Quenching rates were determined from time-resolved measurements of the fluorescence decay. By calibrating the LIF detection system, using spontaneous Raman scattering in H2 in the reactor vessel, the absolute concentration of C2 was determined to be (7.5 ± 1.7) × 1010 cm−3. Observations of the C2 density under varying plasma conditions are reported.

Double pulse technique for the evaluation of the saturation parameter in single-shot laser induced fluorescence measurements: an application to lead in a flame

The enlargement of the probe volume in saturated laser induced fluorescence measurement is a considerable drawback for absolute concentration measurements. For practical purposes, a double pulse technique was developed allowing determination of the saturation parameter, which has a major influence on the probe volume enlargement. The technique is based on the detection of the fluorescence produced by two time-delayed laser pulses of different intensity. The basic theory of the technique is presented, together with the results of an experimental application to Pb in an acetylene/air flame. The implications for absolute concentration measurements are also discussed.