Instrumental Time Constant Research Articles

Catalytic reduction of polar substrates without metals: A thermodynamic and kinetic study of heterolytic activation of hydrogen by vacancies in frustrated Lewis pairs

Time-resolved reaction calorimetry provides a measure of the turnover frequency (TOF), ca. 1.1min−1, and enthalpic driving force, ca. −73kJ/mol, for the metal-free catalytic reduction of an organic imine, tert-butylbenzaldimine, (tBu-IM) with a frustrated Lewis acid-base pair, [2-(dimesitylphosphino)ethyl] bis(pentafluorophenyl)borane (PBCat), at 298K and 13.8bar hydrogen (H2) pressure. Lowering the H2 pressure by a factor of two decreases the TOF (0.6min−1), which is consistent with a pseudo first-order reaction in H2. In the absence of imine, the heat flux measured in the calorimeter provides a measure of the enthalpy for heterolytic splitting of H2, PBcat+H2 --> PBCatH2, ΔH ca. −43(4)kJ/mol. Solution phase 19F nuclear magnetic resonance spectroscopy was used to determine the rate of heterolytic splitting of H2 by PBCat, k=0.7(.3)M−1s−1 and the equilibrium constant for PBCat+H2(soln) ⇔ PBCatH2, Keq(295)=2.2(.5)×105M, providing an estimate of the free energy for heterolytic splitting of H2, ΔG ca. −29.8(1.3)kJ/mol at 295K in toluene. Deconvolution of the instrument time constant from the heat flux using the Tian equation shows the concentration of imine decreases linearly in time (i.e., the substrate imine is not involved in the rate limiting step, suggesting that H2 activation by the Lewis acid-base pair is rate limiting).

SU-E-T-66: Phase-Sensitive Microcalorimetry for Activity and Dosimetry of Low-Level Sources

Purpose: To study the effectiveness of phase-sensitive detection to improve the sensitivity of a microcalorimeter for activity measurements and the dosimetry of compact x-ray sources and brachytherapy seeds. Methods: A commercial isothermal microcalorimeter, originally customized for use as a primary-standard for activity measurements of radionuclides, has been coupled to a periodic Joule-heating source to investigate its thermal frequency response and thereby determine whether better sensitivity could be achieved via periodic modulation of radiation sources being measured. Frequency of input power to the sample cell was varied from 0.2 mHz to 2 mHz, and waveforms acquired over periods ranging from several hours to several days were analyzed both for noise content and attenuation due to intrinsic, instrumental time constants. Results: Power spectral density of the noise floor was found to decrease from a peak of ∼10 mW/sqrt(Hz) at dc to ∼5 nW/sqrt(Hz) at frequencies above 10 mHz. Attenuation of a 400-nW periodic heat input was found to vary from 1 at 0.2 mHz to 0.1 at 2 mHz; at 0.25 mHz, S/N was found to be ∼50:1 after 32 hours of acquisition. Conclusion: When operated as it was originally designed, without modulation, the instrument has been used to measure radioactivity from sources with power output of order 100 mW, yielding a precision of 0.1% over 2–3 days of continuous acquisition; thus, by working at 0.25 mHz, where the noise floor is much lower, we have realized ∼50x higher sensitivity. A further reduction of the noise floor at frequencies above 10 mHz suggests that pW-level sensitivity is achievable if the attenuation can be overcome (by comparison, LDR brachytherapy seeds emit 10–100 mW). Work is underway to eliminate attenuation from inefficient coupling of heating resistors to absorber material and to develop an actuator for controlled movement of sources to/from the sample cell.

Abrupt changes in neon discharge plasma detected via the optogalvanic effect

When a laser is tuned between two excited energy levels of a gas in a Direct Current discharge lamp, the discharge current will experience a temporary disturbance lasting tens or hundreds of microseconds known as the optogalvanic effect. We have carried out extensive studies of optogalvanic effects in neon discharge plasmas for transitions at 621.7nm, 630.5nm, 638.3nm, 650.7nm and 659.9nm. A nonlinear least-squares Monte Carlo technique has been used to determine the relevant amplitude coefficients, decay rates and the instrumental time constant. We discovered an abrupt change in the neon discharge plasma at a discharge current of about 6mA.

Correction for instrument time constant and baseline in determination of reaction kinetics

Rates of reactions can be expressed as dn/dt = kf(n), where n is moles of reaction, k is a rate constant, and f(n) is a function of the properties of the sample. Instrumental measurement of rates requires c(dn/dt) = ckf(n), where c is the proportionality constant between the measured variable and the rate of reaction. When the product of instrument time constant, τ, and k is ≪ 1, the reaction is much slower than the time response of the instrument and measured rates are unaffected by instrument response. When τ k 1, the reaction rate and instrument response rate are sufficiently comparable that measured rates are significantly affected by instrument response and correction for instrument response must be done to obtain accurate reaction kinetics. This paper describes a method for simultaneous determination of τ, k, c, and instrument baseline by fitting equations describing the combined instrument response and rate law to rates observed as a function of time. When τ cannot be neglected, correction for instrument response has previously been done by truncating early data or by use of the Tian equation. Both methods can lead to significant errors that increase as τk increases. Inclusion of instrument baseline as a fitting parameter significantly reduced variability in k and c compared with use of measured instrument baselines. The method was tested with data on the heat rate from acid-catalyzed hydrolysis of sucrose collected with three types of calorimeters. In addition, to demonstrate the generality of this method of data analysis, equations including τ, k, c, and instrument baseline are derived for the relation between the reaction rate and the observed rate for first order, second order (first in each reactant), nth order in one reactant, autocatalytic, Michaelis–Menten kinetics, and the Ng equation. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 53–61, 2011

A calorimetric system based on the LKB 10700-1 flow microcalorimeter

An electrically calibrated and computer-controlled apparatus based on the LKB 10700-1 flow microcalorimeter was set up and tested by determining the enthalpies of dilution of aqueous solutions of sucrose and HCl at 298.15 K. The obtained results are in good agreement with the corresponding reference values recommended in the literature. The quality of the calorimetric signal in terms of noise, linearity and baseline drift, and the instrument time constant were analyzed. Also investigated were the influences of the flow rate and of the viscosity of the calorimetric liquid on the position of the baseline. The new calibration system enables powers ∼25 times smaller than those possible with the original one to be released inside the calorimetric cells. Electrical calibrations carried out with water as the sole calorimetric liquid indicated that the precision of the calibration constant, ε, quickly improved from ∼9% to ∼0.4% with the increase of the input power, P, from ∼2 µW to ∼50 µW, and stabilized at ∼0.1% for P ⩾ 100 µW. The calibration constant was also found to linearly increase with the flow rate of the calorimetric liquid and to vary by less than 1% over a period of eight months under similar experimental conditions. Finally a new experimental procedure for heat of dilution measurements, which combines calibration and experiment in a single run, was evaluated. This method allows a significant economy in experiment time with apparently no accuracy loss relative to the conventional procedure recommended in the literature.

Laser optogalvanic spectroscopy of neon at 659.9 nm in a discharge plasma and nonlinear least-squares fitting of associated waveforms

Laser optogalvanic spectroscopy of neon has been studied in the discharge environment of a hollow cathode discharge lamp. We illustrate the fitting of the time-resolved optogalvanic waveforms by focusing on the 1s 2–2p 2 transition of neon at 659.9 nm and employ a nonlinear least-squares Monte Carlo approach to determine the pertinent amplitude coefficients, decay rates and the instrumental time constant.

On the Time‐Resolved Optogalvanic Spectra of Neon and Krypton

AbstractIn this work time resolved optogalvanic signals associated with transitions excited from the first metastable state of neon and krypton have been studied. These gases have similar energy state configurations and it is of significant interest to study their time resolved optogalvanic waveforms resulting from transitions belonging to the states of same quantum numbers. The experimentally observed optogalvanic signals recorded for different discharge currents have been fitted to a theoretical model to obtain parameters that determine amplitudes, instrumental time constants and decay rates of the 1s levels. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photovoltaic Schottky ultraviolet detectors fabricated on boron-doped homoepitaxial diamond layer

A photovoltaic diode using tungsten carbide (WC) Schottky contact for deep ultraviolet (DUV) light detection is developed using a lightly boron-doped homoepitaxial diamond layer. The photodiode shows a quick dc response time lower than an instrumental time constant of 0.3 s. The responsivity varies linearly with the DUV light intensity. A discrimination ratio of 105 between 210 nm and visible light is achieved at zero bias. The short-circuit photocurrent at 220 nm decreases with increasing the temperature from 300 to 373 K, whereas the Schottky barrier height under illumination remains unchanged. Numerical and experimental analyses reveal that the interface states between diamond and WC play a major role in reducing the zero-bias responsivity at elevated temperatures.

Extreme ultraviolet single-crystal diamond detectors by chemical vapor deposition

High-quality single-crystal diamond films, homoepitaxially grown by microwave chemical vapor deposition, have been used to produce diamond-based photodetectors. Such devices were tested over a very wide spectral range, from the extreme ultraviolet (UV) (20 nm) up to the near IR region (2400 nm). An optical parametric oscillator tunable laser was used to investigate the 210–2400 nm spectral range in pulse mode. In this region, the spectral response shows a UV to visible contrast of about 6 orders of magnitude. A time response shorter than 5 ns, i.e., the laser pulse duration, was observed. By integrating the pulse shape, a minor slow component was evidenced, which can be explained in terms of trapping–detrapping effects. Extreme UV gas sources and a toroidal grating vacuum monochromator were used to measure the device response down to 20 nm in continuous mode. In particular, the extreme UV He spectrum was measured and the He II m, 30.4 nmand He I 58.4 nm emission lines were clearly detected. The measured time response of 0.2 s is totally due to the instrumental readout time constants. In both experimental setups an extremely good stability and reproducibility of the device response were obtained, whereas no persistent photoconductivity nor undesirable pumping effects were observed.

Electrostatics of quantum dots in high magnetic fields and single far-infrared photon detection

Electron transport through a single electron transistor (SET) is studied with and without illumination of far-infrared (FIR) radiation in high magnetic fields. The SET consists of a $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dot (QD). The transport characteristics obtained without the FIR illumination is well analyzed in terms of capacitance matrix by assuming that the QD in strong magnetic fields is split into isolated conductive regions. When a FIR photon is absorbed by a QD upon cyclotron resonance, an excited electron-hole pair induces a charge polarization within the QD, which switches on or off the SET conductance. The absorption of single-FIR photons is thus detected as individual conductance switches of the SET. Experimental results show that the lifetime of the excited state of a QD (with the internal polarization) is longer than the instrumental time constant, 1 ms, in a magnetic field range of $B=3.4\ensuremath{-}4.2\mathrm{T},$ in which the lowest orbital Landau levels are completely occupied while the higher Landau level with a small number of electrons is slightly occupied. The wavelength of the FIR-photon detection, being determined by the magnetic field applied to the QD, ranges from 0.2 mm to 0.17 mm.

Design and initial characterization of an inlet for gas‐phase NOy measurements from aircraft

An understanding of gas‐phase HNO3 transmission through an inlet is necessary to evaluate the quality of NOy measurements from an aircraft platform. A simple, inexpensive, low‐volume Teflon inlet is described and its suitability as an aircraft inlet for gas‐phase NOy is assessed. Aerosol transmission is not characterized, but inlet design and orientation probably discriminates against the majority of aerosol by mass. Laboratory data, in‐flight HNO3 standard addition calibrations, and ambient NOy measurements from the 1997 North Atlantic Regional Experiment aircraft mission are used to characterize inlet transmission efficiencies and time constants. Laboratory tests show high transmission efficiencies for HNO3 which are relatively independent of ambient temperature and humidity. In‐flight standard addition calibrations were carried out at ambient temperatures ranging from −20° to +8°C and relative humidities from 3% to 71%. These data suggest that nearly all the sampled air contacts an inlet surface, with 90% of added HNO3 being transmitted in ∼1.5 s. Ambient data are presented to demonstrate negligible hysteresis in 1‐Hz NOy measurements, relative to variability observed in ozone data, from an air mass where HNO3 is expected to be a large fraction of the total NOy. Power spectra of ambient NOy (at temperatures from −35° to +35°C and relative humidities from 3% to 100%) and ozone measurements suggest an effective NOy instrument time constant of ∼2 s.

The Perkin-Elmer 1020 series thermal analysis system

The performance of a Perkin-Elmer 1020 Series DSC7 Thermal Analyser has been tested between 20 and 700°C with respect to isothermal and dynamic baseline stabilities, sample and instrumental time constants, thermal resistances, and measurement of temperatures, heats and heat capacities. The system includes software which meets the demands of industry, but is unsatisfactory for research. The arrangement of the thermoelectric coolers enables thermal equilibrium to be achieved in a short time, minimizes the isothermal drift, and makes it possible to start programming from subambient temperatures.

Thermal time constants in differential scanning calorimetry

We have examined thermal gradients in a differential scanning calorimeter. We quantified, as a function of sample heat capacity, the thermal lag, Δ T between sample and calorimeter. We found that Δ T = β(τ x + R 0 C s)+ Δ T c; where β is the heating rate, Δ T c is a constant, C s is the total heat capacity of the sample and pan, R 0 is the thermal resistance between sample and calorimeter, and τ x is an instrumental time constant. We observed that the value of Δ T c depended on both the sample size and the thermal resistance between sample and calorimeter. Analysis of changes in Δ T c with changes in R 0 led to an order of magnitude estimate of 3 mcal s −1 for the rate of heat flow between calorimeter and sample when the calorimeter is held at a constant temperature near 500 K. We discuss the effect of this heat flux on the accuracy of the temperature scale.

The dynamic response of instruments measuring instantaneous solar radiation

Conventional thermal pyranometers and pyheliometers used to measure solar radiation are unable to follow rapid changes of radiation associated with clear/cloudy transitions during partly cloudy conditions. As a consequence, significant errors can occur in the measurement of individual instantaneous radiation values. A simple model for the dynamic response of thermal radiation instruments is developed. The model suggests that the readings of a thermal radiometer can be corrected using the rate of change of the indicated reading and the instrument time constant. It is shown experimentally that the error correction applied to the signal of a thermal pyranometer yields almost identical results as a photovoltaic pyranometer. The error correction allows conventional thermal radiation instruments to be used for instantaneous radiation measurements.

Continuous gradient method in flow microcalorimetry: Part I. relation between electrical and chemical calibration

The application of the continuous gradient techniques to flow microcalorimetry, requires the introduction of corrections for the thermal lag due to the high instrumental time constant of heat leakage calorimeters. To this purpose, the system must be properly modeled, with careful consideration of dynamic corrections. Therefore, the calorimetric response was analysed both for electrical and chemical input and Tian's equation parameters were evaluated, when applied to flow microcalorimetry. It is demonstrated how it is possible to calculate the power emitted by a given process, in a steady-state experiment, independently from the electrical calibration, taking into account the residual heat flowing out with the liquid. Application to the protonation enthalpies of d-cycloserine is shown.

Improved Orbit Estimation by Satellite-Mounted Gravity Gradiometers

The feasibility of the use of gravity gradiometers mounted on an Earth satellite in order to improve the estimation of its orbit is studied. Only the high-frequency orbit residuals that are caused by the uncertainty in our knowledge of the Earth's field are considered. Using the principle of minimum variance filtering, critical values of the variance of the mean instrument error, o$T/kT, are calculated at different altitudes under the assumption that harmonics beyond the 8 x 8 field are unknown (af = instrument error variance, T = instrument time constant, T- satellite period, kT- duration of filter window). It is found that instruments presently in development may lead to significantly improved ephemerides, especially at low altitudes.

Bacteriorhodopsin in model membranes. A new component of the displacement photocurrent in the microsecond time scale

A quasi-short-circuit (tunable voltage clamp) measurement method with microsecond time resolution was applied to a bacteriorhodopsin model membrane formed by a novel interfacial technique. A new component (B1) of the displacement photocurrent was recorded: it has no detectable latency at an instrumental time constant of 1.5 museconds, and persists at 5 degrees C. In addition, a slower component (B2) of opposite polarity inhibited by low temperature (5 degrees C) and low pH (pH = 3.0) was recorded. The technique is very sensitive for the study of fast capacitative photoresponses in model membranes, and allows the detection of charge displacements in bacteriorhodopsin associated with distinct stages of the photochemical transformation.

A technical and clinical evaluation of the Disa uroflowmeter.

This investigation reveals that there is a marked reduction in the rotational speed of the transducer disc as the stream flowrate is increased, although the non-linearity is partly reduced by electronic compensation within the instrument. The inaccuracy at low flowrates is probably unimportant in routine clinical applications. However, significant errors are recorded at high flowrates when the stream strikes the grid intersections or when the point of impact is close to the edge of the rotating disc. For these reasons, it is desirable that the patient should avoid micturating directly on to the grid. To obtain acceptable clinical recordings, an instrument time constant of 0-5 sec must be used. Although some characteristic tracings can be identified, the clinical measurements indicate that the effects of the volume voided and stream movement must be taken into consideration when the flow patterns are being interpreted.

Precautions in the measurement of tritium concentrations in air when using flow-through ion chambers

Instruments with flow-through ion chambers are frequently used to measure the concentration of tritium gas in air. However, these simple instruments can give erroneous results because of: cigarette smoke, aerosols, ions, ambient gamma-ray field changes, moisture and condensation, absorption and memory effects, clogged filters, fission product gases, radon, and incorrect instrument calibration. The causes of these errors are explained and chart recordings are presented showing some of them in action. Most problems can be avoided using the proper instrument design and safeguards described and by adequate calibration techniques. A tritium monitor should be checked and calibrated with tritium gas, not gamma-rays or ions. The most accurate calibration is obtained when the instrument operates in a closed loop so that the tritium concentration is constant, a steady meter reading being obtained independent of air flow rate and instrumental time constants.

Regularly spaced instrumental temperature and salinity structures

It is shown that the combined effects of ship-roll, lowering speed and instrumental time constants are important considerations when interpreting small-scale temperature and salinity data. A criterion is established that may help decide which features are real and which are instrumental in oceanographic situations.