Calibration of laser-saturated fluorescence measurements using Rayleigh scattering

Calibration of laser-saturated fluorescence measurements using Rayleigh scattering is presented as an alternative to absorption. This new procedure is advantageous when measuring radical species at concentrations well below the corresponding detection limit for absorption. The calibration accounts for nonuniform laser irradiation by extracting the local fluorescence emission along the laser axis and works equally well for both saturated and near-saturated center-line conditions. The predicted error due to misfocusing of the collection optics is nearly negligible when the measured fluorescence is within 10% of its peak value. Number densities obtained using this method are within 15% of those obtained from absorption measurements.     

Single-shot laser-saturated fluorescence measurements: a new method

A new method of laser-saturated fluorescence shot-to-shot concentration measurement is proposed. The method is based on use of the wing effect of a nonuniform laser irradiance. The measurement is independent of both the collisional transfer rate and variation in laser power. If the laser power is known it is also a method of measurement of the collisional transfer rate at any pressure.     

Calibration of probe volume in fluorescence correlation spectroscopy

In fluorescence correlation spectroscopy (FCS), an accurate evaluation of the probe volume is the basis of correct interpretation of experimental data and solution of an appropriate diffusion model. Poor fitting convergence has been a problem in the determination of the dimensional parameters, the beam radius, omega, and the distance along the optical axis of the probe volume, l. In this work, the instability of fitting during the calibration process is investigated by examining the chi(2) surfaces. We demonstrate that the minimum of chi(2) in the omega dimension is well defined for both converging and diverging data. The difficulty of fitting comes from the l dimension. The uncertainty in l could be significantly larger than that in omega, as determined by F-statistics. A modified calibration process is recommended based on examining two data treatment methods, combining several short data sets into a single long run and averaging the correlation functions of several short data sets. It is found that by using the mean of several converging correlation functions from short data sets instead of a long time correlation, more stable and consistent dimensional parameters are extracted to define the probe volume.     

Comparisons of laser-saturated, laser-induced, and planar laser-induced fluorescence measurements of nitric oxide in a lean direct-injection spray flame

We report quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF), and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) concentration in a preheated, lean direct-injection spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane, and the overall equivalence ratio is unity. NO is excited by means of the Q(2)(26.5) transition of the gamma(0, 0) band. LSF and LIF detection are performed in a 2-nm region centered on the gamma(0, 1) band. PLIF detection is performed in a broad ~70-nm region with a peak transmission at 270 nm. Quantitative radial NO profiles obtained by LSF are presented and analyzed so as to correct similar LIF and PLIF profiles. Excellent agreement is achieved among the three fluorescence methodologies.     

Oxygen plasma flow properties deduced from laser-induced fluorescence and probe measurements

Estimation of the local dissociation degree and the local mass-specific enthalpy of a pure oxygen plasma flow determined mainly from laser-induced fluorescence measurements are reported. Measurements have been conducted for several generator parameters in an inductively heated plasma wind tunnel. Additional probe measurements of total pressure together with the deduced translational temperature are used to estimate the local mass-specific enthalpy. For a reference condition, full dissociation has been measured. The measured translational temperature of atomic oxygen for this condition is T = 3500 K. Subsequently, the local mass-specific enthalpy has been derived using these local density and temperature measurements. For the reference condition the estimated value of h = 27 MJ/kg is in good agreement with the probe measurements and results from diode laser absorption spectroscopy.     

Capacitance probe measurements of brine volume and bulk salinity in first-year sea ice

First-year sea ice plays an important role in the global climate system. It changes the physical properties of the surface of the polar oceans, and modifies the energy and mass transfer between the ocean and the atmosphere. An understanding of the way sea ice affects ocean–atmosphere exchange requires detailed knowledge of the evolution of ice physical properties, which are governed by its temperature and bulk salinity. To this effect, we assessed the utility of commercially available capacitance probes in determining the salinity evolution of first-year sea ice. Measurements of the complex dielectric permittivity, ε = ε′ − iε″, at 50 MHz were carried out in land-fast ice in McMurdo Sound, Antarctica, and in the Chukchi Sea near Barrow, Alaska. For comparison, we also deployed the probes in artificial, young sea ice in an outdoor tank experiment in Fairbanks, AK. The dielectric permittivity data compare well with predictions from a dielectric mixture model.We have derived a simple relation that allows for the derivation of brine volume fraction and bulk salinity in columnar first-year sea ice from the real part of the complex dielectric permittivity. For ice at temperatures below the percolation threshold, the error in the derived bulk salinity is less than 15%. The dependence of dielectric permittivity on brine inclusion morphology needs to be taken into consideration, and measurements indicate that changes in pore morphology are recorded in the capacitance measurements.In this paper we use the real part, ε′, of the complex dielectric permittivity to study the bulk salinity of bubble-free columnar ice. Further investigations, using the imaginary part of the complex dielectric permittivity, ε″, will make it possible to use the same probes to measure the bulk salinity and pore morphology of other types of ice, e.g., frazil, platelet, and multi-year ice.     

Automation of thermal flux measurements with a linear calorimetric probe

A system is developed to automate measurements of specific thermal flux in a plasma jet using a linear calorimetric probe. The effect of turbulence and gas flow rate on thermal flux is studied.Translated from Inzhenero-Fizicheskii Zhurnal, Vol. 41, No. 4, pp. 712–716, October, 1981.     

Fluorescence correlation spectroscopy: incorporation of probe volume effects into the three-dimensional Gaussian approximation

Fluorescence correlation spectroscopy is a valuable tool in many scientific disciplines. In particular, such a spectroscopic technique has received a great deal of attention because of its remarkable potential for single-molecule detection. It is understood, however, that quantitative measurements can be considered reliable as long as molecular photophysics has been well characterized. To that end, molecular saturation and probe volume effects, which can worsen experimental accuracy, are treated here. These phenomena are adequately incorporated into the well-known three-dimensional Gaussian approximation by a novel method applied to interpret saturated fluorescence signals [Opt. Lett. 28, 2016 (2003)]. Comparisons with literature data are given to show the improvements of the suggested method compared with other approaches.     

Plasma measurements in pulse discharge with resistively heated emissive probe

A resistively heated emissive probe has been developed to work in low-pressure air plasma produced by 100 Hz pulsed DC source. The evolution of the discharge and consequent rapid changes in plasma potential and electron temperature are characterized for different fill pressures at constant input voltage of 300 V. The floating point method in the strong emission regime is applied to determine the plasma potential. Emissive probe responds to rapid changes in the discharge current during different stages of the pulse cycle. The electron temperature is determined from the potential difference of hot probe in the strong emission regime and the cold one incorporating the space charge effects of the hot probe. Temporal measurements of V _p and T _e describe the development and characteristics of the emissive probe technique for fast measurements in pulsating discharges.     

Axial resolution limit of a fiber-optic fluorescence probe

We examine the limit of spatial resolution achievable when a sine optical fiber is used for excitation and collection of fluorescence from a bulk specimen. We calculate the probability of detecting a fluorescent particle as a function of its position relative to the fiber face, using excitation wavelength lambda, radius a, numerical aperture N.A., and the particle's fluorescence and absorbance spectra. Treating Rhodamine B as a model fluorescent analyte and using appropriate fiber parameters, we show that the maximum axial resolution (defined as the axial distance in a homogenous solution within which 50% of the detected signal originates) achievable is approximately 10 microm. We experimentally measured the axial resolution for a 500-microM aqueous solution of Rhodamine B with lambda = 543 nm, a = 1.31 microm, and a N.A. of 0.16 and found good qualitative agreement with the calculation.     

Reversible ratiometric probe for quantitative DNA measurements

We have designed a reversible fluorescent DNA probe that can be used to determine the concentration of single-stranded DNA in solution by a ratiometric fluorescence measurement. The probe consists of a single-stranded dual fluorescently labeled DNA molecule that adopts a stem-loop conformation in its nonhybridized state. The stem length and the length of the loop region complementary to the target were chosen to allow for reversible binding. The excitation and emission wavelengths of the two labels Cy3 and Cy5 allow for fluorescence resonance energy transfer in the closed state. Upon hybridization, the probe opens up resulting in a fluorescence intensity increase of the donor and a fluorescence intensity decrease of the acceptor. The ratio of the acceptor-to-donor fluorescence intensities is independent of the amount of probe and provides a quantitative measure of the free target concentration.     

Solgel transition in dye fluorescence measurements

The results of fluorescence quenching measurements, obtained for a dye-substituted aqueous gelatin solution during the sol-gel transition, are presented. Simultaneous optical polarization rotation and fluorescence studies allow us to find the dependence of the fluorescence yield on the amount of helix created in the medium. The exact gel point has been found as the point of inflection on the fluorescence yield versus the helix amount plot. The second-order character of the sol-gel transition is shown, which is in agreement with the theoretical predictions obtained by use of the percolation theory.     

Soot volume fraction and particle size measurements with laser-induced incandescence

Laser-induced incandescence from soot was analyzed with a time-dependent, numerical model of particle heating and cooling processes that includes spatial and temporal intensity profiles associated with laser sheet illumination. For volume fraction measurements, substantial errors result primarily from changes in gas temperature and primary soot particle size. The errors can be reduced with the proper choice of detection wavelength, prompt gating, and high laser intensities. Two techniques for primary particle size measurements, based on ratios of laser-induced incandescence signals from a single laser pulse, were also examined. Compared with the ratio of two integration times, the newly proposed ratio of two detection wavelengths is better suited for simultaneous volume fraction and size measurements, because it is less temperature sensitive and produces stronger signals with, however, a lower sensitivity to size changes.     

Theoretical treatment of fluorescence detection by a dual-fiber-optic sensor with consideration of sampling variability and package effects associated with particles

The characteristics of a dual-fiber-optic sensor for measurements of chlorophyll fluorescence in aquatic environments were evaluated with a theoretical model. Consideration was given to sampling variability and package effects associated with particles (e.g., phytoplankton cells). A numerical simulation was developed to approximate the optical geometry of the dual fiber-optic sensor that permitted a visual representation of the fluorescence distribution within the sensor sampling volume. A Monte Carlo simulation was used to evaluate sampling variability associated with the number and distribution of particles within the sampling volume. Relatively high coefficients of variation were associated with low particle concentrations, although with sufficient signal averaging the coefficient of variation was reduced to less than 20%. The influence of package effects and intracellular absorption of fluorescence was evaluated with a simplified form of the model that treated fluorescence as a linear function of particle density and assumed uniform particle composition, constant fluorescence cross-sectional yield, and sufficient averaging of the fluorescence signal. The model predicted decreasing fluorescence per unit of chlorophyll with increasing values of the product of particle diameter and intraparticle chlorophyll concentration. Experimental trends in size dependence of chlorophyll-fluorescence relationships were compared with predictions of the model.     

Energy dissipation measurements in frequency-modulated scanning probe microscopy

Local dissipation measurements by scanning probe microscopy have attracted increasing interest as a method for probing energy losses and hysteretic phenomena due to magnetic, electrical, and structural transformations at the tip-surface junction. One challenge of this technique is the lack of a standard for ensuring quantification of the dissipation signal. In the following, we explored magnetic dissipation imaging of an yttrium-iron garnet (YIG) sample, using a number of similar but not identical cantilever probes. Typical frequency-dependent dispersion of the actuator-probe assembly commonly approached ± 1 part in 10(3) Hz(-1), much larger than the minimum detectable level of ± 1 part in 10(5) Hz(-1). This cantilever-dependent behavior results in a strong crosstalk between the conservative (frequency) and dissipative channels. This crosstalk was very apparent in the YIG dissipation images and in fact should be an inherent feature of single-frequency heterodyne detection schemes. It may also be a common effect in other dissipation imaging, even down to the atomic level, and in particular may be a significant issue when there are correlations between the conservative and dissipative components. On the other hand, we present a simple method for correcting for this effect. This correction technique resulted in self-consistent results for the YIG dissipation measurements and would presumably be effective for other systems as well.     

Fiber-optic laser-induced fluorescence probe for the detection of environmental pollutants

Laser-induced fluorescence (LIF) spectroscopy in combination with fiber optics is shown to be a powerful tool for qualitative and quantitative diagnostics of environmental pollutants in water and soil. Timeintegrated data accumulation of the LIF signals in early and late time windows with respect to the excitation pulse simplifies the method so that it becomes attractive for practical applications. Results from field measurements are reported, as oil contaminations under a gas station and in an industrial sewer system are investigated. A KrF-excimer laser and a hydrogen Raman shifter can be applied for multiwavelength excitation. This allows a discrimination between benzene, toluene, xylene, and ethylbenzene aromatics and polycyclic aromatic hydrocarbon molecules in the samples under investigation. For a rough theoretical approach, a computer simulation is developed to describe the experimental results.     

Hairpin fluorescence DNA probe for real-time monitoring of DNA methylation

DNA methylation catalyzed by methylase plays an important role in many biological events. However, traditional methods of methylase activity analysis by gel electrophoresis were laborious and discontinuous. In this paper, we report a new strategy to study methylase activity using fluorescent probes coupled with enzyme-linkage reactions. A hairpin DNA probe is prepared with a fluorophore and a quencher linked at the 5'- and 3'-terminus of the probe. A disturbance of the stem sequence by DNA methylation would cause the separation of the fluorophore and the quencher, resulting in the restoration of the fluorescence. We used DNA adenine methylation (Dam) methyltransferase (MTase) and Dpn I endonuclease, both having a 5'-G-A-T-C-3' recognition sequence. Dam MTase catalyzed the methylation of the sequence of 5'-GATC-3', and Dpn I cut the sequence of 5'-G-Am-T-C-3'. The fluorescence of the hairpin probe was restored when it was cleaved by Dpn I endonuclease during the course of methylation. Unlike traditional methods, this assay was done in real time and could be used to monitor the dynamic process of methylation. Our method is easy, simple, and nonradioactive, yet as efficient as gel electrophoresis in detecting the activity of methylase. It also had the potential to screen suitable inhibitor drugs for Dam methylase.     

Improved calibration of a large open-ended coaxial probe fordielectric measurements

An improved method for calibrating an open-ended coaxial sensor is proposed. The calibration utilizes the measurements of an open, a short, and a short-cavity. These terminations are inexpensive to make and result in a calibration that depends only on the geometry of the sensor. The short-cavity is modeled by waveguide modal techniques and the theoretical reflection coefficient determined to any accuracy. Consequently, an improvement in the accuracy of the measurements is obtained over a large bandwidth. The calibration was tested by comparing theoretical and experimental values of the complex permittivity for common liquids, and the discrepancy was seen to be small