L.P. Hart

A comparison of signal-to-noise ratios for single channel methods (sequential and multiplex) vs multichannel methods in optical spectroscopy

Abstract Four major types of spectroscopic systems for quantitative analysis of one or more spectral components we compared, with regard to signal-to-noise ratio for constant analysis time. These four methods are based on sequential-linear scan, sequential-slew scan, multichannel, and multiplex approaches. The multiplex methods can generally be classified into two types, namely Fourier transform spectroscopy and Hadamard transform spectroscopy. It is shown that for the same spectral source, for the same resolving power luminosity product of the optical system, and the same detector, the multichannel approach is the best and the sequential slew scan system is nearly as good for relatively simple spectra in the u.v.- Visible region. Multiplex methods have little to offer in the u.v.- Visible region, where the detector noise limitation seldom applies and where background shot and/or fluctuation noise are dominant but could find considerable use in the u.v.- Visible region for atomic fluorescence or emission spectroscopy especially if the density of spectral lines in the measurement region is not too great and the background intensity is low.

Simultaneous multi-element atomic emission flame spectrometry with an image vidicon detector

The silicon diode array vidicon camera tube and the nitrous oxide-- acetylene flame were used to determine limits of detection for Co, Cu, Er, Ev, Ho, In, Mn, Ni, Sc, Sr, and Yb. (LK)

On the shape of atomic fluorescence analytical curves with a laser excitation source

Abstract The effect of high irradiance levels (approaching saturation) upon the shape of atomic fluorescence analytical curves is considered theoretically for both line and continuum sources. The linear range of the analytical curve may be greatly extended over that obtained with normal (low irradiance) sources, as is experimentally verified using a pulsed dye laser as an excitation source.

Evaluation of argon metastable number densities in the inductively coupled plasma by continuum source absorption spectrometry

Abstract Absolute number densities for the metastable and radiative 4s argon levels in an inductively coupled plasma have been determined for a variety of plasma conditions by the technique of continuum source absorption. As primary source, a 300 W xenon arc was used and much care was taken in screening the optical detection system from the intense background emission of the plasma. A 1.29-m focal length grating monochromator provided a variable bandwidth so that absorption measurements could be carried out, with varying degree of sensitivity, on 19 different argon lines. The number densities were derived from the corresponding curves of growth, calculated for each line. Concentrations ranging from 2.3 × 10 10 to 7.4 × 10 11 cm −3 were obtained for the different levels, depending upon the presence or absence of nebulizing gas and water in the plasma. At observation heights greater than 20 mm above the coil, the number density approaches the value predicted by Boltzman equilibrium for a temperature of 6500 K. The detection sensitivity of the present apparatus is about 5 × 10 9 cm −3 . For seven lines, damping parameter values were also estimated and found to vary from 0.4 to 1.1.

Light Scattering in Turbulent and Laminar Flames

The scattering of light in flames, both turbulent and laminar, is investigated using line and continuum sources of excitation. The wave-length dependence, angular distribution, and polarization of the scattered intensity are measured. Only with the laminar flames does the wave-length dependence of the scattered intensity approximate the λ−4 relationship of Rayleigh scattering. Contrary to the predictions of the theory for Rayleigh scattering, the scattering is not symmetric about 90°. The variation of intensity and polarization of the scattered light at different angles is stressed from the analytical point of view. In atomic fluorescence spectroscopy, the ratio of the fluorescence intensity to the scattered intensity can be considerably improved by the use of polarizers.

Rapid Scanning Constant Energy Synchronous Fluorescence Spectrometry: A Comparison of Flow Cells

Rapid scanning constant energy synchronous fluorescence spectrometry (RSCESFS) is evaluated for flow cell design, scan speed, and constant energy interval choice. Analytical figures of merit for several polycyclic aromatic hydrocarbons (PAHs) are given for several experimental conditions, for several flow cells, and for both RSCESFS and conventional (nonscanning) fluorescence spectrometry. With RSCESFS, absolute detection limits as low as 700 fg, linear dynamic ranges as great as five orders of magnitude and the time to record a complete 350 nm spectrum as short as 17 s have been obtained.

Laser induced stepwise and two-photon ionization studies of strontium in the air acetylene flame

Abstract Using strontium atoms present as a trace constituent in an air-acetylene flame as an example, a rich laser enhanced ionization spectrum was obtained. One pulsed tunable dye laser was tuned to a transition originating from the ground state (460.73 nm) and another scanned over different spectral regions. The lines obtained were spectroscopically characterized as to the type of absorption process, which included non-resonant processes as well as single wavelength, two-wavelength, and two-photon resonant processes. With a maximum irradiance of 100 MW cm −2 , two-photon transitions resulting in collisionally assisted ionization included the 5 s n s and 5 d n d Rydberg series (up to 37 s and 15 d ) together with a strong auto-ionizing transition at 431.10 nm. The complexity of the observed ionization spectrum when the irradiance is high indicates that spectral interferences need to be carefully considered in analytical applications.

Laser-Induced Intermodulated Flame Fluorescence: A New Approach to Scattering Correction in Analytical Atomic Fluorescence

It is shown that the technique of intermodulated fluorescence can effectively correct for scattering problems in analytical flame fluorescence spectroscopy. When two laser beams, amplitude-modulated at different frequencies f1 and f2 and counterpropagated colinearly throughout an atomizer, are tuned to the absorption transition of the element of interest, non-linear mixing of the fluorescence signal results, due to saturation effects. By extraction of the signal at the sum or difference frequency, f2 ± f2, the linear scattering component of the spectrum can be essentially eliminated. This has been demonstrated for a sodium solution nebulized in a premixed, laminar, argon-oxygen-hydrogen flame. Because the modulation signal can be observed only at the intersection volume between the two beams, this technique constitutes a powerful tool for spatially resolved combustion diagnostics.

A Theoretical and Experimental Investigation of the Technique of Laser-Intermodulated Fluorescence for Scattering Correction in Atomic Fluorescence Flame Spectrometry

A theoretical derivation is presented of the signal obtained with the technique of intermodulated atomic fluorescence spectroscopy. One laser beam, tuned at a selected atomic transition, is divided into two beams which are then amplitude-modulated at different frequencies and recombined in a flame containing the vapor of the element investigated. The fluorescence signal at the sum or difference frequency is measured. The derivation is given for both square-wave and sinusoidal modulation. It is shown that the intermodulated fluorescence amplitude depends upon the square of the laser spectral irradiance at low powers and reaches a plateau at high irradiances, but only in the case of square-wave modulation. For sinusoidal modulation, a maximum is reached followed by a roll-off at high irradiances. The theoretical predictions are verified experimentally with a square-wave-modulated cw dye laser for the case of sodium resonance fluorescence in an oxygen-argon-hydrogen flame. The scatter signal has no intermodulation component. Finally, it is shown that when the modulation waveform is not square wave, scattering correction can also be achieved with a single-beam excitation scheme.

Rapid scanning fluorometer based on constant energy synchronous scanning

A rapid scanning fluorometer based on constant energy synchronous fluorescence scanning at 200 nm/s has been developed. Constant energy synchronous fluorescence spectrometry involves measurement of fluorescence as both excitation and emission monochromators are scanned with a constant energy difference being maintained. Spectral information is collected in the forward to reverse directions and is processed in real time. A discussion of the instrumentation and software is given. A theoretical comparison between the photomultiplier tube (PMT) detection system used with this system and a silicon intensified target vidicon (SIT) detection system has been given. Calculations indicate that the signal‐to‐noise ratio S/N for the PMT system should be about 5–40× greater than for the SIT system. A comparison of the experimental figures of merit (detection limits, scan speed, analysis time) of the rapid scan constant energy synchronous fluorescence spectrometer with video fluorometers has also been made, again showi...