Laser-enhanced Ionization Research Articles

Three-Dimensional Atomic Spectra in Flames Using Stepwise Excitation Laser-Enhanced Ionization Spectroscopy

Stepwise excitation laser-enhanced ionization spectroscopy utilizes two independently tunable dye lasers to populate high-lying excited states of atoms in flames. Two atomic resonances are required, with the upper level of the first-step transition coinciding with the lower level of the second-step transition. Efficient population of a high-lying atomic level is achieved, from which a high rate of collisional ionization can take place. The double-resonance aspect of such excitation adds an extra dimension of spectroscopic selectivity to the measurement. A computer-controlled dual-wavelength LEI spectrometer, including a Fizeau wave-meter for wavelength verification, is used to record three-dimensional spectra–ionization signal as a function of both first- and second-step wavelengths. Examples illustrate the accuracy advantage accorded by the three-dimensional survey.

Laser-Enhanced Ionization Detection of Trace Elements in a Graphite Furnace

A graphite furnace is successfully used as an atomizer in laser-enhanced ionization spectroscopy (LEI). The physical origin of the signals is discussed and a comparison is made with LEI measurements in a flame. Extremely small amounts of analyte atoms can be detected using this method. The reproducibility is good, there are almost no memory effects, and the signal output is linear with the amount of analyte over at least 5 orders of magnitude. The experimental results indicate that the method could become an attractive alternative for trace-element analysis.

Elimination of Spectral Interference Using Two-Step Excitation Laser Enhanced Ionization

Spectral interferences from matrix elements in solutions are investigated utilizing Laser Enhanced Ionization (LEI) in flames. Mechanisms which influence the analytical selectivity are discussed and it is shown that the selectivity can be significantly increased by a proper choise of experimental parameters. This is exemplified by determination of low concentrations of Mg in a matrix of Na. The spectral interference, occurring in one-step excitation LEI, is diminished by a factor of 70 when utilizing two-step excitation LEI. The spectral resolution is further improved to a total factor of 1300 by an optimized choice of laser light intensities without any significant loss in sensitivity. It is pointed out that an efficient correction for signal contributions from interfering substances can be made in two-step excitation LEI without scanning the laser wavelengths. Background signals emanating from NO molecules inherent to the flame gases are also detected and virtually eliminated. The very high sensitivity and selectivity achieved by two-step excitation LEI shows that the method is widely applicable for trace element analysis.

A Desolvation Unit for Introducing Microliter Volumes of a Dry Aerosol to an Atomic Spectrometric System

Recent communications from this laboratory describe an impaction-electrothermal atomization atomic absorption spectrometric (I-ETAAS) system for direct and near-real-time determination of metallic compounds in the atmosphere. Calibration was achieved by introducing aqueous standards by a pipet or by using a pneumatic nebulization aerosol deposition technique. This assumes that the transport and final atomization processes are identical for a sample introduced as an ion in solution or as a dry particle from the atmosphere. A more accurate and comparable calibration would be achieved if the standard could be introduced to the I-ETAAS system as a dry aerosol. The introduction of a dry aerosol to an atomic spectrometric system has been achieved with the use of a graphite furnace, although the major advantage of this method of sample introduction was the use of micro-volumes, minimizing sample preconcentration, and reducing interferences. The cost and availability may prevent the widespread use of the graphite furnace for sample introduction of dry aerosol. A dry aerosol has been achieved for laser-enhanced ionization spectrometry using a microcondenser with a heated injection port for introduction of sample, but only a few details of the system have been given. Browner and Boorn have recently reviewed sample introduction techniques for atomic spectrometry, pointing out that pneumatic nebulization for flames and plasmas is the preferred, most widely used, and most readily available approach in the majority of situations. Sample introduction to an electrothermal atomizer using a pneumatic nebulization aerosol deposition technique has been described. The use of pneumatic nebulization is unlikely to change in the near future despite concern over poor transport efficiency.

Determination of Trace Elements in Water Solution by Laser Enhanced Ionization using Coumarin 47

In this paper we wish to demonstrate the multi-element capability of Laser Enhanced Ionization (LEI) in flames by reporting the detection of 15 different elements. All the elements were excited by the fundamental or the first harmonic frequency-doubled light from one single dye in the laser system (Coumarin 47). Detection limits below the ppb region (parts-per-billion, ng/ml) were achieved for 13 elements (Al, Bi, Ca, Cd, Co, Cr, Fe, Ga, Li, Ni, Pb, Sr, Tl). The detection limits reported here are for several elements orders of magnitude superiour to those for existing comparable flame based spectroscopic methods. For 9 elements, the detection limits are found to be lower than any previously reported values for one-step LEI in flames.

Laser-Enhanced Ionization of Refractory Elements in a Nitrous Oxide-Acetylene Flame

Laser-enhanced ionization (LEI) spectrometry using a water-cooled electrode immersed directly in a nitrous oxide-acetylene flame has been examined for the determination of refractory elements. LEI detection limits for refractory elements in aqueous solution are comparable to or better than detection limits obtained by flame atomic absorption, plasma emission, and atomic fluorescence techniques. Only graphite furnace atomic absorption spectrometry exhibits superior detectability for certain refractory elements in aqueous solution over LEI spectrometry using a nitrous oxide-acetylene flame. The successful application of the nitrous oxide-acetylene flame now extends the applicability of LEI spectrometry to include most of the elements in the periodic table which can be determined by other common atomic spectrochemical techniques.

Lifetime Measurements of Metastable Levels of Thallium and Lead in the Air-Acetylene Flame by Laser-Enhanced Ionization Spectrometry

Laser-enhanced ionization spectrometry with two-step excitation has been used to evaluate the collisional lifetime of the metastable P levels of thallium and lead in an air-acetylene flame burning at atmospheric pressure and supported by a three-slot burner head fitted on a conventional nebulizing chamber. A water-cooled molybdenum electrode immersed in the flame was maintained at high negative potential with respect to the burner body. The ionization current resulting after the two-step excitation was amplified and measured with a digital storage oscilloscope and a boxcar averager. The two excimer lasers were triggered externally with two trigger pulses, one being delayed in time with respect to the other one. In this way the second laser photon, tuned at a transition starting from the metatable level under study, could be correspondingly delayed from the first photon tuned at a transition starting from the ground state. The lifetimes measured were found to be 81 ns and 360 ns for T1 and Pb, respectively.

Ion-fraction determination by laser-enhanced ionization spectrometry

Role de la concentration totale en ion et de la mobilite ionique sur le signal d'ionisation renforce par laser. Mesure des fractions ioniques relatives pour les metaux alcalins

Improved theory of laser-enhanced ionization in flames: Comparison with experiment

An improved theory for laser enhanced ionization in flames has been developed for one- and two-step laser excitations. The model gives an analytical expression for the sensitivity of the method for a given transition of any element. The theoretical expression is compared with experimentally measured signals for a number of elements and the agreement is found to be generally good.

Signal detection of pulsed laser-enhanced ionization

A point charge model based upon charge induction theory has been developed to describe the proposed detection mechanisms of laser-enhanced ionization (LEI). The model predictions are in good agreement with experimentally observed LEI electron and ion signal pulses. The predicted effects of alkali metal matrix concentration and laser beam position are also confirmed by experimental results. The development of this model provides a basis for establishing experimental methodology that is necessary for the development of LEI as a technique for trace metal analysis. 23 references, 8 figures, 2 tables.

THEORY OF LASER-ENHANCED IONIZATION IN FLAMES - COMPARISON WITH EXPERIMENTS

Opto-galvanic or laser-enhanced ionization (LEI) spectroscopy has been performed on trace elements in a flame using one- and two-step laser excitations. The sensitivity, defined as the LEI signal divided by the laser pulse energy and the concentration of the trace element in the water solution aspirated into the flame, has been measured for a number of elements. A theoretical model for the LEI process has been developed and tested. The importance of energy levels, not involved in the laser transitions, is emphasized. The agreement between theory and experiment is satisfactory. Possible reasons for discrepancies which arise are discussed.

Trace metal analysis by laser-enhanced ionization in flames.

We review atomic flame spectrometry of metals based on the selective laser-enhancement of the rate of atomic ionization. A discussion of basic principles, prior literature, and instrumentation is followed by an overview of analytical figures of merit for the technique, and a look at its present status and areas for future development.

Laser-enhanced ionization spectrometry with a total consumption burner

Les diverses configurations d'electrode possibles sont comparees en utilisant l'indium comme analyte

Dual Laser Ionization in Flames: A Search for Electrical Interferences

Experimental studies of electrical interferences in the dual laser ionization technique are reported and compared to those commonly encountered in laser-enhanced ionization spectrometry. For the dual laser ionization of sodium, the addition of easily ionized matrix components such as cesium, lithium, and potassium were found to have little or no effect on signal recovery. Similar results were obtained in experiments with lithium and strontium as the analytes. Some chemical interferences were found and discussed.

Evaluation of electrode configurations for high voltage interferent removal in laser-enhanced ionization spectrometry

The laser-enhanced ionization (LEI) signal results from a change in flame impedance produced by collisional ionization of laser-excited analyte atoms. Perturbations in the electrical composition of the flame due to high ion concentrations suppresses the LEI signal. Since the predominant source of electrical interferences is low ionization potential concomitants, high voltage electrodes might be used to remove interfering species prior to signal collection. High voltage interference removal precludes the use of the burner head as the signal collection anode so other electrode configurations were investigated. The optimum electrode geometry was determined and used for experimental evaluation of various removal electrode configurations. The appearance of anomalous signals was investigated and explained. The combination of the optimum signal collection and interference removal electrode geometry produced improvement in LEI signal recovery. The advantages and deficiencies of this system are discussed and compared with other approaches.

Continuous wave excitation in laser-enhanced ionization spectrometry

Continuous wave excitation in laser-enhanced ionization spectrometry

Laser-Enhanced Ionization Flame Velocimeter

The temporal and spatial evolution of the depleted neutral atom density following laser-enhanced ionization in laminar flow flames has been used to characterize the flow velocity of the flame gases. This technique utilizes a low power cw dye laser and a low sodium seed density (1011 to 1012 atoms/ml). The flow velocity can be measured to better than 2% in standard analytical burners.

Mobility measurements of atomic ions in flames using laser-enhanced ionization

Atomic ion mobilities have been directly determined in C 2H 2/air and CO/O 2 flames using optical excitation to create instantaneously a pencil of ions along the laser beam and then measuring ion velocities in an applied electric field. The ions were chosen to provide a wide range of atomic weights (Li, Na, K, Ca, Sr, Ba, Fe, In, Tl, U), thus providing a good comparison with the Langevin theory of ion mobility under several flame conditions. In all cases the Langevin theory provides an upper limit and predicts the measured mobility to within 50%, with best agreement obtained in general for the smaller and less polarizable ions.

A Digital-to-Digital Interface for a Laboratory Microcomputer: Hardware and Software

ABSTRACT This paper describes the design and construction of a digital-to-digital interface for a laboratory microcomputer. In this application, signals from a laser-enhanced ionization experiment are processed with a boxcar signal averager which is interfaced to a Commordore PET 4032 microcomputer. Software has been developed for the microcomputer which performs various operations on the data and prints the values for the average analyte signal, average standard signal, analyte-to-standard ratio, average noise, and analyte signal-to-noise ratio. This interface is also applicable to other laboratory instruments with binary-coded decimal outputs.

Collection of ions produced by continuous wave laser-enhanced ionization in a hydrogen/air flame

Laser-enhanced ionization (LEI) - or the optogalvanic effect in flames - is known to result when a tunable laser is used to significantly populate an excited state of an atomic species in an atmosphere pressure flame. The perturbed ionization rate may be sensed with external electrodes, providing the opportunity for applications to trace metal analysis and combustion research. The present study correlates experimental studies of the spatial and temporal characteristics of the LEI signal with theoretical expectations. The experiments are performed with a c-w dye laser exciting the 3S ..-->.. 3P/sub 3/2/ transition of sodium introduced into a H/sub 2//air flame. Saturation currents with and without laser excitation are found to be consistent with expected ionization rate constants for the Na group and excited states. Vertical spatial profiles - using a unique imaging method - show the physical size of the excess ion region generated by the laser, and the influence of external voltage, flame velocity, diffusion, and Coulombic expansion on the excess ion region. Rise and fall times - measured at different voltages with the laser switched by an acousto-optic modulator - show the dependence on ion mobility, electric field, and excited-state ionization rate constant. The relationships derived andmore » illustrated should be of value for the improvement of precision and accuracy in analytical and diagnostic LEI.« less