O. Axner

Investigation of the Multielement Capability of Laser-Enhanced Ionization Spectrometry in Flames for Analysis of Trace Elements in Water Solutions

One-step Laser-Enhanced Ionization (LEI) spectrometry of 23 different elements in aqueous solutions has been performed in an acetylene/air flame. All elements were detected by light in the ultraviolet region, produced by frequency doubling of the output from the dye Coumarin 153. This was done in order to investigate the multielement capability of LEI in flames that has been made possible by the recent development of commercially available, widely tunable dyes. Among the elements detected, 9 (As, Au, In, Mn, Pb, Sb, Tl, W, Yb) show detection limits which are superior to those reported in the literature for one-step LEI. The lowest detection limit obtained in this investigation was 1 pg/mL for In. Four of the elements (As, Sb, Yb, W) are reported as being detected by LEI for the first time. The multielement capabilities of LEI as a method for trace element analysis are discussed.

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.

Laser-enhanced ionization spectrometry in flames–a powerful and versatile technique for ultra-sensitive trace element analysis

Abstract Analytical applications of Laser-Enhanced Ionization (LEI) spectrometry in flames are reviewed. The experimental features as well as the physical principles of the method are described. Theoretical expressions are given for the signal strength for both one- and two-step excitation LEI as well as for signal enhancement caused by the two-step excitation. A summary of the analytical performance of the LEI method is given with emphasis towards the sensitivity and selectivity of the technique. A detailed description of interference phenomena that are present in the method is presented as well as an estimate of their influence on the analytical performance of the method. Finally, a description of other applications of LEI is presented.

Determination of ionization efficiencies of excited atoms in a flame by laser-enhanced ionization spectrometry

State-specific ionization efficiencies for excited Li and Na atoms in acetylene/air flames have been determined. The ionization efficiencies, i.e., the probability that the excited atoms ionize instead of returning to the ground state, are determined by relating collision-assisted Laser-Enhanced Ionization (LEI) signals from various excited states with laser-induced photoionization signals. The ionization efficiencies are found to decrease (from being one at the ionization limit) almost monotonically as the lower atoms are excited. The most striking feature, however, is that the decrease of the ionization efficiency values is generally found to be less than the decrease of the Boltzmann factor, exp(-δE/kT), when the energy difference, δE, between the excited state and the ionization limit is increased. The ionization efficiencies are found to be close to unity for states with δE < kT and approximately 50% for states with δE ≈ 2.5 kT (np ≈ 6p). For the lower states, the ionization efficiencies are found to be approximately five times larger than the Boltzmann factor.

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.

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.

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.

Direct Determination of Thalliilm in Natural Waters by Laser Induced Fluorescence in a Graphite Furnace

Abstract The thallium content in some natural waters (lakes, rivers and drinking water) in South-West Sweden has been determined by Laser-Induced Fluorescence in a Graphite Furnace (LIF-GF). The high sensitivity of the LIF-GF technique eliminated the need for any sample pre-treatment or sample pre-concentration (except for acidification with small amounts of HNO3), thus reducing the number of sources of contamination. It was found (by the standard addition technique) that the Tl content of the samples investigated ranges from 8 ppt for drinking water up to 56 ppt for water from the Viskan river. No matrix interferences from concomitant elements in the samples could be detected with the present experimental set-up. This allowed for a calibration against matrix-free reference solutions.

Detection of trace amounts of Cr by two laser-based spectroscopic techniques: laser-enhanced ionization in flames and laser-induced fluorescence in graphite furnace

The detectability of Cr in water solutions by two-step laser-enhanced ionization (LEI) in flames and two-step excitation laser-induced fluorescence in graphite furnace (LIF-GF) with nonresonant detection is investigated for what is, to our knowledge, the first time. A thorough investigation of possible excitation and detection routes for Cr for both techniques is given. The detection limit of Cr in water by the LEI technique was found to be 2 ng/mL, while the LIF-GF technique showed a detection limit of 1.4 pg (which corresponds to 0.3 ng/mL, with a 5-microL sample volume), both of which are limited by contamination (from the burner head-nebulizer unit in the flame and from the graphite material in the furnace). A more sensitive two-step LEI excitation scheme than that used here is also proposed. A new technique for reducing fluctuations from blackbody radiation by using sequential detection of the blackbody radiation from one photomultiplier by two boxcar integrators is presented. A possible means of increasing the nonresonant signal in two-step excitation LIF-GF by adding small amounts of quenching enhancing N(2) to the Ar atmosphere gives no positive results. The influence of large amounts of Na on the detectability of Cr by LEI is investigated.

Detection of traces in semiconductor materials by two-color laser-enhanced ionization spectroscopy in flames

In this paper we demonstrate that two-color laser-enhanced ionization (LEI) spectroscopy in flamescan be used for detecting impurities at a sub-ppm level in GaAs. Six elements were investigated(Co, Cr, Fe, In, Mn, Ni), and the content could be determined for three of them (Cr, Fe, Ni), while only an upperlimit below ppm could be given for the other three. The analysis was performed on the bulk material whichwas dissolved in acid and further diluted. In the diluted solution, the detection limits were of the same orderas in pure water solutions. Spectral interferences have been corrected for by using a background correctionmethod, which is performed without scanning the lasers. Furthermore, we discuss the applicability of LEI inflame and graphite furnace to samples with various impurity elements, matrices, and total amounts of sampleavailable for analysis.