Laser enhanced ionization spectrometry in analytical flames

Turk, Gregory C.; Travis, John C.; DeVoe, James R.; O’Haver, T. C.

doi:10.1021/ac50048a003

Cited by 103 publications

(50 citation statements)

References 15 publications

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“…(1) is accommodated in eqn. (3) in the rate constant hi = koPt (4) where the collision rate of X and A species, kg, obviously depends on the number density of X, as well as other factors.…”

Section: Making Ionsmentioning

confidence: 99%

Limits to sensitivity in laser enhanced ionization

Travis¹

1982

J. Chem. Educ.

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Laser enhanced ionization (LEI) occurs when a tunable dye laser is used to excite a specific atomic population in a flame ( 1 4 ) . The thermal ionization rate of the laser-perturbed atomic population is greatly enhanced over that of the thermally distributed population. Resonant (R), non-resonant (N), stepwise (S), and non-resonant stepwise (NS) laser excitation schemes for LEI are illustrated in Figure 1, which also indicates the role of collisional (thermal) processes in LEI. In all cases. the final ionization sten is collisional. ~~ ~~The ions produced hg I.El are decected by impressing a high voltaee across the flamf.. The current in this high-voltace circug is a measure of the volume ionization rate (7) (ions created per cm3 per second), and, hence, changes in this current induced by the laser represent the LEI signal. Under appropriately controlled conditions, the LEI signal varies linearly with the concentration of the resonantly excited element in the flame, yielding the basis for a new method of chemical analysis.Flames have long been used to render free atoms for spectroscopic analysis f& elements (usually metals) in solution (8).For this purpose, a fine mist of solution drawn from a sample heaker is mixed with the fuel and oxidant in the chamber of -~~~ ~ a pre-mix burner (the fuel and oxidant are mixed before combustion). Upon passing into the flame, the tiny droplets rapidly evaporate, leaving a fraction-often near unity-of the desired element in the free atomic state. For a given set of burner adjustments, the ratio of free atom number density in the flame to element concentration in the original solution is constant, but not accurately known. Typically, a 1-ppm solution yields 1010-10" atoms of analytelcdin the flame (9).Free atoms in the flame may be identified by their unique absorptions andlor emissions and quantifird by the strength, or intrnsitv, oi these spectral fttnturrs. Accurate measurrments of solution concentrations require the use of "standard" solutions of known concentration to calibrate the instrument.Flame atomic emission spectroscopy (FAES), flame atomic absorption spectroscopy (FAAS), flame atomic fluorescence spectroscopy (FAFS), and LEI share a common technology up to the point of detecting free atoms in the flame. Each method has unique advantages, and all have a lasting place in atomic spectrometry.A particular advantage of LEI is its extremely high sensitivity (2,lO). Limits of detection (LODs) range from ng/mL (parts-per-billion) to pg/mL (parts-per-trillion), as shown inIn TI Co Sn Figure 1. Excitation schemes for LEI in a 2500 K flame ( k T = 0.22 eV): R = resonant; N = nonresonant; S = stepwise; NS = nonresonant stepwise.

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“…(1) is accommodated in eqn. (3) in the rate constant hi = koPt (4) where the collision rate of X and A species, kg, obviously depends on the number density of X, as well as other factors.…”

Section: Making Ionsmentioning

confidence: 99%

Limits to sensitivity in laser enhanced ionization

Travis¹

1982

J. Chem. Educ.

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“…It has been demonstrated that the acoustic wave measurement can normalize the atomic emission signal generated from the laser plume so that the influence from laser power fluctuations and matrix effects is minimized (15). Another characteristic feature of the laser plasma is that ions can be formed through either thermal ionization (16,17) or laser-induced ionization processes (18)(19)(20)(21)(22). The thermal ionization process under these experimental conditions only induces very small amounts of atomic ions.…”

Section: Introductionmentioning

confidence: 99%

“…These highly excited atoms or molecules can be ionized subsequently by collisions or by absorbing more photons to reach the ionization continuum. The first process has been referred to as laser-enhanced ionization (LEI) (18)(19)(20). LEI combined with an analytical flame, which serves as an atom reservoir, has been used for trace elemental analysis.…”

Section: Introductionmentioning

confidence: 99%

Laser-enhanced ionization as a diagnostic tool in laser-generated plumes

Pang¹,

Yeung²

1989

Anal. Chem.

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“…The outstanding performance of LEI in trace element analysis has been demonstrated in a number of reports previously, both in flames [1,3,4,11,12,19,21,22,28] and in furnaces [-17, 25]. For flame-LEI, detection limits in the 0.001-1 ng/ml range in aqueous solutions are readily obtained with one-step excitations of elements with low or moderate ionization potentials.…”

Section: Detection Limits and Sensitivitiesmentioning

confidence: 93%

Analytical applications of laser-enhanced lonization spectrometry in flames and furnaces

1989

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This paper is concerned with a critical analysis of the analytical performance of laser-enhanced ionization (LEI) spectrometry in flames and furnaces as an ultra-sensitive trace-element technique. Updated tables of LEI detection limits, both in flames and furnaces, are given. Special attention is paid to interference phenomena which are unique to the LEI technique. Ways of reducing their influence on the analytical signals are proposed. Finally, advantages and disadvantages of LEI spectrometry as a tool for ultra-sensitive trace element analysis are discussed.Key words: laser-enhanced ionization (LEI), flame, furnace, trace element.Laser-enhanced ionization (LEI) spectrometry is a very powerful technique for detecting low concentrations of metal atoms in solutions I-1-23]. The major characteristics of the technique constitute of a high sensitivity and a rather high resistance to interferences. The general principle of the LEI technique is that an enhancement of the normal ionization processes (collisions) is obtained by optical excitation of the atoms under study by resonant laser light. The enhanced ionization rate is detected as a change in the current passing through a medium (atomic reservoir) exposed to a voltage by means of an electrode arrangement.When a sample is analyzed by LEI, the atomization can, in principle, be done in any of the conventional atomic reservoirs which previously have been developed for standard analytical techniques (absorption or emission techniques). However, LEI has so far most often been done in atmospheric pressure flames (primarily air/acetylene flames) and only recently LEI has been applied to graphite furnaces [8,16,17,[24][25][26].The atoms under study are excited from low lying, highly populated levels (primarily from the ground level) to high lying excited states by resonant laser light. Solution Boxcar Oscilloscope Fig. l. A typical experimental set-up for two-step LEI in flames. C = 10 nF and R = 50 k~This is almost exclusively done by using pulsed dye-laser systems for production of resonant laser light.Since the atomic reservoirs used for LEI are characterized by high temperatures in fairly dense media the collision rates between analyte atoms and buffer molecules are high. Hence, the atoms have certain probabilities of undergoing ionizing collisions. The probability of such collisional ionization is higher the less energy needed for ionization of the (excited) atoms. Therefore, the ionization probability increases the closer to the ionization limit the atoms are being excited [-27].The excitation of atoms in LEI spectrometry can be carried out in several ways. The most common excitation scheme is to excite the atoms from the ground state by absorption of one photon. By using ultra-violet light or by exciting the atoms in two consecutive steps a high ionization rate (efficiency) can be obtained. The two-step excitation is practically done by illuminating the atoms with light from two simultaneously pumped dye lasers, each tuned to a suitable transition wavelength in the...

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Laser enhanced ionization spectrometry in analytical flames

Cited by 103 publications

References 15 publications

Limits to sensitivity in laser enhanced ionization

Limits to sensitivity in laser enhanced ionization

Laser-enhanced ionization as a diagnostic tool in laser-generated plumes

Analytical applications of laser-enhanced lonization spectrometry in flames and furnaces

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