Laser excited atomic fluorescence spectrometry — a review

Spectrochimica Acta Part B 56 Ž2001. 1565᎐1592

Review

Laser excited atomic fluorescence spectrometry ᎏ a review 夽 Peter Stchur, Karl X. Yang, Xiandeng Hou, Tao Sun, Robert G. MichelU Department of Chemistry, Uni¨ ersity of Connecticut, 55 North Eagle¨ ille Road, Storrs, CT 06269-3060, USA Received 20 April 2001; accepted 7 June 2001

Abstract This review focuses on the development of new instruments, and new applications of laser excited atomic fluorescence spectrometry, LEAFS, in recent years since the last published reviews. Such developments include solid-state tunable lasers, deep UV tunable lasers, the use of charge coupled detectors ŽCCDs., and the applications of LEAFS for trace metal determination in various samples. The advent of diode lasers with their now somewhat improved range of wavelengths and power output, provides opportunities for research and applications in LEAFS. The further development of the coupling of second and third harmonic crystals to pulsed diode lasers shows promise for compact and robust instrumentation. There have been no recent instrumental developments that might provide more isotopic selectivity beyond the elements like uranium where the spectral isotope splitting is greater than most elements, but laser diodes could provide this due to their potential to provide an output with very narrow spectral bandwidth. The advent of optical parametric oscillator-based lasers has enabled LEAFS to be much more practical then in the past when dye lasers were used. This should be the harbinger of more applications of LEAFS to complex real sample analyses that can not be done by other techniques for reasons of sensitivity or selectivity. Array detectors provide an additional degree of freedom by provision of more spectral information more rapidly, which should aid the study of complex samples that might produce complex background problems. The recent literature indicates that

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This article is published in a special honor issue dedicated to Walter Slavin, in recognition of his outstanding contributions to analytical atomic spectroscopy, in appreciation of all the time and energy spent in editing Spectrochimica Acta Part B. U Corresponding author. Tel.: q1-806-486-3143; fax: q1-806-486-2981. E-mail address: [email protected] ŽR.G. Michel.. 0584-8547r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 2 6 5 - 8

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the sensitivity, selectivity and ease of method development of LEAFS is well-established, and that there are no substantial analytical disadvantages to the technique beyond the instrumental limitations associated with the single element at a time mode of operation and the complexity of the laser systems. Laser technology continues to develop rapidly, which heralds a bright future for LEAFS. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Lasers; Atomic fluorescence; Optical parametric oscillator; Dye lasers; Trace metals; Background correction; Laser diodes; Charge coupled detectors; Laser ablation; Glow discharge; Graphite furnace; Plasmas; Wavelength modulation; Zeeman effect

1. Introduction Laser excited atomic fluorescence spectrometry ŽLEAFS. is based on the excitation of gaseous atoms by optical radiation of suitable wavelength and the measurement of the resultant fluorescence radiation w1,2x. LEAFS has significant advantages for the determination of trace element concentrations in a wide variety of matrices by use of a variety of atom cells. Its advantages stem from the inherently high sensitivity, and the high selectivity that results from the double selection process involved with the choice of energy levels in the selected excitation and detection scheme w1,2x. Compared to atomic emission spectrometry ŽAES., the spectral interferences of LEAFS are far fewer, since elements do not fluoresce unless they are excited by the laser. Ultimately, LEAFS is only limited in sensitivity by the onset of saturation of the atomic energy levels, at which point the signal is no longer proportional to the laser irradiance. However, the laser irradiance is typically set at or above the point of saturation, because this minimizes both quenching and the effects of fluctuations in laser irradiance. Another significant advantage of LEAFS is the long linear dynamic range of the calibration curves, that can cover 5᎐7 orders of magnitude in concentration. The disadvantages of LEAFS include the technical complexity of the tunable lasers required to excite a wide variety of energy levels, and the fact that only one element at a time can be determined, i.e. it is a sequential multielement technique rather than a simultaneous multielement technique. By now, approximately 40 elements have been determined by LEAFS, most of which are the

metallic elements w1,3x. Non-metallic elements are infrequently measured by LEAFS w4,5x, because wavelengths for most of their single photon transitions lie within the VUV region, which is very difficult to reach by modern laser technology. This review focuses on the development of new instruments, and new applications of LEAFS in recent years since the last published reviews. Such developments include solid state tunable lasers, deep UV tunable lasers, the use of charge coupled detectors ŽCCDs., and the applications of LEAFS for trace metal determination in various samples. Some of the more recent review papers w1x review of electrothermal include Sjostrom’s ¨ ¨ atomizer ŽETA. LEAFS, in which fundamental considerations for LEAFS were thoroughly discussed. Also, Sjostrom ¨ ¨ et al. w2x have reviewed the detection limits, and applications of LEAFS, and compared those with other laser atomic spectroscopic techniques; while Hou et al. w3x have reviewed the principles, instrumentation and applications of LEAFS.

2. Theory There have been no significant new developments in the theory of LEAFS in recent years. In this section, only key issues such as optical saturation, excitation and detection schemes, interferences, self-absorption, and background correction techniques will be summarized. For more details regarding the theory of LEAFS, readers are referred to the literature w1,6x. Most of the content discussed in this section can be found in the above references unless they are cited otherwise.

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2.1. Optical saturation Before proceeding to a discussion of optical saturation it is useful to define the terms that are properly used to describe the ‘intensity’ of a laser w7x has discussed these terms, beam. Demtroder ¨ and a summary is provided in Table 1. When an atom is excited by a light source, many physical and chemical processes occur, which include spontaneous emission, stimulated emission, elastic and inelastic collisions. Atomic fluorescence is included in the category of spontaneous emission of radiation. When the energy density of the light source is relatively low, the fluorescence signal is linearly proportional to the source energy density, since the de-excitation rate of an atom is faster than the excitation rate. As a result, most of the atoms are in the ground state ready to be pumped to an excited state. As the energy density of the light source increases, the fluorescence signal becomes independent of the source energy density. This occurs when the deexcitation rate of the atoms is close to that of the excitation rate, and therefore fewer ground state atoms are available. This stage is called optical saturation. A prototypical plot of fluorescence signal vs. source energy density is shown in Fig. 1 w3x. In this figure, when the source energy density is significantly lower than the saturation energy density, the fluorescence signal increases linearly with increasing source energy density. However, the relative slope decreases as the source ap-

proaches the saturation energy, beyond which the slope becomes less than 1, indicating that the source energy is higher than the saturation energy. When the laser energy falls within the linearly increasing region, higher laser energy provides higher signals, and thus higher sensitivity. However, having the laser energy density significantly greater than the saturation energy density not only gives progressively less increase in sensitivity, but it causes spectral broadening which leads to an increased chance of spectral interference. The exact theoretical explanation for the spectral broadening is discussed rigorously by w7x. It is assumed that the atoms abDemtroder ¨ sorb radiation from a source that has a relatively broad spectral profile compared to the atomic absorption profile. Since absorption is wavelength dependent, and the de-excitation rate is not, saturation is strongest at the line center. This causes the signal in the wings of the line to increase with laser energy density, while the center of the line stabilizes to give an apparent broadening of the line and the possibility of increased spectral interferences. It should be noted that Demtroder re¨ gards power broadening as another viewpoint on saturation broadening, although some regard the two types of broadening to be different. One advantage of saturation is that fluctuations in the laser energy density are not reflected strongly in the resultant fluorescence signal because the slope of the fluorescence calibration curve is reduced. Furthermore, quenching is a possible de-excita-

Table 1 Definitions of quantities of emitted and absorbed radiation Term

Units

Definition

Radiant energy

J

Radiant power Žradiant flux. Radiant energy densitya Radiance

W

Total amount of energy emitted by a light source, transferred through a surface, or collected by a detector. Radiant energy per second

J my3

Radiant energy per unit volume of space

W my2 sterady1

Irradiance Žor intensity .

W my2

Power emitted per unit surface element into a unit solid angle Radiant flux incident on the unit detector area

a

Abbreviated in this review as ‘energy density’ for convenience.

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Fig. 1. An idealized saturation curve in laser excited atomic fluorescence spectrometry.

tion mechanism, but as long as the excitation rate dominates the rate of population, which is the case under saturation, then quenching is no longer important. Therefore, in order to obtain the highest signal-to-noise ratio ŽSNR., the optimal laser intensity is the minimum radiant energy density necessary to saturate the transition, which maximizes the signal, and minimizes both spectral broadening and quenching.

escence wavelength is shorter than the excitation wavelength, which provides a better SNR than the Stokes resonance scheme. This detection scheme is useful for elements with excited states close to the ground states. Two-step schemes, such as two photon and two color or double resonance excitationrdetection schemes can be employed for elements whose excitation wavelengths are in the deep UV region where a single laser photon can not be provided by an extant laser. Usually, one tunable laser is set to a wavelength to pump the atoms to an intermediate excited state while the atom is simultaneously pumped from that state to the target excited state by a second laser. This is an efficient scheme for pumping atoms into high-lying excited states w1x. However, the method is elaborate due to the requirement of two tunable lasers. In a thermally assisted fluorescence scheme, atoms are pumped to an excited state by one laser beam, and the subsequent fluorescence is detected from a nearby excited level which is reached through thermal collisions caused by the high temperature of the atom cell. 2.3. Optical interferences

2.2. Excitation and detection schemes There are several types of atomic transition schemes used in the excitation and detection of the analyte, which include resonance, Stokes, and anti-Stokes fluorescence, thermally assisted excitation, and double resonance excitation w3x. A specific type of fluorescence detection scheme is chosen based on a consideration of the atomic structure of the analyte, the atomization temperature in the atom cell, the availability and feasibility of particular excitation and detection wavelengths, spectral interferences, linear dynamic range ŽLDR., and the required limit of detection ŽLOD.. Resonance detection schemes provide a strong fluorescence signal, but they suffer interference from the laser light scattered at the same wavelength. The Stokes non-resonant scheme can be efficient for those elements that fluoresce directly from the atomic levels to which they are pumped by the laser. This scheme provides good sensitivity, and rejection of scattered background. In an anti-Stokes non-resonant scheme, the fluor-

There are several possible optical interferences involved in LEAFS, including stray light from the laser, blackbody radiation from atom cells, nonanalyte atomic fluorescence, molecular fluorescence, and concomitant scattering of laser radiation by the matrix w1,3x. Stray light and concomitant scattering are the limiting sources of spectral interferences for the resonance detection scheme. The scattering of the laser beam generates stray light when striking different parts of the instrument, while concomitant scattering is generated when the light is scattered from particles in the matrix. In a nonresonance detection scheme, if the wavelength difference between fluorescence and excitation radiation is larger than the bandwidth of a monochromator, the stray light can be substantially reduced. A double resonance scheme can be advantageous with regard to the rejection of stray light and scattering, since the fluorescence wavelength is shorter than the excitation wavelength. Linearly polarized laser radiation is another ef-

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ficient way to reduce the scattered light because the scattered light retains the polarization from the laser source, while the fluorescence is nonpolarized. Blackbody radiation, or thermal radiation, results from the high temperatures of sample particles in the atom cell, and from the high temperatures of the walls of atom cells. This radiation is generally in the visible region. To reduce the influence of blackbody radiation, fluorescence wavelengths in the UV region can be used to avoid overlap between the fluorescence wavelength and blackbody radiation, but also requires analyte atoms to be excited to higher transition levels w8x. Modulating the laser light source through the use of an optical chopper, or other means, can subtract the magnitude of blackbody radiation, but does not remove noise on the blackbody radiation unless further noise filtering is performed w9x. Molecules that are present in samples or formed by the reactions of analyte atoms and other species can give spectrally structured or continuous interferences that impose a background on the atomic fluorescence. Also, the presence of molecules may prevent the formation of analyte atoms of interest causing a reduction in the analyte signal. High temperature atomization cells such as graphite furnaces, plasmas, and glow discharge devices can, to a large extent, break bonds and release analyte atoms, which reduces molecular spectral interferences. In the case of plasmas, the high temperatures increase signal sizes, but usually decrease SNR because the plasma background radiation levels are very high. This is a primary reason why the inductively coupled plasma has not been a particularly successful atom cell for atomic fluorescence. 2.4. Self-absorption, prefilter effect, postfilter effect and calibration cur¨ es Self-absorption refers to the re-absorption of the fluorescence by the atoms in the same cell, which can be severe, and leads to a reduction in the fluorescence signal when atom densities are high. In optical saturation, further absorption of light and self-absorption is no longer possible,

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and such a state is considered to be ‘optically transparent’. The LDR is extended by up to two orders of magnitude or so, because the reduction in signal at high concentrations is mitigated when the laser excitation rate dominates self-absorption w10x. The postfilter effect is a special category of self-absorption where the atoms illuminated by the laser produce fluorescence, but atoms outside the illuminated volume are in the ground state, and are able to absorb fluorescence emitted by the excited atoms. In a side-view detection scheme, the fluorescence must go through the un-illuminated area, in which case the postfilter effect is severe w11x. The postfilter effect occurs in both resonance and non-resonance excitationr detection schemes, and can cause significant reduction of the linear dynamic range. Physically, the postfilter effect is non-existent in front-view mode, where illumination of the atom cell and detection of fluorescence are at 180⬚ to each other, because the atoms are completely irradiated in the direction of the detection of the fluorescence. In the prefilter effect, there is a reduction in fluorescence intensity due to the absorption of the source light by atoms near the front part of the illuminated volume from which fluorescence is not detected. The prefilter effect causes early calibration curve roll-off, and consequent reduction in the linear dynamic range. Both postfilter and prefilter effects can be eliminated when optical saturation is reached and front-view mode is employed, due to the absence of ground state atoms in both the illuminated volume and the fluorescence detection path. Most optical spectroscopic methods do not provide absolute measurements, therefore, a calibration curve, constructed from standard solutions, is needed to calculate the sample concentration from a measured signal. The reliability of the calibration curve affects the precision and accuracy of the concentration measured. The linear dynamic range is the concentration range over which the calibration curve is linear, and determines the concentration range over which measurements can be made without inordinate amounts of dilution to bring the signals into a

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useful region of the calibration curve. In atomic absorption spectrometry, the LDR of the calibration curve extends to only approximately two orders of magnitude due to both the inherent low sensitivity of the absorption measurement, bending of the curves caused by stray light, and the non-linearity of the overlap between the source light and the atomic absorption profile. In LEAFS, the LDR of a calibration curve can extend up to seven orders of magnitude. Such a long LDR is achieved due to the inherent high sensitivity of fluorescence measurements, and the control of pre- and post- filter effects by use of optical saturation, and the front detection scheme. Other factors include careful alignment of the instrument, to minimize stray light. Furthermore, it is possible to obtain an even greater LDR by choice of different non-resonance fluorescence lines for low-end and high-end concentrations. 2.5. Background correction Several background correction techniques for LEAFS include Zeeman background correction w12᎐14x, detuning of the laser wavelength w15᎐18x, harmonic saturation spectroscopy w6x, multichannel background correction w6x, time-resolved fluorescence, and variation of the excitation spectral profile. Only Zeeman background correction and detuning of the laser wavelength will be discussed here, as they are the most reliable and easy to implement. Readers are referred to the literature for details of other background correction methods w6,12᎐19x. Zeeman background correction is a very effective background correction method for fluorescence detection. The atomic energy levels of an analyte are split by the application of a strong magnetic field to prevent absorption of the laser light by the atoms. The energy levels of matrix components can be split in the case of a few molecules such as O 2 , NO and ClO 2 and NO 2 , due to the interaction of the magnetic field with unpaired electrons or unquenched orbital angular momenta, but these splittings are only of the same order of magnitude as the rotational energies and can be neglected, which means that molecules that cause background signals still absorb the same amount of laser light when the

magnetic field is on. When the magnetic field is not applied, both atoms and molecules absorb the laser light. Therefore, a background-corrected measurement can be obtained when the signal measured with the applied magnetic field is subtracted from that measured without it. The key to the technique is to have a laser of narrow linewidth and a strong magnetic field. However, only half the laser shots are used for the signal measurement, and thus the signal is sampled for half the time compared to the signal that would be sampled without the use of Zeeman background correction, which results in a slightly lower SNR. When employing the technique of detuning of the laser wavelength, also called wavelength modulation, the laser wavelength is automatically and rapidly oscillated in and out of resonance with the analyte atomic transition by piezo-electrically driving a wavelength-tuning mirror in the tunable laser w20x. Unstructured background and scattered light can be effectively subtracted because the measurement off the excitation wavelength is usually an adequate measurement of the background at the line itself. The amount of shift in wavelength can be as small as a few tens of picometers. This background correction technique provides high temporal and spectral resolution yet is relatively easy to construct, and should be able to correct for all of the major background interferences such as stray light, scattering, flame emission in flame-LEAFS and blackbody radiation in graphite furnace-LEAFS. However, detuning of the laser wavelength is not applicable for correction of structured background where the background at the same wavelength as the resonant line can not be estimated by measurement of the background slightly to one or both sides of the analyte line. There are no systematically documented instances of this type of background in the context of wavelength modulation, although it is probable that some reported interferences may be of this type. In such cases Zeeman background correction would be more effective. Wavelength modulation can be done manually by moving the laser wavelength to another close wavelength for a separate measurement of the

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background, which is probably the most common technique.

3. Instruments 3.1. Lasers As an excitation source for LEAFS, a laser should be reliable and possess characteristics such as wide wavelength tuning range from the visible to the deep UV, narrow spectral line width of the same order of magnitude of the linewidth of the atomic absorption line, high pulsed repetition rate or continuous wave ŽCW., high output energy for saturation of atomic transitions, ease of use, compactness, and a relatively low price. Various lasers, such as dye lasers, diode lasers, and optical parametric oscillator- ŽOPO. based lasers, have been used as excitation sources for LEAFS. Among those lasers, dye lasers have been most frequently used, but OPO lasers are rapidly gaining popularity due to their wide tunable range, high pulsed energy, and cost that is comparable to tunable dye lasers. Yet, none of these lasers has been convincing as an ideal excitation source for LEAFS. This is one of the reasons that LEAFS has not had commercial success, despite its ultrahigh sensitivity and selectivity that has been demonstrated for most elements and many matrices. 3.1.1. Dye lasers While dye lasers are most frequently used in LEAFS, there have been no significant advances in dye lasers in recent years. Dye lasers have a tunable range of approximately 30 nm with each dye, while typically 15᎐20 dyes are required to encompass the entire visible wavelength range. Frequency conversion devices are used for the wavelength range of 180᎐320 nm, where most excitation wavelengths lie for LEAFS. The disadvantages of a dye laser include the limited tunable wavelength range for each dye, their limited operating lifetime, and the toxicity of the dyes or their solvents. Therefore, the messy and timeconsuming task of changing of dyes is inevitable, especially when performing multielement analyses. Dye-doped polymer materials have been de-

Fig. 2. Experimental arrangement for dye laser calibration with an electrodeless discharge lamp.

veloped w21x with laser-damage resistant polymers that have durability as high as most of their inorganic glass and crystal counterparts, but the problem of dye degradation still exists. Before a dye laser can be used for LEAFS, its wavelength must be calibrated, which is commonly achieved by aspirating a concentrated solution of the element of interest into a flame, after which the laser wavelength is tuned to the excitation wavelength of the standard solution. Cheam et al. w22x have developed a simple, safe, and inexpensive non-flame method for wavelength calibration. This method utilizes a pierced mirror and a commercial electrodeless discharge lamp ŽEDL., as shown in Fig. 2. For lead analyses, a lead EDL lamp is shone onto the pierced mirror and focussed into a spectrometer system designed for LEAFS. The dye laser beam is then passed through the atomizer and into the EDL. The dye laser is first roughly calibrated using a previously calibrated monochromator, and then finely tuned by manually dialing the dye laser until the maximum fluorescence of atoms in the EDL is obtained. Cheam et al. w22x did not show the complete optical arrangement for collection of fluorescence from the graphite furnace in Cheam et al. w22x. However, they showed a more detailed representation of the instrumentation in another paper concerned with the direct determination of lead in Great Lakes water w23x. Overall, they demonstrated that it was possible to monitor the

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stability of the dye laser during analytical measurements, and take corrective action if necessary. While Raman shifting has been used predominantly for frequency conversion in the spectral range around 200 nm, the advent of commercial ␤-barium borate ŽBBO. crystals has provided a more efficient means of frequency conversion into this region. Heitmann et al. w24x reported the design and performance characteristics of a tunable dye laser system for sum frequency mixing ŽSFM. in a BBO crystal. Compared with the Raman shifting method, higher conversion efficiency of the generation of tunable UV laser radiation below 205 nm was achieved. The laser system consisted of two tunable pulsed dye lasers pumped synchronously by the second harmonic of a commercial Nd:YAG laser. The output of the first dye laser was frequency doubled, and then mixed by SFM in the BBO with the output radiation from the second dye laser. With a pump energy of 67 mJ from the Nd:YAG laser, the output of the SFM was 100 ␮J at 196 nm. This laser system was designed as an excitation source for the determination of selenium in human blood samples. Overall, the laser system was elaborate because two dye lasers were involved, but the output energy of 100 ␮J at 196 nm was usefully high. A dual-wavelength dye laser system for the UV has been developed and used for simultaneous multielement measurements by LEAFS w25x. This laser system was composed of oscillator and amplifier stages, and pumped by the second harmonic of a commercial Nd:YAG laser. Also, the design included a frequency doubling unit. Fig. 3 shows the oscillator layout. Two tuning mirrors were used in order to achieve dual-wavelength operation. These two mirrors were shifted by 30 mm to take both upper and lower parts of the diffracted beam. This geometrical division of the laser beam between the two wavelengths was maintained inside the dye cell to limit mode competition. By driving the shifted reflection mirror out of the diffracted order, this laser system could be switched between dual- and single-wavelength operation. After exiting the oscillator stage, the laser beam was directed to the amplifier stage

Fig. 3. Oscillator layout of a dual-wavelength dye laser system.

and then to a frequency doubler which was composed of two KDP crystals, placed one after the other and rotated in opposite directions to compensate for beam shift. To maintain long-term wavelength stability, the oscillator and the KDP crystals were temperature-stabilized at 33 " 0.05⬚C. Finally, in order to measure four elements simultaneously, namely lead, manganese, nickel, and cadmium, two dual-wavelength dye lasers were constructed by sharing one Nd:YAG laser, using 355 nm and 532 nm as pump beams. The four independently tunable UV output wavelengths, were combined through a long-pass filter and then directed into a graphite furnace for excitation. The resultant atomic fluorescence of the four elements was collected with an off-axis ellipsoidal mirror and focussed onto a quartz glass fiber bundle, which replaced the monochromator entrance slit. An array detector was used for signal detection. Fig. 4 shows the simultaneous

Fig. 4. Simultaneous excitation spectra of four elements in ETA-LEAFS. Two dual-wavelength dye lasers were used and detection was achieved with an array detector Žreproduced with permission from Cheam et al. w23x..

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excitation spectra of those four elements under an atomization temperature of 2600⬚C. By exploiting the array detector’s simultaneous multielement detection capabilities, LODs at femtogram levels for these elements were easily attainable. However, array detectors are limited by their readout rate and have an inherently lower sensitivity in the UV compared to a photomultiplier tube, consequently single element detection limits are usually poorer. Su et al. w20x constructed an excimer pumped grazing incidence dye laser with wavelength modulation capability for LEAFS background correction. To achieve wavelength modulation, a piezoelectric pusher was used to drive a wavelengthtuning mirror in a laboratory-constructed grazing incidence dye laser. The pusher was capable of operation up to a modulation frequency of 1 kHz, which was accomplished by a sine wave applied to an amplifier. The sine wave was synchronized with the laser pulses so that alternate laser pulses measured data at the analytical line Žon-line. and at a wavelength displaced to one side of the analytical line Žoff-line.. The background-corrected signal was then obtained by subtracting the off-line ‘background’ from the on-line ‘signal plus background’. Effective and quantitative background correction by wavelength modulation was demonstrated by measurement of sodium resonance fluorescence in an air᎐acetylene flame and thallium non-resonance fluorescence in a graphite furnace. 3.1.2. Diode lasers Diode lasers are attractive for LEAFS and atomic absorption because of their potential for low cost, narrow spectral linewidth, and small size. Hence, there have been a number of papers that have explored the use of diode lasers in atomic absorption and atomic fluorescence. The active lasing region, pn junction, of a diode laser is a spatially confined layer in the form of a stripe, which is a few micrometers wide, and less than 1 ␮m thick. When an injection current is applied to the active region of the diode between the n- and p-type cladding layers, electrons and holes will move to the pn junction. The recombination of these electrons and holes at the pn

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junction results in emission of radiation. When the density of electrons and holes are large enough, this radiation can stimulate the recombination of electrons and holes. The lasing action can be achieved if the amplification of radiation exceeds the loss; hence population inversion is achieved. The edges of the laser substrate act as resonant mirrors because of the high refractive index of the semiconductor material. The performance characteristics of diode lasers include compactness, narrow line width, limited wavelength tunability, ease of wavelength modulation, and cost efficiency. However, the main disadvantages of diode lasers are low peak power, limited wavelength range, limited wavelength tunable range of each diode Žapprox. 20 nm., and the temperature-dependence of the output wavelength Ža temperature stability of a few mK is required.. Many types of diode lasers are needed to cover the wavelength range of 625᎐1600 nm. While the output wavelengths can be frequencydoubled into ultraviolet region, the output power is generally low. Nichia Chemical Industries offers a violet diode laser with an output wavelength typically at 405 nm with a nominal optical output power of approximately 5 mW. Niemax et al. w26x recently characterized this gallium nitride ŽGaN. laser diode which provides an alternative to other means of obtaining blue and near-UV radiation. These lasers, however, are still very early in their developmental stage, and Niemax et al. point out that frequency doubling of longer wavelength diode lasers is still more competitive. While diode lasers have been used as narrow line sources mostly for atomic absorption spectrometry, they have also been used to excite atomic fluorescence. The applications of diode lasers in atomic spectroscopy have been reviewed by Niemax and co-workers w26᎐28x, and Imasaka et al. w29x. Several other reviews or book chapters have also briefly covered this topic w30,31x. A single mode wavelength modulated diode laser has been used for the determination of rubidium by LEAFS w32x. The fluorescence of rubidium was excited either at 780.023 or 794.67 nm. A detection limit of 0.2 ng mly1 was achieved for rubidium in a hydrogen᎐air flame. A wavelength reference system was necessary for the

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elimination of noise originating from phase shifting in the lock-in amplifier detection system due to random output wavelength fluctuations of the diode laser. Nevertheless, it was concluded that diode lasers are superior to conventional excitation sources, and could compete very effectively with much more expensive and complicated laser systems in many branches of spectroscopy. For spectroscopic applications, the most useful advantages of diode lasers are compactness, relatively low price, and ease of wavelength tunability through temperature and electrical manipulation, albeit over a relatively small wavelength range. Hence, several diode lasers can be arranged for simultaneous measurements, which is difficult or impractical for other types of lasers. Zybin et al. w33x have demonstrated this capability by the simultaneous measurements of rubidium and lithium by LEAFS. The analytes were excited in a commercial graphite furnace atomizer by CW diode lasers emitting at 670.776 and 780.027 nm, respectively, while a simple photodiode without interference filters or a polychromator was used to detect the fluorescence. The diode lasers were tuned and temperature-stabilized, with a longterm wavelength stability of better than "0.4 pm. A low-frequency chopper, approximately 10 Hz, alternately chopped the beams of both lasers, and the time constant of the lock-in amplifier was 10 ms. The preliminary detection limits for lithium and rubidium were 10 and 20 fg, respectively, which were at least two orders of magnitude better than those achieved with laser atomic absorption spectrometry. However, diode lasers are still far from ideal lasers for LEAFS, because of their limited tunable range, unavailability in the UV range where most excitation wavelengths lie, and insufficient output power to saturate most atomic transitions. 3.1.3. Optical parametric oscillator-based lasers In recent years, a new type of all-solid-state laser, the OPO-based laser, has been explored for use as an excitation source for LEAFS w31,34᎐36x. OPO lasers have been discussed in detail by Hou and co-workers w31,36x. However, the principles of OPO lasers are summarized here, as they are relatively new lasers. The OPO principle is basi-

Fig. 5. Ža. The sum frequency of signal, ␻ s , and idler, ␻ i , is equal to the pump frequency, ␻ p . Energy conservation is observed. Žb. Momentum conservation has to be observed to fulfill phase matching.

cally the reverse of frequency mixing. The parametric interaction is that between the electric field of a pump source and a birefrigent non-linear material, usually a ␤-barium borate ŽBBO. crystal, which occurs when a sufficient intensity of the pump laser is focused into the crystal. Unlike conventional lasers, an OPO laser does not involve population inversion, as it does not depend on any form of atomic or molecular transition. The electric field of the pump laser directly drives the electrons in the non-linear crystal in such a way that the pump photon at frequency ␻ p is split into two photons. The signal photon at a frequency of ␻ s and the idler photon at a frequency of ␻ i are depicted schematically in Fig. 5. The sum of the frequencies of signal and idler must equal that of the pump frequency, ␻ p s ␻ s q ␻ i , to maintain conservation of energy. There are infinite pairs of wavelengths available to achieve this, which is the origin of the tunability of an OPO laser. Phase matching of the momentum vector, K p s K s q K i , determines a specific frequency pair. The phase matching condition can be achieved through the manipulation of the temperature, or the orientation of the BBO crystal. The latter is called angle tuning, and it is more commonly used due its ease of manipulation. A 10-Hz commercial OPO laser that has been characterized is shown in Fig. 6 w34x. This laser is composed of a master oscillator, a power oscillator, and a frequency doubling section. It has a

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Fig. 6. Layout of a typical commercial OPO laser ŽSpectraPhysics MOPO built around 1995..

wide tunable wavelength range, and high output pulse energy throughout the tuning spectrum ŽFig. 7.. The advantages of this OPO laser include narrow spectral linewidth Ž0.1᎐0.3 cmy1 ., computer-controlled wavelength tuning, and relative ease of use. Compared with dye lasers, there is no apparent degradation in the active medium, which is usually a BBO crystal. In addition, the OPO process can produce wavelengths that are difficult for traditional lasers to generate, because it does not depend on atomic or molecular transitions. The main disadvantages of the OPO laser include the requirement for stabilization of the environ-

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mental temperature usually within at least "2.5⬚C, and the replacement of flashlamps for the Nd:YAG pump laser and subsequent, fairly complicated, optical alignment of the entire OPO laser system. The flashlamps have an average lifetime of 25᎐30 million shots, which usually lasts up to 3 or 4 months. This is a shorter lifetime than the flash lamps that are used to pump dye lasers, as the threshold flash lamp energy required for pumping the BBO crystal is higher than that for pumping dye solutions. Furthermore, OPO lasers tend to be operated at low repetition rates, typically 10᎐30 Hz, which degrade the precision and detection limits of all measurements in all atom cells. The rapid tuning over a wide wavelength range has made rapid sequential, multielement measurements possible by LEAFS. A flame LEAFS instrument, based on the OPO laser mentioned above, allowed approximately 640 measurements to be made in approximately 6 h, with triplicate measurements of all sample solutions and the construction of aqueous calibration curves w34x. Accurate analytical results for a NIST 2704 river sediment standard reference material were obtained for cobalt, copper, lead, manganese, and thallium. Cadmium, cobalt, lead, manganese, and thallium in the same sample were determined by

Fig. 7. Pulse energy output vs. wavelength of the OPO laser of Fig. 7.

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graphite furnace LEAFS in sequential multielement mode w35x. The excitation wavelengths ranged from 228 to 304 nm, which were scannable in 15 min. This allowed the analysis time to be limited primarily by the slow heating cycle of the graphite furnace rather than the laser wavelength tuning. The significance of these experiments is that multiple elements can be measured in a sequential mode, which is very difficult for a dye laser to perform due to the messy, time-consuming work of changing dyes. Although simultaneous multielement measurements could be done by use of a multi-wavelength dye laser system, this type of dye laser system would be complicated, and the number of measurable elements would be limited. For multielement LEAFS by use of multiple diode lasers, the number of diode lasers that can be used simultaneously is only limited by the available number of power supplies. For sequential measurements, almost all elements previously determined by LEAFS can be covered by the output wavelengths of an OPO laser. For elements such as phosphorus, arsenic, antimony, selenium, tellurium, chromium, and dysprosium, for which the excitation wavelengths either fall below 220 nm, or fall in the degeneracy region of the OPO laser, various frequency mixing techniques could be used, although this has not yet been demonstrated in OPO-LEAFS. 3.2. Detection optics In LEAFS, the arrangement of optics should lead to efficient collection of fluorescence photons with concurrent discrimination against unwanted background radiation. Yuzefovsky et al. w37x summarized the main requirements for an optical arrangement for a fluorescence detection system as follows: Ž1. maximal collection efficiency over a wide range of wavelengths, 190᎐800 nm; Ž2. maximal discrimination between background and analytical signal without significant sacrifice in the latter; Ž3. minimal losses of analytical signal along the optical path due to reflection from different surfaces and apertures; Ž4. ease of alignment; and Ž5. commercial availability and relatively low price of the optical components. A computer program based on a rigorous ray-

tracing algorithm has been developed by Farnsworth et al. w38x to calculate the collection efficiency of a combination of lenses and stops as a function of position in Cartesian space. This numerical method has been used to model the atomic fluorescence from a graphite furnace. For front surface illumination, they compared optical collection efficiencies for biconvex, plano-convex, and achromatic lenses. It was concluded that the most efficient arrangement consisted of one flat mirror and a pair of achromatic lenses, while a pair of plano-convex lenses provided a useful compromise in cost and efficiency. Differences in the collection efficiency were reduced when the signal was integrated over the 2-mm diameter of the excitation laser beam and the length of the graphite furnace. In experiments, this resulted in an LOD for thallium of 0.1 fg w39x. The ray-tracing calculation method could be used more extensively to optimize such instrumental parameters as slit width and height, and laser beam diameter. A single 90⬚ off-axis ellipsoidal mirror fragment has been used to collect fluorescence in a dispersive detection system for graphite furnace LEAFS, in the front surface illumination mode w37x. The performance of this new optical arrangement was compared with that which employed a plane mirror together with biconvex or plano-convex lenses. BEAM-4 w40x, an optical ray tracing program, was used for the calculations of spatial ray distribution and optical collection efficiency for the optical configurations. The SNR and the fluorescence collection efficiency were also studied as a function of the optical components. For cobalt and phosphorus, the SNR with the ellipsoidal mirror was stable within 10᎐20% during "8 mm shifts in the position of the detection system from the focal plane of the optics. The best experimental collection efficiency was obtained with the ellipsoidal mirror, and was in qualitative agreement with the results of simulations. The lower optical aberrations allowed more efficient spatial discrimination between the fluorescence and blackbody radiation, which resulted in better LODs. The best LOD of cobalt was found to be 20 fg, which was improved by a factor of 5, compared with that obtained with conventional optical arrangement with otherwise the same instrumenta-

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tion. Overall, the use of an ellipsoidal mirror offered high collection efficiency, excellent sensitivity, and facile optical alignment. 3.3. Detection systems A detection system consists of a wavelength selective device and a photon detection device. A monochromator or a narrow band filter w41x is usually applied as a wavelength selective device. A photomultiplier tube is the most common photon detection device in LEAFS due to its excellent noise properties, high amplification, fast response, and low price. Readers are referred to other sources w42x for more information because there are very few recent publications in the study of this traditional detection system for LEAFS. In the meantime, the use of CCDs in LEAFS is being studied. An intensified charge coupled device ŽICCD. was first used by Marunkov et al. to replace a PMT in a detection system for LEAFS w43x. Compared to a PMT, there are several advantages for the utilization of an ICCD in LEAFS. Its ability to simultaneously detect several wavelengths results in a more rapid search for the most sensitive excitation-fluorescence wavelengths. This is markedly evident when elements with complicated electronic structures are determined, including such elements as vanadium, chromium, manganese, iron, cobalt and nickel, etc. Absolute sensitivity is also improved by simultaneous multi-wavelength detection. The study of background signals caused by matrix interferences, blackbody radiation and laser scattered light both at and around the wavelength of detection are easily obtainable, which could lead to an improvement in selectivity. Also, studies can be done to achieve a better understanding of the time and wavelength dependence of matrix interferences. Furthermore, the height distribution during atomization and diffusion processes in a graphite furnace can be studied by use of a two-dimensional ICCD. The disadvantages of an ICCD include high price, the slow read-out rate, and low sensitivity in the UV, which depends on the photocathode sensitivity of the intensifier. In the same paper, nickel was studied in an ICCD-ETA-

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LEAFS system. The most sensitive excitationr fluorescence wavelength scheme was identified. The detection limit was found to be 15 fg with the ICCD and 10 fg with a PMT. Two-dimensional imaging of flame species such as oxygen, nitrogen and hydrogen atoms has been implemented by two-photon LEAFS coupled with a CCD w44x. Single-shot visualization of oxygen and hydrogen atoms was demonstrated. Proper gating and filtering minimized spectral interferences. Different approaches for compensating for non-linear laser intensity dependence were compared. The experiment revealed the potential for qualitative measurements of the spatial distributions of species in flames. Masera et al. w45x combined a tunable dye laser with a pulsed ICCD camera to obtain time and spectrally resolved mapping of species atomized in a graphite furnace. The main molecular interference was identified in the determination of gold in a silver matrix. Formation of condensed diatomic molecules during the atomization process results in strong spectral interferences due to the fluorescence overlap of the molecules with analyte atoms. Both absorption and LEAFS methods were utilized to record images of the furnace during the atomization step with the probe laser wavelength tuned to a gold transition, 242.8 nm. The gold atom distribution in the furnace was not disturbed for up to 200 ␮g of silver added as a matrix. In a later paper w46x, the same experimental setup was employed to identify other matrix interferences namely the formation of AgH and Ag 2 , which strongly inhibits the analyte signal. The origin and spatio-temporal behavior of the condensation were studied. Optimization of such parameters as the use of neon gas as a purge gas, a preliminary evaporation of part of the matrix prior to atomization, and the use of a transverse heated atomizer were applied to minimize the matrix interference, resulting in a strong fluorescence signal and improved limits of detection of precious metals. As a result, gold, iridium, palladium, platinum and rhodium were determined in a silver matrix. This study represents one of the more challenging matrices that have been encountered in LEAFS.

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3.4. Atom cells for LEAFS Atom cells must first vaporize analytes into a gaseous phase before a laser can excite the analyte atoms. Any device that generates a transient or continuous atom cloud can be considered for use as an atom cell for LEAFS. The most frequently used atom cells are flames, graphite furnaces, inductively coupled plasmas, cathodic sputtering cells, and laser ablation-induced plumes. 3.4.1. Flames Flames were the earliest atom cells used for LEAFS due to their popularity in atomic absorption spectrometry ŽAAS., and they are still popular atom cells in LEAFS due their simplicity and reliability. A flame provides a continuous source of atoms, and hence it can also be used as a wavelength calibration device for the excitation laser. Although burner heads used in flame AAS could be easily adopted for LEAFS w19x, specially designed burner heads have been employed in many cases. Round or square burner heads, which consist of many small holes or capillary tubes, are designed to minimize the pre- and post-filter effects. In order to minimize the quenching of the fluorescence signal and reduce the flame background, a sheath of inert gas may be used to surround the flame to minimize the interaction of the flame gases with air w47x. Butcher et al. w6x reviewed the detection limits of approximately 50 elements obtained by flame LEAFS, most of which remain unchanged because of the limited amount of work done on flame LEAFS in recent years. Multielement flame LEAFS has been used to characterize a tunable optical parametric oscillator laser system w34x. Rapid, sequential, multielement analyses were demonstrated by slew scan of the laser. Analytes such as cobalt, copper, lead, manganese and thallium in Buffalo River Sediment were determined with high accuracy. Compared to the flame LEAFS literature, the detection limit of cobalt was improved by two orders of magnitude to 2 ng mly1 . Walton et al. w48x coupled high-performance liquid chromatography with flame LEAFS to study organomanganese and organotin compounds in rat urine. A study of the signal and noise charac-

teristics of various detection schemes, such as dispersive, non-dispersive, and front surface was reported. Transverse resonance detection of the fluorescence of manganese at 280 nm was found to provide the best detection limit, 0.07 ng mly1 . The dominant noise in non-resonance Ž280r403 nm. dispersive and non-dispersive fluorescence was primarily flame emission. The signal size observed for a long path flame Ž100 mm. was approximately four times larger than that observed for a shorter flame Ž10 mm. by the front view arrangement. This suggested that such an arrangement was unable to collect the entire fluorescence signal emitted from the full length of the long path flame. The detection limits for various organomanganese species by HPLC-flame LEAFS were two orders of magnitude better than HPLC-UV and HPLC-continuous source excited flame atomic fluorescence spectrometry. Anwar et al. w49x used an air᎐H 2 flame as an atom cell to determine tin by LEAFS. The effects of organic content and acids on the accuracy of the analysis were discussed. Some elements, such as arsenic, have strong absorption lines only below 200 nm. Resonance excitation of such elements requires more complicated tunable laser systems. However, Hueber et al. w50x detected arsenic atoms in a hydrogen᎐air flame by taking advantage of the coincidence that a fixed wavelength argon fluoride ŽArF. excimer laser, emitting at 193.0᎐193.2 nm, somewhat overlaps the arsenic absorption line at 193.7 nm. A detection limit of approximately 20 ng mly1 was achieved, with limiting noise from scattering of laser radiation. Turk et al. w51x studied the correlation between random fluctuations of simultaneously detected laser-enhanced ionization ŽLEI. and LEAFS signals with respect to each other and laser power for spectral background correction. The correlation obtained indicated that the major sources of noise for both LEI and LEAFS are of similar origin, and therefore must be associated with nebulization, flame atomization, laser photoexcitation, or other processes that determine the population of excited Na atoms in the flame after the laser pulse. The effect of laser energy on both LEI and LEAFS signal produced a strong corre-

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lation, from which it can be concluded that atom density fluctuations are an equivalent or greater source of noise than laser power fluctuations. The correlated LEAFS signal was then used to reduce the noise level of the sodium LEI signal. This gave a factor of 1.9 signal-to-noise improvement over the uncorrelated LEI signal. The use of simultaneous detection of LEAFS for background correction and noise reduction of broadband interference on LEI was successfully demonstrated, but this technique was only effective for noise reduction at wavelengths less than approximately 0.2 nm from the peak resonance. Cignoli et al. w52x proposed the basic theory and application of a technique for single-shot determination of the saturation parameter in LEAFS, which is particularly important for turbulent combustion diagnostics where the usual procedures can not be applied to the determination of the saturation curve. Using a flame as the atom cell, the laser pulse was split into two pulses that probed the atoms in the flame at different delay times and intensities. A saturation curve was constructed that agreed with theoretical predictions. Discrepancies in the intensity of the laser profile, the so-called ‘wing effect’, were corrected in two widely separated zones on the saturation curve. The possibility of absolute determination of lead was also discussed. 3.4.2. Electrothermal atomizers The combination of electrothermal atomizers ŽETA. with LEAFS has been shown to be a rather successful technique for ultra-trace element determination. The ETA-LEAFS technique has been demonstrated to detect trace elements in solid, liquid and gaseous samples w1x, and only a small volume of sample is required for analysis. Detection limits in the sub-femtogram range, corresponding to concentrations at the sub-pg mly1 level and linear dynamic ranges up to seven orders of magnitude have been reported w53x. In general, for ETA-LEAFS, there are three types of electrothermal atomizers: graphite cups, graphite rods and graphite tubes. Graphite cups and graphite rods are open type atomizers and graphite tubes are closed type atomizers. Open type graphite atomizers may lead to loss of ana-

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lyte atoms by molecular formation and condensation due to the temperature inhomogeneity of the observation zone. Graphite cups may suffer from matrix interference for real sample analysis. On the other hand, graphite tubes, especially when a graphite platform is inserted in the tube, provides a more stable and homogeneous temperature environment within the observation zone and fewer matrix interferences for real sample analysis. Hence, graphite tube atomizers are generally considered to be superior to graphite cups and graphite rod atomizers, although graphite cup atomizers are useful for volatile elements in simple matrices. Many applications for real sample analysis by LEAFS have been conducted in graphite tubes adapted from atomic absorption and are discussed later in the applications section. The readers are referred to two review articles written by Butcher et al. w6x and Sjostrom ¨ ¨ w1x for technical details about ETA-LEAFS. Two-photon LEAFS has continued to provide alternative excitationrfluorescence schemes for some elements for which excitation wavelengths are unreachable by single photons. Resto et al. w54x used a two-photon scheme to excite mercury atoms in a graphite tube. The two excitation wavelengths were set at 253.7 and 435.8 nm, respectively. The best detection limit, 90 fg, was achieved when the fluorescence was collected at 546.1 nm with a LDR of five orders of magnitude. A poorer detection limit of approximately 9 pg was obtained when the fluorescence was collected at 407.8 nm but with an extended LDR of seven orders of magnitude. Vera et al. w55x used two dye lasers which were pumped by a Nd:YAG laser to perform two-photon ETA-LEAFS of several elements. Collisional coupling between the highest energy level pumped by laser radiation and the nearby levels resulted in several fluorescence transitions in the ultraviolet region for each element. The detection limits of indium, gallium and ytterbium were 2, 1 and 220 fg, respectively. Sjostrom ¨ ¨ et al. w56x improved the detection limit of vanadium to picogram levels by two photon ETA-LEAFS in a side-heated, integrated contact, graphite tube furnace. The fluorescence was detected in the ultraviolet region by a solar-blind photomultiplier to eliminate scattering of visible

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laser photons. The limiting factor for the detection of vanadium was the contamination from the graphite furnace, which caused fluctuations of the background level of the vanadium signal. Petrucci et al. w57x used two-photon anti-Stokes LEAFS to determine gold atoms in a graphite furnace and in a flame. The fluorescence was detected at 200 nm, which produced the best detection limit of gold at 3 fg. This sensitivity was adequate for the determination of gold in size-segregated, atmospheric particulate samples. Chekalin et al. w58x employed a low pressure ETA-LEAFS system to study atomization mechanisms of silver in a graphite furnace. They found that the integrated fluorescence signal was proportional to the pressure, which indicated the absence of gas phase reactions influencing the creation of free silver atoms. The temperature for the first appearance of the fluorescence and the optimum temperature needed to produce the maximum fluorescence decreased with decreased pressure, which indicated repeated desorption and adsorption processes occurring between the analyte atoms and the wall of the graphite atomizer. The activation energy of atomizing silver was found to be strongly dependent on the pressure, the amount of silver and the heating rate. Lonardo et al. w59x developed a model which predicted that an optimal pressure would minimize the gas phase interactions between the analyte and the matrix, while prolonging the analyte residence time. However, this model was unable to predict whether this pressure would be below or above atmospheric pressure. Their experiments to atomize tellurium and cobalt under reduced pressures indicated that atmospheric pressure seemed to be close to the optimal working pressure. Liang et al. w60x used an anisotropic graphite furnace, heated by capacitive discharge, as an atom cell in LEAFS. The use of a capacitive discharge furnace allowed for a shorter integration time because the furnace pulses were temporally narrower, hence less background noise was integrated and detection limits improved in relative terms. Some of the improvement in detection limits was mitigated by the short, 7᎐8 mm, length of the capacitively heated furnace. A detection limit of 5 fg of thallium was obtained with

an integration time of 80 ms and a laser repetition rate of 500 Hz. Thallium concentrations were determined with good accuracy in NIST biological samples. Matrix interferences from calcium, sodium chloride, and potassium chloride on the thallium signal were also investigated and found to be largely similar to LEAFS in normal atomic absorption furnaces. Irwin et al. w61x applied transverse Zeeman background correction to ETA-LEAFS. The LDR was not adversely affected by the Zeeman ETALEAFS system. The detection limits of lead and cobalt were within a factor of 2 of those without the correction. Transverse Zeeman background correction was found to correct for ETA blackbody radiation and backgrounds caused by the addition of 20 ␮g of aluminum chloride to cobalt aqueous standards. Bolshov et al. w62x determined trace amounts of cadmium by LEAFS in a graphite cup. The results from two excitation wavelengths were compared and the best detection limit was 3.5 fg. The limiting factors for the detection limits were also discussed. The sources of background and the excitationrfluorescence wavelength pairs in the detection of bismuth were investigated w63x. A detection limit of 2.5 fg was reported. 3.4.3. Glow discharge cells Glow discharges ŽGDs. in some respects are considered to be ideal atom cells for LEAFS. As atoms are produced through sputtering, GDs may be less affected by matrix variation. The low pressure and inert gas environment of the discharge provide reduced absorption line broadening and less fluorescence quenching. Solid samples can be analyzed directly by glow discharge cells, which eliminates sample dissolution and dilution processes, so that sample contamination and the time for analysis are minimized. For GD-LEAFS, the GD can be operated in a direct current mode or a pulsed mode. In the pulsed mode, the background is minimized if the fluorescence is observed when the discharge is turned off w64x. Although GD cells have such advantages, the number of publications of their applications in LEAFS is very limited. Glick et al. w65x studied lead and iridium by LEAFS in a

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pulsed hollow-cathode glow discharge cell. The detection was made 100 ␮s after the discharge was turned off. The atom population was large during the dark period while the background emission was negligible. Dashin et al. w66x studied a glow discharge cell theoretically and experimentally for LEAFS in both continuous and pulsed operation modes. Lead in solid copper standards was studied with a detection limit of 40 ng gy1 . An improved glow discharge cell, a planar magnetron sputtering device, was later reported by the same authors w67x. The detection limits of silicon in a high-purity indium matrix and silicon in a high-purity gallium matrix were found to be 0.4 and 1 ng gy1 , respectively. Womack et al. w68x studied a GD-LEAFS system that consisted of a copper-vapor laser pumped dye laser and a graphite electrode glow discharge atom cell. Lead solution was deposited on the electrode and the detection limit was calculated to be 15 pg. Deavor et al. w69x used a similar system but with a planar copper rod as the cathode. No difference in SNR was found for a continuous discharge vs. a pulsed discharged, or when a stopped argon flow vs. a continuous argon flow were compared. The best SNR was obtained when measured beneath the anode. The detection limit of lead was found to be 2 pg for peak height measurement and 60 pg for peak area measurement. Davis et al. w70,71x studied lead, thulium, europium and yttrium by LEAFS in a miniature glow discharge in order to improve the detection limits by increasing the atom density in the atom reservoir. A nickel electrode and nanoliter-sized solution aliquots were used. The optimum chamber pressure and current were found for each element, and the detection limits for lead, europium, yttrium, and thulium were 0.03 pg, 2 fg, 1.2 pg and 0.08 fg, respectively. 3.4.4. Plasmas The plasmas that have been used as atom cells for LEAFS include inductively coupled plasmas ŽICPs., microwave induced plasmas ŽMIPs. and direct current plasmas ŽDCPs.. Strong background emission from ICPs poses a serious problem for the detection of the fluorescence, which is especially true for resonant detection LEAFS.

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Butcher et al. w6x and Sjostrom ¨ ¨ et al. w2x listed the detection limits of elements by the use of ICPLEAFS, and few improvements have been reported since then. Hobbs et al. w72x utilized the LEAFS method to study fluorescence signals from strontium ions in an ICP. The fluorescence was investigated as a function of nebulizer gas flow rate and applied power in the presence and absence of concomitant species. The results, when compared with those obtained by ICP-MS, indicate that the matrix effects in ICP-MS seem to originate in the plasma. However, theoretical consideration of ambipolar diffusion shows the absence of mass dependence, therefore massdependent matrix effects do not originate in the plasma. The authors indicate that simultaneous examination of the same plasma by fluorescence and MS is required to accurately determine the relative contribution of matrix effects originating in the plasma and those observed by ICP-MS. Simeonsson et al. w73x used single and two-step excitation LEAFS to study metals in an ICP. The best detection limits for silver, gold, cobalt, nickel, lead, palladium, platinum and scandium were reported to be 0.8, 4.2, 15, 3, 5, 0.7, 3.9, 3.3 and 0.2 ng mly1 , respectively. Vera et al. w74x applied ICP-LEAFS to determine concentrations of uranium isotopes, 235 U and 238 U, in a concentrated and complex matrix. The non-resonant fluorescence detection proved to be free of spectral interference and the detection limit of uranium was estimated to be 2 ␮g mly1 . However, the detection limit was poorer than those obtained by either ICP-MS or high-resolution ICP-AES. A very limited amount of work has been done on DCP-LEAFS and MIP-LEAFS. Hendrick et al. w75x used LEAFS to study analyte population enhancement caused by easily ionized elements ŽEIEs. in a two-electrode DCP by spatially profiling the atom density via resonance fluorescence of calcium with and without the addition of potassium. The small variations in atom density caused by the EIE were unable to account for marked enhancements in atomic emission signals. The effect of lithium as an EIE on excited state populations were then studied by probing barium ions via direct line fluorescence which indicated that fluorescence signal enhancement at low laser

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powers disappeared at laser powers which were sufficient to saturate the atomic transitions. Although the findings did not lend insight to the DCP excitation mechanism, it reinforced fundamental postulates that the analytical region of the DCP is not in local thermodynamic equilibrium and EIE enhancement proceeds by modulating the rates of power distribution among various plasma zones. Later, LeBlanc et al. w76x used a similar experimental arrangement with a threeelectrode DCP. Their results showed that the spatial distribution of barium ion ground states are dictated primarily by flow dynamics around the argon plasma jet. Sodium was added as an EIE and the spatial distribution of barium ions was not altered significantly, but an ionization suppression of barium ions was observed. The combination of an MIP in atmospheric helium with a filament vaporization system was used as an atom cell in LEAFS by Oki et al. w77x. Both calibration and saturation curves were measured for sixteen aqueous metal and non-metal solutions. Detection limits were calculated to be less than 1 ng mly1 , which provides similar sensitivity to traditional flame LEAFS. 3.4.5. Laser ablation sample plumes In laser ablation LEAFS ŽLA-LEAFS., a focused laser beam is used to ablate a sample, and after a delay time, a second laser, which is tuned to the resonant wavelength of the analyte atoms of interest, excites the atoms in the ablation plume and the fluorescence signal is detected. The advantages of laser ablation sampling include w78x: Ž1. solid sampling, for which minimal sample dissolution and contamination is encountered; Ž2. suitability for both conducting and non-conducting samples; Ž3. localized analysis for which the ablation laser beam can be focused down to several micrometers to conduct local analysis; Ž4. depth profiling where analyte from different depths can be investigated; Ž5. suitability for both atmospheric and reduced pressures; and Ž6. close matching between the volume occupied by the sample atoms, and the volume probed by the laser. The possible disadvantages include Ž1. fractionation due to inhomogeneity or differing relative volatilities in the sample. The high tempera-

ture of the ablation plume, especially when an IR laser is used for ablation, could cause melting and boiling point-dependent diffusion and evaporation of the material and structural changes in the solid, which can cause poor accuracy of results. Ž2. Redeposition of the ablated material on the rest of sample would cause errors on non-homogeneous samples. In such cases single-shot measurements might be necessary for laser ablation sample analysis. For better precision, laser ablation requires internal standardization or integration of signals from multiple shots in order to compensate for pulse-to-pulse variation of the mass of ablated sample w79x. Pesklak et al. w80x studied the spatial and temporal distribution of atomic species in a laser microprobe plume using non-resonance atomic fluorescence to provide three-dimensional resolution. The laser microprobe plume favors extended lifetimes of free atoms which allows for increased observation time. However, matrix effects are pronounced without the use of an internal standard and multiple sampling or simultaneous internal standards must be used to improve precision. The microplume was studied in helium, neon, and argon atmospheres at various pressures for the analysis of iron, titanium, zirconium and hafnium in steel and niobium samples. It was demonstrated that the interaction of the microplume with the atmosphere was strongly affected by atmospheric pressure. At a constant pressure, pre- and post-filter effects are more severe in higher density gases, although the prefilter effect was reduced when optical saturation was reached. Gornushkin et al. w81x used laser ablation LEAFS to study cobalt in solid sample matrices: soil, graphite and steel. Cobalt atoms, which were excited from a low lying, thermally excited state yielded a better SNR than excitation from the ground state. The optimal time interval between the ablation laser and the probe laser was found to be in the microsecond range with an ablation laser power density of several GW cmy2 . Detection limits in the ppb to ppm range and linearity over four orders of magnitudes were obtained, and accurate measurements were obtained in a soil standard. They also studied lead in metallic reference materials by LA-LEAFS w82x. A UV

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laser was used to ablate metallic samples, and a dye laser that was tuned to the resonant excitation wavelength of lead, 283.3 nm, was used to probe the lead atoms in the sample plume. Calibration curves were constructed from various metallic samples that contained a wide range of concentrations of lead, from 0.15 to 750 ␮g gy1 . The amount of sample ablated by each laser pulse was estimated to be approximately 20 ng. The optimal time interval between the ablation laser and the probe laser was found to be 100 ␮s and an integration time of 200 s was used to improve the precision. Matrix interference-free calibration curves were constructed from all of the samples. Sdorra et al. w83x studied chromium, boron, and silicon via LEAFS in laser ablated steel samples. The samples were ablated normal to the surface with a Nd:YAG laser operating at 1064 nm and an energy of 5 mJ per pulse. Selective excitation was then accomplished by an overlapping a dye laser, which was shone through the laser-induced plasma parallel to the surface. A gated optical multichannel analyzer was used to detect subsequent fluorescence. Optimization of the pressure of the buffer gas in the ablation chamber strongly influenced the resulting fluorescence intensities from which the ideal value was determined to be approximately 140 mbar. Absolute detection limits of approximately 270 fg were obtained for single laser shots. In a later paper, Quentmeier et al. w84x presented a solution in which internal standardization was employed using the same experimental arrangement. This was accomplished by measuring the ratio of fluorescence intensities of line pairs of analytes with approximately the same excitation energy. It was demonstrated that the ratios of ground state atoms of silicon, chromium, manganese, and magnesium probed by LEAFS were independent of the matrix, the amount of ablated material and the plasma temperature. Smith et al. w85x developed a method for the isotopically selective determination of 235 U and 238 U in UO 2 , ablated with the 1064-nm output from a Nd:YAG laser. A pulsed diode laser was used as a probe of the laser ablation plume. The isotopically selective measurements were done using two different experimental approaches. In the first set of experiments, the wavelength of the

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diode laser was scanned across the spectral interval between the 235 U and 238 U isotopes, which is approximately 17.7 pm. The other was a timeintegrated separate detection of fluorescence signals from the individual isotopes in sequential experiments. There are factors for both approaches that must be considered. In the case of scanning the diode laser wavelength, attention must be paid to the expanding plasma and exponential decay of the uranium population density within the measurement volume. Likewise for the time-integrated approach, the slow degradation of the stationary sample surface with the increasing number of laser shots must be considered. However, the spatial probe efficiency between the volume occupied by the sample atoms and the volume probed by the laser is very good.

4. Limits of detection The limit of detection ŽLOD. is defined as the amount of an analyte in a sample that produces a signal extrapolated from a calibration curve, that is equivalent to three times the standard deviation of the noise associated with the blank. A compilation of the best-reported detection limits of 41 elements so far investigated is listed in Table 2. Most of the elements in the table were found to have the best detection limits by ETALEAFS, among which lead and thallium had detection limits in the attogram level. GD-LEAFS holds its edge over ETA-LEAFS when elements with low-volatility are analyzed. Three elements with low volatility, europium, yttrium, and thulium, were found to have the best detection limits by this method. For routine analysis of radioactive elements such as lanthanides, ICPLEAFS has been found to be advantageous when minimum exposure to the elements can be arranged. Other methods, such as ICP-MS, may lead to the spread of contamination to the mass spectrometer and create a hostile environment when maintenance procedures are required. Elements demanding deep UV excitation wavelengths can now be analyzed due to the push of tunable laser technology into the deep UV region, where arsenic, selenium, and antimony have

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Table 2 Best reported limits of detection by laser excited atomic fluorescence spectrometry Element LOD Žfg.

Atomizer

Ref

Element

LOD Žfg.

Atomizer

Ref

Element

LOD Žfg.

Atomizer

Ref

Ag Al As Au B Ba Bi Ca Cd Clb Co Cs Cu Eu Fb

ETA ETA ETA ETA ICP ETA ETA ICP ETA ETA ETA ETA ETA GD ETA

w86x w41x w88x w90x w2x w91x w63x w2x w62x w95x w96x w97x w16x w70x w99x

Fe Ga Hg In Ir Li Lnc Mn Mo Na Ni P Pb Pd Pt

70 1 90 2 ; 104 1 ; 106 90 ; 105 60 10 8000 0.2 700 50

ETA ETA ETA ETA ETA,GD ETA ICP ETA ETA ETA ETA ETA ETA ETA ETA

w41x w55x w54x w55x w16x w33x w2x w41x w94x w91x w43x w98x w41x w87x w100x

Rb Rh Sb Se Sia Sn Te Ti Tl Tm V Y Yb

20 2 5 15 ; 104 30 20 ; 103 0.1 0.08 ; 103 1200 220

ETA ETA ETA ETA GD ETA ETA ETA ETA GD ETA GD ETA

33 87 89 88 67 41 92 93 39 70 56 70 55

8 100 54 1 ; 104 ; 104 2.5 ; 104 3.5 ; 104 4 1200 60d 2 300

A sample volume of 20 ␮l was assumed for the calculation of the detection limits for ICP atomizers in aqueous solutions. ETA, electrothermal atomizer; GD, glow discharge; ICP, inductively coupled plasma. a Solid sampling, based on a 20-mg sample. b Analysis was performed via laser excited molecular fluorescence ŽLEMOFS.. c Lanthanides. d Recalculated from Butcher et al. w15x ŽTable 2..

been successfully detected in the low femtogram range.

5. Real sample analysis A significant number of applications have been conducted in graphite furnaces LEAFS. Bolshov et al. w101x determined the bismuth content in polar snow and ice samples. The measured bismuth concentrations in snow samples ranged from 0.07 to 0.6 pg mly1 . Strong matrix interferences in snow samples were encountered while the main source of background was molecular fluorescence from unidentified species. In a second paper w102x, they described detailed procedures for every stage of sample analysis. Iridium was found to be a better matrix modifier than palladium due to its high purity and thermal stability. Concentrations of bismuth were determined in two sections of ice core. Enger et al. w89x employed ICCD-ETA-LEAFS to detect antimony at pg mly1 levels in various biological and environmental samples. The detec-

tion limit of antimony in aqueous solutions was 5 fg. The antimony contents in certified samples were determined with good accuracy. Measurements of some non-certified environmental and biological samples were also conducted. Antimony concentrations in human blood were found to be much lower than previous values in the literature. Enger et al. w103x used the same setup to study trace element contents of aluminum and lead in size-fractionated aerosol samples from the Norwegian Arctic. The sensitivity and selectivity of the setup was sufficient to detect the elements in aerosols with concentrations of 1᎐50 ng my3 . Olesik et al. w104x used an ICCD to record laserinduced fluorescence and emission from an ICP. This technique was able to detect isolated, single clouds of atoms or ions as a function of time. The ratios of emission to fluorescence intensities were used to investigate excitation processes in the plasma. Ljung et al. w93,105x conducted detailed studies of excitationrfluorescence wavelength pairs of titanium with ICCD-ETA-LEAFS. A number of fluorescence lines were found at each excitation wavelength due to the complex elec-

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tronic structure of titanium and collisional redistribution processes among the excited levels. A program was developed to calculate atomic wavelengths from existing energy levels, and suitable excitation and fluorescence wavelength combinations were given. However, contamination of titanium in the graphite furnace was found to be the limiting factor for the detection limit. Tungsten furnaces showed no memory effect, but yielded a weaker signal than in a graphite furnace, because of incomplete atomization processes. Barker et al. w106x applied laser enhanced ionization ŽLEI. and LEAFS techniques to measure gas flow velocities. A sodium solution was injected into the gas flow first; the flow was then tagged by the use of LEI to deplete a substantial fraction of the neutral sodium atoms in a well-defined upstream region of the flow. After a certain time delay, the inverse image of the depleted tagged region down stream of the gas flow was taken by a CCD-LEAFS. This method was tested in air᎐acetylene flames and an accuracy of 10% was reported. Petrucci et al. w107x utilized ETA-LEAFS for size-segregated analysis of lead and gold in ultrafine 0.02᎐0.2 ␮m particles. The isomobility aerosols were transported and deposited in a graphite furnace under the influence of an electric field. The particle diameter dependence of the collection efficiency of this process was discussed, and the total lead concentration in an air sample was estimated. The detection limits of lead and gold were estimated to be 5 and 3 fg, respectively. The time required to complete such analyses varied from minutes for lead to weeks for gold. Neuhauser et al. w108x set up an on-line and size-segregated detection of lead in ultrafine aerosols by flame or laser induced plasma LEAFS. The detection limit of lead was found to be 47 ng my3 by flame LEAFS and was independent of the particle size. By laser induced plasma LEAFS, the detection limits increased with increase in particle size. Liang et al. w92x determined the concentrations of tellurium and antimony in nickel alloys by ETA-LEAFS. Both dissolution and solid sampling methods were employed successfully, resulting in a detection limit of 20 fg for thallium. Antimony was successfully determined by the dissolution

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method with a detection limit of 10 fg, but incomplete recovery was found by the solid sampling method. Lonardo et al. w109x determined the concentrations of phosphorus in polymers by ETALEAFS. Both dissolution and solid sampling methods were used. Compared to ETA-AAS and ICP-AES, ETA-LEAFS was the only method that could cover the concentration range of phosphorous from 2 to 3000 ␮g gy1 due to the high sensitivity and wide linear dynamic range of ETA-LEAFS. This technique yields limits of detection typically between 100 and 1000 times lower than those obtained with ETA-AAS. A new dissolution method, which utilized trifluoroacetic acid and toluene, was also discussed. The detection limit of phosphorus was 8 pg. Yang et al. w110x determined tin in nickel-based alloys by ETALEAFS. Nickel was found to behave as a permanent chemical modifier that remained in the graphite tube during analyses. The results obtained by ETA-LEAFS were as accurate as those obtained by ETA-AAS and ICP-MS. Yuzefovsky et al. w111x utilized ETA-LEAFS with semi on-line, flow injection, preconcentration to determine ultra-trace amounts of cobalt in ocean water. The conditions for preconcentration were discussed. Compared to ICP-MS, ETA-LEAFS required much less of the seawater sample. The detection limit of cobalt was calculated to be 0.08 and 1 ng ly1 in aqueous standards and seawater, respectively. Wagner et al. w112x detected ultratrace levels of lead in whole blood samples by ETALEAFS. In order to exploit the high sensitivity of ETA-LEAFS, the use of a matrix modifier was eliminated due to its relatively high level of lead contamination of approximately 8 ppb and the blood samples were diluted to a 1:21 ratio. During the atomization step, smoke was generated in the graphite furnace resulting in a loss of fluorescence intensity. This was corrected by normalizing the fluorescence signal with respect to the attenuated laser intensity. Thus resulted in good agreement between their results and certified values. The detection limit of lead in blood was estimated to be 10 fg mly1 . Butcher et al. w113x used ETA-LEAFS to determine thallium, manganese, and lead in food and agricultural standard reference materials. Slurry sampling was

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used to combine the advantages of dissolution and solid sampling with reduced risk of contamination while permitting reproducibility of furnace introduction. ETA-LEAFS afforded a linear calibration curve 5᎐7 orders of magnitude, compared to the 2᎐3 orders of magnitude obtained with ETA-AAS and higher sensitivity Ž10y1 6 to 10y14 g, compared to 10y1 3 to 10y11 g.. Analyses of sediment samples require complex sample preparation involving hot or microwave acid digestion followed by tedious steps of separation and preconcentration. However, Cheam et al. w114x have developed a much simpler sample preparation technique for analysis of thallium in such samples. The preparation includes a cold dissolution step using a nitrous-hydrofluoric acid mixture at room temperature followed by dilution with water. The solution was analyzed directly by LEAFS and compared with its hotplate-digestion counterpart. The results of 10 different sediment reference materials showed excellent agreement using both hot and cold dissolution techniques with a detection limit estimated to be 0.5 ng gy1 . Based on ETA-LEAFS, Aucelio ´ et al. w100x developed a method for the detection of platinum by employing a high-repetition rate laser source coupled with a carefully optimized furnace, whereby urine samples could be analyzed directly, while blood samples only required a simple dilution. The experimental arrangement used is similar to that shown in Fig. 2. Absolute limits of detection at femtogram levels were achieved. The application of ETA-LEAFS has also been investigated by Swart et al. w115x for the determination of selenium in human bone marrow serum. By utilizing this highly selective technique for such analyses, interferences of species such as iron and phosphate in ETA-AAS, interferences from argon adducts and other molecular ions in ICP-MS, are avoided or reduced. An excimerpumped dye laser was used to produce a beam near 468 nm, which was then frequency doubled to 234 nm using a BBO crystal. The output was then propagated collinearly with the primary beam and directed into a Raman converter filled with H 2 . By using the 4155 cmy1 shift of H 2 , the second anti-Stokes shift of the dye laser output produced a beam of 196 nm. The experimental

Fig. 8. Experimental arrangement for the production of 196nm light using a Raman converter.

arrangement is depicted in Fig. 8. The accuracy of the technique was verified by analysis of a NIST SRM bovine serum where the experimental and certified values were 42 " 3 and 42.4" 3.5 ng gy1 , respectively. Swart et al. w116x later utilized the aforementioned LEAFS apparatus for the determination of arsenic in human blood serum which demonstrated its superiority over previously used methods including chromatography and hydride generation techniques coupled with AAS or ICP-MS. Hydride generation ŽHG. is commonly used for the determination of hydride-forming elements, including antimony, arsenic, selenium, and tellurium, to provide better sensitivity than conventional techniques due to the efficiency of analyte transport. This method allows speciation information to be determined which gives insight into the bioavailabilty of a species in studies to determine the essentiality or toxicity of an element. The application of this technique coupled to ETALEAFS and ICP- LEAFS has been studied for the determination of arsenic and selenium in aqueous solutions by Pacquette et al. w117x. LEAFS measurements were accomplished by excitation at 193.696 and 196.026 nm for arsenic and selenium, respectively. Notably, HG coupled to various LEAFS techniques yielded poor LODs relative to theoretically less sensitive counterparts such as graphite furnace atomic absorption. A conclusive analysis of these problems was not

P. Stchur et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1565᎐1592

given by Pacquette et al. Possibly the poor detection limits may be explained due to losses in hydride transport andror trapping processes or the atomic transitions may not have been saturated. Determination of rare earth elements has always been problematic. Spectra produced by ICP emission spectrometry are plagued with spectral interferences that make identification difficult. Problems with graphite electrothermal atomizers include loss of analyte and delayed atomization due to graphite’s porosity; non-iosothermality of the atomization process due to low rate of atomization; reactions between analyte and graphite lead to formation of thermally stable carbides; and high background emission from the graphite surface. In order to minimize these effects, Aucelio ´ et al. w118x evaluated various linings for laser excited atomic fluorescence in a graphite furnace, such as pyrolytic graphite, tungsten, tantalum, and rhenium, as well as coating treatments with noble metal solutions. The rhenium lining provided the best results in terms of fluorescence intensity, lifetime, and reproducibility. Thulium determination in urine and coal fly ash yielded recoveries of 95.2 and 93%, respectively with LODs of 200 fg. Although lower LODs were achieved using a miniature glow discharge, as discussed earlier, this method provides a simple modification to existing pyrolytic graphite furnaces. Beissler et al. w119x have explored the presence of femtogram-levels of gold in graphite atomization tubes and its effects on blank determination. They have shown that although the gold signal is absent upon initial dry firings of the atomization tube, there does exist signal spikes due to gold both during and after the addition of aqua regia. To further prove the presence of gold, the signal disappeared if either the laser beam was blocked or slightly detuned several picometers. It was shown that the boron nitride-coated furnaces produced no gold signal spikes, but their use with real analytical samples has not been validated. Borgmann et al. w120x have applied LEAFS to monitor environmental pollution by study of the uptake and bioaccumulation of thallium and cadmium in an invertebrate, Hyalella azteca, to

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determine acute and chronic toxicity. The significance of this research lies in the ability to discriminate between bioavailable metal and total metal. The studies showed that the sites of maximum metal concentration were not necessarily the sites of maximum metal bioavailability. The authors point out that total metal concentrations in the sediments are poor predictors of biological effects. However, more accurate prediction can be obtained by correcting for the amount of sulfide, which binds and precipitates metals in the sediment, or by measuring pore water concentrations.

6. Conclusions The advent of diode lasers with their now somewhat improved range of wavelengths and power output, provides opportunities for research and applications in LEAFS w26x. The further development of the coupling of second and third harmonic crystals to pulsed diode lasers shows promise for compact and robust instrumentation. There have been no recent instrumental developments that might provide more isotopic selectivity beyond the elements like uranium where the spectral isotope splitting is greater than most elements, but laser diodes could provide this due to their potential to provide an output with very narrow spectral bandwidth. The advent of optical parametric oscillator-based lasers has enabled LEAFS to be much more practical then in the past when dye lasers were used. This should be the harbinger of more applications of LEAFS to complex real sample analyses that can not be done by other techniques for reasons of sensitivity or selectivity. Array detectors provide an additional degree of freedom by provision of more spectral information more rapidly, which should aid the study of complex samples that might produce complex background problems. The recent literature indicates that the sensitivity, selectivity and ease of method development of LEAFS is well-established, and that there are no substantial analytical disadvantages to the technique beyond the instrumental limitations associated with the single element at a time mode of operation and

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