Lasers in atomic spectroscopy: Selected applications

MICROCHEMICAL

JOURNAL

45, 1-35 (1992)

REVIEW Lasers in Atomic

Spectroscopy:

Selected

Applications

TERRY L. THIEM,* YONG-ILL LEE,? AND JOSEPHSNEDDON?' *Air Force Phillips Laboratory, Spacecraft Interactions Branch, Hanscom Air Force Base, Bedford, Massachusetts 01731; and TDepartment of Chemistry, University of Massachusetts-Lowell, Lowell, Massachusetts 01854

Received September 28, 1991; accepted October 22, 1991 The use and results of the laser in selected areas of atomic spectroscopy are discussed, in particular as a light source in atomic absorption and fluorescence spectrometry, as a sample introduction system, as an atomizer, and in laser-enhanced ionization spectrometry. 0 1992 Academic Press. Inc.

INTRODUCTION

In 1958, Schalow and Townes published a paper on “Infrared and Optical Masers” (1). This was closely followed by Maimans’ classical paper on the ruby laser (2). These papers and other papers on lasers which quickly followed caught the imagination of scientists (and the community at large) and led to a tremendous increase in the development and use of the laser in science. Analytical chemists (excluding Raman spectroscopists), and atomic spectroscopists in particular, were somewhat slower to embrace this new technology. However, in the 1980s the use of the laser in analytical atomic spectroscopy increased markedly and it is reasonable to assume that this trend will continue in the 1990s and beyond. This is due, in part, to the production of laser systems that are more reliable, are of a reduced size, and have improved performance. This (and other factors) has led to increasing use of the laser in more applications and techniques. This paper focuses on selected areas of the use of lasers in atomic spectroscopy, in particular as light sources, sample introduction systems, and atomizers/excitation sources, and for laser-enhanced ionization spectrometry. A secondary objective of this review is to present a discussion on the merits of the laser in an area of atomic spectroscopy and why it has improved this area. Consequently, where appropriate, a discussion on the atomic spectroscopic technique is presented in order to understand the ways in which the laser has improved this area compared to traditional sources. It is beyond the scope of this review to describe in detail the laser and its properties. Numerous textbooks and articles are available on this subject and these authors recommend the book by O’Shea et al. (3) and an article by Ciurczak (4) for those who would like a simplified introduction to lasers. Important properties of lasers from the atomic spectroscopists’ point of view are tunability, ’ To whom correspondence should be addressed. 1 OO26-265W92$1.50 Copyright Q 1992 by Academic Press, Inc. AU rights of reproduction in any form reserved.

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power, repetition rate, monochromaticity, beam diameter and divergence, wavelength, and pulse width. THE USE OF THE LASER AS A LIGHT SOURCE 1 .l. Atomic Fluorescence Spectroscopy Atomic fluorescence spectroscopy (AFS) as a method of chemical analysis is based upon the absorption of radiation by an atomic vapor, thereby producing atoms in an excited electronic state, and the measurement of the light emitted when a fraction of the excited atoms undergo deactivation. AFS is discussed in detail elsewhere (5). There are five basic types of atomic fluorescence: resonance fluorescence, direct-line fluorescence, stepwise fluorescence, two-photon absorption-single-photon fluorescence, and sensitized (energy transfer) fluorescence. The first three types can also involve thermal activation. Resonance fluorescence involves the same two levels in the excitation and deexcitation processes, whereas the other processes do not. Resonance fluorescence involving the first excited state has been of greatest analytical use, although Stokes and anti-Stokes direct-line fluorescence has shown limited analytical use, particularly in cases where scatter of exciting radiation by particles within the atomizer has been appreciable (6). It is well known that the fluorescence signal does not respond linearly to very high values of excitation source irradiance. Instead, the signal reaches a plateau or saturation point at which more u-radiance yields little or no increase in the signal. This saturation level differs for each atom, ranging from 10” to 10” W/cm* (7). In contrast to conventional light sources, the use of the laser as an excitation source enables saturation to be reached for both atomic absorption and atomic fluorescence. Optical saturation occurs when the rate of the induced (or stimulated) emission dominates the combined rate of spontaneous emission and collisional deexcitation of the laser-excited level (8). Another emerging field is atomic fluorescence dip spectrometry which uses the decrease in the fluorescence signal caused by the depletion of the population of the fluorescent level by laser-induced relaxation (9). For normal AFS, an optimum signal-to-noise ratio may be expected when the excitation just achieves saturation for the transition under study. The maximum temperature achieved with thermal excitation sources for spectral analysis is about 10,000 K but by using a laser for excitation of atoms, the population of the upper level may correspond to 100,000 K excitation temperature (10). Due to the dye laser’s properties of tunability, monochromicity, pulsability, and spectral intensity, it has found wide use as an excitation source in the field of AFS. A comparison between detection limits obtained by five different light sources in AFS is shown in Table 1. It should be noted that these detection limits are not directly comparable, as different instrumentation systems were used to achieve the results (this is true for detection limits in Tables 2 through 6 as well). Nevertheless, the detection limits obtained using the laser as the light source are superior in almost all cases and in the picogram per milliliter range for many elements. Tunable dye lasers have been used in combination with AFS since early 1970 (11). The use of a pulsed dye laser makes it possible to obtain sufficiently

LASERS

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SPECTROSCOPY

TABLE 1 Detection Limits (&ml) for Atomic Fluorescence Spectrometry Using Various Light Sources Element

& Al As Au Ba Be Bi Ca Cd Ce co Cr cu Eu Fe Ga Gd Ge & In Li Mg Mn MO Na Nb Ni OS Pb Pr Pt Rh Ru Sb SC Se Si Sn Sr Te Ti Tl V Zn

Line source 0.1 70 IO 5

Continuum source O.-l

200

Laser 0.4 pg/ml” 0.2”

ICP 3

150

2 8 2 0.3 0.001

40 1

2 0.3 0.3

15 1.5 1.5

15 10

0.6 7

10 140

140 0.2 10

1800

0.1 0.5 200 100,000

25 0.1 2 100 8

1

25

10

10

150 3000 5000

700

w.11 [0.06] 600 30 30 [0.081 0.6 70 0.02

20 0.9 790 200 6 30 0.5

Source. Ref. (5) Table 11.5. L2Ref. (II). b Weeks, S., et al. Anal. Chem., 1976, SO(2)360. ’ Horvath, J., et al. Anal. Chem., 1981, 53(l) 6. d Ref (.38), p. 80A. e Kachin, S., et al. Appl. Spectrosc., 1985;39(4)587.

40 pg/ml” 0.08 0.9 pg/ml” 500 lb 1 3 pg/ml” 1.5” 5 pg/ml” 0.5” 800 1 pg/ml” 0.5 0.01 pgc 4 pglml” 5” 30 pg/ml” 1000d 50 pg/ml” 150,000 0.025 pg/ml” 1000 6” 0.5” 2’ 50 10

4 0.7

0.1 5 15 0.3 50 0.2 2 0.4 0.1

11 2 2

2 0.6 0.2

6

0.3

0.09 2

50

400 5 20 0.4 0.3 1 8 0.3 0.4 5

0.3 250

25” 0.75 pg’ 2 4 pg/ml” 30

HCL

0.035 0.5

9 40 200 2 4 25 30 25 0.1

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intense radiation over the wavelength range 220-l 100 nm. Resonance lines of over 60 elements fall within this range. Owing to the capability of gating the signal due to the pulsed nature of a laser, a considerable decrease in intrinsic noise of the detection system and background radiation from the atomizer can be achieved. Laser atomic fluorescence has become a very important method of analysis and has been applied to many systems including atmospheric pressure flames, graphite furnaces, inductively coupled plasmas, and other types of atom cells. Early work in the field showed that while theoretically the method should be sensitive, in many cases the detection limits were inferior to those of other methods, including atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES). In recent years, continued study and experimentation combined with the development of dye lasers capable of working in the ultraviolet (UV) region have brought about the superior detection limits first theorized. It has been shown that laser atomic fluorescence has the capability of single atom detection (SAD), whereas other spectroscopic methods based on absorption, emission, and photothermal processes are not capable of achieving this level (22). In order to detect at this level, many problems such as water and standard purity must be taken into account. 1.1 .I. Laser-Excited Atomic Fluorescence Flame Spectroscopy The fluorescence of atoms in flames was first reported in 1924 by Nichols and Howes (23). The flame itself is not regarded as a sufficiently intense source of fluorescence and is rather limited in the number of elements that can be determined because of incomplete atomization (flame temperature is too cool). However, it is high enough to ionize easily ionizable species (14). Studies of rare earth elements revealed stable diatomic monoxide molecule formation, which turned out to be a major controlling factor in limiting the production of free absorbing and emitting atoms. It is well known that the flame contains species (C,, CH, OH, and CN) giving rise to a molecular fluorescence background in the spectral range 200-660 nm when irradiated by a continuum source (15). These fluorescence species contribute to the noise in the analytical signals in flame AFS and may cause spectral interferences even if nonresonance fluorescence transitions are used (25). Higher flame temperatures, such as nitrous oxide-acetylene flame, have been used to improve the atomization efftciences (26). However, oxygensupported flames produce a higher background noise. Both line (hollow cathode lamps, pulsed hollow cathode lamps, and electrodeless discharge lamps) and continuum sources were initially used to induce atomic fluorescence of analytes in flames with limited success. The ideal source for use in flame AFS would be a higher power source with a low duty cycle. The low duty cycle is required to reduce the noise inherent in flames. The pulsed dye laser with a fast response photomultiplier or diode array has been shown to fulfill these requirements. To increase the sensitivity, and thus reduce detection limits, multipass cells have been constructed for use in Stokes direct-line atomic fluorescence. Detection limits as low as 0.2 rig/ml have been achieved for iron with a

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multipass configuration (17). Use of the flame as the atomization source limits this technique to a selected number of elements (9). 1.1.2. Laser-Excited Atomic Fluorescence Graphite FurnacelQuartz Cell Spectroscopy Laser-excited atomic fluorescence spectroscopy (LEAFS) is more efficient with graphite furnace atomizers than that of the flame (18). Using graphite furnaces, the atomization can be performed in a cylindrical tube or in a hollow cup (9). When the cup is used, the fluorescence is observed over the cup, whereas in the tube, it is observed at 90” with respect to the excitation axis. The simplicity of the graphite rod makes it attractive for use with volatile elements. The graphite cup offers improvement over the graphite rod because the sample is in a semienclosed environment, which gives a more “furnace” effect. However, the atoms still must emerge from the furnace into the cooler atmosphere before being excited by the laser, which allows them time to react with interferences such as oxygen. Therefore the best atomizer is one that contains the atoms in the hot environment while they are being excited by the laser (19). The low scattering level of the exciting radiation in the analytical zone and the absence of organic compounds (removed in the ashing or pyrolysis step) could quench the fluorescence of the analyte and provide considerable background by molecular fluorescence. Removal permits an increase in sensitivity. When the method was applied to pure aqueous standards combined with sample atomization in an inert atmosphere such as argon, elemental detection limits of 10-i to lo-’ &ml were realized. In Table 2 the detection limits for laser AFS using flame, graphite furnace (cup and tube), and conventional solution inductively coupled plasma atomic emission spectrometry (ICP-AES) are shown. Matrix components drastically decrease the sensitivity. Matrix effects in a graphite furnace can be attributed to several factors including (but not limited to) (a) thermochemical reactions at the surface, (b) gaseous phase chemical reactions during the transport of the atomized matrix components from the surface to the analytical zone, and (c) quenching of the optically excited levels of the analyte atom by matrix components. One way to reduce or minimize gaseous phase reactions and quenching is to atomize under vacuum rather than the more common use of an inert argon atmosphere (15). Graphite furnaces also allow the direct analysis of solid samples which decrease sample dilution in comparison to traditional flame and plasma methods. Graphite furnaces do have limitations because of matrix interferences in the graphite furnace. Matrix interference elimination or reduction in graphite furnace atomic spectroscopy is a fertile research area. Despite this limitation, furnace atomization is preferred to flame and/or plasma atomization when sensitivity is the goal to be achieved. 1.I .2.1. Comparison of laser systems using graphite furnace atomization. A recent study compared three different types of pumping lasers and their characteristics to determine which system was best suited to AFS studies (20). The systems were a nitrogen-pumped dye laser, a Nd:YAG-pumped dye laser, and a

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TABLE 2 Detection Limits for Laser-Induced Fluorescence Using Different Atomization Sources vs Traditional Inductively Coupled Plasma Atomic Emission Spectrometry Graphitefurnace Element AI3 Al AU Ba Bi Be Ca Cd Ce co Cr cu DY Er Eu Fe :: Hf Ho In Ir Li LU Ml2 Mn MO Na Nb Nd Ni OS Pb Pd PI Pt Rb Ru Sb SC

Se Sm Sn EJ Ti Tl Tm V Yb

CUP Flame atomization CDL kdml) ADL (~8) wml) 5 0.1 0.004 45 0.0006 2250 0.004 0.008 0.003 40 0.8 0.05pg 0.008 0.5 0.001 0.001 0.001 0.3 0.5 0.02 0.06&ml 0.007 0.8 loo 0.1 0.0008 0.009 0.0005 3 0.01pg 0.04&ml 0.012 0.0001 1 2 0.002 150 0.9 P8 0.001 1 0.0007 0.1 0.002

Tube CDL WmO 0.4 200

0.9

3

0.06

1500 5 3500

300 0.1 70

5 300

0.1 6

10 5ooo 30

0.2 100 0.6

50

1

0.038 35 6ooo so00

7.5 x 10-d 0.7 120 100

20

500

1 2.4 x Iti

4

AIlL W 0.008 4

0.018

0.2 0.2 0.9 0.01 10 0.003 0.0001 0.07

0.4

0.1 0.08 0.04

10

1 0.06 0.09 0.6

0.02 475

0.08

0.75pg 0.5 0.002 0.004 0.1 0.03 0.01

0.4

0.003 0.01 0.2 0.1 0.2

0.025

500

5 x 10-d

10

1 0.9 10 0.4

0.01 0.1

Conventionalsolution ICP (&ml)

2.5 x 104

0.035 8.5 x lo4

Note.

CDL is Concentration Detection Limit. ADL is Absolute Detection Limit.

500

0.002

7 x 10-d

0.1

1700

12 x 104

0.002 2200

3 0.06

LASERS IN ATOMIC

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7

copper vapor dye laser. For the comparison, lead was chosen as the analyte because it possesses a very sensitive direct-line fluorescence detection system in which the lead atoms are excited at 280.30 nm and fluorescence emission is collected at 405.78. The experiment used graphite tube atomization for all three lasers. The nitrogen-pumped dye laser was operated at 20 Hz. The dye laser output had a spectral linewidth of 0.15 nm, a pulse width of 5 ns, and a pulse energy of 0.8-I. 1 mJ in the visible region (520-620 nm) and 25 CJ when the visible output was frequency doubled. The frequency-doubled Nd:YAG pumping laser operated at 30 Hz with output energies of -250 mJ per pulse with a pulse duration of 12 ns. The copper vapor pumping laser system operated at 6 kHz with an average power of 20 W. Output energies following frequency doubling were 200500 nJ, the pulse width was -20 ns, and the spectral bandwidth was 0.02 nm. If limit of detection (LOD) is used as a measure for the best system, the copper vapor-pumped dye laser had the lowest at 0.5 fg. Both the Nd:YAG- and the nitrogen-pumped dye lasers showed an LOD of 3 fg. This increase in sensitivity by a copper vapor laser was probably due to the high repetition rate producing the best excitation rate. Since both processes, excitation and relaxation, are extremely fast, dead time (time between excitation/relaxation) should be kept to a minimum. All three of these LODs were comparable to the best detection limits for lead at the time of the experiment and much better than the best LOD using graphite furnace AAS of 2000 fg. I .I .3. Laser-Excited

Atomic Fluorescence

Plasma Spectroscopy

The plasma sources that have been investigated as atomization sources for LEAFS are the inductively coupled plasma (ICP), direct current plasma (DCP), and microwave-induced plasma (MIP). The addition of an intense radiation source such as a tunable dye laser when combined with a plasma as an atomization or ionization cell allows sensitive AFS to be performed (9). The ICP, for example, has many advantages over a flame source in that it has fewer chemical and ionization interferences and better detection limits for the refractory elements (21). Further, spectral interferences, a major disadvantage in normal ICP analysis, are reduced due to the narrow bandwidth of the laser output. Both pulsed and continuous wave (CW) lasers have served as excitation sources in laser-excitedplasma AFS. In addition to normal fluorescence spectroscopy, the LE-ICP-AFS method has also been applied to measurements of spatial distribution of plasma support gas, argon, and analyte species and determination of temperatures in small areas of the plasma. For analytical uses, the LE-ICP-AFS method has been performed on plutonium in the nuclear fuel reprocessing industry with a detection limit of 50 &liter (22). Due to the lower temperature, only volatile elements such as the alkali and alkaline earth elements can be efficiently studied by the MIP. Laser-excited AFS has had limited use in the MIP. The detection of sodium atoms atomized by a helium MIP using a tungsten filament vaporization technique was on the order of picograms per cubic centimeter (23). Continued research is needed in this field to

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better evalute the use of the MIP as an atomization source for laser-excited AFS studies. The use of the DCP as an atom cell in AFS has also seen limited use. Most LE-DCP-AFS work has been confined to use as a diagnostic tool for studying the effects within the plasma. One such study investigated the enhancement effects of analyte emission in matrices containing easily ionized elements (24). LE-DCPAFS was used because emission measurements cannot be resolved along the observation axis due to the DCP’s lack of symmetry and therefore cannot be evaluted mathematically. However, LEAFS is capable of probing the atom or ion population in a nonsymmetrical source with spatial resolution dependent upon the overlap of the laser beam with the observation angle of the monochromator. This method has also been used to obtain spatially resolved data for the ICP (22, 25). 1.I .4. Laser-Excited Atomic Fluorescence Glow Discharge Sputtering Spectroscopy The use of the glow discharge lamp (GDL) as a method of atomization for AFS is finding increasing use. GDL can be used for the direct analysis of materials such as metals and alloys (26-28). Although the method works best with conducting materials, nonconducting materials can be analyzed if the sample is ground into powder and mixed with a conducting material such as copper powder and pressed into pellets (29, 30). Direct vaporization from a solid sample is desirable because of rapid analysis with minimal or no sample pretreatment or preparation (31). The use of glow discharge in a pulse mode has exhibited high signal-to-background when excited by a dye laser for AFS (32, 33). The AFS signal reproducibility on repetitive reignition of the discharge source over time is very good (26). However, in the sputtering process, some molecular species which include dimers as metal monoxides are also produced (34). Although detection limits are adequate for many analyses, they are not as good as those of graphite furnace AFS. For example, indium and lead have absolute detection limits of 8 ng (26) and 20 pg (33) using GDL, whereas with graphite tube AFS, detection limits are 0.02 pg and 0.5 fg, respectively (35). 1.l S. Laser Atomic Fluorescence Dip Spectroscopy The decrease of the fluorescence signal caused by the laser-induced depletion of the atomic population has been proposed as another tool for the diagnostic study of atmospheric pressure flames and plasmas (36). Rate equations describing the population of fluorescence levels have shown that the technique of fluorescence dip spectroscopy provides the same information as that obtained by measuring the fluorescence from the excited level. Fluorescence dip is produced using two lasers. The first laser will excite the atoms in the initial excitation level. A second laser then irradiates the sample, stepping it up to the second and higher excitation levels. This second excitation results in a detectable decrease in signal intensity or “dip” from the first fluorescence level. If the second excitation energy level is near ionization, the second laser can be wavelength scanned, resulting in a twostep laser-enhanced ionization spectrum simultaneously recorded with the fluorescence dip spectrum. When the second excitation step results in ionization of the atom, the absolute magnitude of the fluorescence dip will reflect the ionization

LASERS

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yield (37). This technique offers high spatial resolution since it probes the intersection of the two focused laser beams. The relative fluorescence dip is independent of the total number of atoms. A theoretical treatment based on the rate equation approach also provides information such as quantum efficiency and absorption oscillator strength in much the same way as the conventional saturated fluorescence method. When steadystate conditions are achieved, the fluorescence dip can be plotted as a function of the spectral irradiance of the second excitation laser so that a complete saturation curve can be obtained from which the saturation parameter can be derived. In time-resolved fluorescence experiments, the method should have the advantage that the transient dip will reflect directly the absorption oscillator strength of the second excitation step which will provide a significant advantage over conventional fluorescence and ionization spectrometry. Further study in the field of laser-induced fluorescence dip spectroscopy is needed to better define how this method can be optimized and applied in analytical chemistry. This area is discussed in more detail in Section 4, Laser-Enhanced Ionization Spectrometry. 1.2. Atomic

Absorption

Spectroscopy

In atomic absorption spectrometry, the hollow cathode lamp (HCL) is the most widely used and accepted radiation source. More intense sources such as pulsed HCLs and electrodeless discharge lamps (EDLs) have been used with some degree of success (38), but these sources lack the tunability for true multielement analysis. The use of the laser to replace the conventional light source in AAS has several advantages. The laser allows the use of a simple photodiode detector to discriminate the laser beam intensity from the background radiation from a flame or graphite furnace atomizer. The method may also be used to extend the upper limit of the dynamic calibration curve by using the line wings of the major absorption lines instead of choosing another analyte line. Absorption measurements can also be extended to lower concentrations by an appropriate mirror arrangement (multipass cell) being measured by the photodiode. Switching the wavelength of the modulated laser beam from the absorption line to the background next to the line allows background absorption to be recorded. The absorption line can be corrected for the background. The use of a CW dye laser for a narrow bandwidth radiation source is possible but too expensive and bulky for routine analysis. One possible solution is the use of an inexpensive, tiny, and easy to run semiconductor diode laser (39, 40). Due to the small size of diode lasers, simultaneous application of several could allow multielement determination. The output wavelength of diode lasers is a strong function of both diode current and junction temperature and requires that both be precisely controlled. Diode lasers also have a limited wavelength range (660-860 nm). Application of the second harmonic generation in nonlinear crystals allows the extension of diode laser wavelength (-33w30 nm) at the expense of about three orders of magnitude decrease in laser power. The use of the semiconductor diode laser and application of power modulation and Fourier analysis offer the possibility of performing background-corrected multielement AAS without a spectrometer. Laser AAS using diode lasers has the

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potential to surpass “classical” AAS for routine analysis because its application has clear advantages over the established method. Improvements in the lower limit of wavelengths achievable by diode lasers are currently in progress and should only improve on the results that have already been obtained using this method. 2. THE USE OF THE LASER AS A SAMPLE INTRODUCTION METHOD The introduction of the sample to an atomic spectroscopy system has been referred to as “the Achilles’ heel of the technique” (42). Following the invention of the laser by Maiman, it was realized that high-powered lasers could be used to volatilize, ablate, or vaporize small amounts of material to be transported to an atomization or excitation source (42). The first theoretical considerations for the use of the laser for plasma production and heating were presented by Basov and Krokhim in 1963 (43). Since that time, a considerable amount of work, both theoretical and experimental, has been devoted to the study of the interaction of a laser beam with solid surfaces. Much of the characterization of the energetics of the ablated material has been achieved by mass spectroscopy (44). Much of the theoretical work is found in the physics and physics-related literature. The use of the laser for sample introduction in atomic spectroscopy is closely related to the work on laser plasmas discussed in Section 3 of this review. Laser ablation for sample introduction in atomic spectroscopy has many advantages for the direct analysis of solid samples, including little or no sample preparation, applicability to hard conducting or nonconducting samples, micromass sampling, and high spatial resolution allowing analysis of small areas of the sample (essentially nondestruction of the sample). In addition, laser ablation can be and has been coupled with most common methods of atomic spectroscopy, including flame, graphite furnace, and plasmas. A recent book edited by Sneddon (45) describes several of the more common methods of sample introduction in atomic spectroscopy. Included in this book is a chapter devoted to laser ablation as a method of sample introduction (46). Other reviews of this topic are available (47, 48). In general, laser ablation is a very complex process and is difficult to model successfully at the fundamental level. The condition of the crater, depth, size, and shape as well as the amount of sample removed depend on the properties of the sample, condition of the surface, and properties of the laser. All these factors undergo complex, time-dependent changes during the laser pulse. The atmosphere under which the ablation occurs determines the characteristics of the plasma created. Studies conducted have shown that both the pressure and the chemical composition of the atmosphere above the sample at the time of analysis can influence the free atom concentration in the vapor plume (49). Both lifetime (50) and spatial expanse (51) of the plume have been shown to be greater at reduced pressure (10 Tot-r). Pioneering work in this area used low-repetition rate-pulsed ruby lasers leading to a transient signal. This results in signal reproducibility being directly related to the pulse-to-pulse stability of the laser. The higher repetition rate of currently available laser systems coupled with flow systems suitable for transport of the

11

LASERS IN ATOMIC SPECTROSCOPY

ablated sample into an atom reservoir has resulted in a continuous signal with improved precision and duty cycle. In addition, extremely low absolute detection limits (-lo4 atoms) can be achieved with the use of mass spectrometric detection of the vaporized sample (52). Similar detection limits have been reported for a laser microplume which was optically interrogated with the use of resonance ionization detection methods (53). Some general results of the study of laser ablation are available in the literature (54-60) and are discussed in more detail in Section 3. 2.1. Atomic

Absorption

Spectroscopy

Flames and graphite furnaces are the normal and most common means of solution sample atomization in atomic absorption spectroscopy. Flame atomization efficiencies are very small for most elements, even in the fuel-rich oxygenacetylene flame (51, 61). Solid samples present the problem of long digestion processes that can introduce contamination into an analysis. Laser ablation has been used as a sample introduction or vaporization method for solid samples (62). Initially problems were encountered when the laser was used as the sample introduction method and prevented widespread acceptance of this method. These problems included reproducibility and background correction of emission spectra produced by the laser-generated plume when it is used as the absorption path. Recent work has reduced many of these problems and at present multielement analysis can be performed using this method. In Table 3 the detection limits for selected elements in various solid samples by laser ablation AAS are shown (62). A comparison of the detection limits obtained by conventional solution flame AAS is also shown in Table 3. Typical results are in the low parts per million range and working ranges one to two orders of magnitude. Kantor et al. (63) combined a Nd:YAG laser with flame AAS to directly determine iron in the concentration range of 8 to 92% in nickel-based alloys. The laser was used in the normal mode and generated pulses of 500~l.~.s duration, with each pulse consisting of approximately 250 laser spikes. Further work on laser ablaTABLE 3 Detection Limits for Several Elements Using Laser Atomic Absorption Spectrometry Detection limit Element

Type sample

Cont. (ppm)

Al Cr cu

Steel Steel Steel Al alloy Brass Al alloy Steel Al alloy Brass Steel Steel

1.3 2.7 7.2 32 1.9 11 2.1 20 4.7 5.7 16

Fe Mn Ni V Source.

Ref. (62).

Weight (g) 6.7 x 2.4 x 6.5 x 2.6 x 1.9 x 8.9 x 1.9 x 1.6 x 2.4 x 5.1 x 1.4 x

lo-‘* 10-l’ IO-‘* lo-l2 lo-‘2 lo-l2 lo-‘* 10-l’ 10-l’ lo-‘2 lo-=

Cont. range (%) 0.012-0.070 0.13-0.21 0.017-0.062 0.055-0.20 0.044-0.26 0.075-0.48 0.374.95 0.13-0.30 0.043J3.16 0.069-0.24 0.11-0.33

Flame AAS Wml) 340 40 25 50 20 50 500

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tion-flame AAS by Kantor et al. (64) used a passive switch ruby laser to determine the thickness of silver, gold, or nickel on electroplated copper. Matousek and Orr (65) used a pulsed CO, laser, with 100 mJ per pulse, a half-width of 300 ns, and a peak power of 0.5 MW interfaced with a graphite furnace, to directly determine silver in copper alloys. Ishizuka et al. (62) used a Q-switched ruby laser combined with a graphite furnace system to determine several metals in brasses, carbon steels, and alloys. Dittrich and Wenmich (66) used a Q-switched ruby laser combined with the graphite furnace to determine silver in solid samples. Further work by these authors involved a dual-channel AAS system for simultaneous determination of two metals in synthetic powders (67). Sumino et al. (68) used a Nd:YAG laser with a maximum pulse energy of 2 J for the localization of cadmium in human kidney cortex. Schron and Bomback (69) detected silver in geological samples using a laser ablation-graphite furnace AAS method. At this time there is no indication that this technique of laser ablation AAS can match the detection limits achieved by conventional flame and graphite furnace atomization techniques. The major advantage of the technique would appear to be its ability to directly analyze solid samples with no sample pretreatment. 2.2 Atomic Emission Spectroscopy In conventional quantitative atomic emission spectroscopy, sample preparation of solids usually involves the dissolution or digestion of solids and the introduction of an aerosol or solution into a flame, graphite furnace, or plasma. Laserinduced breakdown emission spectroscopy (LIBS) is a technique which has shown promise for obtaining analytical atomic emission directly from solid, liquid, and gaseous samples. The technique involves focusing the laser into a gaseous sample or the surface of a solid or liquid to produce a transient plasma where sample analytes emit their characteristic radiation. The technique is discussed in more detail in Section 3, Laser-Induced Plasma. This section primarily deals with laser-ablated plasma atomic emission spectrometry and, where appropriate, related topics of mass spectrometry. 2.2.1. Laser Ablation-Microwave-Induced Plasma Atomic Emission Spectrometry The low-power microwave-induced plasma (MIP) has been used as an excitation source in AES. In almost all cases, the sample is introduced in aqueous form. To maintain a stable plasma, much of the aqueous phase must be removed prior to introduction to prevent extinguishing the MIP. In the case of solid samples, they are converted to solutions by a digestion process prior to analysis. As the introduction of a large amount of materials may cause significant changes in the plasma’s impedance, a method of introducing a microsample of a solid material into the plasma would be of great benefit, in time savings and possibly in the reduction of detection limits due to dilution (70). Such a system coupling an MIP with laser vaporization for a solid sample introduction system has been constructed. A total of 10 different metals in different sample types were analyzed. Detection limits, using both peak height and peak

13

LASERS IN ATOMIC SPECTROSCOPY

area, and concentration ranges for working curves can be seen in Table 4 (70). A wide variety of both elements and sample types can be analyzed with this method with respectable results even though the MIP is much lower in temperature than an ICP or DCP. More work is needed in this field to fully unlock its analytical potential. Leis and Laqua (71, 72) used a Q-switched ruby laser to ablate aluminum and zinc alloys. They investigated the influence of argon lines on spectral lines and obtained several detection limits in the parts per million range for selected elements. 2.2.2. Laser Ablation-Inductively Emission Spectrometry

Coupled Plasma Atomic

The inductively coupled plasma has found large-scale use in the field of trace metal analysis for a wide variety of materials. A limitation of ICP methods has been the use of pneumatic nebulization to introduce samples into the plasma (42). Laser ablation as a sample introduction source offers significant potential for the direct introduction of a large variety of solid and powdered sample forms into the ICP. The first information about the use of laser ablation as a sample introduction source for ICP-AES was presented at the Pittsburgh Conferences in 1977 (73) and 1979 (74). Several different types of lasers including ruby (75) have been used for sample introduction of solids into an ICP. The laser plume contains significant amounts of solid particulate material, and thus a more complete atomization cell such as an ICP is needed to completely analyze the entire amount of material ablated. Several of the parameters of laser ablation ICP-AES are of significance. First, the connection of the sampling chamber to the torch of the ICP must be kept as short as possible to minimize condensation, diffusion, and dilution of the ablated sample. Increasing the length of the connection causes peak heights to decrease and results in peak broadening due to dilution of the sample. A simultaneous multichannel measurement system is desirable and critical since a transient emission signal is generated by the ICP as the material is swept through the plasma discharge. It has been found that the emission intensities in the ICP are directly TABLE 4 Detection Limits, Using Both Peak Height and Peak Area and Concentration Working Ranges for Laser Ablated-Microwave-Induced Plasma Atomic Emission Spectrometry Detectionlimit RSD (%) Element Al Cr CU Fe Mn Ni Zn

Sample type Steel Al alloy Steel Brass Al alloy Al alloy Al alloy

Source.Ref (70).

Cont. Pm

Peak height

Peak area

0.07 0.031 0.16 0.088 0.13 0.20 0.035

13.8 7.2 4.3 3.5 2.4 3.8 7.3

8.5 8.0 2.3 5.9 4.4 5.4 6.3

Peakheight Cont. (wm) 9.3 13 2.4 2.7 5.4 13 0.9

Weight (pg) 8.4 10 2.2 2.7 4.3 10 0.7

Peakarea Cont. (wm) 14 17 4.2 3.8 4.7 9.0 1.2

Weight (Pg) 13 14 3.8 3.8 3.8 7.2 1.0

Cont. rangein working curve (%) 0.017-0.16 0.044-0.26 0.026-0.30 0.012-2.01 0.035-0.20

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related to the laser energy and how the energy is focused. Detection limits using two types (Q-switched and normal) of lasers for several elements in differing materials are shown in Table 5. Thompson and co-workers (76, 77) used a ruby laser delivering 1 J per pulse to directly analyze soil samples and stream sediment pebble coatings. Kawaguchi et af.(78) used a Nd:YAG system with a maximum of 100 mJ per pulse and a commercial optical system to determine metals in low-alloy steels. Cat-r and Horlick (42) used a ruby laser combined with a simultaneous multichannel analyzer, and time resolution was used with a photodiode array detector and a spectrograph. At present, the laser-ICP-AES system shows excellent potential for the direct analysis of solid samples if several problems can be resolved, including the optimization of the sample introduction/interface between the laser and the ICP. 2.2.3. Laser Ablation-Direct

Current Plasma Atomic Emission Spectrometry

Several studies have used laser ablation as a sample introduction method for direct current plasma atomic emission spectrometry (DCP-AES). One study involved the direct determination of copper in ore samples (59, 79-81). They found that the laser pulse frequency, different copper compounds (copper sulfate, copper carbonate, copper hydroxide, and copper nitrate), and particle size all played important parts in the intensity level of the DCP emission signal. Emission results as well as precision were improved by integrating the signal over a period of 30 s as a larger portion of an inhomogeneous sample was ablated. By using these parameters, a detection limit of 20 p&g for copper was achieved. As with all techniques, the optimization of the sample introduction was important to achieve reproducible results. 2.2.4. Laser Ablation-Inductively

Coupled Plasma Mass Spectroscopy

As with most other techniques discussed in this section, inductively coupled plasma mass spectroscopy (ICP-MS) has primarily used aqueous type solutions for which the technique has demonstrated high sensitivity for a large number of elements and greatly simplified spectra compared to ICP-AES. Solid sample inTABLE 5 Detection Limits for Several Elements in Steel Using Laser Ablation-Inductively Atomic Emission Spectrometry Q-switch

Cr CU Mn MO Ni V

Normal

Analytical

Cont.

Weight

Cont.

line

(mm)

(Pg)

(ppm)

20 8 10 9 3 20 20 20

2 0.6 1 0.3 0.3 2 1 1

Element AI co

laser

I II II I II II II II

3%.1x? 228.616 267.716 324.754 257.610 281.615 231.604 310.230

Source. Ref. (75)

20 8 10 9 3 20 20 20

Coupled Plasma

laser Weight (PP)

Conventional solution ICP (&ml)

60 20 30 9 9 60 30 30

0.2 0.1 0.08 0.04 0.01 0.2 0.2 0.06

15

LASERS IN ATOMIC SPECTROSCOPY

troduction for ICP-MS has consisted of arc nebulization (82), glow discharge (83), electrothermal atomization (84), and both low (85) and high (86) pulse rate laser ablation. The higher repetition rate laser ablation results on various solid samples for LA-ICP-MS are shown in Table 6. A comparison to the detection limit obtained by conventional solution ICP-AES is also shown in this table. Detection limits are in the low to submicrogram per gram levels. Additional research in the area of LA-ICP-MS is needed, but the introduction of a commercial system several companies may rapidly realize its potential. 2.2.5. Laser Ablation Mass Spectroscopy Laser mass spectroscopy (LMS) or laser microprobe mass spectroscopy (LMMA) can be used for trace analysis of most elements. Traditionally, the spark source technique of ionization has been used for inorganic analysis (87) but is unable to analyze insulators directly. The first laser mass spectrometer was reported by Honig and Woolston using a ruby laser (88). A commercially available instrument is the LAMMAlaser microprobe mass spectrometer system. One disadvantage of this system is its requirement of a thin sample through which the laser light can pass. Other research laser mass spectrometers utilize direct ablation of the sample (89) and therefore can be used for semiquantitative bulk analysis of any type of sample (solid, powder, or liquid). Information can be obtained about the concentration of all elements simultaneously with a dynamic range of 10 ppb to 100%. The primary advantage of the use of the laser for sample introduction/ionization is its ability to rapidly analyze both organic and inorganic samples with high sensitivity (-lo-*’ g). 2.3. Atomic

Fluorescence

Spectrometry

Laser ablation combined with AFS has been proposed by Lewis et al. (90). They used time resolution to avoid the strong stray light in the laser plume. Mayo et al. (91) proposed an ultramicro and ultratrace technique called trace element TABLE 6 Detection Limits for Several Elements by Laser Ablation-Inductively Mass Spectrometry

Isotope

Sample type

% abundance

Detection limit (3(r) b.dd

“As

NBS steel Copper Std NBS steel NBS steel Copper Std NBS steel NBS steel Copper Std Copper Std

100 100 30.9 42.8 57.3 23.8 51.5 51.8 28.9

2 6 2 2 4 0.9 0.3 0.3 0.2

65Cu lz3Sb “‘Sb 98Mo 9oZr “‘Ag “.‘Cd

Source. Ref (86).

Coupled Plasma Detection limit conventional solution ICP (&ml) 2 0.04 10 0.2 0.006 0.2 0.07

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analysis based on laser ablation and selectively excited radiation (TABLASER). Laser ablation combined with AFS has not been used as often as AAS and AES. 3. LASER-INDUCED PLASMA The interaction of high-power laser light with a target or solid sample has been an active topic not only in plasma physics but also in the field of analytical chemistry. From a practical standpoint, the use of lasers to vaporize, dissociate, excite, or ionize species on solid surfaces has the potential of becoming a powerful analytical tool. This section describes the basic characteristics of laser-induced plasmas, including the laser-target interaction, the influencing factors in plasma formation, the emission characteristics of laser-induced plasma, excitation temperatures and electron densities of the plasma, and the application of the laserinduced plasma in atomic spectroscopy. In order to understand the analytical utility of the laser-ablated or laser-induced plasma it is necessary to present a discussion of its production and characterization. 3.1. The Interaction of a Laser Beam with Target Materials When a high-power laser pulse is focused onto a solid target, the irradiation in the focal spot can lead to rapid local heating, intense evaporation, and degradation of the material. Descriptions of laser-solid interaction and comprehensive reviews of analytical techniques have been published by several authors (47, 92-94). The interaction between a laser beam and a metal is a complicated process dependent on many characteristics of both the laser and the metal. Numerous factors affect ablation, including the laser pulse properties, such as pulse width, spatial and temporal fluctuations of the pulse, and power fluctuations. The mechanical, physical, and chemical properties of the sample also influence the ablation process. The phenomena of laser-target interaction have been reviewed by several authors (95, 96). Ready (96) gave a comprehensive description of melting and evaporation at metal surfaces. Anisimov and his co-workers (97, 98) related the thermal conductivity mechanism to the boundary condition of free vaporization of the solid into a vacuum. Caruso et al. (99) found three different regions existing in a metal and the hot plasma formed on the outer surface expanding toward the light source at a supersonic speed. The hot vapor plasma interacts with the surrounding atmosphere in the two ways illustrated in Fig. 1 (92) and involves (i) the expansion of the high-pressure vapor driving a shock wave into the atmosphere, and (ii) energy being transferred to the atmosphere by a combination of thermal conduction, radiative transfer, and heating by the shock wave. The subsequent plasma evolution depends on irradiante, size of vapor plasma bubbles, target vapor composition, ambient gas composition and pressure, and laser wavelength. The history of important quantities such as radiative transfer, surface pressure, plasma velocity, and plasma temperature is strongly influenced by the nature of the plasma, as is the final steady-state nature of the plasma. The three major types of laser absorption wave are (i) laser-supported combustion (LSC) waves, (ii) laser-supported detonation (LSD) waves, and (iii) laser-supported radiation (LSR) waves (100). The difference in the

LASERS IN ATOMIC SPECTROSCOPY

17

HOT, HIGH-PRESSURE, !Xl?ONGLY ABSORBING VAPOR PlASMA TEMPERATURE PROFILE AMBIENT ATMOSPHERE CONDUCTION SHOCK WAVE RADIATION FIG. 1. Features of the interaction between the vapor plasma and the ambient gas.

waves arises from the different mechanisms used to propagate the absorbing front into the cool transparent atmosphere. The characteristics distinguishing the waves are velocity, pressure, and the effect of radial expansion on the subsequent plasma evolution. At low irradiation, laser-supported combustion waves are produced. Razier (201) examined the long-time propagation of the laser-supported combustion waves at 1 atm. Thermal conduction was assumed to be the primary propagation mechanism. Subsequent authors (102, 103) suggested that radiative transfer could contribute. Several workers (204-106) studied the one-dimensional propagation with radiation using a variety of transport models. The major mechanism causing LSC wave propagation is radiative transfer from the hot plasma to the cool highpressure gas created in the shock wave. The plasma radiation is primary in the extreme ultraviolet and it is generated by photorecombination of electrons and ions into the ground-state atom. At intermediate it-radiance, the precursor shock is sufficiently strong that the shocked gas is hot enough to begin absorbing the laser radiation without requiring additional heating by energy transport from the plasma. The laser absorption zone follows directly behind the shock wave and moves at the same velocity. This is the analog of the chemical detonation wave and has been modeled by Ramsden and Savic (107) and Razier (108). The propagation of the laser-supported detonation wave is entirely controlled by the absorption of the laser energy. Several workers (109412) theoretically and experimentally studied the ignition and propagation of the LSD wave off metal surfaces. Plasma energy transfer to metal surface and the breakdown times were calculated and modeled. At sufficiently high it-radiance, the plasma radiation is so hot that, prior to the arrival of the shock wave, the ambient gas is heated to temperatures at which laser absorption begins. In the idealized configuration, laser absorption is initiated without any density change, and the pressure profile results solely from the strong local heating of the gas rather than a propagating shock wave. This configuration is an example of an overdriven absorption wave (108). These supersonic waves

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were modeled numerically by Bergel’son et al. (109). Their numerical results confirm the basic structure that once the transient plasma initiation and formation process are completed the quasi-steady approximation is suitable. The lasersupported radiation wave velocity increases much more rapidly with irradiance than that of the LSC and LSD waves. The temperature and pressure increase, conversely, is quite slow. This behavior illustrates that the LSR wave is effective in channeling the absorbed energy into heating a large amount of gas rather than increasing the local enthalpy. 3.2. Laser-Induced

Plasma-Production

By increasing the energy deposited into the sample surface, temperatures reach a point at which material transfer across the surface becomes significant. In this type of experiment, target erosion appears in the form of craters. The theoretical considerations on plasma production and heating by means of laser beams have been proposed by several workers (11~223). These models produce essentially similar solutions for the plasma temperature, density, and expansion velocity and are broadly in agreement with experimental results. Cottet and Romains (114) studied the formation and decay of laser-generated shock waves by a hydrodynamic model. Measurements of shock wave velocities were performed on copper foils for incident intensities between 3 x 10” and 3 x lo’* W/cm*, with the use of piezoelectric detectors. Balazs ef al. (225) calculated the time development of density, velocity, temperature, and pressure profiles below and above the plasma ignition threshold. Below the plasma ignition threshold, the temperature of the expanding plume never exceeds the surface temperature, and in the vapor, thermal ionization is almost completely absent. The plume expands into the vacuum, and its flow becomes supersonic. In the high-fluence case, the energy delivered to the plume through electron-neutral inverse Bremsstrahlung processes was enough to elevate the temperature close to the surface value. This gives rise to high electron density as well as intense light absorption. Between the creation of a localized vapor plasma and the steady-state lasersustained plasma, a plasma evolves through several transient phases. The initiation of a plasma over a target surface begins in the hot target vapor. Absorption generally commences via electron-neutral inverse Bremsstrahlung, but when sufficient electrons are generated, the dominant laser absorption mechanism makes a transition to electron-ion inverse Bremsstrahlung. Photoionization of excited states can also contribute to short wavelength interactions. The same absorption processes are also responsible for the absorption by the ambient gas. The basic progression of interaction (from absorption through compression) is, however, preserved. In recent years, research has revealed a strong dependence of absorption and scattering processes on the laser wavelength (116, 117). The aforementioned trend toward short-wavelength research thus implies investigation of plasma processes at a much higher density, where collisional effects will be emphasized. The critical densities for some common laser wavelengths are shown in Table 7 (118).

19

LASERS IN ATOMIC SPECTROSCOPY TABLE 7 Critical Densities for Some Important Lasers Laser

co* Nd glass (q,) Nd glass (30,) KrF

Wavelength (pm)

n, (cmm3)

10.6 1.06 0.35 0.25

10’9 IO*’ 9 x 102’ 1.6 x lo**

Source. Ref. (118).

3.3. Factors Influencing

Plasma Formation

3.3.1. Laser Parameters 3.3.1.1. Influence of irradiation wavelength. In practice, different types of lasers, mainly Nd:glass resonators (A = 1064 nm), ruby laser (h = 694 nm), Nd:YAG laser (A = 1064 nm), sometimes CO,-TEA (carbon dioxide-transversely excited atmospheric pressure laser) (A = 10.6 pm), nitrogen laser (A = 337 nm), and dye laser s (A = 22CL740nm), are used in the production of laser plasmas. The interaction of near- and midinfrared pulsed laser with metals has been studied extensively. The use of the UV excimer laser is of growing interest because of the low UV reflectivity for most metals, with improved energy coupling efficiency, and the high optical resolution offered by short wavelengths (119-121). The influence of the laser wavelength on the material removal was studied by several authors. Eloy (122) researched the laser evaporation using different wavelengths and reported that the thickness of a car paint sample can be reduced many times by the use of a UV wavelength compared to red laser light to obtain analytical signals of the same magnitude in mass spectrometry. Bingham and Salter (54) researched the ion production in mass spectrometry using three different lasers, CO,, ruby, and Nd:YAG. They obtained the highest sensitivity for the elements P, S, Ti, V, Cr, Mn, Ni, Co, Cu, As, Zr, MO, Nb, S, Ta, and W with a steel standard using ruby (A = 694 nm) laser ablation. However, the CO, laser (A = 10.6 rJ.m)showed poor sensitivity for the high boiling point elements (Ti, V, Zr, MO, Nb, Ta, and W). Fabbro et al. (57) used a Nd:YAG laser that was frequency doubled and quadrupled to give wavelengths of 1064, 532, and 266 nm to study the effect of wavelength and postulated the following equation for the mass ablation rate, m (kg/s cm’), its dependence on wavelength, A, and the absorbed flux, F, (W/cm2): m = 110 (F,‘/3)/(10’4) A-4/3. They found that the mass ablation rate would increase strongly at shorter wavelengths. Measurements of the ablation pressure generated by the ablating plasma have been carried out at a number of laboratories (223-125). These results confirmed the expected higher ablation pressure with shorter wavelength laser irradiation. Kwok et al. (226) investigated the optical emission produced by laser ablation of YBa,Cu,O, targets using a wide range of laser wavelengths and showed that

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193-nm radiation produced mostly neutral atomic species while 10&l- and 532~nm radiation produced mostly ionic species. 3.3.1.2. Influence of irradiation energy. There are two main mechanisms for electron generation and growth. The first mechanism involves absorption of laser radiation by electrons when they collide with neutrals. If the electrons gain sufficient energy, they can impact and ionize solids. The electron concentration will increase exponentially with time due to the cascade breakdown. The second mechanism, called multiphoton ionization (MPI), involves the simultaneous absorption by an atom or molecule of a sufficient number of photons to cause its ionization. Multiphoton ionization is important only at short wavelengths (
LASERS

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the frequency-quadrupled Nd:YAG (A = 266 nm) laser was always the largest, while for the CO, laser it was the lowest. This is in accordance with the widely known observation that UV lasers produce sharply etched craters in the target, while increasing laser wavelength creates a molten crater rim. Laqua (133) distinguished two different cases of vaporization depending on the irradiation, at an irradiation of less than lo8 W/cm* and at one higher than lo9 W/cm*. In the first case, the stream of vapor leaves the surface at a velocity of lo4 cm/s. After the initial vaporization, the process changed to a melting-flushing mechanism as a result of the heat conduction of the material. At higher irradiation, the temperature of the vapor leaving the surface is higher than the boiling point of the target material. The gas molecules above the target were ionized with a velocity of lo6 cm/s near the surface. Hwang ef al. (58) studied quantitative plasma emission with increasing laser energy by an ArF-excimer laser with the selected metals Zn and Cu and Ni-alloy and iron-alloy. The results show that the emission intensity from a laser-induced plasma with increasing laser energy can be quantified both theoretically and experimentally and a potential correlation obtained. Based on a heat conduction mechanism, they derived the equation m(t) = A(d)

+ B(aZ)*tQ.

This equation shows that the mass removal, m(t), is proportional to the metal thermal properties (A, B), energy coupling factor (a), laser irradiance (Z), and laser pulse duration (t). 3.3.1. Physical Properties of the Target Material The physical properties of the target have an important influence on the shape and size of craters in target materials. The reflection of part of the laser energy is an important consideration in determining the fraction of laser energy absorbed by sample materials. The change in reflectivity may be due in part to the result of phase changes that occur during intense heating. In any case reflectivity measurements indicate that laser energy can be coupled effectively to a target that is initially highly reflective, if the irradiance is high enough (234). Recognition of the fundamental differences of the interaction of burst (single) and Q-switched modes of operation with materials led to the conclusion that craters produced by the latter mode might be less material-dependent than those for the former case (135, 236). However, Klocke (137) and Baldwin (138) found that the sample by a laser beam is strongly target material-dependent, whether the laser is Q-switched or not. Allemand (239) showed that the reflectivity of the sample surface, density, specific heat, and boiling point of the pure metal target have an important influence on the shape and size of the craters and derived the relationship to the physical constants of pure materials, D = A(1 - R)IpCT,, where

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D = diameter of total splash (crater), A = proportionality constant (in energy per unit area), R = reflectivity of the surface at 1 mm, p = density, C = specific heat, T,, = boiling temperature. Ishizuka (140) studied the size and depth of the crater in samples of rare earth oxides, aluminum oxide, and sodium salts by using a Q-switched ruby laser. The crater ‘produced by a laser shot was about 1 mm in diameter regardless of the composition of the matrix, but the depth of the crater depended on the type of matrix. A comparison of the crater size of the homogeneous material revealed that the thermal conductivity is an important parameter. The depth of the craters increased with this value. The volume heated depended on the thermal conductivity of the material under the same laser conditions (241). On the other hand, heating of material around the crater increased with incident light intensity because evaporation depends only on the boiling point of the material at fixed pressure. Dimitrov et al. (142, 243) investigated the substance evaporation processes and the kinetics of plasma plume development depending on target orientation with respect to the laser radiation source direction. When the metal target is irradiated by laser radiation, the erosion products emerge nearly perpendicular to the target surface. When the target surface is inclined with respect to the direction of laser radiation, the path length of the radiation in the plasma is shortened, which results in decreased absorption of the laser-produced plasma. Lee et al. (144) studied the laser-induced plasma by space-resolved spectroscopy with an ArF-excimer laser using an energy of 100 mJlpulse and different target species, Cu and Pb, in air. They found that the size and emission characteristics generated by Cu and Pb target species were quite different. The optimum position for spectrochemical analysis (highest line-to-background ratio) also differed in the plasma using the two target metals. 3.3.3. Ambient Conditions The laser is focused on the surface and when the temperature is sufficient, a plasma will form. The plasma starts expanding rapidly, inducing a shock wave in the ambient gas. The expansion slows down when the pressure in the explosion center decreases to the level of the ambient pressure. Peipmeier and Olsten (145) investigated the effect of atmosphere on the spectra, on the crater size, and on the amount of sample vaporized, with changing ambient pressure in air. They clarified the role of the atmosphere in the plasma generation process by considering the absorption of the laser beam by the atmospheric plasma. Treytl et al. (246) investigated the effect of various atmospheres including argon, air, oxygen, nitrogen, and helium on the atomic emission intensities of laserinduced plasmas. The strongest signal was produced in argon atmosphere and the largest signal-to-background ratio occurred in a vacuum with its lower back-

LASERS

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23

ground continuum. Kagawa et al. (147, 148) also reported that a characteristic plasma, emitting sharp atomic line spectra with a negligibly low background signal, was generated by bombardment of high-power nitrogen laser light on a solid target when the pressure was reduced to 1 Tot-r in argon. Yasuo (149) researched the emission of the laser-induced plasma, with the use of a Q-switched ruby laser of energy 1.5 J, in an argon atmosphere at reduced pressure. In the argon atmosphere at reduced pressure, the emission period of the plasma is elongated to over a 100 ms, and the emissive region expands to more than a few tens of millimeters above the target surface. The emission intensities of atomic lines increased severalfold in an argon atmosphere, in comparison with those obtained in air at the same pressure. Moderate confinement of plasma and a resultant increase of emission intensities were achieved at 50 Tort-. These results were explained by the chemical inertness and the thermal characteristics of the argon atmosphere and the decrease in absorption of the laser pulse by the plasma plume. Recently, Grant and Paul (150) studied the laser-induced plasma by irradiation of a steel target with an XeCl-excimer laser (A = 308 nm) with an energy of 40 mJ/pulse in an atmosphere of air, argon, nitrogen, and helium at pressures from 0.5 to 760 Tot-r. The maximum spectral intensity and line-to-background ratio occur in an atmosphere of argon at a pressure of 50 Ton-. Beenen and Peipmeier (49) studied the spatial and the temporal properties of binary molecules formed by the reaction of the sample vapor with the surrounding atmosphere, using optical spectroscopic detection methods. The experiments were performed at 150 Torr in 100% O,, 50% 0,/50% Ar, and 100% Ar. The initial concentration of the atmosphere gas in the region of the plume was found to be less than the average bulk concentration in the sample chamber. Complete mixing of the plume with the atmosphere took at least 100 ms. A significant quantity of metal monoxide was formed in an atmosphere containing only trace amounts of oxygen. Mason and Goldberg (151) investigated the interaction of pulsed magnetic field on laser-induced plasma and found an increase in line-broadening, neutral atom self-reversal, and minor constituent emission intensities with the magnetic field. Both ion and neutral atom emission were confined close to the sample surface by the magnetic field, indicating compression of the laser-induced plasma. 3.4. The Emission Characteristics of the Laser-Induced Plasma Experimental observations of the emission from laser-induced plasmas were studied by temporally and spatially resolved spectroscopy (60, 152, 153). The emission from a Q-switched mode plasma showed that initially an intense continuum is emitted close to the target surface, corresponding to the emission of blackbody radiation from the dense plasma. As the plasma cools and expands away from the target surface, line emission tends to dominate the radiation processes, with first the highly ionized lines being emitted close to the target (superimposed on the plasma continuum) and then the atomic lines appearing in the higher regions of the plume. Line emission from the multiply ionized species occurs around the same time,

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whereas singly ionized and neutral species do not appear until about 250 and 300 to 400 ns, respectively. They were found at a greater height above the target surface. Similar behavior exists for normal mode plasmas, but the radiant emission occurs in pulses which coincide temporally with the relaxation oscillation spikes of the laser pulse. Kagawa and Yokoi (247) found that the laser-induced plasma using a nitrogen laser (A = 337.1 nm) consists of two distinct regions. One region (primary plasma) emits an intense continuous emission spectrum for a short time just above the surface of the target and the other region (secondary plasma) expands with time around the primary plasma, emitting sharp atomic line spectra with a negligible low background signal. They observed a similar appearance of the dual plasma regions using an XeCl-excimer laser (A = 308 nm) and a 58%nm dye laser (148). However, the appearance of the plasma generated by a Nd:YAG laser is quite different (145). The secondary plasma was not observed in the plasma formed by the Nd:YAG laser. 3.5. Excitation Temperatures and Electron Densities of the Plasma Temperature is one of the more important properties of any excitation source. Knowledge of the temperature of an excitation source is vital to the understanding of the dissociation, atomization, ionization, and excitation processes occurring in the source and is helpful in attempts to utilize the source to its maximum analytical potential. For it-radiances just above threshold, evaporation from the liquid boundary layer proceeds at the normal boiling temperature of the material. Such low-density vapors are virtually transparent to the incident laser beam. Consequently, little heating of the vapors occurs and the expansion velocity and temperature of the plasma are dependent only on the thermal properties of the material, and not on the laser it-radiance. At irradiance well above threshold, extremely dense, hightemperature plasmas are formed (154, 255) and are primarily of interest in analytical studies of a laser-induced plasma. The methods most frequently used for determination of excitation temperatures are the two-line method (156) and the Boltzmann plot method (257, 258). Several possibilities for the determination of the laser plasma plume temperature are described in the literature. Zahn and Dietze (159) calculated the temperature of Nd:YAG laser-induced plasmas by measuring the numbers of atoms, electrons, and ions with a mass spectrometry for different elements. They found plasma temperature to range from 3300 K in the case of indium to 8200 K in the case of molybdenum. The temperature of laser-produced plasmas is dependent on the target materials. Grant and Paul (150) also reported the electron temperature and density of XeCl-excimer laser-induced plasma. The relevant atomic contents of the 11 Fe(I) lines used in the Boltzmann plot determination of electron temperature were used. The temperature ranged from 9000 to 22,000 K depending on the ambient conditions. Temperature decreases with distance from the surface and with decreasing ambient pressure. Electron densities were calculated according to the Saha equa-

LASERS

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25

tion and the fitted values of temperature with the assumption of LTE (local thermodynamic equilibrium). The density profile exhibited features similar to those for temperature, ranging from 3 x 1019to -1016 cmV3. Kagawa ef al. (148) calculated the excitation temperature in a high-power nitrogen laser-induced plasma with the two-line method by the line pair of Zn(1). The temperature ranged from 8000 to 9000 K, and the region of maximum temperature was at a point some distance from the center of the plasma, rather than at the center. Ursu et al. (160) studied the optical breakdown plasma in a gas in front of various solid samples by a CO,-TEA laser source. They measured the energy absorbed into the blade calorimeter, placed at various distances from the center of the plasma. An upper limit of the vapor temperature of -14,000 K was inferred from the characteristic darkening curve of the photographic film. They found that the initial maximum corresponding to the breakdown plasma in gas having a temperature of -20,000 K is followed by a luminescence tail due to vapors acting with the fireball and having temperatures of -10,000 K. Radziemski et al. (261) and Cremers et al. (262) measured the temporal variation of temperature and electron density in an air plasma induced by a CO, laser operating 0.5 and 0.8 J/pulse. The excitation temperature was determined spectroscopically by a Boltzmann plot, and ranged from 19,000 K at 1 ms to above 11,000 K at 25 ms. The electron density was measured at 500 mJ/pulse and determined from 3.6 x 10” cmp3 at 1 ms and 4 x 1016cmm3. Lee et al. (244) calculated the excitation temperature of ArF-excimer laserablated plasma by the Boltzmann plot method with Cu(1) and Pb(1) lines. The temperatures of the excimer laser-ablated plasma were quite high, ranging from 13,200 to 17,200 K in the plasma formed with copper and from 11,700 to 15,300 K for the plasma formed with lead depending on the location in the plasma. Hwang et al. (163) calculated a temperature of 14,000 K for an excimer (X = 193 nm) ablated plasma. Measurement of precision and accuracy using this method is highly dependent on sample composition, homogeneity, surface condition, and particle size generated. Precision is typically 5 to 20% but values of less than 1% have been achieved under certain conditions (57). There are several advantages to LIBS, including the high spatial resolution provided by the focused laser pulses, the small (ng-pg) sampling size, the ability of the laser-induced plasma to vaporize and excite the solids in one step without extensive sample preparation, and the ability to analyze through a window or at a distance from the sample. The method by which the plasma emission is collected is important because of the small size of the spark and the inhomogeneous distribution of material within it, which require that the same part of the plasma be monitored for maximum signal reproducibility. Maximum sensitivity is achieved by time resolving the emission. Time resolution is useful for discriminating against a strong background continuum radiation occurring immediately after the spark formation and for avoiding spectral interferences between species that emit at different times during the plasma decay (58).

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3.6. Applications

The use of laser-induced breakdown emission spectroscopy has been pioneered by Radziemski and Cremers and co-workers. An early introduction to LIBS by Radziemski and Loree (164) was followed by an in depth study of the LIBS plasma (160-262, 165) and application to the detection of chlorine and fluorine in air. Detectable masses of 80 ng for chlorine and 2000 ng for fluorine in air and 3 ng in helium for both gases were achieved (166). LIBS has been used to detect beryllium on filters (267). A detectable mass of 3.6 ng was obtained with a precision of 4%. Belliveau et al. (268) detected trace levels of chromium, manganese, and nickel in steels using a Nd:YAG laser and echelle optics system. Levels as low as 0.01% could be detected. Mallard et al. (269) detected beryllium in beryllium-copper alloys with a linear range from 0.0001 to 0.22% and a detection limit of 0.0002% (2 ppm). Cremers (170) used LIBS to detect metals at distances of between 0.5 and 2.4 m from focusing the lens and optics. The detection of eight metals over a 40-nm wavelength range was achieved. Wachter and Cremers (271) determined uranium in solution with a precision of l-2% for a 4.2 g/liter solution. A calibration curve in the range 0.1 to 300 g/liter was obtained. Cadmium, lead, and zinc in aerosols were detected by LIBS by Essien et al. (272). Detection limits of 0.019, 0.21, and 0.24 g/g, respectively, were obtained. Grant and co-workers (173, 174) detected calcium, silicon, magnesium, aluminum, and titanium in iron ore with detection limits around 0.01%. Cheng et al. (175) detected Column III and V hydrides with detection limits of -1-3 ppm. 4. LASER-ENHANCED

IONIZATION

SPECTROMETRY

The most widely used and desirable atomizers in atomic spectroscopy will efficiently create atoms. However, depending on the ionization potential of a particular species, they can create some level of ionization of these atomic and molecular species. If a laser is used to resonately excite the atomic species, the laser-excited (laser-induced) atom population may have a lower effective ground state population and may be more readily ionized by thermal collisions. This process is the basis of an analytical technique most frequently called laserenhanced ionization spectrometry (LEI). The general class of this technique is called the optogalvanic effect. The basic principle of LEI involves applying a voltage across the atomizer and monitoring the resulting current. The LEI signal is a change in the current due to the concentration of species in the atomizer. A more detailed discussion of the theory, including signal production and detection, is provided elsewhere (176). Although used and developed by approximately 20 research groups, the technique was promoted and developed with the pioneering work in the late 1970sand early 1980s by Travis, Turk, and co-workers at the National Institute of Science and Technology, Gaithersburg, Maryland (formerly National Bureau of Standards), and Green and co-workers at the University of Arkansas, Fayetteville, Arkansas. Early reviews of LEI spectrometry are available (177, 178).

LASERS IN ATOMIC

4.1. Laser Systems

SPECTROSCOPY

27

Used in LEI Spectrometry

Various laser systems have been used in LEI spectrometry. In general, dye lasers have been found to be the most widely used and successful due to the wide wavelength coverage and high peak power with pulsed systems resulting in improved performance over continuous wave (CW) systems. CW lasers produce low power in the UV region. This region is where many resonance (strongest) transitions occur. The UV region transitions also terminate near the ionization limit. Ordinarily, this would produce the largest (most sensitive) LEI signals. There is a significant difference in the performance of the pulsed dye laser depending on the pumping system. Flashlamp-pumped, laser-pumped, nitrogen-pumped, excimer-pumped, and Nd:YAG-pumped dye lasers have been investigated and compared for LEI spectrometry (signal intensity). The flashlamp-pumped dye laser was considered the best pulsed system because of its relatively long (-1 t.~s)pulse duration, whereas the use of the laser-pumped dye laser was poorer due to the relatively short pulse width of 6 to 20 ns. Nitrogen-pumped systems are the poorest systems due to lower peak powers, narrow pulse width, and the potential radiofrequency (RF) interference from the nitrogen-laser system. Nd:YAG- and excimer-pumped dye laser systems give LEI signals comparable to those of the flashlamp-pumped systems. Copper vapor-pumped dye lasers give poor LEI signals due to the RF problem and low peak power. However, the kilohertz repetition rates and high pulse-to-pulse correlation of the copper vapor laser RF noise allow the use of relatively sophisticated noise correction and reduction strategies such as boxcar systems to improve the signal. Rutledge ef al. (179) improved the detection power by 35-fold with such an approach. The diode laser has been recently applied to LEI spectrometry by Lawrenz et af. (180). The laser produced single longitudinal CW modes from 760 to 805 nm with a maximum power of 5 mW and 810 to 860 nm with a maximum power of 15 mW. The system was used to measure isotope ratios of barium. The relatively low cost, small size, ease of use, and reliability make the diode laser attractive in all spectrometry (as well as LEI). 4.2. Instrumentation

for LEI Spectrometry

The laser systems used in LEI spectrometry are discussed separately in this review. The first and still most widely used source of ions is the flame. Turk et al. (181) demonstrated the sensitivity and detection limit (in &ml) of the LEI flame method for copper (IOO), magnesium (O.l), manganese (0.3), sodium (O.l), and lead (3) using transitions (resonance lines) normally used for conventional flame atomic absorption spectrometry (FAAS) and atomic lines not normally used in FAAS of manganese (5 at 280.0 nm), sodium (0.05 at 285.3 nm), and lead (0.6 at 280.2 nm). Further work by Turk et al. (182) improved the sensitivity of the LEI flame method using two tunable dye lasers for stepwise photoexcitation. This occurs due to the increase in the energy level of the photoexcited level which promotes and enhances thermal ionization. The enhancement could be very large (>lOOO) at low wavelength powers with the degree of enhancement due to the laser powers of the two wavelengths of the laser and the degree of optical and electrical saturation.

28

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SNEDDON

Early work in flame LEI used the conventional premixed burners. Hall and Green (183) investigated the use of the total consumption burner for LEI spectrometry and concluded than an effective desolvation system is necessary to reduce the cooling effect of the flame but that increasing the path length and/or confining the sample to a smaller volume will increase the sensitivity. Further work by Turk et al. (284) gave detection limits for 18 elements, potential interferences (ionization interference of 10 and 20 l&ml of sodium), and the application of the technique to nickel-based high-temperature alloy samples. Travis et al. (285) developed a model to explain the production of electron-ion pairs by a combination of optical and collisional processes. The model yielded an analytical expression for the relative enhanced ionization sensitivity of a given transition of any element. Havrilla and Green (286) developed and investigated an electrode positioner in LEI which was useful for optimizing the signal. The laserrelated increase in ionization was detected with electrodes held at a negative high voltage and the burner as the anode and measured with conventional electronics. Turk (187) investigated matrix ionization interferences and developed a new water-cooled cathode for insertion in the flame. This led to an improved tolerance from less than 300 to over 3000 l&ml of sodium in an air-acetylene flame. Van Dijk et al. (188) used a dye laser pumped by nitrogen laser to detect ionization signals of sodium ions in a cool HZ-O,-argon flame. The ionization signals were detected with a pair of biased nichrome probes suspended in the flame in the immediate vicinity of the irradiated region. Concentrations of 0.1 &ml were detected. Turk et al. (189) used laser stepwise excitation, with two electronic transitions connected by a common intermediate level, to populate high-energy electronic levels in an atom. This approach yielded improved detection limits over the conventional single step method for seven elements. Nippoldt and Green (290) investigated pulsed LEI to minimize electrical interferences but concluded no improvement could be made. Time-resolved signal recovery proved useful for discriminating against electrical interferences. Cm-ran et al. (292) investigated a closely related technique of dual laser ionization (DLI) spectrometry using a flame reservoir and nitrogen-pumped dye laser. In this technique, two laser beams that overlap both temporally and spatially are employed. The first is a dye laser tuned to resonance with an excited state of the analyte atom, and the second beam produced by the nitrogen laser is used to produce photoionization from the excited state. Some schemes produce no enhancement, which was attributed to the decline of photoionization cross-sections with increasing energetic overshoot into the electronic continuum and the competition between photoionization and collisional ionization. Turk and Watters (192) investigated LEI in an ICP and reported poor sensitivity, but found it quite sensitive for detecting transitions to high-lying Rydberg levels. Positioning of the electrodes to avoid an interference from RF was critical. Messman et al. (193) detected previously difficult (using an air-acetylene flame) to detect refractory elements using a nitrous oxide-acetylene flame. Stepwise excitation involves the use of one laser beam whose wavelength is resonant with a transition from the ground state; the other beam, temporally and spatially coincident with the first and tuned to a resonant transition from the level populated by the first beam, can then efficiently

LASERS

IN

ATOMIC

SPECTROSCOPY

29

promote atoms to electronic levels close to the ionization potential. These levels are subject to rapid collisional ionization in a flame and result in an increase in sensitivity over single wavelength ionization. When the spectral it-radiance is sufficiently high, multiphoton ionization can also occur. In this case, an excited and bound state is reached by the simultaneous absorption of two or more laser photons via intermediate virtual levels. Hart ef al. (194) used stepwise and multiphoton LEI to study strontium in an air-acetylene flame. Smith et al. (19.5)used single-step and two-step excitation to measure the ionization yield of lithium using two pulsed tunable dye lasers, pumped simultaneously by an excimer laser. The ionization yield for the singlestep excitation at 670.784 nm was found to be 0.26%, increasing to 58% for two-step excitation (670.784 + 460.286 nm). Havrilla and Choi (196) detected zinc as low as 1 r&ml using single and stepwise excitation involving seven zinc transitions. In this system a dye laser pumped by a Nd:YAG was used. Axner et al. (197) used single-step LEI in the UV region produced by frequency doubling of the output from a tunable dye laser to demonstrate the multielement capability (using 23 elements). Turk et al. (198) investigated LEI in an ICP and concluded that ionization can be found in the normal analytical zone of the ICP, laser-induced ionization is - 14%, decay measurements were solely due to recombination, and the rate of recombination was slower in the higher regions of the ICP studies despite higher ion fractions. They proposed that this was a result of higher kinetic distribution of electrons. Magnusson (199) developed a theoretical model of the observed signal with the charged species produced in LEI. Bekov et al. (200) used a combination of resonant photoionization of atoms with thermal atomization of a sample in a vacuum to detect ultralow concentrations of gallium in germanium and rhodium in environmental samples. Detection limits were 3 ppt for gallium and 5 ppt for rhodium and linearity extended over six orders of magnitude. These authors used a nitrogen laser pumped by an excimer laser. Sekreta et al. (201) discussed various spectroscopic applications of laser ionization spectroscopy, including laser ionization time of flight mass spectrometry, vibrationally resolved laser photoelectron spectroscopy, rotationally resolved laser photoelectron spectroscopy, and the use of LEI to probe excited state dynamics. Magnusson (202) investigated LEI in a graphite furnace and found high sensitivity which could be further improved using a two-color laser excitation system. LEI was detected in UV nitrogen-ablated laser plasma by Cache et al. (203). They concluded that LEI detection was not feasible under atmospheric conditions because ionization was strong and collection was weak. Seltzer and Green (204) used direct laser ionization (DLI) produced by resonant excitation of an analyte followed by photoionization o the excited-state atoms in analytically useful flames. Lin and Duh (20.5) used DLI to investigate ion enhancement in an air-acetylene flame. Axner and Rubinsztein-Dunlop (206) further investigated LEI in flames as a powerful technique for ultrasensitive trace element analysis. Axner and Berglind (207) determined the ionization efficiencies of excited atoms in a flame by LEI. Butcher et al. (208) investigated LEI after atomization from a probe inserted in a graphite furnace. The detection limits obtained by LEI spectrometry are shown in Table 8.

30

THIEM,

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TABLE 8 Detection Limits for Laser-Enhanced Ionization Spectrometry Laser-induced detection limits 1st or single step wavelength

2nd step wavelength

Element

(nm)

(nm)

Lasef

Flameb

Ag

328.1 309.3 278.0 242.8 307.2 306.8 422.6 228.8 252.1 252.0 455.5 324.8 302.1 294.4 451.1 766.5 670.8 308.2 285.2 279.5 319.4 589.0 300.2 283.3

421.1

E F E Y F F Y Y Y E N Y E E N K E F E Y F E Y Y N E F F Y E F E F F E F E Y

A N A A A A A A A A P A A A A H A N A A N A A A P A N N H A N A N N A N A A

Al As Au Ba Bi Ca Cd co Cr cs CU Fe Ga In K Li LU Mg Mn MO Na Ni Pb Rb Sb SC

Si Sn Sr Ti Tl Tm V W Y Yb Zn

287.8 301.9 288.2 284.0 293.2 320.0 291.8 297.3 318.4 283.1 298.4 267.2 213.9

479.3 585.7 466.2 591.7

453.1

571.0 460.3 521.5 568.3 576.5 600.2

597.0

377.57

396.6

LOD’ Wml) 0.07 0.2 3000 1 0.2 2 0.03 0.1 0.08 0.02 0.004 0.7 0.08 0.04 0.0009 0.1 0.0003 0.2 0.003 0.02 10 0.003 0.08 0.09 0.1 50 0.2 40 0.3 0.01 1 0.008 200 0.9 300 10 2 1

Source. Ref. (176). n F, flashlamppumped dye laser; Y, Nd: YAG-pumped dye laser; N, nitrogen-pumped dye laser; K, krypton ion-pumped continuous-wave dye laser; and E, excimer-pumped dye laser. b A, air-acetylene; N, nitrous oxide-acetylene; H, air-hydrogen; and P, propane-butane-air. ’ Lit of detection based on three times the standard deviation of a single measurement for a signal equivalent to the background.

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IN ATOMIC

SPECTROSCOPY

31

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