Recent advances in the field of laser atomic spectroscopy

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22 D. Sicilia, S. Rubio and D. Perez-Bendito, Analyst, 115 23 24 25 26 27 28 29 30

(1990) 1613. D. Sicilia, S. Rubio and D. Perez-Bendito, Anal. Chem., 64 (1992) 1490. N. Lacy, G. D. Christian and J. Ruzicka, Anal. Chim. Actu, 224 (1989) 373. D. Sicilia, S. Rubio and D. Perez-Bendito, Anal. Chim. Actu, 266 (1992) 43. L. Lunar, S. Rubio and D. Perez-Bendito, Anal. Chim. Actu, 237 (1990) 207. L. Lunar, S. Rubio and D. Perez-Bendito, Tulanta, 39 (1992) 1163. D. Sicilia, S. Rubio and D. Perez-Bendito, Fresenius J. Anal. Chem., 342 (1992) 327. K. Hayakawa, M. Kanda and I. Sataki, Bull Chem. Sot. Jpn., 52 (1979) 3171. E. Athanasiou-Malaki and M. A. Koupparis, Anal.

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Chim. Acta, 219 (1989) 295. 31 H. A. Archontaki, M. A. Koupparis and C. E. Efstathiou, Analyst, 114 (1989) 591. 32 C. A. Georgiou, M. A. Koupparis and T. l? Hadjiioannou, Tulanta, 38 (1991) 689. 33 L. Lunar, S. Rubio and D. Perez-Bendito, Anal. Chim. Actu, 268 (1992) 145. 34 D. Sicilia, S. Rubio and D. Perez-Bendito, Talc&u, 38 (1991) 1147. 35 V. Gonzalez, B. Moreno Cordero, D. Sicilia, S. Rubio

and D. Perez-Bendito, unpublished results. 36 D. Sicilia, PhD Thesis, University of Cbrdoba, 1992. Drs. D. PBrez-Bendito and S. Rubio are at the Deparfment of Analytical Chemistry, University of Cdrdoba, Cdrdoba, Spain.

Recent advances in the field of laser atomic spectroscopy Terry L. Thiem Hanscom AFB, MA, USA

Yong-III Lee Lowell, MA, USA

Joseph Sneddon* Lake Charles, LA, USA An overview of recent advances in the field of analytical laser atomic spectroscopy will be discussed, in particular the laser’s use as a light source in atomic absorption spectroscopy, as an excitation source in laser atomic fluorescence and laser enhanced ionization spectroscopy, as a method of solid sample introduction, and as a diagnostic tool. Selected current research in these fields will be highlighted together with their results.

Introduction Since the construction of the first laser in the late fifties, their application in the field of analytical *To whom correspondence

should be addressed.

chemistry has gradually increased. Recent advances in producing more reliable and less expensive laser systems has brought about growth in the field of laser atomic spectroscopy and has allowed the power of laser spectroscopy to move from the research laboratory to the analytical chemist’s bench-top [l]. The properties of the laser that make it most attractive to the analytical chemist include tunability, especially that of the dye and diode lasers, monochromaticity, wavelength capability (UV to IR coverage by differing types of lasers), power, and pulse width. Of course, the type of laser used by the chemist will depend on the application. For example, in laser-enhanced ionization (LEI), a high intensity, pulsed dye laser is used due to tunability, although somewhat limited, of the dye system and the need for high laser intensity. On the other hand, solid state lasers (Nd:YAG and Ruby) have been used extensively for sample introduction systems due to their ruggedness and reliability coupled with the ability to achieve high power densities. This article is an overview of recent advances in such fields as sample introduction and LEI but will expand to include several different areas of analytical laser spectroscopy research in the field of atomic spectroscopy. Selected and representative applications of the recent advances are summarized in Table 1.

Q 1993 Elsevier Science Publishers B.V. All rights reserved

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TABLE 1. Selected

use of the laser in atomic spectroscopy. Use

Comments

Ref.

Radiation source in flame AAS

The multiple wavelength characteristics of the laser allowed peak ratio measurements and possible background correction by monitoring the absorption spectrum with a photodiode array spectrometer. Detection limits for lithium were comparable to traditional hollow cathode lamp flame AAS

7

XeCl pulsed excimer at 308 nm, 70 mJ per pulse, pulse width of 25 ns, and 100 Hz repetition rate

Sample introduction system of ICP-AES and ICP-MS

Detection limits of 12 mg/g (ICP/AES) and 4 rig/g (ICP-MS) with values attributed to low power producing a more uniform and finer spray. Precision improved by 2

9

Excimer laser at 193 nm, 100 mJ per pulse pulse width of 20 ns, and 1 Hz repetition rate

Formation of laser ablated plasma as an atomization/ excitation source for AES and AAS

The highest L/B ratio obtained in a He gas at atmospheric pressure. The shape and size of the plasma are dependent on the surface and environment. Highest L/B required optimization of the plasma

11,12

Frequency doubled Nd: YAG operated at 10 Hz which pumps two dye lasers whose beams are directed into an analytical flame

Element specific detector for LC

LC separated organotins and resolves spectral interferences from the analytes. Tributylintin (TBT) is extracted from sediments into 1-butanol. A detection limit 3 ng/ml tin as TBT or 0.6 ng of tin was obtained

18

Solid-state at1064nm

Simultaneous measurement of droplet size and velocity distributions produced by five common nebulization systems

Sauter mean diameter of tertiary aerosols produced by Frit-type nebulizer was smaller than those of ultrasonic and pneumatic nebulizers. The method was rapid and direct

19

Laser-system Multiple-mode diode

laser

laser

Light sources Atomic fluorescence spectroscopy The use of the laser as a light source in atomic fluorescence spectroscopy (AFS) was demonstrated in the early 1970’s [2]. In fluorescence spectroscopy the signal obtained is directly proportional to the intensity of the excitation source up to a saturation level and therefore the property of laser intensity and tunability can be used to great advantage. A recent study by Vera et al. [3] compared the characteristics of three different pumping lasers to determine those pumping laser system properties which were most important in achieving the lowest detection limits for lead in an aqueous solution. The three pumping lasers were a nitrogen, Nd:YAG, and copper vapor. A graphite tube atomization source was used. It was shown that high repetition rate copper vapor-pumped dye laser had the lowest detection limit of the three, being 0.5 fg compared to 3 fg for the other two laser

systems. The decrease of the fluorescence signal caused by the laser-induced depletion of the atomic population of a fluorescent level has been proposed as another tool for diagnostic studies of atmospheric pressure flames and plasmas [4]. Rate equations describing the population of fluorescence levels have shown that the technique of fluorescence dip spectroscopy provides the same information as that obtainable by measuring the fluorescence from the excited level. Fluorescence dip is brought about by the use of two lasers, the first to excite the atoms to the initial excitation level. A second laser then irradiates the sample stepping it to a second, even higher energy excitation level. This second excitation results in a detectable decrease in signal intensity or “dip” from the first level fluorescence. If the second excitation energy level is near ionization the second laser can be scanned in wavelength resulting in a two-step laser enhanced ionization spectrum recorded together with a fluorescence dip spectrum. When the

20

second excitation step results in ionization of the atom, the absolute magnitude of the fluorescence dip will reflect the ionization yield [5]. This technique offers high spatial resolution since it probes the intersection of the two focussed laser beams. The relative fluorescence dip is independent of the total number of atoms. Theoretical treatment based on the rate equation approach also gives information such as quantum efficiency and absorption oscillator strength in much the same way as the conventional saturated fluorescence method. When steady-state 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 when compared to conventional fluorescence and ionization spectroscopy. Further study in the field of laser-induced fluorescence dip spectroscopy is needed to better define how this method can be optimized for analytical uses. Atomic absorption spectroscopy The use of a diode laser has been proposed as an alternative to the traditional hollow cathode lamp (HCL) in atomic absorption spectroscopy (AAS) by Lawrenz and Niemax [6] and others [7]. The advantages of the diode laser over the HCL for AAS include: the use of a simple photodiode detector to discriminate the laser beam intensity from the background radiation of a flame or graphite furnace atomizer; increased intensity allowing higher concentration to be measured by using the wings of a saturated absorption line; the ability of the laser to be tuned at wavelengths slightly off the resonance line for background correction of the absorption signal (similar to the Smith-Hieftje background correction method). Another advantage of the tunable diode laser is the possibility of simultaneous multi-element analysis with a single light source. Continued experimentation in the field of diode lasers is needed to lower the wavelength capabilities into the UV region where many of the transition metal resonance lines are.

trends in analytical chemistry, vol. 12, no. 1, 1993

Sample introduction Introduction of a solid sample for analysis has always been advantageous in the field of atomic spectroscopy because minimal sample preparation is required. Arc and spark methods have been used in the past but were limited to conducting samples. The laser offers an efficient system of vaporization similar to an arc or spark but allows the analysis of both conducting and non-conducting samples. For the most part, solid state laser systems, such as the Nd:YAG [S] and ruby laser, have been used but gas excimer lasers (XeCl, ArF, and KrF) have shown potential [9]. The power density requirements for sample vaporization depend largely on the thermal properties of the sample itself. Power densities of 106-lo8 W cme2 are typical for most applications. Power densities of lo9 W cmd2 and greater usually result in excitation and ionization of the vaporized species. Recently, commercial systems have become available for solid sample introduction using laser vaporization for both mass spectroscopy (MS) and inductively coupled plasma mass spectroscopy (ICP-MS) (Fisons, Danvers, MA, USA and Perkin-Elmer, Norwalk, CT, USA). Considerable research is still needed in this field, particularly with regard to the effects of laser parameters and the fundamental vaporization process.

Atomization/excitation

source

When a laser beam of sufficient energy (lo9 W cmb2 and greater) is focussed onto the surface of solids a hot plasma is formed at and directly above the sample surface. This laser ablated plasma [lO121 or laser spark [ 131 can be used either as an atomization source as described above, or can be probed spectroscopically for the direct analysis of the solid material. Recent work has shown the potential of an ArF excimer laser (wavelength 193 nm) to form a plasma near the surface of solid metals that is spectroscopically usable for elemental determinations [ 11,121. These authors found that the thermal properties of the metal, position in the plasma, surrounding atmosphere (both composition and pressure) all had a large contribution to the line-to-background ratio as well as the shape and size of the plasma itself. Fig. 1 depicts the shape of the plasma formed at a copper metal surface under an atmospheric condition of 760 Torr He. The plasma, in this

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Outer Sphere Plasma

E

T A R G E T

I

Inner Sphere Plasma

/ Optimum Region of Observation

Fig. 1. ArF excimer laser ablated plasma from a copper surface in a helium environment and at atmospheric pressure [12].

case, consists of two distinct regions, an inner and outer sphere plasma. The optimum region for the best signal-to-noise ratio exists in the outer edge of the inner sphere where ionization effects are less pronounced and relaxation or quenching effects due to the surrounding atmosphere are minimized. Fig. 2 shows the results of two spectra taken from a solid lead surface at different distances. Clearly the spectrum obtained at 1.6 mm (A) has a lower background continuum and narrower emission lines than that obtained at 3.6 mm (B) from the sample surface. In addition to solid sampling, if the laser is focussed beyond the dielectric constant of air or a dry aerosol, emission spectra of the air or aerosol can be obtained. The method is frequently referred to as laser spark or more commonly as laser-induced breakdown emission spectroscopy (LIBS) and has the potential to be used for the direct determination of metals in air. In fact, several laser-based techniques are currently being developed by Ottesen et al. [14] to provide in situ determinations of size, velocity, and elemental compositions for individual particulates in a coal combustion environment. Many of the properties of LIBS are similar to those of conventional arc or spark plasmas. An important advantage of the LIBS method (or arc or spark) is the ability to obtain spectral data in environments where electrodes can not be introduced. Although most elemental analysis using LIBS is still in the semiquantitative state, the outlook for additional advances is promising.

Laser-enhanced (LE9

ionization spectroscopy

When a laser is used to resonately excite an atomic species in an atomization source such as a flame, the

laser-excited atom population has a lower effective “ground state” population and therefore is more readily ionized by thermal conditions. LEI spectroscopy exploits this condition by applying a voltage across the atomizer (i.e., flame, furnace, plasma, etc.) and monitoring the current that is produced by the ionized species. Because the number of ionized species is directly proportional to the concentration of the species in the atomizer, the current will vary with concentration. As discussed in the introduction, the dye laser is most commonly used. Copper vapor pumped dye lasers have been used but suffered poor signal levels due to radio-frequency problems and low peak power. Rutledge et al. [ 151 found a 35fold increase in signal level using a sophisticated noise reduction strategy which included a boxcar integration system. Although the flame is the most commonly used atomization source, an argon inductively coupled plasma (ICP) has also been investigated as an atom

01

, 400

WAVELENGTH

, 450 (nm)

, 500

B

j

400

WAVELENGTH

450

(nm)

Fig. 2. Spectrum of an ArF excimer laser ablated plasma formed with lead in an atmosphere of air at 1.6 mm (A) and at 3.6 mm (B), from the surface [ll]. For line assignment see ref. 11.

22

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reservoir by Turk et al. [ 161. An extended-torch ICP is used due to the requirement of taking LEI measurements in a region of lower electron population, to reduce radio-frequency (rf) interference, and to avoid arcing from the plasma to the LEI electrodes. In addition, the ICP was rfpower modulated for a period of approximately 1 ms before each pulse of the laser and restored after the laser-induced ionization was detected. Using this method, limits of detection (signal-to-noise ratio = 3) was determined to be 80 pg/l for Fe and 20 pg/l for Ga, using a l-s signal averaging time constant and a plasma power of 400 W. Although this is a dramatic improvement over their first measurement, the sensitivity still falls far short of traditional flame LEI. Graphite furnace atomization has also been studied by Butcher et al. [ 171 and LEI has been used as a selective detector for the liquid chromatographic determination of alkyltins in sediment [ 181.

Diagnostic tool The shape, size, and velocity of aerosols produced by the most commonly used sample introduction devices (nebulizers) in atomic spectroscopy can be studied using laser spectroscopy to determine particle size and transport efficiency to the atomizer. As a diagnostic tool, laser-diffraction has been used to study droplet size and velocity distributions of aerosols produced in nebulizers [ 191.

Conclusion The previous discussion has given a brief overview of some of the exciting research that is currently taking place in the field of analytical laser atomic spectroscopy. Although many of the methods currently being developed have not gained in the way of detectability, they have stimulated experimentation that will hopefully someday bring about newer and better analytical instrumentation for the analytical chemist.

References T.L. Thiem, Y.I. Lee and J. Sneddon, Microchem. J., 45 (1992) 1. N. Omenetto, Spectrochim. Acta, 44B (1989) 13 1. J.A. Vera, M.B. Leong, N. Omenetto, B.W. Smith, B. Womack and J.D. Winefordner, Spectrochim Acta,

vol. 12, no. 7, 1993

44B (1989) 939. N. Omenetto, G.C. Turk, M. Rutledge and J.D. Winefordner, Spectrochim Acta, 42B (1987) 807. 0. Axner, M. Norberg and H. Rubinsztein-Dunlop, Spectrochim Acta, 44B (1989) 693. J. Lawrenz and K. Niemax, Spectrochim Acta, 44B (1989) 155. K.C. Ng, A.H. Ali, T.E. Barber and J.D. Winefordner, Appl. Spectros., 44(5) (1990) 849. P.G. Mitchell, J. Sneddon and L.J. Radziemski, Appl. Spectros., 41(l) (1987) 141. H. Pang, D.R. Wiederin, R.S. Houk and E.S. Yeung, Anal. Chem., 63 (1991) 390. 10 Z.W. Hwang, Y.Y. Teng, K.P. Li and J. Sneddon,Appl. Spectros., 45(3) (1991) 435. 11 Y.I. Lee, S.P. Sawan, T.L. Thiem, Y.Y. Teng and J. Sneddon, Appl. Spectros., 46(3) (1992) 436. 12 Y.I. Lee, T.L. Thiem, G.H. Kim, Y.Y. Teng and J. Sneddon, Appl. Spectros., 46 (11) (1992) 1597. 13 D.A. Cremers and L.J. Radziemski, Appl. Spectros., 39(l) (1985) 57. 14 D.K. Ottesen, J.C.F. Wang, andL.J. Radziemski,Appl. Spectros., 43(6) (1989), 967. 15 M.J. Rutledge, M.E. Trembley and J.D. Winefordner, Appl. Spectros., 41(l) (1987) 5. 16 G.C. Turk, L. Yu, R.L. Watters, Jr. and J.C. Travis, Appl. Spectros., 46(8) (1992) 1217. 17 D.J. Butcher, R.L. Irwin, S. Sjostrom, A.P. Walton and R.G. Michel, Spectrochim. Acta, 46B (1991) 9. 18 K.S. Epler, T.C. O’Haver, G.C. Turk and W.A. MacCrehan, Anal. Chem., 60 (1988) 2062. 19 R.H. Clifford, I. Ishii, A. Montaser and G.A. Meyer, Anal. Chem., 62 (1990) 390. Terry L. Thiem is a Captain in the U.S. Air Force basedat Hamscom Air Force Base, MA 01731, USA. He is a Ph.D. candidate in the Chemistry Depatfment at University of Massachusetts, Lowell. He anticipates graduating in Spring 1993 and will be a chemistry instructor at the Air Force Academy in Colorado Springs in Fall, 1993. His research interests are in laseratomic andmolecularspectroscopy. Yong-Ill Lee graduated with a Ph.D. in December 1992 from the Chemistry Deparfment at the University of Massachusetts, Lowell, MA 01854, USA, and joined the Chemistry Faculty at Kunyang University, Nonsan, Cheungnan, South Korea in January 1993. His research interests are in laser atomic and molecular spectroscopy Joseph Sneddon is Professor and Head of the Chemistry Department at McNeese State University in Lake Charles, LA 70609, USA. Prior to this recent move he was on the faculty at University of Massachusetts, Lowe//. He is the author or co-author of over eighty publications in the area of atomic spectroscopy He is a/so the editor-in-chief of the Microchemical Journal.