Anal. Chem. 1994,66, 2964-2975
Detector for Trace Elemental Analysis of Solid Environmental Samples by Laser Plasma Spectroscopy Richard Wisbrun Institute for Hydrochemistry, Technical University of Munich, Marchioninistrasse, 17, D-8 1377 Munich, Germany Israel Schechter' Department of Chemistry, Technion- Israel Institute of Technology, Haifa 32 000, Israel Reinhard Niessner Institute for Hydrochemistry, Technical University of Munich, Marchioninistrasse, 17, D-8 1377 Munich, Germany Hartmut Schriider and Karl L. Kompa Max Planck Institute for Quantum Optics, 0-85748 Garching, Germany Analysis of heavy metals in soils, sand, and sewage sludge samples has been studied by means of time-resolved optical emission spectrometry from laser-induced plasma. The instrumental setup has been developed as a fast screeningdetector for future application as an on-lineand in situ method. Several experimentalparametershad to be optimized. Various factors affecting the detection limits and the quality of the analysis have been investigated. These include aerosol production, crater formation, size effects, timing effects, laser intensities, and humidity. In order to improve reproducibility of elemental analysis, a special data analysis program has been developed. It consists of a data analysis program and of the principal component regression calibration technique, which utilizes many spectral lines for each element. A special renormalization algorithm bas been tested for internal calibration. The computer programprovides good calibrationplots and detection limits in the 10 pg/g range, which are usually below the ecological requirements. The technique of spectrochemical analysis by using laser plasma excitation is well established in analytical chemistry and has been reviewed previo~sly.l-~In this method, a laser pulse is focused on the substratte to create a plasma spot and to ablate material from its surface. The ablated material is (1) Scott, R. H.; Strasheim, A. In Applied Atomic Spectroscopy; Grove, E. L., Ed.; Plenum: New York, 1978; Vol. I, Chapter 2, pp 73-118. (2) Piepmeier, I., Edward, H., Eds. Analytical Applications of Lasers; Wiley: New York, 1986; pp 31-74, 627-670. (3) Cremers, D. A.; Radziemski, L. J. In LaserSpecz?oscopyandi:s Applications;
Radziemski, L. J., Solarz, R. W., Pasincr, J. A., Eds.; Marcel Dekker: New York, 1986; Chapter 5, pp 3 5 1 4 1 5 . (4) Moenke-Blankenburg, L. Laser Microanalysis; Wiley: New York, 1989. ( 5 ) Leis, F.; Sdorra, W.; KO,J.-B.; Niemax, K. Mikrochim. Acta 1989.2.85-199. (6) Sdorra, W.; Quentmeier, A,; Niemax, K. Mikrochim. Acto 1989,2,201-219. (7) Radziemski, L. J.; Cremers, D. A. In Laser-InducedPlasmas and Applicationr; Radziemski, L. J., Cremers, D. A., Eds.; Marcel Dekker: New York, 1989; Chapter 7, pp 295-325. (8) Kim, Y. W. In Laser-Induced Plasmas and Applications; Radzicmski, L. J., Cremers, D. A., Eds.; Marcel Dekker: New York, 1989; Chapter 8, pp 327346. (9) Bcauchemin, D.; Le Blanc, J. C. Y.; Peters, G. R.; Craig, J. M. Anal. Chem. 1992.64.4421-467R.
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excited in the hot plasma, and its spectral emission is recorded and analyzed in order to identify and quantify its elemental composition. Detailed investigation of the various processes involved in this sort of analysis has been carried 0ut.5fjJG22 The method has been applied for the analysis of g a s e ~ , 2 3 , ~ ~ and solids (usually metal surfaces).' 1~12,26,27 Several applications to soil^,^*^*^ to mineral analysis,30 and even to kidney stone analysis31 have been reported. This method has always been tempting to the chemical analysts because of its main advantaged It is a simple method, because the ablation and excitation processes are carried out by the laser pulse in a single step. Thus, no preparation of the sample is required, and the risk of contamination of the sample by an improper treatment is prevented. The method provides, in principle, simultaneous multielement analysis Beenen, G. J.; Piepmeier, E. H. A w l . Sci. 1984, 38, 851-857. Cremers,D.A.;Romero,D. J.Proc.SPIEInt.Soc.Opr.Eng. 1986,644,7-12. Cremers, D. A. A d . Sci. 1987, 41, 572-579. Hein, S. J.; Piepmeier, E. H. Trends Anal. Chem. 1988, 7, 137-142. Ursu, I.;Stoica, M.; Mihailmu, I. N.; Hening, AI.; Prokhorov,A. M.; Nikitin, P. I.; Konov, V. I.; Silenok, A. S.J . Appl. Phys. 1989, 66, 5204-5215. (15) Sdorra, W.; Qucntmeier, A.; Nicmax, K. 2.Phys. D 1989, 13, 95-99. (16) KO,J. B.; Sdorra, W . ;Nicmax, K.Fresenius 2.Anal. Chem. 1989,335,648-
(10) (11) (12) (13) (14)
651. (17) Leis, F.; KO, J. B.; Nicmax, K. Fresenius Z . Anal. Chem. 1989, 334, 649. (18) Catcs, M. C. Proc. SPIE 1nt. Soc. Opt. Eng. 1990, 1279, 102-1 11, (19) Thompson, M.; Chenery, S.; Brett, L. J . Anal. At. Spectrom. 1990,5,49-55. (20) Niemax, K.; Sdorra, W. Appl. Opt. 1990, 29, 5000-5006. (21) Sdorra, W.; Niemax, K. Spectrochim. Acta 1990, 45B, 1917-1926. (22) Timmer, C.; Srivastava, S. K.; Hall, T. E.; Fucaloro, A. F. J . Appl. Phys. 1991, 70, 1888-1892. (23) Ottesen, D. K.; Wang, J. C. F.; Radzicmski, L. J. Anal. Sei. 1989, 43, 967976. (24) Casini, M.; Harith, M. A.; Pallcschi, V.; Salvetti, A.; Singh, D. P.; Vaselli, M. Laser Part. Beams 1991, 9, 633-639. ( 2 5 ) Loree, T. R. Laser Insl. Am. (ICALEO) 1983,42, 38-45. (26) Radziemski, L. J.; Millard, J. A.; Dalling, R. H. Proc. SPIE Int. SOC.Opt. Eng. 1986, 644, 13-15. (27) Quentmeier, A.; Sdorra, W.; Niemax, K. Spectrochim. Acta 1990,458,537546. (28) Han, M. Y.; Cremers, D. A., submitted for publication in In:. J . Enuiron. Anal. Chem. (29) Wisbrun, R.; Niessner, R.; SchrBder, H. Anal. Methods Inrrrum. 1993, 1 , 1-5. (30) Grant, K. J.; Paul, G. L.; ONeill, J. A. AMI. Sei. 1990, 44, 1711-1714. (31) Meyer, W.; Engelhardt, R.; Hering, P. LIBS of Kidney Stones. In Laser Lithotripsy; Steiner, R., Ed.; Springer: Berlin, 1988.
0003-2700/94/03662964$04.50/0
0 1994 American Chemical Society
without increased instrumental complexity and cost. It has the potential of providing both qualitative and quantitative results. In addition, it can provide spatial resolution when needed, and only optical access to the sample is required, which may be of importance in some applications. However, in spite of the above-mentioned advantages, the method has never been developed as afield instrument for soil and sediment analysis. Thevariety of current applications is limited to somewhat homogeneous samples, handled under laboratory conditions. However, a successful automated system for detection of airborne berylliumcollected on filters has been rep~rted.~ZThe required experimental equipment is fairly standard, but this method had never been developed for these applications due to several unsolved problems, which will bedescribed in the following. Thesolution that wesuggest is based on a combination of an optimized experimental setup, plus a set of especially designed chemometric tools. The restriction to a laboratory analysis limits the main potential advantages of the method, namely, its speed and simplicity. Nevertheless, the reasons for this restriction are not accidental but are forced by some of the intrinsic problems of this method. In order to achieve a reasonable reproducibility of the results, many experimental factor must be controlled. For instance, the exact location of the breakdown point relative to the sample and to the collecting optics. It is well-known that the spectral intensities depend on both the plasma and ablation profiles. Minor changes in the experimental conditions have considerable influence on these profiles, and thus, they influence the spectral intensities. Moreover, each spectral line has its own characteristics and is affected in a different way by the various factors. For example, the spectral lines’ dependence on the plasma’s temperature buildup or on the self-adsorption mechanisms varies from line to line. Thus, the main problem that one faces when attempting to develop a field instrument is a poor reproducibility. Actually, a direct application of the method to soil analysis, without specially designed methods, provides no useful results. In principle, much information could be gained from the related ICP method, which also utilizes plasma spectroscopy for multielemental analysis. Unfortunately, the substantial differences between these two methods prevent the usage of the same technical or numerical tools. In contrast to the ICP signals, ours are unstable and vary significantly for each single event (laser pulse). Nevertheless, this experimental system is so much simpler (and cheaper) that the effort is worthwhile. Our goal was to investigate the main experimental factors in order to develop a system that is able to provide good analysis of heavy metals in soft environmental samples, in an automated mode of operation. Our main motivation was the gain of the precious benefits of the method in this sort of analysis. Due to ecological considerations, there is an increasing demand for heavy metals analysis in soils, sludges, and similar industrial wastes. The actual analysis consistsof four steps: (a) sampling procedure; (b) transportation of the samples to an analytical laboratory; (c) total digestion of the samples in acids (usually overnight); (d) analysis, usually by atomic absorption spectroscopy (AAS). After carrying out these steps, the analytical chemist faces several new questions: What are the borders of the contami(32) Cremcrs, D. A.; Radziemski, L. J. Anal. Sci. 1985, 39, 57-63.
nated area? Are there any gradients in the results pointing out the source of the contamination? In order to answer such questions, a new sampling strategy must be designed and the whole procedure (steps a-d) must be carried out again. Such methods are time consuming and expensive. The major advantage of our method is its potential to provide a fast, in situ scanning of heavy metals in sand, soils, and industrial wastes. We succeeded in gaining useful information, although the results are less accurate as compared to AAS. This method is devoid of most of the time-consuming and costly steps of the current analyses. Moreover, we want to raise the question of whether the results obtained by the currently used method, which is based on total digestion in acid, really provides the relevant results. In other words, the question is whether the total concentration of heavy metals in soils represents the ecological danger, which is usually thequestion of interest. We shall demonstrate that, in some cases, due to the nature of the contamination processes, the results provided by our method are more related to the ecological exposure to heavy metals, as compared to the classical method. In the following we describe the experimental setup, which has been designed for the analysis of heavy metals in soils, sludges, and similar materials. Then, the main findings, regarding the most important factors that influence theanalysis are presented. These include the aerosol production mechanism, the particle size effect, the various effects of the measurement timing, the energy of the laser, and the humidity of the samples. A separate section is dedicated to the data analysis program, which is described briefly. This program, which consists of several algorithms, was of a major importance in achieving stable and reproducible results in this sort of analysis. It contributed to overcoming the particular problem that arises in application of the method to materials such as sand and soil. Finally, some of the calibration results are shown. These include detection limits, which are usually below the requirements, and two calibration models, which are evaluated by known chemometric parameters.
EXPERIMENTAL SECTION The experimental arrangement is shown in Figure 1. Most of the experiments have been carried out with a Q-switched Nd:YAG laser (Quanta-Ray GCR 11, X = 1064 pm, fwhm = 8-9 ns, E,,, = 300 mJ/pulse). A XeCl excimer laser was applied for testing the effect of the laser in use (Lambda Physik 201 MSC, X = 308 nm, fwhm = 20 ns, E,,, = 250 mJ/pulse). An optical system guided the desired laser beam to the surface of the sample (inside of the detection head). The light was focused with a planoconvex quartz lens o f f = 400 mm, in the case of the Nd:YAG laser, or o f f = 150 mm for the excimer laser. The samples were positioned -50 mm above the focal point. This configuration was found to produce a stable breakdown, while maximizing the interaction area. An optical fiber (quartz and silica PCS 600 B multimode, step index, -20 m long) was placed -10 mm apart the primary interaction regime in order to collect the emitted light. This geometry provides a wide collection angle that covers most of the spark and also prevents damaging the entrance Analytical Chemistty, Vol. 66,No. 18, September 15, 1994
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The emission intensity produced within a short time window of a single laser pulse is weak; thus many pulses were integrated. Moreover, for statistical reasons, many pulses must be applied for an accurate analysis. The data acquisition program determined the number of pulses integrated on the target (analogue integration) and the number of pulses integrated on the computer (digital integration).
I
v
Flgure 1. Experimental setup. The analyzing unit is separated from the detector head. For some applications, the laser light will be transferred through an optical fiber: thus the small detector head will be easily manipulated. The inset shows schematically the laser spark and the collecting fiber positioning. The detector head consists of the focusinglens, the collectingfiber (adjustedby a mechanicalmanipulator), and the sample holder. The sample is continuously rotated, and its surface is flatted.
surface of the fiber and reduces its dusting by ablated material. The light from the fiber was passed to a circular to rectangular bundle (1OX quartz fibers, inner diameter, 200 pm, step index, SI) and imaged with a quartz lens (biconvex,f= 10 mm) onto the entrance slit of a spectrometer (ARC Spectra Pro, 6X = 2 A). The spectra were recorded and temporally resolved by a gated optical multichannel analyzer (OMA). (Usually OMA I11 EG&G/PARC. For some experiments, when a better UV sensitivity was required, OSMA/SI was used.) Four spectral regions (of -50 nm) were analyzed, starting at 21 7 . 6 , 320.6, 3 8 9 . 5 , and 4 7 5 . 1 nm. Software for data analysis and evaluation were written in FORTRAN 77, and compiled by MS-FORTRAN 4.1 compiler. Data acquisition and analysis were carried out on a PC. A variety of samples were analyzed with this experimental arrangement, These included clay-rich soil with little organic material, marshy soil with no clay and a lot of organic fibers, marine sediments, and locally collected sands and soils as well as a variety of standard samples. The locally collected samples consisted of several sands and agricultural soils from various regions. The same procedure has been carried out for all samples, regardless of their different internal composition. However, contamination processes and the level of homogeneity were examined by electron microscopy. Several standard samples were measured to assess accuracy. The contamination of the locally collected samples with heavy metals was carried out by mixtures of standard solutions. The samples were dried at 95 "C (to avoid water boiling) and then ground with a mortar. For the investigation of the effect of the water contents, samples were wetted again with a measured quantity of bidistilled water and were kept sealed until the measurements were carried out. In some of the experiments, samples were pelletized at a pressure of 1 MPa. Each sample was analyzed by digestion in aqua regia, according to DIN 3841444 and AAS measurements as a reference method. (33) Huang,P. M. Adsorption ProccssesinSoil. In TheHandbookofEnuironmental Chemistry; Hutzinger, O., Ed.; Springer: Berlin, 1980; Vol. ZA. (34) Sdorra, W.; Niemax, K. Mikrochim. Acta 1992, 107, 319-327.
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MAIN EFFECTS Persistent Aerosols. As has already been mentioned previously, the signal was integrated over many pulses. Such integrations were regularly used in previous applications of the m e t h ~ d . ~However, -~ in previous applications, the repetition rate of the laser was optimized for delivery of maximum energy, -10 Hz. A similar application to sand and soil samples introduces a new factor that must be taken into account. The laser breakdown produces persistent amount of aerosols above the sample. As the laser's repetition rate is higher, the aerosol production rate is higher and the steadystate aerosol concentration above the sample is higher. A low concentration of aerosol, originating at the surface of the sample, increases the signal and contributes to better results. The reason is that some of the sample's material is already ablated and available (for the next pulse) in the plasma spot location, prior to the plasma production. In other words, the process of aerosol production supplies material into the hot plasma. This is of considerable importance because the ablation process is one of the bottlenecks in this method. Nevertheless, production of aerosols beyond a certain concentration decreases the signal. This happens because laser light is absorbed by the aerosol along the laser pass, and the energy available at the focal location is reduced. This effect can sometimes be observed even by eye. At high repetition rates, the laser pass is lighted by many emitting points, thus indicating the presence of large particles. Therefore, the expected overall effect of these two opposing mechanisms is a function with a maximum. The signal as a function of the repetition rate increases at the beginning and then decreases again. However, there is an additional effect that must be considered, namely, the dependence of the delivered energy as a function of the repetition rate. Some experimental data to illustrate this point are shown in Figure 2. The average line intensities (over -50 spectral lines, collected from 10 laser pulses) are presented as a function of the repetition rate for a sand sample and for a soil sample. The laser output energy is shown for reference. The laser's (35) Chen, G.; Yeung, E. S.Anal. Chem. 1988,60, 2258-2263. (36) Press, W. H., Flannery, B. P., Teukolsky, S. A., Vetterling, W. T., Eds. Numerical Recipes; Cambridge University: Cambridge, England, 1989. (37) Dennis, P. N . J. Photodetertors-An Introduction to Current Technology; Plenum: New York, 1987. (38) Hutcheson, L. D., Ed. Integrated Optical Circuits and Components-Design and Application; Marcel Dekker: New York, 1987. (39) Wilson, J.; Hawkes, J. F. B. Optoelectronics-An Introduction; Prentice Hall: New York, 1989. (40) Gnanadesikan, R. Methods for Statistical Data Analysis of Multiuariate Observations; Wiley: New York, 1977. (41) Takeuchi, K.; Yanai, H.; Mukherjee, B. N . The Foundations ofMultioariate Analysis; Wiley Eastern: New Delhi, 1983. (42) Martens, H.: Naes, T. Multiuariate Calibration;Wiley: Chichester, England, 1989. (43) Sharf, M. A.; Illman, D. L.;Kowalski, B. R. Chemometrics; Wiley: New York, 1986. (44) Aufschluss mit Kdnigwasser zur nachfolgendenBestimmungdcs s8urel&islichen Anteils von Metallen (S7), DIN 38414, part 7. Vom Wasser 1982,58,395397.
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An interesting observation is a slight increase of the signal for the second pulse. This is not the effect of aerosol production, since in these experiments the repetition rate was very low. This effect might be explained by the enrichment of the site with larger particles, as discussed in the following. (The increase shown in this figure is not very significant, relative to the precision of the measurements; however, it has always been observed in many sets of experiments.) Calculations of the relative standard deviations (RSD) as a function of pulse number for the above measurements were carried out (Figure 3b). RSD values were obtained from many repeated sets of experiments (e.g., results of the first pulse in all set were collected for RSD of the first pulse). In the first series of measurements (all sparks at the same site), we observed an increase of the relative standard deviation after the second pulse. Thus, we conclude that, for the case of particulate material analysis, one needs to supply a fresh site for each laser pulse. This can be achieved by rotating the sample (and smoothing the surface after the spark) or by sliding the detector head, which is placed directly on the ground (as designed in Figure 1). In addition to the above-mentioned side influences, changing the analyzed site is of importance due to statistical arguments. Unlike most of the alloy analyses, our samples may be contaminated in a nonhomogeneous way. Moreover, usually the global concentrations are of more importance that the local (
