Comparative study of two new commercial echelle spectrometers equipped with intensified CCD for analysis of laser-induced breakdown spectroscopy

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1 Comparative study of two new commercial echelle spectrometers equipped with intensified CCD for analysis of laser-induced breakdown spectroscopy Mohamad Sabsabi, Vincent Detalle, Mohamed A. Harith, Walid Tawfik, and Hisham Imam The purpose of this paper is to provide the reader with comparative information about two new commercial echelle spectrometers equipped with intensified CCD ICCD detectors for laser-induced breakdown spectroscopy analysis. We carried out a performance comparison between two commercial ICCD echelle spectrometers ESA 3000 LLA Instruments GmbH, Berlin-Adlershof, Germany and a Mechelle 7500 Multichannel Instruments, Stockholm, Sweden for the determination of the concentrations of Be, Mg, Si, Mn, Fe, and Cu in the same Al alloy samples adopting the same experimental conditions. The results show that both systems, despite their differences in terms of resolution, have similar performance in terms of sensitivity and precision of measurements for these elements in an Al alloy matrix at least for the range of wavelength nm studied in this work Optical Society of America OCIS codes: , , M. Sabsabi mohamad.sabsabi@nrc.ca and V. Detalle are with the National Research Council Canada, Industrial Materials Institute, 75 de Mortagne Boulevard, Boucherville, Quebec J4B 6Y4, Canada. M. A. Harith, W. Tawfik, and H. Imam are with the National Institute of Laser Enhanced Science, University of Cairo, Giza, Egypt. Received 17 January 2003; revised manuscript received 12 May $ Optical Society of America 1. Introduction Laser-induced plasma spectroscopy, also known as laser-induced breakdown spectroscopy LIBS, isa form of atomic emission spectroscopy AES. LIBS is being used as an analytical method by an increasing number of research groups. The growing interest in LIBS, due to progressive research and improvements in the technology of lasers and photodetectors, particularly in the last decade, has led to an increasing number of publications on its applications, both in the laboratory and in industry. Undoubtedly, the advent of high-quality solid-state detectors is revolutionizing the field of atomic spectroscopy. 1 3 New optical technologies, when coupled with new generations of optical detectors, provide powerful tools for plasma diagnostics and spectrochemical analysis. Unlike inductively coupled plasma AES or other techniques based on optical emission spectroscopy OES, the LIBS technique requires time-resolved detection owing to the transient nature of the plasma. In the initial moments, the plasma emission consists of an intense radiation continuum superimposed with very broadened lines. Thus temporally gating off the earlier part of the plasma is essential for spectrochemical analysis. This dictates an important difference based on timegated detection of the atomic emission between the detector requirements for LIBS and other techniques based on OES. Since the early 1990s, echelle-based systems coupled with two-dimensional CCD for simultaneous measurements of analyte lines at different wavelengths have been developed and commercialized in the field of inductively coupled plasma AES 4 9 and used in microwave-induced plasma analysis. 10 In the past seven years few were used in LIBS, and it is presently commercialized, to our knowledge, by three companies For analytical spectrochemistry by LIBS, the appropriate choice for the experimentalist is based on the combination of the spectrometer and the detector, which requires a compromise between wavelength coverage, spectral resolution, read time, dynamic range, and detection limit. The combination of spectrometer and detector is an important factor to consider in OES for any plasma characterization or analytical spectrochemistry experiments. The instrumental choice of the researcher depends on the type of measurements to be made. Based on that, 6094 APPLIED OPTICS Vol. 42, No October 2003

2 the appropriate system can be designed. For example, using nonresonance lines of nonmetals such as F, Cl, Br, I, S, N, and O requires working in the nearinfrared region 940 nm, while the resonance lines of these elements are located in the vacuum ultraviolet below 185 nm. The requirements for an ideal spectrometer detector system to furnish simultaneous determination of any combination of elements in the spectrum include: First, a high resolution of nm to resolve the lines of interest and avoid interferences. Second, wide wavelength coverage, typically from 165 to nm to be able to detect simultaneously several elements. Third, a large dynamic range to provide the optimum signal-to-noise ratio SNR for a large range of elemental concentrations; the detector needs a wide dynamic range, typically 6 7 orders of magnitude. Fourth, a high sensitivity and a linear response to radiation. The detector has to have high quantum efficiency, particularly in the near-infrared and UV regions, and low noise characteristics. Furthermore, for rapid analysis the readout and dataacquisition time should be shorter, at least less than the time lap between the laser pulses. At present, these requirements are in part fulfilled by a combination of echelle spectrometer intensified CCD ICCD detector that is on the market. In this work we report for the first time, to the best of our knowledge a comparison study between the performances of two new commercial echelle spectrometers equipped with ICCD detector LLA Instruments GmbH ESA 3000 and Multichannel Instruments Mechelle 7500 for the quantitative analysis of Al alloys in air at atmospheric pressure in terms of sensitivity. Both systems were optimized by the manufacturers for the UV-visible range. We chose the Al samples because of the availability of experimental data in our laboratory, making the comparison easier. Furthermore, the poor spectrum of Al does not require a spectrometer with high resolution, making it ideal for this study, since the two systems can be used in our case. The purpose of this paper is to help the LIBS users to choose the appropriate system for a selected application in terms of sensitivity. The performances related to the resolution or the rate of transmission will be only mentioned briefly here; details of these parameters are available from the manufacturer and can be easily compared by the reader. 2. Experimental Setup To evaluate the performance of the two-echelle spectrometer detector systems LLA System A and Mechelle System B for LIBS analysis, we examined them in the same experimental conditions. However, LLA was used in the National Research Council laboratory setup, and the Mechelle was used in the National Institute of Laser-Enhanced Science experiments. Both experimental setups were fully described elsewhere 18,19 and only brief overview will be given here to make the comparison easier for the reader. Figures 1 and 2 show the experimental Fig. 1. Experimental setup at NRC with System A ESA 3000 from LLA. L, lenses. setup used with System A and System B, respectively. The ESA 3000EV from LLA has a focal length of 25 cm with a numerical aperture of 1:10. The operating diffraction orders range from 30 to 120. The flat image plane is mm 2. This system is a compromise that offers maximum resolution in the wavelength range between 200 and 780 nm resolution power is above 10,000. The linear dispersion per pixel ranges from nm at 200 to nm 780. The detector is an ICCD camera, comprised of a Kodak KAF 1001 CCD array of pixels m 2 and a microchannel plate type BV2562 of 25-mm diameter from Proxitronix coupled with a UV-enhanced photocathode. The Mechelle 7500 has a focal length of 17 cm with f-number of 5.2. It provides a constant spectral resolution of 7500 corresponding to 4 pixels FWHM, over a wavelength range nm displayable in a single spectrum. A Fig. 2. Experimental setup at NILES with System B Mechelle 7500 from Multichannel. PC, personal computer. 20 October 2003 Vol. 42, No. 30 APPLIED OPTICS 6095

3 gateable ICCD camera, DiCAM-Pro-PCO from Computer Optics, with a high-resolution sensor with pixels 9 9 m 2 coupled to the spectrometer, was used for the detection of the dispersed light. The 25-mm microchannel plate is from the DiCAM with a UV-enhanced photocathode. The overall linear dispersion of the Mechelle spectrometer camera system ranges from at 200 nm to nm pixel at 1000 nm. In both cases a 2-m fused-silica optical fiber 600- m diameter mounted on a micro xyz-translation stage is used to collect the emission light from the plasma plume and feed it to the echelle spectrometer detector system. In the experimental setups for System A and System B the plasma was produced by focusing 60 mj of Nd:YAG laser pulses at 1064 nm with 6-ns duration on Al alloy samples. The composition of the samples can be found in Ref. 18. The laser pulses were suitably focused on the Al alloy sample in order to generate plasma of 800- m spot diameter in both cases. This helps in comparing the performance of both spectroscopic systems at nearly the same irradiance. It should be noted that there are particles and aerosols generally present above the sample that are caused by the ablation of material owing to the lasersample interaction. Since the threshold of the breakdown on these particles is lower than that of the air, the likelihood of the plasma generation in the air above the sample will be increased. Depending on the size of these plasmas and of the laser beam, the latter can be partially or completely absorbed by these plasmas and prevented from reaching the sample. Consequently, both the variation of laser energy density on the sample and the presence of the breakdown on the particles in the laser path beam will affect the light emitted by the plasma. This increases the one shot-to-shot variation and decreases the reproducibility of the measurements. To avoid these problems, a low repetition rate of 0.2 Hz was used in both setups. Optical emission from the plasma plume was collected directly by a fibre optic of 600- m diameter positioned close to the plasma Figs. 1 and 2, which delivers the light to the entrance of the echelle spectrometer. The acquisition of the ICCD of System A was delayed by 3 s after the laser pulse was fired. However, to avoid saturation of the detector, the integration time was limited to 1 s, and the electron gain was set to a minimum. Similarly for System B, the delay and integration times were 1.5 and 3 s, respectively. The choice of the delay time and the integration time is based on the best SNR of the two systems. Figures 3 and 4 show a spectrum obtained by Systems A and B, respectively. 3. Results and Discussion Quantitative spectral analysis generally involves a linear relation from the emission line intensity from an element in the transient plasma from its concentration in the target. We investigated a set of eight standard samples of aluminum alloy to establish calibration curves for five elements Be, Mg, Si, Mn, and Fig. 3. Single-shot spectrum of aluminum sample obtained by System A with zoomed segment showing the beryllium line in the UV region. Cu by the two systems. We reproduced the measurements at five locations on the sample surface in order to avoid problems linked to sample heterogeneity. Twenty shots were fired at each location, and Fig. 4. The spectrum is an accumulation of five shots fired on an aluminum sample obtained by System B, with the zoomed segment showing the beryllium line in the UV region APPLIED OPTICS Vol. 42, No October 2003

4 Table 1. Limit of Detection for Various Elements in the Same Aluminum Alloys Obtained by Systems A and B LOD Element Wavelength of the Spectral Line Used nm System A LLA Echelle ICCD ppm System B Mechelle ICCD ppm Be Mg Si Mn Cu Fig. 5. Calibration curve of Mn obtained by System A. one measurement is the result of the average for five locations. The first 10 shots were used for cleaning the surface, and the last 10 shots were taken into account for establishing calibration curves to be used for the calculation of the limit of detection LOD of each of the five elements in the Al sample. For this task we chose the strongest line for each element and based the calculation on the 3 International Union of Pure and Applied Chemists definitions. However, we did not use an analytical blank to determine the standard deviation, because a region near the line where the spectrum is free of emission lines has been chosen. Using this approach, we determined the analytical capability of both systems in terms of the obtained limit of detection. Figure 5 shows the calibration curve of Mn obtained using System A, whereas Fig. 6 depicts the calibration curve of the same element adopting System B. The Mn content in the Al samples varies from % to 1.09%. In both cases the Mn atomic line at nm normalized to the Al line at nm has been exploited to build up the calibration curves. The aluminum line was chosen because it is not self-absorbed. As shown in the figures, the two curves are reasonably linear and comparable in terms of the sensitivity slope and the repeatability. In the case of Be, the calibration for both systems uses the Be nm normalized to the Al nm line. A comparison between the LOD obtained via the two spectroscopic systems for the five elements mentioned above, in the same samples, is given in Table 1. Our results indicate that the LOD of these elements obtained by System A are similar to those obtained by System B. This is mainly because the two systems were optimized for the range of wavelength nm. Outside this range, which is not studied in our conditions, when optimization is made for each system separately in terms of light collection and SNR some differences could be expected. It should be pointed out that the sensitivity of each system depends on several parameters related to the echelle spectrometer and the detector. For example, the detector is composed of the image intensifier coupled to the CCD. The image intensifier comprises three main components: a photocathode, a microchannel plate MCP, and a phosphor screen. The photocathode converts the incident photons into electrons. These electrons are then accelerated toward the MCP, where multiplication takes place up to an amount dependent on the gain voltage across the MCP. After the MCP stage, the multiplied electrons are accelerated further toward the phosphor screen, where they are converted back into photons ready for the CCD to detect. Similarly, the input output flux of photons through the echelle spectrometer is affected by its optical components, namely the dispersive elements and the mirrors. The output signal S for a given wavelength can be written as S S ph QE pc tg VE p QE CCD, Fig. 6. Calibration curve of Mn obtained by System B. where S ph is the average photon flux incident on the ICCD photocathode in units photons per second per pixel, QE pc is the average quantum efficiency value across the photocathode in units of photoelectrons per photon, t is the signal integration time, G is the current gain amplification of the MCP, typically 1 to 3000, V is the difference between the output and phosphor voltage, E p is the average phosphor efficiency for converting electrons to photons, and QE CCD is the average conversion efficiency of phosphor photon signal to CCD electrons. There is also some dependence on the transmission efficiency of phosphor light emitted through two optical fiber coupler windows. 20 October 2003 Vol. 42, No. 30 APPLIED OPTICS 6097

5 Since the response of these parameters is not equal throughout the spectral range, one system will be more sensitive than the other for some regions in the spectrum. Furthermore, for an element that has a rich spectrum iron, for example, a system with high resolution will be more appropriate for the application. Another parameter to be considered is the dynamic range, which presents some limits related to the well of the CCD detector. The dynamic range of System A 16 bits was better than that of System B 12 bits. A high dynamic range will be preferred in LIBS analysis, particularly with echelle spectrometer ICCD, which provides a large spectrum, including simultaneously the strongest lines of the major elements and the weak lines of the trace elements. The dynamic range is less problematic in the case of Czerny Turner ICCD configuration than the echelle spectrometer ICCD. This is because the spectral window can be adjusted to avoid the simultaneous detection of the strong lines with weak lines. 4. Conclusion In summary, we have carried out a comparative study between two commercial echelle spectrometers equipped with ICCD UV enhanced detectors in terms of spectrochemical analysis by LIBS of five trace elements in Al alloy samples. The two systems are exploited in two typical LIBS setups running under the nearly the same experimental conditions. The results showed that both systems have similar limit of detection of the five investigated elements when spectral lines in the UV range nm are used in the analysis. References 1. J. M. Harnly and R. E. Fields, Solid-state array detectors for analytical spectrometry, Appl. Spectrosc. 51, 334A 351A F. M. Pennebaker, D. A. Jones, C. A. Gresham, R. W. Williams, R. E. Simon, M. F. Schappert, and M. B. Denton, Spectroscopic instrumentation in the 21st century: excitement at the horizon, J. Anal. At. Spectrom. 13, Q. S. Hanley, C. W. Earle, F. M. Pennebaker, S. P. Madden, and M. B. Denton, Charge-transfer devices in analytical instrumentation, Anal. Chem. 68, 661A 667A M. J. Pilon, M. B. Denton, R. G. Schleicher, P. M. Moran, and S. B. Smith, Evaluation of new array detector for atomic emission spectrometer for inductively coupled plasma atomic emission spectroscopy, Appl. Spectrosc. 44, T. W. Barnard, M. J. Crockett, J. C. Ivaldi, and P. L. Lundberg, Design and evaluation of echelle grating optical system for ICP-OES, Anal. Chem. 65, T. W. Hieftje, The future of plasma spectrochemical instrumentation, J. Anal. At. Spectrom. 11, A. T. Zander, Continual improvement of instrumentation for analytical spectrochemistry, J. Anal. At. Spectrom. 13, A. T. Zander, R-L. Chien, C. B. Cooper, and P. V. Wilson, An image-mapped detector for simultaneous ICP-AES, Anal. Chem. 71, S. Luan, R. G. Schleicher, M. J. Pilon, F. D. Bulman, and G. N. Coleman, An echelle polychromator for inductively coupled plasma optical emission spectroscopy with vacuum ultraviolet wavelength coverage and charge injection device detection, Spectrochim. Acta Part B 57, L. Hiddemann, J. Uebbing, A. Ciocan, O. Dessenne, and K. Niemax, Simultaneous multielement analysis of solid samples by laser ablation-microwave-induced plasma optical emission spectrometry, Anal. Chim. Acta 283, H. E. Bauer, F. Leis, and K. Niemax, Laser induced breakdown spectrometry with an echelle spectrometer and intensified charge coupled device detection, Spectrochim. Acta Part B 53, H. Becker-Ross and S. V. Florek, Echelle spectrometers and charge-coupled devices, Spectrochim. Acta Part B 52, C. Haisch, U. Panne, and R. Niessner, Combination of an intensified charge coupled device with an echelle spectrograph for analysis of colloidal material by laser-induced plasma spectroscopy, Spectrochim. Acta Part B 53, P. Lindblom, New compact Echelle spectrographs with multichannel time-resolved recording capabilities, Anal. Chim. Acta 380, R. G. Scott, S. L. Morgan, R. Hoskins, and A. Oxsher, Identifying alloys by laser-induced breakdown spectroscopy with a time-resolved high resolution echelle spectrometer, J. Anal. At. Spectrom. 15, P. Fichet, P. Mauchien, J. F. Wagner, and C. Moulin, Quantitative elemental determination in water and oil by laser induced breakdown spectroscopy, Anal. Chim. Acta 429, S. Florek, C. Haisch, M. Okruss, and H. Becker-Ross, A new, versatile echelle spectrometer relevant to laser induced plasma applications, Spectrochim. Acta Part B 56, V. Detalle, R. Héon, M. Sabsabi, and L. St-Onge, An evaluation of a commercial echelle spectrometer with intensified charge-coupled device detector for materials analysis by laserinduced plasma spectroscopy, Spectrochim. Acta Part B 56, B. Charfi and M. A. Harith, Panoramic laser-induced breakdown spectrometry of water, Spectrochim. Acta Part B 57, Multichannel Instruments AB, Sweden, http: Catalina Scientific Corporation, USA, http: com. 22. LLA Instruments GmbH, Germany, http: APPLIED OPTICS Vol. 42, No October 2003