Quantitative analysis of low-alloy steel by microchip laser induced
breakdown spectroscopy
C. Lopez-Moreno,bK. Amponsah-Manager,aB. W. Smith,aI. B. Gornushkin,a N. Omenetto,aS. Palanco,bJ. J. Lasernaband J. D. Winefordner*a
a
University of Florida, Gainesville, FL 32611, USA. E-mail: [email protected]fl.edu; Fax:þ1 352 392 4651
b
University of Malaga, Campus Teatinos s/n, 29071 Malaga, Spain
Received 23rd December 2004, Accepted 22nd March 2005 First published as an Advance Article on the web 14th April 2005
The development of a compact laser induced breakdown spectroscopy (LIBS) system increases the possibilities of applying the technique in industrial arenas, field applications and process monitoring. Significant progress has been achieved in miniaturization of optical detectors and lasers, allowing portable, low-cost LIBS equipment to be devised. Conventional lasers for LIBS, like actively Q-switched Nd:YAG lasers are limited by their bulkiness, the need for a cooling system and high power consumption. The use of a miniature solid state microchip laser overcomes these drawbacks and offers further advantages of good beam quality, high pulse repetition frequency and less damage to target. In this work we studied the quantification of elemental composition of low alloy steel samples using a higher power microchip
(‘‘powerchip’’) laser. The possibility of real time,in situquantification of such materials by powerchip LIBS enhances the applicability of the technique to process monitoring in the steelmaking industry. The
performance of the LIBS technique based on a powerchip laser and a portable non-intensified, non-gated detector for elemental quantification is evaluated and compared to that obtained using an intensified detector. Calibrations were achieved for Cr, Mo, Ni, Mn and Si with linear regression coefficients between 0.98–0.99 and limits of detection below 100 ppm in most cases.
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is an elemental analytical tool that has been extended to a wide range of applications. The technique is based on the generation of an emitting plasma1on solid, liquid or gaseous samples and the collection of emission from the plasma which is used to determine the composition of the sample qualitatively or quantitatively. The technique provides numerous advantages such as rapidity, multielemental analysis, minimal sample preparation and low cost. It is non-invasive and minimally destructive. Recently, the development of techniques such as remote-LIBS in open-path atmospheric conditions,2 portable LIBS equipment,3double-pulse4systems and ultra-short laser pulses has increased the number of possible applications of the technique. Application areas include industrial processing, analysis of archaeology and art works, radioactive and hazar-dous material determinations, environmental monitoring, space exploration, military explosive detection and biomedical studies.
Significant progress has been achieved in the miniaturization of analytical devices and components resulting in portable analytical systems, an important requirement in instrument development. The need to obtain results in real-time is essential in industrial processes, but the advantages of real-time LIBS lose their importance unless the technique is accompanied by anin situcapability. Hand-held optical detectors with impress-ive resolution have been developed but such a level of minia-turization has only recently been attained in commercial laser sources which are suitable for LIBS.
Conventional LIBS setups use actively Q-switched Nd:YAG lasers which are bulky due to complex electronics and the need for cooling systems and have high power consumption. These drawbacks are overcome by the use of a passively Q-switched microchip laser capable of delivering peak pulse powers up to a
megawatt at high repetition rate. Microchip lasers consist of a small piece of solid-state gain material polished and parallel on two sides.5 The cavity mirrors are dielectrically deposited onto the polished surfaces and the laser is pumped with a semiconductor diode laser. The short cavity lengths of Q-switched microchip lasers allow them to produce pulses with very short width (sub-nanosecond), but with a high repetition rate (several kilohertz). The energy delivered by powerchip lasers is nearly ten times higher than that delivered by micro-chip lasers, achieving 50 mJ per pulse. The plasma plume generated has a short lifetime which minimizes the shielding effect associated with plasmas induced by longer pulses, and the consequent increased damage to the sample. This makes powerchip LIBS a suitable technique to analyze fragile or highly valuables samples, like archaeological artefacts or artworks.
LIBS is now more widely challenged as a quantitative analytical tool. However, difficulties presented by various matrix effects limit the applicability and accuracy of quantita-tive analysis. In recent years, quantitaquantita-tive analysis of steels by LIBS have been performed successfully by several authors3 achieving exceptional figures of merit using both conventional equipment and remote LIBS systems. We have reported pre-viously an evaluation of a microchip laser induced plasma emission system characterized by a marginal sensitivity in the detection of magnesium in a graphite matrix.6The aim of this work is to assess the feasibility of using a powerchip laser for the quantitative analysis of low alloy steel samples by LIBS. The elements of interest in this work are Cr, Mn, Mo, Ni, and Si. The results obtained using a portable spectrometer as the detector were compared with those obtained using a conven-tional intensified CCD array spectrometer. The influence of sample temperature was also studied as it relates to the applicability of quantitative measurements in an industrial environment.
A R T I C L E
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2. Experimental
A schematic diagram of the complete powerchip LIBS system is shown in Fig. 1. It consists of four primary components: A powerchip laser, a focusing microscope objective, a detection fiber optic and the spectrometer. The laser used was a passively Q-switched powerchip laser (JDS Uniphase PowerChip Nano-laser). The powerchip laser is a solid state Nd: YAG laser that operates at 1064 nm with pulse repetition frequency of 1 kHz. The output pulse has a temporal length of 500 ps with 50mJ pulse energy. The 2 mm diameter laser beam was focused by a 50microscope objective (Mitsutoyo, M plan NIR 50) onto the sample to produce a focal spot diameter of 8–10mm.
Low alloy steel samples (Czechoslovakia Spectrometric re-ference materials, Cast Iron C18.8) were used for analysis. The steel samples were placed in a motorized holder tilted at approximately 451 to the laser beam and were rotated con-tinuously in the plane of the laser pulse in order to sustain plasma formation on fresh sample surface. The motion of the sample is essential at this high repetition rate in order to avoid probing the melted material formed by the previous laser shot, which requires much higher energy to overcome the break-down threshold than the solid phase material. In order to minimize the overlap of pulses and to provide a fresh solid surface for every pulse, the plasma was generated in the proximity of the outer edge of the sample discs.
Plasma emission was collected with a 600mm diameter fiber optic coupled to a high resolution miniature fiber optic spectro-meter (HR 2000, Ocean Optics Inc., USA) with 0.03 nm FWHM spectral resolution and a grating with 2400 grooves mm1. No optics were used to collect the plasma emission. The fiber tip was simply placed within a few mm of the plasma. The data were collected and processed by the OOI Base32 software running on a laptop computer. The experiments were repeated with a gated, intensified CCD spectrometer, SpectroPro-500i (Acton Research Corp.) with 0.02 nm spectral resolution and a 40 mm entrance slit operating in a free running (non-gated) mode. For these experiments, spectral segments of 8 nm for the plasma emission were recorded using a grating with 2400 groove mm1. Spectral data were collected and processed with Winspec 32 software running in a desktop computer.
3. Results and discussion
Quantitative analysis of low alloy steel
The stability of the signal during the laser sampling was characterized first. One of the main noise contributions7 influ-encing the response stability is the fluctuation noise due to random changes in the plasma intensity from pulse to pulse. A repetition rate of 1000 Hz was used to perform the measure-ments. Although the sample was always in continuous rota-tion, the microplasma created at such a high repetition rate could be partially hidden in the crater depth, reducing the
absolute emission intensity. For this reason, peak intensities were normalized to the intensity of a suitably selected Fe emission line. The emission lines of the elements of interest assessed in this work and the Fe lines chosen for the normali-zation are Cr (I) 425.43: Fe (I) 425.07, Mn (I) 403.07: Fe (I) 400.52, Mo (I) 386.41: Fe (I) 387.25, Ni (I) 341.47: Fe (I) 342.71, and Si (I) 288.16: Fe (I) 287.41. The normalization lines were carefully selected taking into account the features that characterize a suitable internal standard while avoiding the interference of emission lines in such a complex steel matrix, previously studied by other authors.8
For the present experiments, an acquisition time of 1000 ms was used, effectively adding up the emission from 1000 sequen-tial plasmas. Fig. 2 shows three graphs: (a) the fluctuation of the emission intensity of Mn (I) 403.1 nm, where each point represents the sum of 1000 shots; (b) the standard deviation of this signal obtained by averaging the collected data points progressively (moving average of previous data points); and (c) the standard deviation of the signal by averaging only ten data points and moving this average along the following data point. As can be seen in line (a), the fluctuations in the first ten collected data points are higher in amplitude than the subse-quent points, where the variation is more regular and this fact is reflected in the observed precision. The other elements studied behaved similarly. As mentioned in previous papers9, the samples need to be exposed to a certain number of preparation laser shots before performing the measurements in order to eliminate any surface impurities and also to ensure that the surface of the target is representative of the bulk composition. In every experiment, ten surface cleaning laser shots and 15 laser shots for data acquisition (all integrated over
Fig. 1 Experimental setup.
Fig. 2 Various noise contributions to the data, (a) fluctuation of the
emission intensity of Mn (I) 403.1 nm; (b) standard deviation of this signal obtained by averaging the collected data points progressively (moving average of previous data points); and (c) standard deviation of the signal by a moving average of successive ten data points.
1000 ms) were fixed as the best acquisition conditions to minimize the signal fluctuation.
Fig. 3a shows the calibration curve for Mo using the compact spectrometer. Similar curves were obtained for Cr, Mn and Mo were correlation coefficient of 0.98 or better and accuracy in terms of relative standard deviation of 3.6, 6.3, 5.5%, respectively. The RSD values were obtained with the samples with the lowest concentration of the analyzed element.
Comparison of the portable spectrometer with the intensified spectrometer
The quantitative analysis of the same steel samples was per-formed using a conventional spectrometer fitted with an in-tensified CCD as detector. Fig. 4 shows an 8 nm spectral region of 15 averaged data points using (a), SpectraPro 500 Intensified spectrometer which allows 8 nm acquisition window and (b), HR 2000 portable non-intensified spectrometer which allows 90 nm acquisition window. The acquisition data parameters for both experiments were the same: a free running mode for both detectors and each data point is an average of 1000 laser shots. Resolution is slightly better for the ICCD than the portable spectrometer except that a few spectral details appear in the spectra obtained with the portable spectrometer for the same sample. For instance, the line 426.42 nm, appearing at the shoulder of the Fe 426.04 nm does not appear in the spectrum with the ICCD. In the data acquisition mode employed in this work, the most significant difference between the two detection systems is the CCD. As expected, the signal and background in the intensified spectrometer were much higher than when the non-intensified CCD was used. The intensified spectrometer was capable of detecting more elements in the steel samples than the portable one because of the flexibility in the selection of wavelength windows. On the other hand, the narrow spectral window provided by this spectrometer presents a problem in performing a real-time multielemental analysis. Fig. 3b and 3c show the calibration curves for Ni and Si, respectively, species that could not be detected in the fixed wavelength window of the portable spectrometer.
Table 1 summarizes the limits of detection (LOD) and the accuracy of the detection for the species studied using the two spectrometers. The highlighted values are those obtained using the non-intensified portable spectrometer. For Cr, the LOD obtained with the portable spectrometer was lower than that obtained with the ICCD but the data obtained for Mo and Mn are comparable for the two spectrometers. Precision of the two detection systems was comparable except that it was several times better in the case of Mn with the ICCD than the HR 2000. Earlier work10 in this research group showed that the signal to noise ratio and sensitivity of the intensified CCD can be several times higher than those obtained with the non-intensified CCD. It is not clear at the moment the reason for the comparable sensitivity for the two detectors with the Powerchip laser systems. It should be noted that the intensified CCD used in this work was operated in a non-triggered, non gated mode. In the earlier work referred to above, the detector was operated with gate delay with respect to the plasma-initiating laser pulse. Generally, LIBS data conventionally collected this way have higher signal to background and sensitivity than those obtained in the mode used in this work.
Effect of the sample temperature on plasma emission
The effect of the sample temperature on plasma emission was assessed for two main reasons: to approximate the experimen-tal set up to actual field situation in the steel making process where moving, hot and molten material has to be analyzed in real time, and second, to study whether the heating of the target could lead to an increase in the sensitivity of the technique by reducing the fraction of the laser pulse energy
that is wasted through thermal dissipation. The effect of target temperature on plasmas generated with nanosecond laser pulses has been studied by other authors.11They have shown that in plasmas generated using laser pulses shorter than 50 ps, the processes leading to plasma formation are quite different from those that occur using nanosecond pulse lasers. In the latter, inverse bremsstrahlung absorption processes play an important role during plasma development.1With short pulse duration, the plasma formation process is dominated by multi-photon ionization, and the temperature of the sample does not affect the plasma formation process significantly.12The laser source used in this work delivers pulse widths on the order of 500 ps, which might not belong to either of these two extremes. A study of the potential relevance of the sample temperature on the emission spectra was carried out using the portable
non-Fig. 3 Calibration curves for some of the elements studied using two
different spectrometers: (a) Mo (I) 386.41 nm normalized to Fe (I) 387.25 nm using the portable spectrometer as detector; (b) Ni (I) 341.47 nm normalized to Fe (I) 342.71 nm using the intensified detector and (c) Si (I) 288.16 nm normalized to Fe (I) 287.41 nm using the intensified detector.
intensified spectrometer as detector. The temperatures selected were 251C, 3001C, and 5501C. Fig. 5 shows the increase in the emission lines of some elements of interest in the steel when the temperature increases. The net intensity was normalized to the intensity at room temperature. Although there was an overall increase in the spectral line intensity, no new species were observed, nor was there any significant change in peak position or shape.
In order to ascertain whether the increase in the emission intensity was due to a change in the plasma physical conditions or to an increase in mass removal, the plasma temperature was determined by a Boltzmann plot using the relative intensity of iron lines. No significant variation of plasma temperature over the target temperature studied was found. The plasma temperature obtained for the target at 300 1C and 550 1C
were 11 200 K and 11 400 K, respectively. We did not consider the 200 K temperature difference between the two measure-ments as significant, considering the errors associated with these measurements. The increase in the emission intensity can therefore be considered to be due to increase in mass removal rather than a change in plasma temperature or selective ablation of species.
Table 1 Analytical figures of merit of the elements analyzed using
both the portable high resolution spectrometer; the intensified spectro-meter as detector
Emission line SLR (%)a RSD (%) LOD (%)
Cr (I) 425 nm 0.018–1.92 4.6 0.01
3.6b 0.003
Mn (I) 403.1 nm 0.09–1.86 1.2 0.003
6.3 0.004
Mo (I) 384.4 nm 0.007–1.34 5.2 0.008
5.5 0.009
Ni (I) 341.42 nm 0.052–2.42 1.3 0.009 Si (I) 288.16 nm 0.27–2.02 4.6 0.02 a
SLR denotes the studied linear range.bThe data in bold were obtained for the portable spectrometer.
Fig. 5 Behaviour of the net intensity of the emission lines for Cr, Mn
and Mo with changes in target temperature. The values are normalized to that obtained at room temperature for each element.
Fig. 4 Superposition of 8 nm spectral window acquired for pure Fe standard and steel samples containing 1.92% Cr using (a) intensified detector
SpectraPro 500i; and (b) portable detector OOI HR 2000.
Fig. 6 shows images of typical craters formed on the rotating steel targets by single laser pulses. Generally, no significant visible difference was seen among different targets. This is because the concentration of iron in all targets was more than 90% and the targets could be considered just as iron samples. In other cases we have shown that crater depth and diameter could vary significantly from target to target.13
4. Conclusions
The feasibility of microchip laser induced plasma as an analy-tical tool has been studied recently,6but the detection limits were unsatisfactory. It was shown here that the detection capabilities for a quantitative analysis have been improved using a powerful microchip laser that delivers an energy up to 50 mJ pulse1. The miniaturization of the detector and the simplicity of the new configuration confer to the technique the advantage of field portability and real-time application. Using this new setup, analysis of the composition of low alloy steel has been performed and characterized by remarkably good figures of merit. The results were compared with those ob-tained using a conventional intensified spectrometer. The sensitivity obtained with the portable non-intensified spectro-meter was generally comparable to that obtained with an intensified detector. The noise contributions to the signal associated with both the portable and intensified spectrometers are discussed in the paper. The applicability of the lab proto-type setup to the real industrial arena was carried out by assessing the signal obtained with hot and mobile samples. The intensity of the emission lines of the elements of interest increased in the hot steel, but the ratio with the Fe emission lines remain constant in the temperature range studied. These results were corroborated by calculating the plasma ture which was nearly the same irrespective of target tempera-ture, suggesting that selective ablation did not result from the increase in the target temperature. However, the SNR was
improved, which has the potential to increase the sensitivity in quantitative analysis based on this prototype.
Acknowledgements
The authors would like to thank to the Ministry of Science and Technology (Spain) and United Stated Department of Energy for the financial support of this work. We would also like to thank Aerodyne Research Inc. (USA) for providing the power chip laser. The help of Kathleen Herrera in the data acquisition is gratefully appreciated.
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