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An evaluation of the analytical performance of collinear multi-pulse laser induced breakdown spectroscopy
Microchemical Journal 97 (2011) 255–263
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Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
An evaluation of the analytical performance of collinear multi-pulse laser induced breakdown spectroscopy N. Jedlinszki, G. Galbács ⁎ Department of Inorganic and Analytical Chemistry, University of Szeged, 6720 Dóm tér 7, Szeged, Hungary
a r t i c l e
i n f o
Article history: Received 17 September 2010 Accepted 19 September 2010 Available online 24 September 2010 Keywords: Multi-pulse Laser induced breakdown spectroscopy LIBS Signal enhancement Calibration curve
a b s t r a c t A detailed study of the relevant analytical figures of merit for time- and spatially integrated multi-pulse laser induced breakdown spectroscopy (MP-LIBS) was performed. Laser bursts containing up to 6, ns-range duration collinear pulses, separated by 20–40 μs interpulse gaps were used in the experiments, and the effect of the number of pulses within the burst on the analytical parameters was investigated. Signal enhancement, repeatability, matrix effects, background signals, sensitivity, linear dynamic range and limits of detection were studied for 20 lines of 11 elements in different solid matrices. It was established that all analytical figures of merit significantly improved with respect to those of single- or double-pulse LIBS as a result of the use of multiple laser pulses. For example, six-pulse limits of detection values were found to be with a factor of 4.2– 16.7 lower than for double-pulses and calibration plots were found to be linear up to several tens of percents concentrations in some alloys. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It has been demonstrated by a number of studies in recent years, that the use of double laser pulses in laser induced breakdown spectroscopy (LIBS) is a very efficient approach for the improvement of the analytical capabilities of LIBS. Double pulse LIBS (DP-LIBS) can provide enhanced emission intensities, improved precision, lower limits of detection and longer sustained emission. Double-pulse LIBS research has been reviewed in recent book chapters and review papers (e.g. [1–3]). Double-pulse LIBS (also called dual-pulse LIBS) can be carried out basically in two different pulse configurations called orthogonal and collinear (also called coaxial), referring to the relative directions of the two laser beams. In the orthogonal arrangement, one beam is perpendicular to the sample surface, whereas the other one propagates in parallel with and close to the sample surface. In this arrangement, only one pulse is ablative, and by controlling the timing of the pulses the non-ablative pulse can help either the ablation or the excitation process. In the coaxial configuration, both the first and second pulses are focused onto the same location on the sample surface. This construction is the most practical, especially if the two laser pulses are originating from the same laser; nevertheless the plasma processes are largely complicated by the fact that both pulses can, and in most cases do, contribute to the ablation. To date, dual-pulse LIBS has been studied in many technical variations, including different combinations of relative laser beam direction, pulse timing, pulse duration, energy distribution and wavelength. Nevertheless, the use of collinear funda-
⁎ Corresponding author. Tel.: + 36 62 544013; fax: + 36 62 420505. E-mail address: [email protected] (G. Galbács). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.09.009
mental Nd laser pulses of ns duration and high speed gated spectrometers equipped with iCCD detectors, with the spectral integration time positioned with some delay after the second laser pulse, is still the most popular arrangement [1–3]. Tentative mechanisms, supported by modeling and studies of plasma dynamic, were suggested to explain the observed enhancement effects [4–11]. The enhancements were attributed to either or both of the following processes: 1) direct coupling between the plasma and the second laser pulse (“plasma reheating”), 2) increased sample ablation, which is caused by either sample modification (e.g. heating, recrystallization, etc.) from the first pulse or the reduced pressure behind the shockwave front from the first plasma. The interplay of these effects and the added relevance of the interpulse time separation and the energy distribution of the two laser pulses make dual pulse LIBS processes very complex. Multi-pulse LIBS (MP-LIBS), that is when more than two laser pulses are used, is another approach with which only a few research groups have experimented. This approach was originally proposed by Piepmeier and Malmstadt [12] and Scott and Strasheim [13]. MP-LIBS can only be practically realized in the collinear mode, using a single laser source set up to release multiple pulses. In the recent literature, three distinct classes of experimental MP-LIBS approaches can be identified, which primarily differ in the way the synchronization of spectral data acquisition with the laser pulses is done. One setup uses controlled bursts of up to three laser pulses with sophisticated high speed, gated spectrometers and μs range integration times positioned with some delay after the last laser pulse [14–18]. Another approach uses a train of pulses of uncontrolled length with kHz range repetition rate from a microchip (powerchip) laser and a non-gated spectrometer
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[19–22]. The approach that our own group also uses represents the third MP-LIBS way. Our setup employs a Nd-doped gadolinium gallium garnet (Nd:GGG) laser releasing bursts containing up to 11 adjustable number of pulses, and the spectra from all pulses within the burst are accumulated after a controlled initial delay by the gated, ms range integration time CCD spectrometer [23–25]. All three MP-LIBS approaches demonstrated for some selected elements and spectral lines that it is analytically advantageous to use more than two laser pulses in LIBS, as they provide increased material ablation and enhanced signal emission. For example, we reported about signal enhancements of up to 129 relative to the single pulse case, improved repeatabilities of 2–5% RSD [23], analytical results for gold alloys accurate to a few ‰ [24], and multiple times greater material ablation [25] using sixtuple laser pulses.
The aim of our present work was to systematically assess the analytical performance of MP-LIBS in our collinear configuration, for several metallic elements and spectral lines, in different matrices. For this purpose, the repeatability, signal enhancement, limits of detection and calibration curve characteristics were studied for MP-LIBS as a function of the number of laser pulses, for up to 6 pulses. 2. Experimental 2.1. Instruments and methods The multi-pulse LIBS system used had been described in detail elsewhere [23–25], hence only the most important features will be repeated here. The system is built around a converted stereo metallurgical
Fig. 1. Net (●) and energy normalized net (○) emission intensities of some neutral spectral lines as a function of the number of laser pulses in a burst. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
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microscope (SP80, Brunel), and is based on a fast triggerable, 2048 pixels, fiber optic CCD spectrometer (AvaSpec 2048FT, Avantes) and a flashlamp pumped compact Nd:GGG laser (MP/G-Q-005, Technoorg-Linda) equipped with a solid state LiF passive Q-switch. The 1055 nm laser light is guided with the use of a short wave pass dichroic laser beam splitter through the microscope objective (WD = 22 mm, magnification: 4×), which focuses it onto the surface of the sample in a perpendicular direction. The sample surface is observed via the same objective (in reversed path) by a digital camera attached to the ocular of the microscope. Reproducible sample-to-objective distance positioning and optimal light collection was aided by a HeNe laser beam. During setup, this laser beam was used to illuminate the focal spot of the focused ablation laser beam on the sample surface; this illuminated spot allowed the accurate positioning of the sample and the collection lens. The plasma light emission is collected and coupled into the deep-UV resistant 200 μm diameter optical fiber by a lens placed at an angle of 45° with respect to the normal of the sample surface. Data collection of the CCD spectrometer (integration time: 2 ms) is triggered by the signal of a fast photodiode (DET36A, Thorlabs) side-viewing the plasma. The delay time between the onset of plasma and the start of data collection was 3.4 μs. The pulse structure of the laser source was monitored using the same photodiode and an oscilloscope (TDS-1002, Tektronix). The system allows us to directly control the number of pulses (and thus the total energy) delivered to the sample in a burst. The energy of each pulse is practically the same, ca. 18 mJ. The interpulse time delay between pulses of a burst is about 28 μs for six-pulse bursts and increases slightly as the number of pulses in the burst decreases [23]. Time- and space-integrated emission spectra were collected both in the UV (198–318 nm, 0.09 nm optical resolution) and visual (345–888 nm, 0.4 nm optical resolution) ranges. All spectra were collected in ambient
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air. All net intensities reported were calculated using two point background correction. To allow comparison of repeatibility data, we kept the total number of pulses delivered to the sample in the experiments a constant 24 pulses. This meant measuring with 24 single-pulse bursts, 12 doublepulse bursts, 8 three-pulse bursts, etc. In the case of five-pulse bursts, a total of 25 pulses were delivered. The sample surface was renewed between the laser bursts by laterally moving the sample. The spectral lines of collected LIBS spectra were assigned using a spectroscopy software (Peax v2.0, Systematix AB), that uses the NIST spectral line database. The on-line Kurucz's spectroscopy database was also used for cross-reference. All further data evaluation was done in Origin (Version 7.5, OriginLab Corp.). 2.2. Materials Analytical purity Al, Cu, Mg, Si and Zn metals, series of certified gold alloys and steel samples, commercially available soldering tin samples, as well as pyrolithic graphite and polytetrafluoroethylene (PTFE) samples were used as test targets. The gold alloy series was a ninemember, quaternary gold alloy series. The gold content of the samples were determined by the cupellation method in two standard assay offices in Hungary. The average of the two independent certified concentrations were 334.1‰, 412.7‰, 506.3‰, 586.8‰, 674.2‰, 756.8‰, 838.1‰, 912.2‰ and 994.3‰, respectively. The samples in the certified steel sample series were prepared by the Research Institute for Ferrous Metallurgy (Budapest, Hungary). The six samples were labeled as A3, A2, A12, A1, A16 and A11 and contained 0.37%, 0.66%, 1.25%, 1.46%, 1.75% and 2.16% chromium, respectively. The manufacturer, product number and composition of the employed soldering tin
Fig. 2. Net (●) and energy normalized net (○) emission intensities of some ionic spectral lines as a function of the number of laser pulses in a burst. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
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samples are CFH 52340 No. 3 (Sn 97%, Cu 3%), CFH 52330 (Sn 40%, Pb 60%), Multicore 419590 (Sn 5%, Pb 93.5%, Ag 1.5%), Lux 539066 (Pb 70%, Sn 30%). The high purity, highly oriented pyrolithic graphite (HOPG) samples were manufactured by Advanced Ceramics Corp. (Lakewood, OH, USA). The PTFE sample was a commercial, 80 μm thick, 12 mm wide insulating tape (Schlaefer, Germany). No sample preparation, other than wiping the surface of the samples clean using pure acetone or ethanol prior to the measurements was done. 3. Discussion 3.1. Signal behaviour 3.1.1. Net signal enhancement Background corrected net and energy normalized net signal intensities were studied as a function of the number of pulses in a laser burst in metallic, polymer and graphite samples. Our findings are illustrated in Figs. 1 and 2. for various spectral lines of neutral atoms and ions. It can be clearly seen, that generally both the net and the energy normalized net intensities increase with the increase of the number of pulses in the burst and that this increase is significantly stronger than linear. The increasing normalized intensity curves indicate that the net emission signal generated by each laser pulse is larger than the signal originating from the former laser pulse, because if the contribution of each laser pulse within the burst to the emission signal was approximately the same, the normalized net intensity curve would be constant. The above described behaviour holds to all studied neutral lines (Fig. 1.). The behaviour of ionic lines, however, is a mixed one, as is illustrated in Fig. 2. Some lines, like the Mg (II) 448.1 nm and Si (II) 412.8 nm lines, behave analogously to neutral lines and display net intensity and normalized net intensity curves with a strong monotonous increase, whereas other lines, such as the Au (II) 312.7 nm and Al (II) 422.7 nm lines for example, behave differently. For the latter ionic lines, the net intensity curves are still monotonously increasing, which is important from the analytical point of view, but the energy normalized net intensity curves are either leveling up or are practically constant. This indicates that the net signal intensity for these lines which originate from high-lying excited levels, generated by each pulse is similar or increases only slightly. In a recent DP-LIBS study, Gautier et al. [8] found that the signal enhancement correlates with the excitation potential (EP, calculated for ionic lines as the sum of ionization and excitation potentials) and that this effect becomes more pronounced as the interpulse delay time increases from sub-microseconds to tens of microseconds. This suggests that in our case, when the interpulse delay is relatively long (ca. 25 μs), the signal enhancement should be most pronounced for
ionic lines and atomic lines with high excitation energies. Data in our present and former studies [23,25] do not support the existence of such direct correlation, at least under the multi-pulse collinear experimental conditions of our system. This is illustrated in Fig. 3. that graphically shows our findings with the spectral lines included in the present study. In fact, our experience is that net signal enhancements are greatest for “soft” spectral lines, for which the excitation potential is low (EP ≤ ca. 7 eV). These soft lines experience conditions that promote an elongated period of excitation, which leads to a significantly longer emission lifetime in double- or multiple pulse LIBS. On the other hand, emission from high EP spectral lines appear to be short lived and rekindled every time a new pulse in a multi-pulse collinear configuration arrives to the sample. The curves in Figs. 1 and 2 also provide evidence for this. Based on Fig. 3, it can be stated with more or less confidence that the relative net signal enhancement when increasing the pulse number from one to six is greatest for low EP lines and is consistently low for high EP lines. The signal enhancement appears to be maximum for lines with 5–7 eV EP values. It is also worth pointing out to the fact that high EP lines still show some intensity enhancement when using time-integrated detection. Table 1 summarizes the net and the energy normalized net signal enhancements for the spectral lines studied, relative to the single-pulse signal. It can be clearly seen that the unnormalized net signal enhancement, which is the more important of the two from an analytical point of view, is always significantly larger for more pulses in the burst, and in several cases it is over a hundred. We would like to emphasize however, that the actual signal enhancement values strongly depend on the geometry and timing of the emission data acquisition, thus may not
Table 1 Wavelength and excitation energy data, along with signal enhancement results for all spectral lines used in this study. Missing excitation energy data indicate that energy level data is unavailable in both used spectral databases. Signal enhancement data marked with an asterisk (*) indicates that single-pulse signals were too weak for reliable evaluation, hence double-pulse signals were used as reference in those instances. Matrix
Pure elemental form
Steel Alloy
Fig. 3. Net signal enhancement obtained with six-pulse laser bursts for all spectral lines used in this study as a function of excitation energy. Signal enhancement was calculated using the single-pulse net signal as reference.
PTFE
Species
Al I Al I Al II Au I Au II CI Cr I Cu I Cu I Cu II Mg I Mg II Si I Si II Sn I Sn I Zn I Zn I Zn I Cr I Fe I Au I Cu I Cu I Cu II Sn I Sn I Zn I CI
λ (nm)
Eexcitation (eV)
394.4 396.2 422.7 291.5 312.7 247.8 520.8 510.5 521.8 203.7 470.3 448.1 390.5 412.8 270.7 452.5 468.0 481.0 307.6 520.8 414.4 291.5 510.5 521.8 203.7 270.7 452.5 481.0 247.8
3.14 3.14 23.99 – – 7.69 3.32 3.82 6.20 16.64 6.99 19.28 5.08 21.00 4.79 4.87 6.66 6.66 4.03 3.32 4.55 – 3.82 6.20 16.64 4.79 4.87 6.66 7.69
Signal enhancement Net signal
Normalized net signal
2-pulse
6-pulse
2-pulse
10.5 10.6 2.1 3.6 2.7 2.2 10.4 12.1 30.1 2.3 9.2 1.0⁎ 1.0⁎ 1.0⁎
89.3 85.1 13.2 13.3 8.2 13.3 115.1 114.1 341.1 7.4 111.7 10.4⁎ 5.3⁎ 6.6⁎
5.2 5.3 1.1 1.8 1.4 1.1 5.2 6.1 15.0 1.2 4.6 1.0⁎ 1.0⁎ 1.0⁎
5.0 108.2 839.2 679.3 15.4 27.5 48.1 6.5 67.8 171.6 7.3 5.6 99.0 572.4 5.8
1.1 5.4 69.1 47.4 1.5 0.8 1.5 1.2 6.3 13.4 1.3 0.9 4.3 38.4 1.0
2.2 10.8 138.1 94.8 3.1 1.6 3.0 2.4 12.5 26.7 2.6 1.9 8.6 76.8 2.0
6-pulse 14.9 14.2 2.2 2.2 1.4 2.2 19.2 19.0 56.9 1.2 18.6 3.5⁎ 1.8⁎ 2.2⁎ 0.8 18.0 139.9 113.2 2.6 4.6 8.0 1.1 11.3 28.6 1.2 0.9 16.5 95.4 1.0
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Fig. 4. Background (●) and energy normalized background (○) intensities in the vicinity of some UV and Vis spectral lines as a function of the number of laser pulses in a burst. Background signal intensities were averaged over several nm ranges on both sides of the lines.
be directly compared to values provided by other studies. The reasons were discussed in detail by several studies (e.g. [26,27]), now we would just like to point out to the fact that the side-viewing two-lens collimating arrangement used by so many LIBS setups is sub-optimal for analytical DP- or MP-LIBS, as it attempts to collect light only from a well defined small spot within the plasma. For DP- and MP-LIB plasmas, which expand significantly faster and reach a larger volume than SP-LIB plasmas, spatial-integration with the use of a single collimation lens or even without lens is more suitable in order to realize large signal enhancements. 3.1.2. Background signal All observations described throughout the manuscript were made for background corrected net emission signals, but in addition, we also studied the influence of the time-integration nature of our experimental setup on the background signal. The curves in Fig. 4. show the behaviour of the average background signals recorded in the UV and Vis range in the neighborhood of some neutral and ionic spectral lines as a function of the number of the pulses in the burst. As it can be seen, the average background emission signal only slightly increases with the number of pulses, and energy normalized curves are practically constant or show saturation as a function of pulse number. Fig. 4. can be interpreted as that the background signal generated by each pulse in the burst is short lived and its contribution is about the same, from the second pulse on. The contribution of the first pulse is significantly lower than that of later pulses, which is consistent with the 3.4 μs acquisition (gate) delay time used – by this time, the plasma temperature is lower and the continuum emission from the plasma drops to a low level [25]. These findings indicate that the use of time integrated spectral data acquisiton in MP-LIBS does not worsen the S/B ratio of signals. This is further supported by microchip/powerchip MP-
LIBS applications [19–22], which employ ungated (non-synchronized) time-integrated detection. 3.1.3. Matrix effects The effect of the sample matrix on the net signal enhancement was also studied. In Fig. 5, the net signal enhancement curves for spectral lines of Cu, Au, Cr and C in a pure form and in a different matrix (alloy or polimer) can be compared. In this experiment, the net enhancement values for six laser pulses were found to be ca. 13.3 and 6.5 for the Au (I) line, 341 and 171 for the Cu (I) line and 7.4 and 7.2 for the Cu (II) line, 2.2 and 1.0 for the C (I) line and 6.8 and 4.3 for the Cr (I) line, in the pure and the alloy matrix, respectively. It can be stated that the trend of the curves is similar for any given spectral line in the two matrices, what suggests that the transitions and the nature of species have more influence on the enhancement than the matrix. The relative net signal enhancement that can be realized is different for the same line in different matrices, but seems to be consistently lower in the more complex matrix. The signal enhancement also shows a small variation for the different species within the same matrix. Obviously, further, more extensive experiments with many matrices will need to be performed before a general conclusion can be drawn with respect to the matrix dependence of the signal enhancement. 3.2. Signal repeatability The repeatability of the signal is an important analytical figure of merit, as it directly influences the limit of detection and the precision of analysis. LIBS is primarily used as a microanalytical solid sampling technique, hence it suffers from relatively poor repeatability – as do other similar techniques such as SS-ETAAS or LA-ICP-MS. In the case of LIBS this is the result of both instrumental (e.g. shot-to-shot fluctuation
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of laser pulse energy, perturbations of plasma characteristics) and sample related effects (e.g. sample micro-heterogeneity). As a consequence, typical SP-LIBS signal repeatability values (RSD%) are consistently reported to be about 15–25% (e.g. [28–30]), when perfoming a small number of repetitions (10–50). These values can only be generally improved either by using a large number (e.g. 500–2500) of spectra for averaging [28] or by employing some signal normalization method [31,32]. A factor of a 2–3 improvement in precision was also reported for DP-LIBS (e.g.[14,33]). Our observation with collinear MP-LIBS using time- and spatially integrated signals that the signal repeatability largely improves as the number of pulses in a burst increases. As it can be seen in Fig. 6. for various spectral lines in different metallic matrices, the relative standard deviation on net line intensities steadily decreases as a function of number of pulses; starting from tens of percents it drops down to 5–10%. We would like to emphasize that these RSD% values were obtained with a relatively small number of laser bursts, and in such a way that the total number of pulses (number of repetitions× number of pulses in the burst) delivered was kept constant, as described in detail in the experimental section. This way the effect of total number of pulses on the repeatability data could be eliminated. No signal normalization method or extensive averaging was performed. We would like to note that the obtained RSD% values are comparable or better than the values reported for DP-LIBS for 50 or more shots [14,15,33]. We reported about similiar repeatability values earlier also for other lines and matrices [23–25]. As to explain this large improvement in the repetability (precision), the relevance of the use of time integrated detection in our setup has to be emphasized. The integration time extends over the entire useful lifetime of the plasma, as opposed to typical time resolved LIBS setups where the spectral integration time is on the order of microseconds. Additionally, as described in the preceeding section, the signal-to-background ratio also significantly increases under the MP-LIBS conditions used here. The improvement in signal repeatability is therefore a combination of these effects which significantly improve the signal statistics. 3.3. Calibration curves In Fig. 7, calibration plots obtained for some neutral lines of Cu, Zn, Cr and Sn in some of their alloys can be seen. For comparison, single-pulse and six-pulse curves are both plotted. (For the Zn line, single pulse emission was too weak for reliable calibration, hence the double-pulse plot is shown.) It is apparent that the linear dynamic range of the sextuple-pulse plots is much wider. In SP-LIBS, the upper concentration limit for the linearity of calibration curves for strong neutral lines is usually around a few percents. Although the availability of suitable calibration samples restricted the range of concentrations for which we could record the calibration plots, it can be seen that our six-pulse plots can be well fitted with a straight line for up to several tens of percent concentration, perhaps even higher. We would also like to mention that the spectral lines used in Fig. 7 are relatively strong neutral lines, with transition probabilities on the order of 107 per second and excitation energies of only a few electronvolts. Considering that the literature primarily attributes the saturation characteristics of LIBS calibration plots to self absorption ([e.g. 34,35]), our data suggest that self-absorption is advantegously suppressed in MP-LIBS. The observed reduced-level self-absorption in the plasma can be probably accounted for by the lower number density of emitters in the plasma, caused by the fast expansion rate of double- and multi-pulse LIB plasmas, mainly caused by the pressure drop behind the shockwave front. This reasoning is supported by the fact that some SP-LIBS studies
also reported about significantly extended linear dynamic ranges in reduced pressure atmospheres [36,37]. Pressure conditions also influence collisional excitation and deexcitation processes, thus if only the „dilution” of emitters is considered, and with a light collection setup employing spatial resolution, it could even result in decreased light emission signals. However, due to the spatial- and time-integration employed in our multi-pulse experiments, which collects emitted light from practically over the whole plasma volume and lifetime, the integrated emission signal does not show a decrease with respect to the single-pulse case. In fact, as described in earlier sections of the present study, we consistently observed no signal decrease. In most cases, a significant signal increase was observed as a function of the number of pulses in a laser burst. In quantitative analytical applications, this signal enhancement directly transforms into a sensitivity increase (increase of the slope of calibration plots).
3.4. Quantitative figures of merit Based on the data shown above, it can be expected that detection capabilites improve significantly with the use of multiple pulses in LIBS. As a combined effect of the decreasing standard deviation (improving repeatability) and increasing sensitivity, limits of detection (LOD) calculated by the IUPAC three-sigma definition indeed improve significantly, not only over the single-pulse but also over double-pulse LIBS results. In order to illustrate the latter improvement, in Table 2 we listed both the two-pulse and six-pulse LOD values found for the data in Fig. 7. Limits of quantitation (six-sigma definition) are also shown. As can be seen, the six-pulse MP-LIBS figures of merits show a 4.2–16.7 times improvement over the DPLIBS values. We would also like to point out to the fact that this improvement was realized by using only a small number of laser shots (4 shots consisting of 6 laser pulses each). In quantitative bulk analytical applications, one typically uses average signals originating from a large number of laser shots delivered to a translating or rotating sample, thus attempting to reduce signal scatter even further. Lower limits of detection can also be achieved by using higher laser pulse energies and/or a CCD detector of higher sensitivity. Therefore, further improvements in the absolute quantitative figures of merit can potentially be realized in some MP-LIBS applications.
4. Conclusions We have shown that the analytical performance of LIBS can be significantly improved if, instead of single or double laser pulses, bursts of multiple collinear laser pulses are used for the plasma generation. This improvement was documented for twenty spectral lines of a total of eleven elements. Data presented suggest that the use of spatial and time integration of the plasma emission helps to realize not only enhanced sensitivities but also improved repeatabilities. Our findings suggest that emission signal enhancement is present even when the laser interpulse delay is on the order of 20–40 μs. Although our present experimental setup does not allow the direct control of the interpulse delay, and the number of pulses in the burst can also not be increased further than 6, but it can be assumed that these two parameters have a strong influence on the analytical performance of MP-LIBS. Our future research efforts will be concentrating on constructing a new setup that would allow us testing the effect of these parameters.
Fig. 5. Comparion of the net (●) and energy normalized net (○) emission intensities of the same spectral lines in two different sample matrices (in pure elemental form on the left and in an alloy/compound on the right) as a function of the number of laser pulses in a burst. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
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N. Jedlinszki, G. Galbács / Microchemical Journal 97 (2011) 255–263 Table 2 Comparison of the limits of detection for single-pulse and sextuple-pulse LIBS based on the three-sigma formula and calibration data shown in Fig. 7. For the Zn line, single pulse emission was too weak for reliable calibration, hence the double-pulse plot is shown. Species
λ (nm)
LOD (mg/g) 2 pulses
6 pulses
2 pulses
6 pulses
Cu I Zn I Cr I Sn I
510.5 481.0 520.8 452.5
34.69 6.94 5.19 20.74
2.65 0.51 0.31 4.87
69.37 13.87 10.37 41.49
5.30 1.02 0.62 9.75
LOQ (mg/g)
Acknowledgements The authors cordially thank Dr. József Pallósi (Dunaferr Rt., Hungary) and László Túri (Festék Bázis Kft., Hungary) for helping our research by providing us with the certified steel and gold alloy samples, respectively. The authors kindly acknowledge the financial funding received under No. TAMOP-4.2.1/B-09/1/KONV-2010-0005. References
Fig. 6. Relative standard deviation (repetability) for some spectral lines as a function of the number of laser pulses in a burst. All data were calculated for the same total number of laser pulses delivered, as described in the experimental section.
[1] J. Pender, B. Pearman, J. Scaffidi, S.R. Goode, S. Michael Angel, LIBS using sequential laser pulses, in: A.W. Miziolek, V. Palleschi, I. Schechter (Eds.), Laser-induced breakdown spectroscopy: fundamentals and applications, Cambridge University Press, 2006, pp. 516–538. [2] J. Scaffidi, D.A. Cremers, S.M. Angel, Dual-pulse LIBS, in: J.P. Singh, S.N. Thakur (Eds.), Laser-induced breakdown spectroscopy, Elsevier, 2007, pp. 137–150. [3] V.I. Babushok, F.C. DeLucia Jr., J.L. Gottfried, C.A. Munson, A.W. Miziolek, Double pulse laser ablation and plasma: laser induced breakdown spectroscopy signal enhancement, Spectrochim. Acta 61B (2006) 999–1014.
Fig. 7. Comparison of single-pulse (●) and sextuple-pulse (○) calibration curves obtained for some neutral spectral lines. Fitted lines and curves are only meant to guide the eye. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
N. Jedlinszki, G. Galbács / Microchemical Journal 97 (2011) 255–263 [4] J. Scaffidi, S.M. Angel, D.A. Cremers, Emission enhancement mechanisms in dualpulse LIBS, Anal. Chem. 78 (2006) 24–32. [5] A. Bogaerts, Z. Chen, D. Autrique, Double pulse laser ablation and laser induced breakdown spectroscopy: a modeling investigation, Spectrochim. Acta 63B (2008) 746–754. [6] R. Noll, R. Sattmann, V. Sturm, S. Winkelmann, Space- and time-resolved dynamics of plasmas generated by laser double pulses interacting with metallic samples, J. Anal. At. Spectrom. 19 (2004) 419–428. [7] P.A. Benedetti, G. Christoforetti, S. Legnaioli, V. Palleschi, L. Pardini, A. Salvetti, E. Tognoni, Effect of laser pulse energies in laser induced breakdown spectroscopy in double-pulse configuration, Spectrochim. Acta 60B (2005) 1392–1401. [8] C. Gautier, P. Fichet, D. Menut, J.-L. Lacour, D. L'Hermite, J. Dubessy, Main parameters influencing the double-pulse laser-induced breakdown spectroscopy in the collinear beam geometry, Spectrochim. Acta 60B (2005) 792–804. [9] F. Colao, V. Lazic, R. Fantoni, S. Pershin, A comparison of single and double pulse laser-induced breakdown spectroscopy of aluminum samples, Spectrochim. Acta 57B (2002) 1167–1179. [10] X. Mao, X. Zeng, S.-B. Wen, R.E. Russo, Time-resolved plasma properties for double pulsed laser-induced breakdown spectroscopy of silicon, Spectrochim. Acta 60B (2005) 960–967. [11] C.D. Gehlen, P. Roth, Ü. Aydin, E. Wiens, R. Noll, Time-resolved investigations of laser-induced plasmas generated by nanosecond bursts in the millijoule regime, Spectrochim. Acta 63B (2008) 1072–1076. [12] E.H. Piepmeier, H.V. Malmstadt, Q-switched laser energy absorption in the plume of an aluminum alloy, Anal. Chem. 41 (1969) 700–707. [13] R.H. Scott, A. Strasheim, Time-resolved direct-reading spectrochemical analysis using a laser source with medium pulse-repetition rate, Spectrochim. Acta 26B (1971) 707–719. [14] R. Sattmann, V. Sturm, R. Noll, Laser induced breakdown spectroscopy of steel samples using multiple Q-switch Nd:YAG laser pulses, J. Phys. D Appl. Phys. 28 (1995) 2181–2187. [15] V. Sturm, L. Peter, R. Noll, Steel analysis with laser-induced breakdown spectrometry in the vacuum ultraviolet, Appl. Spectrosc. 54 (2000) 1275–1278. [16] A. Löbe, J. Vrenegor, R. Fleige, V. Sturm, R. Noll, Laser-induced ablation of a steel sample in different ambient gases by use of collinear multiple laser pulses, Anal. Bioanal. Chem. 385 (2006) 326–332. [17] C. Hartmann, A. Gillner, Ü. Aydin, R. Noll, T. Fehr, C. Gehlen, R. Poprawe, Investigation on laser micro ablation of metals using ns-multi-pulses, J. Phys. Conf. Ser. 59 (2007) 440–444. [18] K.Y. Yamamoto, D.A. Cremers, T.M.J. Ferris, L.E. Foster, Detection of metals in the environment using a portable laser-induced breakdown spectroscopy instrument, Appl. Spectrosc. 50 (1996) 222–233. [19] G. Christoforetti, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni, P.A. Benedetti, F. Brioschi, F. Ferrario, Quantitative analysis of aluminium alloys by low-energy, high-repetition rate laser-induced breakdown spectroscopy, J. Anal. At. Spectrom. 21 (2006) 697–702. [20] C. Lopez-Moreno, K. Amponsah-Manager, B.W. Smith, I.B. Gornushkin, N. Omenetto, S. Palanco, J.J. Laserna, J.D. Winefordner, Quantitative analysis of low-alloy steel by microchip laser induced breakdown spectroscopy, J. Anal. At. Spectrom. 20 (2005) 552–556. [21] I.B. Gornushkin, K. Amponsah-Manager, B.W. Smith, N. Omenetto, J.D. Winefordner, Microchip laser-induced breakdown spectroscopy: a preliminary feasibility investigation, Appl. Spectrosc. 58 (2004) 762–769.
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Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
An evaluation of the analytical performance of collinear multi-pulse laser induced breakdown spectroscopy N. Jedlinszki, G. Galbács ⁎ Department of Inorganic and Analytical Chemistry, University of Szeged, 6720 Dóm tér 7, Szeged, Hungary
a r t i c l e
i n f o
Article history: Received 17 September 2010 Accepted 19 September 2010 Available online 24 September 2010 Keywords: Multi-pulse Laser induced breakdown spectroscopy LIBS Signal enhancement Calibration curve
a b s t r a c t A detailed study of the relevant analytical figures of merit for time- and spatially integrated multi-pulse laser induced breakdown spectroscopy (MP-LIBS) was performed. Laser bursts containing up to 6, ns-range duration collinear pulses, separated by 20–40 μs interpulse gaps were used in the experiments, and the effect of the number of pulses within the burst on the analytical parameters was investigated. Signal enhancement, repeatability, matrix effects, background signals, sensitivity, linear dynamic range and limits of detection were studied for 20 lines of 11 elements in different solid matrices. It was established that all analytical figures of merit significantly improved with respect to those of single- or double-pulse LIBS as a result of the use of multiple laser pulses. For example, six-pulse limits of detection values were found to be with a factor of 4.2– 16.7 lower than for double-pulses and calibration plots were found to be linear up to several tens of percents concentrations in some alloys. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It has been demonstrated by a number of studies in recent years, that the use of double laser pulses in laser induced breakdown spectroscopy (LIBS) is a very efficient approach for the improvement of the analytical capabilities of LIBS. Double pulse LIBS (DP-LIBS) can provide enhanced emission intensities, improved precision, lower limits of detection and longer sustained emission. Double-pulse LIBS research has been reviewed in recent book chapters and review papers (e.g. [1–3]). Double-pulse LIBS (also called dual-pulse LIBS) can be carried out basically in two different pulse configurations called orthogonal and collinear (also called coaxial), referring to the relative directions of the two laser beams. In the orthogonal arrangement, one beam is perpendicular to the sample surface, whereas the other one propagates in parallel with and close to the sample surface. In this arrangement, only one pulse is ablative, and by controlling the timing of the pulses the non-ablative pulse can help either the ablation or the excitation process. In the coaxial configuration, both the first and second pulses are focused onto the same location on the sample surface. This construction is the most practical, especially if the two laser pulses are originating from the same laser; nevertheless the plasma processes are largely complicated by the fact that both pulses can, and in most cases do, contribute to the ablation. To date, dual-pulse LIBS has been studied in many technical variations, including different combinations of relative laser beam direction, pulse timing, pulse duration, energy distribution and wavelength. Nevertheless, the use of collinear funda-
⁎ Corresponding author. Tel.: + 36 62 544013; fax: + 36 62 420505. E-mail address: [email protected] (G. Galbács). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.09.009
mental Nd laser pulses of ns duration and high speed gated spectrometers equipped with iCCD detectors, with the spectral integration time positioned with some delay after the second laser pulse, is still the most popular arrangement [1–3]. Tentative mechanisms, supported by modeling and studies of plasma dynamic, were suggested to explain the observed enhancement effects [4–11]. The enhancements were attributed to either or both of the following processes: 1) direct coupling between the plasma and the second laser pulse (“plasma reheating”), 2) increased sample ablation, which is caused by either sample modification (e.g. heating, recrystallization, etc.) from the first pulse or the reduced pressure behind the shockwave front from the first plasma. The interplay of these effects and the added relevance of the interpulse time separation and the energy distribution of the two laser pulses make dual pulse LIBS processes very complex. Multi-pulse LIBS (MP-LIBS), that is when more than two laser pulses are used, is another approach with which only a few research groups have experimented. This approach was originally proposed by Piepmeier and Malmstadt [12] and Scott and Strasheim [13]. MP-LIBS can only be practically realized in the collinear mode, using a single laser source set up to release multiple pulses. In the recent literature, three distinct classes of experimental MP-LIBS approaches can be identified, which primarily differ in the way the synchronization of spectral data acquisition with the laser pulses is done. One setup uses controlled bursts of up to three laser pulses with sophisticated high speed, gated spectrometers and μs range integration times positioned with some delay after the last laser pulse [14–18]. Another approach uses a train of pulses of uncontrolled length with kHz range repetition rate from a microchip (powerchip) laser and a non-gated spectrometer
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[19–22]. The approach that our own group also uses represents the third MP-LIBS way. Our setup employs a Nd-doped gadolinium gallium garnet (Nd:GGG) laser releasing bursts containing up to 11 adjustable number of pulses, and the spectra from all pulses within the burst are accumulated after a controlled initial delay by the gated, ms range integration time CCD spectrometer [23–25]. All three MP-LIBS approaches demonstrated for some selected elements and spectral lines that it is analytically advantageous to use more than two laser pulses in LIBS, as they provide increased material ablation and enhanced signal emission. For example, we reported about signal enhancements of up to 129 relative to the single pulse case, improved repeatabilities of 2–5% RSD [23], analytical results for gold alloys accurate to a few ‰ [24], and multiple times greater material ablation [25] using sixtuple laser pulses.
The aim of our present work was to systematically assess the analytical performance of MP-LIBS in our collinear configuration, for several metallic elements and spectral lines, in different matrices. For this purpose, the repeatability, signal enhancement, limits of detection and calibration curve characteristics were studied for MP-LIBS as a function of the number of laser pulses, for up to 6 pulses. 2. Experimental 2.1. Instruments and methods The multi-pulse LIBS system used had been described in detail elsewhere [23–25], hence only the most important features will be repeated here. The system is built around a converted stereo metallurgical
Fig. 1. Net (●) and energy normalized net (○) emission intensities of some neutral spectral lines as a function of the number of laser pulses in a burst. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
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microscope (SP80, Brunel), and is based on a fast triggerable, 2048 pixels, fiber optic CCD spectrometer (AvaSpec 2048FT, Avantes) and a flashlamp pumped compact Nd:GGG laser (MP/G-Q-005, Technoorg-Linda) equipped with a solid state LiF passive Q-switch. The 1055 nm laser light is guided with the use of a short wave pass dichroic laser beam splitter through the microscope objective (WD = 22 mm, magnification: 4×), which focuses it onto the surface of the sample in a perpendicular direction. The sample surface is observed via the same objective (in reversed path) by a digital camera attached to the ocular of the microscope. Reproducible sample-to-objective distance positioning and optimal light collection was aided by a HeNe laser beam. During setup, this laser beam was used to illuminate the focal spot of the focused ablation laser beam on the sample surface; this illuminated spot allowed the accurate positioning of the sample and the collection lens. The plasma light emission is collected and coupled into the deep-UV resistant 200 μm diameter optical fiber by a lens placed at an angle of 45° with respect to the normal of the sample surface. Data collection of the CCD spectrometer (integration time: 2 ms) is triggered by the signal of a fast photodiode (DET36A, Thorlabs) side-viewing the plasma. The delay time between the onset of plasma and the start of data collection was 3.4 μs. The pulse structure of the laser source was monitored using the same photodiode and an oscilloscope (TDS-1002, Tektronix). The system allows us to directly control the number of pulses (and thus the total energy) delivered to the sample in a burst. The energy of each pulse is practically the same, ca. 18 mJ. The interpulse time delay between pulses of a burst is about 28 μs for six-pulse bursts and increases slightly as the number of pulses in the burst decreases [23]. Time- and space-integrated emission spectra were collected both in the UV (198–318 nm, 0.09 nm optical resolution) and visual (345–888 nm, 0.4 nm optical resolution) ranges. All spectra were collected in ambient
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air. All net intensities reported were calculated using two point background correction. To allow comparison of repeatibility data, we kept the total number of pulses delivered to the sample in the experiments a constant 24 pulses. This meant measuring with 24 single-pulse bursts, 12 doublepulse bursts, 8 three-pulse bursts, etc. In the case of five-pulse bursts, a total of 25 pulses were delivered. The sample surface was renewed between the laser bursts by laterally moving the sample. The spectral lines of collected LIBS spectra were assigned using a spectroscopy software (Peax v2.0, Systematix AB), that uses the NIST spectral line database. The on-line Kurucz's spectroscopy database was also used for cross-reference. All further data evaluation was done in Origin (Version 7.5, OriginLab Corp.). 2.2. Materials Analytical purity Al, Cu, Mg, Si and Zn metals, series of certified gold alloys and steel samples, commercially available soldering tin samples, as well as pyrolithic graphite and polytetrafluoroethylene (PTFE) samples were used as test targets. The gold alloy series was a ninemember, quaternary gold alloy series. The gold content of the samples were determined by the cupellation method in two standard assay offices in Hungary. The average of the two independent certified concentrations were 334.1‰, 412.7‰, 506.3‰, 586.8‰, 674.2‰, 756.8‰, 838.1‰, 912.2‰ and 994.3‰, respectively. The samples in the certified steel sample series were prepared by the Research Institute for Ferrous Metallurgy (Budapest, Hungary). The six samples were labeled as A3, A2, A12, A1, A16 and A11 and contained 0.37%, 0.66%, 1.25%, 1.46%, 1.75% and 2.16% chromium, respectively. The manufacturer, product number and composition of the employed soldering tin
Fig. 2. Net (●) and energy normalized net (○) emission intensities of some ionic spectral lines as a function of the number of laser pulses in a burst. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
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samples are CFH 52340 No. 3 (Sn 97%, Cu 3%), CFH 52330 (Sn 40%, Pb 60%), Multicore 419590 (Sn 5%, Pb 93.5%, Ag 1.5%), Lux 539066 (Pb 70%, Sn 30%). The high purity, highly oriented pyrolithic graphite (HOPG) samples were manufactured by Advanced Ceramics Corp. (Lakewood, OH, USA). The PTFE sample was a commercial, 80 μm thick, 12 mm wide insulating tape (Schlaefer, Germany). No sample preparation, other than wiping the surface of the samples clean using pure acetone or ethanol prior to the measurements was done. 3. Discussion 3.1. Signal behaviour 3.1.1. Net signal enhancement Background corrected net and energy normalized net signal intensities were studied as a function of the number of pulses in a laser burst in metallic, polymer and graphite samples. Our findings are illustrated in Figs. 1 and 2. for various spectral lines of neutral atoms and ions. It can be clearly seen, that generally both the net and the energy normalized net intensities increase with the increase of the number of pulses in the burst and that this increase is significantly stronger than linear. The increasing normalized intensity curves indicate that the net emission signal generated by each laser pulse is larger than the signal originating from the former laser pulse, because if the contribution of each laser pulse within the burst to the emission signal was approximately the same, the normalized net intensity curve would be constant. The above described behaviour holds to all studied neutral lines (Fig. 1.). The behaviour of ionic lines, however, is a mixed one, as is illustrated in Fig. 2. Some lines, like the Mg (II) 448.1 nm and Si (II) 412.8 nm lines, behave analogously to neutral lines and display net intensity and normalized net intensity curves with a strong monotonous increase, whereas other lines, such as the Au (II) 312.7 nm and Al (II) 422.7 nm lines for example, behave differently. For the latter ionic lines, the net intensity curves are still monotonously increasing, which is important from the analytical point of view, but the energy normalized net intensity curves are either leveling up or are practically constant. This indicates that the net signal intensity for these lines which originate from high-lying excited levels, generated by each pulse is similar or increases only slightly. In a recent DP-LIBS study, Gautier et al. [8] found that the signal enhancement correlates with the excitation potential (EP, calculated for ionic lines as the sum of ionization and excitation potentials) and that this effect becomes more pronounced as the interpulse delay time increases from sub-microseconds to tens of microseconds. This suggests that in our case, when the interpulse delay is relatively long (ca. 25 μs), the signal enhancement should be most pronounced for
ionic lines and atomic lines with high excitation energies. Data in our present and former studies [23,25] do not support the existence of such direct correlation, at least under the multi-pulse collinear experimental conditions of our system. This is illustrated in Fig. 3. that graphically shows our findings with the spectral lines included in the present study. In fact, our experience is that net signal enhancements are greatest for “soft” spectral lines, for which the excitation potential is low (EP ≤ ca. 7 eV). These soft lines experience conditions that promote an elongated period of excitation, which leads to a significantly longer emission lifetime in double- or multiple pulse LIBS. On the other hand, emission from high EP spectral lines appear to be short lived and rekindled every time a new pulse in a multi-pulse collinear configuration arrives to the sample. The curves in Figs. 1 and 2 also provide evidence for this. Based on Fig. 3, it can be stated with more or less confidence that the relative net signal enhancement when increasing the pulse number from one to six is greatest for low EP lines and is consistently low for high EP lines. The signal enhancement appears to be maximum for lines with 5–7 eV EP values. It is also worth pointing out to the fact that high EP lines still show some intensity enhancement when using time-integrated detection. Table 1 summarizes the net and the energy normalized net signal enhancements for the spectral lines studied, relative to the single-pulse signal. It can be clearly seen that the unnormalized net signal enhancement, which is the more important of the two from an analytical point of view, is always significantly larger for more pulses in the burst, and in several cases it is over a hundred. We would like to emphasize however, that the actual signal enhancement values strongly depend on the geometry and timing of the emission data acquisition, thus may not
Table 1 Wavelength and excitation energy data, along with signal enhancement results for all spectral lines used in this study. Missing excitation energy data indicate that energy level data is unavailable in both used spectral databases. Signal enhancement data marked with an asterisk (*) indicates that single-pulse signals were too weak for reliable evaluation, hence double-pulse signals were used as reference in those instances. Matrix
Pure elemental form
Steel Alloy
Fig. 3. Net signal enhancement obtained with six-pulse laser bursts for all spectral lines used in this study as a function of excitation energy. Signal enhancement was calculated using the single-pulse net signal as reference.
PTFE
Species
Al I Al I Al II Au I Au II CI Cr I Cu I Cu I Cu II Mg I Mg II Si I Si II Sn I Sn I Zn I Zn I Zn I Cr I Fe I Au I Cu I Cu I Cu II Sn I Sn I Zn I CI
λ (nm)
Eexcitation (eV)
394.4 396.2 422.7 291.5 312.7 247.8 520.8 510.5 521.8 203.7 470.3 448.1 390.5 412.8 270.7 452.5 468.0 481.0 307.6 520.8 414.4 291.5 510.5 521.8 203.7 270.7 452.5 481.0 247.8
3.14 3.14 23.99 – – 7.69 3.32 3.82 6.20 16.64 6.99 19.28 5.08 21.00 4.79 4.87 6.66 6.66 4.03 3.32 4.55 – 3.82 6.20 16.64 4.79 4.87 6.66 7.69
Signal enhancement Net signal
Normalized net signal
2-pulse
6-pulse
2-pulse
10.5 10.6 2.1 3.6 2.7 2.2 10.4 12.1 30.1 2.3 9.2 1.0⁎ 1.0⁎ 1.0⁎
89.3 85.1 13.2 13.3 8.2 13.3 115.1 114.1 341.1 7.4 111.7 10.4⁎ 5.3⁎ 6.6⁎
5.2 5.3 1.1 1.8 1.4 1.1 5.2 6.1 15.0 1.2 4.6 1.0⁎ 1.0⁎ 1.0⁎
5.0 108.2 839.2 679.3 15.4 27.5 48.1 6.5 67.8 171.6 7.3 5.6 99.0 572.4 5.8
1.1 5.4 69.1 47.4 1.5 0.8 1.5 1.2 6.3 13.4 1.3 0.9 4.3 38.4 1.0
2.2 10.8 138.1 94.8 3.1 1.6 3.0 2.4 12.5 26.7 2.6 1.9 8.6 76.8 2.0
6-pulse 14.9 14.2 2.2 2.2 1.4 2.2 19.2 19.0 56.9 1.2 18.6 3.5⁎ 1.8⁎ 2.2⁎ 0.8 18.0 139.9 113.2 2.6 4.6 8.0 1.1 11.3 28.6 1.2 0.9 16.5 95.4 1.0
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Fig. 4. Background (●) and energy normalized background (○) intensities in the vicinity of some UV and Vis spectral lines as a function of the number of laser pulses in a burst. Background signal intensities were averaged over several nm ranges on both sides of the lines.
be directly compared to values provided by other studies. The reasons were discussed in detail by several studies (e.g. [26,27]), now we would just like to point out to the fact that the side-viewing two-lens collimating arrangement used by so many LIBS setups is sub-optimal for analytical DP- or MP-LIBS, as it attempts to collect light only from a well defined small spot within the plasma. For DP- and MP-LIB plasmas, which expand significantly faster and reach a larger volume than SP-LIB plasmas, spatial-integration with the use of a single collimation lens or even without lens is more suitable in order to realize large signal enhancements. 3.1.2. Background signal All observations described throughout the manuscript were made for background corrected net emission signals, but in addition, we also studied the influence of the time-integration nature of our experimental setup on the background signal. The curves in Fig. 4. show the behaviour of the average background signals recorded in the UV and Vis range in the neighborhood of some neutral and ionic spectral lines as a function of the number of the pulses in the burst. As it can be seen, the average background emission signal only slightly increases with the number of pulses, and energy normalized curves are practically constant or show saturation as a function of pulse number. Fig. 4. can be interpreted as that the background signal generated by each pulse in the burst is short lived and its contribution is about the same, from the second pulse on. The contribution of the first pulse is significantly lower than that of later pulses, which is consistent with the 3.4 μs acquisition (gate) delay time used – by this time, the plasma temperature is lower and the continuum emission from the plasma drops to a low level [25]. These findings indicate that the use of time integrated spectral data acquisiton in MP-LIBS does not worsen the S/B ratio of signals. This is further supported by microchip/powerchip MP-
LIBS applications [19–22], which employ ungated (non-synchronized) time-integrated detection. 3.1.3. Matrix effects The effect of the sample matrix on the net signal enhancement was also studied. In Fig. 5, the net signal enhancement curves for spectral lines of Cu, Au, Cr and C in a pure form and in a different matrix (alloy or polimer) can be compared. In this experiment, the net enhancement values for six laser pulses were found to be ca. 13.3 and 6.5 for the Au (I) line, 341 and 171 for the Cu (I) line and 7.4 and 7.2 for the Cu (II) line, 2.2 and 1.0 for the C (I) line and 6.8 and 4.3 for the Cr (I) line, in the pure and the alloy matrix, respectively. It can be stated that the trend of the curves is similar for any given spectral line in the two matrices, what suggests that the transitions and the nature of species have more influence on the enhancement than the matrix. The relative net signal enhancement that can be realized is different for the same line in different matrices, but seems to be consistently lower in the more complex matrix. The signal enhancement also shows a small variation for the different species within the same matrix. Obviously, further, more extensive experiments with many matrices will need to be performed before a general conclusion can be drawn with respect to the matrix dependence of the signal enhancement. 3.2. Signal repeatability The repeatability of the signal is an important analytical figure of merit, as it directly influences the limit of detection and the precision of analysis. LIBS is primarily used as a microanalytical solid sampling technique, hence it suffers from relatively poor repeatability – as do other similar techniques such as SS-ETAAS or LA-ICP-MS. In the case of LIBS this is the result of both instrumental (e.g. shot-to-shot fluctuation
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of laser pulse energy, perturbations of plasma characteristics) and sample related effects (e.g. sample micro-heterogeneity). As a consequence, typical SP-LIBS signal repeatability values (RSD%) are consistently reported to be about 15–25% (e.g. [28–30]), when perfoming a small number of repetitions (10–50). These values can only be generally improved either by using a large number (e.g. 500–2500) of spectra for averaging [28] or by employing some signal normalization method [31,32]. A factor of a 2–3 improvement in precision was also reported for DP-LIBS (e.g.[14,33]). Our observation with collinear MP-LIBS using time- and spatially integrated signals that the signal repeatability largely improves as the number of pulses in a burst increases. As it can be seen in Fig. 6. for various spectral lines in different metallic matrices, the relative standard deviation on net line intensities steadily decreases as a function of number of pulses; starting from tens of percents it drops down to 5–10%. We would like to emphasize that these RSD% values were obtained with a relatively small number of laser bursts, and in such a way that the total number of pulses (number of repetitions× number of pulses in the burst) delivered was kept constant, as described in detail in the experimental section. This way the effect of total number of pulses on the repeatability data could be eliminated. No signal normalization method or extensive averaging was performed. We would like to note that the obtained RSD% values are comparable or better than the values reported for DP-LIBS for 50 or more shots [14,15,33]. We reported about similiar repeatability values earlier also for other lines and matrices [23–25]. As to explain this large improvement in the repetability (precision), the relevance of the use of time integrated detection in our setup has to be emphasized. The integration time extends over the entire useful lifetime of the plasma, as opposed to typical time resolved LIBS setups where the spectral integration time is on the order of microseconds. Additionally, as described in the preceeding section, the signal-to-background ratio also significantly increases under the MP-LIBS conditions used here. The improvement in signal repeatability is therefore a combination of these effects which significantly improve the signal statistics. 3.3. Calibration curves In Fig. 7, calibration plots obtained for some neutral lines of Cu, Zn, Cr and Sn in some of their alloys can be seen. For comparison, single-pulse and six-pulse curves are both plotted. (For the Zn line, single pulse emission was too weak for reliable calibration, hence the double-pulse plot is shown.) It is apparent that the linear dynamic range of the sextuple-pulse plots is much wider. In SP-LIBS, the upper concentration limit for the linearity of calibration curves for strong neutral lines is usually around a few percents. Although the availability of suitable calibration samples restricted the range of concentrations for which we could record the calibration plots, it can be seen that our six-pulse plots can be well fitted with a straight line for up to several tens of percent concentration, perhaps even higher. We would also like to mention that the spectral lines used in Fig. 7 are relatively strong neutral lines, with transition probabilities on the order of 107 per second and excitation energies of only a few electronvolts. Considering that the literature primarily attributes the saturation characteristics of LIBS calibration plots to self absorption ([e.g. 34,35]), our data suggest that self-absorption is advantegously suppressed in MP-LIBS. The observed reduced-level self-absorption in the plasma can be probably accounted for by the lower number density of emitters in the plasma, caused by the fast expansion rate of double- and multi-pulse LIB plasmas, mainly caused by the pressure drop behind the shockwave front. This reasoning is supported by the fact that some SP-LIBS studies
also reported about significantly extended linear dynamic ranges in reduced pressure atmospheres [36,37]. Pressure conditions also influence collisional excitation and deexcitation processes, thus if only the „dilution” of emitters is considered, and with a light collection setup employing spatial resolution, it could even result in decreased light emission signals. However, due to the spatial- and time-integration employed in our multi-pulse experiments, which collects emitted light from practically over the whole plasma volume and lifetime, the integrated emission signal does not show a decrease with respect to the single-pulse case. In fact, as described in earlier sections of the present study, we consistently observed no signal decrease. In most cases, a significant signal increase was observed as a function of the number of pulses in a laser burst. In quantitative analytical applications, this signal enhancement directly transforms into a sensitivity increase (increase of the slope of calibration plots).
3.4. Quantitative figures of merit Based on the data shown above, it can be expected that detection capabilites improve significantly with the use of multiple pulses in LIBS. As a combined effect of the decreasing standard deviation (improving repeatability) and increasing sensitivity, limits of detection (LOD) calculated by the IUPAC three-sigma definition indeed improve significantly, not only over the single-pulse but also over double-pulse LIBS results. In order to illustrate the latter improvement, in Table 2 we listed both the two-pulse and six-pulse LOD values found for the data in Fig. 7. Limits of quantitation (six-sigma definition) are also shown. As can be seen, the six-pulse MP-LIBS figures of merits show a 4.2–16.7 times improvement over the DPLIBS values. We would also like to point out to the fact that this improvement was realized by using only a small number of laser shots (4 shots consisting of 6 laser pulses each). In quantitative bulk analytical applications, one typically uses average signals originating from a large number of laser shots delivered to a translating or rotating sample, thus attempting to reduce signal scatter even further. Lower limits of detection can also be achieved by using higher laser pulse energies and/or a CCD detector of higher sensitivity. Therefore, further improvements in the absolute quantitative figures of merit can potentially be realized in some MP-LIBS applications.
4. Conclusions We have shown that the analytical performance of LIBS can be significantly improved if, instead of single or double laser pulses, bursts of multiple collinear laser pulses are used for the plasma generation. This improvement was documented for twenty spectral lines of a total of eleven elements. Data presented suggest that the use of spatial and time integration of the plasma emission helps to realize not only enhanced sensitivities but also improved repeatabilities. Our findings suggest that emission signal enhancement is present even when the laser interpulse delay is on the order of 20–40 μs. Although our present experimental setup does not allow the direct control of the interpulse delay, and the number of pulses in the burst can also not be increased further than 6, but it can be assumed that these two parameters have a strong influence on the analytical performance of MP-LIBS. Our future research efforts will be concentrating on constructing a new setup that would allow us testing the effect of these parameters.
Fig. 5. Comparion of the net (●) and energy normalized net (○) emission intensities of the same spectral lines in two different sample matrices (in pure elemental form on the left and in an alloy/compound on the right) as a function of the number of laser pulses in a burst. Error bars indicate standard deviations based on using a comparable total number of laser pulses, as described in the experimental section.
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N. Jedlinszki, G. Galbács / Microchemical Journal 97 (2011) 255–263 Table 2 Comparison of the limits of detection for single-pulse and sextuple-pulse LIBS based on the three-sigma formula and calibration data shown in Fig. 7. For the Zn line, single pulse emission was too weak for reliable calibration, hence the double-pulse plot is shown. Species
λ (nm)
LOD (mg/g) 2 pulses
6 pulses
2 pulses
6 pulses
Cu I Zn I Cr I Sn I
510.5 481.0 520.8 452.5
34.69 6.94 5.19 20.74
2.65 0.51 0.31 4.87
69.37 13.87 10.37 41.49
5.30 1.02 0.62 9.75
LOQ (mg/g)
Acknowledgements The authors cordially thank Dr. József Pallósi (Dunaferr Rt., Hungary) and László Túri (Festék Bázis Kft., Hungary) for helping our research by providing us with the certified steel and gold alloy samples, respectively. The authors kindly acknowledge the financial funding received under No. TAMOP-4.2.1/B-09/1/KONV-2010-0005. References
Fig. 6. Relative standard deviation (repetability) for some spectral lines as a function of the number of laser pulses in a burst. All data were calculated for the same total number of laser pulses delivered, as described in the experimental section.
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