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Comparison of Single Pulse and DoubleSimultaneous Pulse Laser InducedBreakdown SpectroscopyZuhaib Haider a , Yusof Bin Munajat a , Raja Kamarulzaman a &Nurhidayu Shahami aa Advanced Photonics Science Institute (APSI), Department ofPhysics, Faculty of Science , Universiti Teknologi Malaysia , JohorBahru , MalaysiaAccepted author version posted online: 01 Oct 2014.Publishedonline: 03 Nov 2014.

To cite this article: Zuhaib Haider , Yusof Bin Munajat , Raja Kamarulzaman & Nurhidayu Shahami(2015) Comparison of Single Pulse and Double Simultaneous Pulse Laser Induced BreakdownSpectroscopy, Analytical Letters, 48:2, 308-317, DOI: 10.1080/00032719.2014.940532

To link to this article: http://dx.doi.org/10.1080/00032719.2014.940532

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Atomic Spectroscopy

COMPARISON OF SINGLE PULSE AND DOUBLESIMULTANEOUS PULSE LASER INDUCEDBREAKDOWN SPECTROSCOPY

Zuhaib Haider, Yusof Bin Munajat, Raja Kamarulzaman, andNurhidayu ShahamiAdvanced Photonics Science Institute (APSI), Department of Physics,Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Malaysia

A simple and cost-effective variant of laser induced breakdown spectroscopy is presented that

involves a double simultaneous pulse configuration employing a single laser source. Its per-

formance is compared with conventional single pulse configuration. Double simultaneous

pulses were accomplished by splitting a Nd:YAG laser (1064 nm, 6 ns, 360mJ) beam into

two components that were focused on the sample surface to produce two concurrent break-

downs. Experiment was repeated for single pulse and double simultaneous pulses under

different ambient pressures. The performance was evaluated on the basis of self-absorption,

signal-to-noise ratio (SNR), and relative standard deviation (RSD) of the Mg II doublet

(280.2704 nm, 279.553 nm). Optically thin emission lines of better profiles with higher

signal-to-noise ratio resulted from double simultaneous pulses. The lowest relative standard

deviations obtained by single pulse and double simultaneous pulse configurations were

18.89% and 12.01%, respectively. In fact, double simultaneous pulses have performed better

than single pulse in all respects within the studied regime.

Keywords: Ambient atmosphere; Heavy metals; LIBS; Repeatability; Self-absorption; Signal to noise

ratio; Strobilanthes cripus

INTRODUCTION

Laser induced breakdown spectroscopy (LIBS) is a widely employed potentialtechnique that developed following the invention of the laser and is still widelystudied (Baudelet and Smith 2013). One of its major advantages is its ability to per-form simultaneous multi-elemental analysis (Death et al. 2011). A high power pulsedlaser atomizes and excites the sample into a plasma whose spectrum is recorded. Theemission wavelengths are characteristic of elements and provide information about

Received 28 February 2014.

Address correspondence to Zuhaib Haider, Advanced Photonics Science Institute (APSI),

Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai 81310, Johor Bahru,

Malaysia. E-mail: zuhaibhaider@gmail.com

Color versions of one or more of the figures in the article can be found online at www.tandfonline.

com/lanl.

Analytical Letters, 48: 308–317, 2015

Copyright # Taylor & Francis Group, LLC

ISSN: 0003-2719 print=1532-236X online

DOI: 10.1080/00032719.2014.940532

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the sample composition and plasma condition (Cremers and Radziemski 2013).Advantages of LIBS analytical applications are related to laser sampling and thesimplicity of the design (J. P. Singh and Thakur 2007), feasibility of producing lasersparks on solids and in gases permits the analysis of a wide range of materials frommetals and alloys to fluids and gases (Cremers, Multari, and Knight 2011) andrequirement of little or even no sample preparation (J. P. Singh and Thakur2007). Its applications cover a variety of disciplines such as concentration monitor-ing (Monfort Guy, Vanderheyden, and Tusset 2012; Palanco, Conesa, and Laserna2004), heritage processing, art-piece cleaning (Cremers and Chinni 2009; Michel2010), study of biological samples (V. K. Singh and Rai 2011), and so forth.

In spite of its many advantages, LIBS suffers from low accuracy and repeat-ability (Hou et al. 2013). There are several studies in which its accuracy and precisionare addressed (Guo et al. 2013; Axente et al. 2014) and some techniques, forexample, Double Pulse LIBS have actually contributed in improving these aspects(Harilal, Diwakar, and Hassanein 2013; Effenberger and Scott 2010; Khalil 2013).

Ambient atmosphere plays an important role in confinement, radiation emis-sion, and life span of laser induced plasma (Cremers and Radziemski 2006). Also,nature of the ambient atmosphere affects emission intensities of elements (Ma et al.2010; Cremers and Radziemski 2006). Therefore, the optimization of ambientenvironmental conditions is of key importance for accurate, stable, and repeatablemeasurements.

In this work, a simple and cost-effective configuration of double pulse LIBS isintroduced. It is a double simultaneous pulse configuration that is technically differ-ent from conventional double pulse LIBS. It involves one laser source and collectsdata from two distinct sites of the sample simultaneously. Intense emissions linesresult from larger mass evaporated into two plasmas. There would be more massablation without significant increase in plasma density. It enhances line emissionswithout increase in self absorption and the background continuum radiation(Cremers and Radziemski 2006). The performance of double simultaneous laserpulses is explored and compared with the single pulse under various ambientpressures. Self-absorption (SA), signal to noise ratio (SNR), and relative standarddeviation (RSD) are determined to assess the performance for single and doublesimultaneous pulses in similar experimental conditions.

EXPERIMENTAL

The experimental setup consists of a stainless steel sample chamber for control-ling ambient conditions, an XM100 (Beijing Anchorfree Technologies) Nd:YAGlaser (1064 nm, 360mJ, 6 ns), ocean optics HR4000 spectrometer, a personal com-puter, and optical components (for beam collimation, splitting, and focusing). Sche-matic diagram of the experimental setup is given in Figure 1. Collimated laser beamentering through the quartz window in the chamber, was subsequently split by a50T=50R fused silica beam splitter. Fifty percent of the beam was reflected verticallydownward while the remainder was transmitted and subsequently reflected down-ward by a dielectric mirror. Both of these beams were focused by a horizontallyaligned plano-convex lens into the sample surface where the distance between twospots was kept at 2mm. Sample was a pallet (4 cm dia., 0.5 cm thick) made from

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�9 grams of powdered leaves of Strobilanthes crispus. Fresh leaves were washed withde-ionized water and then dried in a furnace at �110�C for 5 h. Dried leaves werepulverized and then sieved through 75micron mesh. Pallets were made by pressingthe powder under 40 kN for 6min. The target sample was placed on an electricallycontrolled xyz-translation stage inside the sample chamber. Plasma radiations werecollected by a miniature collecting lens attached in front of a fiber optic cable(600 mm core diameter) and transferred to the spectrometer.

Experiment was repeated for SP and DSP at eight different ambient pressures(i.e., 220, 260, 330, 400, 460, 530, 590, and 1000mbar). SP was accomplished byblocking path of the transmitted beam right after the beam splitter. Hence, both con-figurations were tested in the identical optical alignments.

RESULTS AND DISCUSSION

A LIBS spectrum, an average of 10 single pulse spectra, obtained from Strobi-lanthes crispus with 180mJ of 1064 nm Nd:YAG laser is presented in Figure 2. Mostprominent lines observed in the spectrum are originated from Magnesium andCalcium. Mg II doublet (280.2704 nm, 279.553 nm) is used in further analysis.

Since quantity of the ablated mass varies with the laser energy (Haglund et al.1997) fluctuations in the laser energy cause fluctuations in intensity of the spectra. Laserenergy is constantly monitored during the experiment. A relative standard deviation(RSD) of 6.72% is observed in energy of 100 consecutive laser pulses. A part of theRSD observed in emission spectra can therefore be attributed to RSD in laser energy.

Self Absorption

It is important to check for self-absorption before selecting an emission line forcalculation of plasma parameters or calibration of the LIBS system. Photons emitted

Figure 1. Schematic diagram of LIBS experimental setup.

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by species in the plasma plume may encounter absorption on their way out (J. P.Singh and Thakur 2007). Consequently, the emission intensity reaching the detectorwill be less than that was originally emitted. It raises error in quantitativemeasurements.

A simple way to determine self absorption of emission lines is throughrelative intensities of multiplet lines (Cremers and Radziemski 2006). Theoreticalintensity ratio of a multiplet can be obtained with following formula (Hegazyet al. 2013).

I1I2

¼ A1g1k2A2g2k1

ð1Þ

Here I, A, g and k are emission intensity, transition probability, degeneracy of upperenergy level, and wavelength of the emission lines, respectively. Subscripts 1 and 2represent their association with the corresponding line.

Experimental line intensity ratios of the Mg II lines (279.553 nm, 280.2704 nm)are calculated and compared with theoretical value that is 2.03 (from Eq. 1).Self-absorption starts with affecting the most intense line first, by reducing its inten-sity (Cremers and Radziemski 2006). It leads to the deviation of experimental lineintensity ratio from its theoretical value. With increasing self-absorption the mostintense line profile becomes flat topped, further it shows a dip in the middle whichis the extreme case of self-absorption termed as ‘‘self-reversal’’ (J. P. Singh andThakur 2007). The greater the self-absorption is, the greater is the deviation of theexperimental intensity ratio from its theoretical value.

Figure 3 shows intensity ratios of the Mg II doublet [i.e., I1(279.553 nm)=I2(280.2704 nm)] against ambient pressure. It determines the optical thickness ofthe plasma or self-absorption of the emission lines. Red horizontal line representsthe theoretical intensity ratio (i.e., 2.03) of the doublet. Experimental ratios thatare equal to theoretical ratio stand for optically thin emission lines, that is, emissionlines free from self-absorption. However, the experimentally obtained ratio less

Figure 2. LIBS spectrum (avg. of 10 single pulse spectra) of Strobilanthes crispus obtained with Nd:YAG

laser and HR4000 spectrometer (Gate width: 5msec).

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than theoretical intensity ratio represents self-absorbed lines. With single pulse, it isobserved that in the pressure range 460–530 mbars, line intensity ratios are nearest totheoretical value with acceptable error bars. It signifies no self-absorption in thisparticular range of ambient pressure. With an increase, emission lines suffer fromself-absorption and intensity ratios fall below the theoretical value.

For double simultaneous laser pulses, the experimental intensity ratio hasnever decreased below the theoretical value even at atmospheric pressure. The emis-sion lines appear to be safe from the self-absorption. Double simultaneous pulses aresupposed to give rise to better emissions with self-absorption equal or less than singlepulse by generating more emitting species distributed among two plasmas.

Intensity Evolution and RSD

Figure 4 shows evolution of the Mg II doublet (i.e., 279.553 nm, 280.2704 nm)with increase in ambient pressure for single pulse and double simultaneous pulses.Each one is average of 10 spectra.

At 220mbar the intensity of emission lines is very weak with single pulse;whereas, with double simultaneous pulses, the emissions lines are in reasonableshape even at pressure 220mbar, as depicted in Figure 4b. The peaks are sharperand background level is low. It is indicative of low electron density, which causes lesscollisions and bremsstrahlung instances that are responsible for line broadening andcontinuum emission respectively (J. P. Singh and Thakur 2007; Effenberger andScott 2010; De Giacomo et al. 2010). However, when implying double simultaneouspulses, the energy imparted to the sample is doubled that causes more material toablate and provide greater number of radiating species in two plasmas. The emission

Figure 3. Intensity ratio of Mg II doublet at various pressures with SP and DSP configurations.

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lines are reasonably intense even at low pressures, which is due to the fact that twolaser pulses have excited more species into the plasma which gives rise to morecounts of intensity upon de-excitation. These results justify the assumption of moreablation and less density because of distribution of the ablated material among twoplasmas.

Evolution of emission line MgII 279.55 nm and RSD in measurement, at eachpressure, are presented in Figure 5.

At lower pressures, with SP (Figure 5a), RSD of the signal intensity is highand, hence, repeatability is low. It improves with increasing pressure and the lowestRSD of 18.90% is obtained at 590mbar that increases to 24.77% at 1000mbar. The

Figure 4. Line intensity profile of Mg II doublet obtained with (a) Single Pulse configuration (b) Double

Simultaneous pulse configuration.

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variations in RSD are similar to the variations in SD (Standard Deviation) repre-sented by error bars of line intensity ratio measurements in Figure 3. Figure 5bshows increase in Mg II line intensity and variation in the RSD of measurementswith change in pressure for the double simultaneous pulses. Significant improvementin RSD is observed. The highest RSD shown by the measurements is 39.16% at460mbar while the best is obtained at 590mbar, that is, 12.01% that increases to13.22% at 1000mbar, which is far lower than what is obtained at the same pressurewith single pulse operation.

Figure 5. Emission intensity of Mg II (279.55 nm) line and RSD in its measurement at various pressure

with (a) Single Pulse configuration (b) Double Simultaneous Pulse configuration.

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Contrary to the case in single pulse operation, the trend of RSD observed withdouble simultaneous pulses is different from the standard deviation of intensity ratioas shown in Figure 3. By considering identical fluctuations in all emission lines of thespectrum originated from one plasma, the difference of variation trends in RSD andSD can be attributed to the contribution of the two plasmas in emission intensity ofMgII 280.2704 nm line. Line intensity ratios accumulate standard deviations inintensity measurements of both 280.2704 nm and 279.553 nm emission lines. Fromthese differences it can be assumed that the contributions of two plasmas to emissionlines are different from one another due to ablation from distinct points and=orslight differences in energy of split beams.

Signal to Noise Ratio (SNR)

Figure 6 shows the variation in the signal-to-noise ratio (SNR) of the Mg II279.553 nm line as a function of ambient pressure. At low pressures, the peak inten-sity and signal-to-noise ratio are low. The signal-to-noise ratio increases from thelowest value of 3.60 at 220mbar in single pulse mode with pressure (Figure 6),reaches a maximum value of 214 at 460mbar, and decreases with further increasein pressure. As the pressure rises, the increase in collisional excitation enhancesplasma emission (Yalcin, Tsui, and Fedosejevs 2004; J. P. Singh and Thakur2007). Also, ambient pressure confines the plasma for longer time, resulting in longeremission instances. With further increase in pressure, the bremsstrahlung dominates,which contributes to the continuous background radiation or noise that conse-quently reduces overall SNR.

Figure 6. Signal-to-Noise Ratio of Mg II (279.55 nm) line at various pressures obtained with single pulse

and double simultaneous pulse configurations.

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In Figure 6, red circles represent the SNR ofMg II 279.553nm line obtained withdouble simultaneous pulses. Lowest and highest values of SNRs are always higherthan those obtain with single pulse. It starts with SNR of 129.81 at 220mbar andshows a gradual increment with pressure until 288.11 at 1000mbar. The lowestSNRwith double simultaneous pulses is even higher than themaximum SNRobtainedwith single pulse. Double simultaneous pulse configuration, hence, presents betterSNR below and at atmospheric pressures than single pulse configuration of LIBS.

CONCLUSIONS

Laser breakdown spectroscopy was performed in single pulse and doublesimultaneous pulse modes over a range of ambient air pressures using a simple con-figuration that is cost-effective and capable of better performance. Self-absorption,signal-to-noise ratio, and relative standard deviation measurements were estimatedfor the Mg II doublet obtained by the breakdown of a pellet prepared from leavesof Strobilanthes crispus. The results show that the double simultaneous pulse modeoutperforms single pulse LIBS. At lower pressures, the former provides better lineprofiles, signal-to-noise ratios, and relative standard deviations. Self-absorption ofthe emission lines was not observed with double simultaneous pulses even at atmos-pheric pressure. Hence, the double simultaneous pulse mode provides better and morestable LIBS measurements than single pulse. However, investigations continue inour laboratory on quantitative analytical capability of this technique and will bepublished in the future.

FUNDING

The authors would like to thank Ministry of Higher Education Malaysia andUniversiti Teknologi Malaysia for their financial support toward this project ERGS(R.J130000.7826.4L069).

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