Anal Bioanal Chem (2006) 385: 316–325DOI 10.1007/s00216-006-0363-z

SPECIAL ISSUE PAPER

Marwa A. Ismail . Gabriele Cristoforetti .Stefano Legnaioli . Lorenzo Pardini .Vincenzo Palleschi . Azenio Salvetti .Elisabetta Tognoni . Mohamed A. Harith

Comparison of detection limits, for two metallic matrices,of laser-induced breakdown spectroscopy in the singleand double-pulse configurationsReceived: 31 October 2005 / Revised: 6 February 2006 / Accepted: 13 February 2006 / Published online: 11 March 2006# Springer-Verlag 2006

Abstract Limits of detection have been studied for severalelements in aluminium and steel alloys, at atmosphericpressure in air, by use of the single and collinear double-pulse configurations of laser-induced breakdown spectros-copy. For this purpose, calibration plots were constructedfor Mg, Al, Si, Ti, Cr, Mn, Fe, Ni, and Cu using a set ofcertified aluminium alloy samples and a set of certifiedsteel samples. The investigation included optimization ofthe experimental conditions to furnish the best signal-to-noise ratio. Inter-pulse delay, gate width, and acquisitiondelay were studied. The detection limits for the elements ofinterest were calculated under the optimum conditions forthe double-pulse configuration and compared with thoseobtained under the optimum conditions for single-pulseconfiguration. Significantly improved detection limits wereachieved, for all the elements investigated, and in bothaluminium and steel, by use of the double-pulse config-uration. The experimental findings are discussed in termsof the measured plasma conditions (particle and electrondensity, and temperature).

Keywords Laser-induced breakdown spectroscopy .Double-pulse . AES . UV–visible . Metals/Heavy Metals

Introduction

Detection limits in laser-induced breakdown spectroscopy(LIBS) are typically poorer than those obtained by otherelemental analysis techniques [1] and the LIBS signal for aspecific element, even for fixed experimental conditions, maydepend on the general composition of the sample investigated[2]. In a previous investigation based on single-pulse LIBS,large differences were observed between detection limits forMg, Mn, and Cu, depending on the kind of the alloy(aluminium or steel) [3]. In this work we have extended theinvestigation to the double-pulse LIBS configuration, tocheck whether detection limits can be improved by use of thisconfiguration rather than single-pulse.

The analytical benefits of using double-pulse irradiationin LIBS have been reported elsewhere [4, 5]. Since then,many efforts have been devoted to characterization of themechanisms of the observed signal enhancement. Differentdouble-pulse approaches have been proposed, rangingfrom collinear [5–13] to orthogonal geometry [4, 14–19],from infrared [4–6, 8–16] to visible and ultraviolet laserwavelengths [7, 17], and from nanosecond [4–18] tofemtosecond pulse duration [19, 20]. In these differentmodes, a large variety of results has been obtained.Intensity enhancements (compared with delivery of thesame total energy in single-pulse mode) reported in theliterature range from a factor of two [5, 13] to a factor of30–40 [7, 14–16]. The relative standard deviation of thesignal has been found to be improved by use of the double-pulse configuration [5, 6]. Comparison of plasma temper-ature and electron density measured in the double andsingle-pulse configurations has provided different resultswhich depend on the measurement conditions [5, 6, 8, 11,16, 18]. The change in mass ablation has also beenevaluated by several groups [5, 7, 8, 14, 21].

As far as we are aware, claims of signal enhancementhave usually been based on comparison of the line intensitymeasured after single and double-pulse irradiation, underthe same conditions of acquisition gate delay and width.It has, however, been demonstrated that time and

M. A. Ismail . M. A. HarithNational Institute of Laser Enhanced Sciences (NILES),Cairo University,Giza, Egypt

G. Cristoforetti . S. Legnaioli . L. Pardini . V. Palleschi .A. Salvetti . E. Tognoni (*)Applied Laser Spectroscopy Laboratory,Institute for Chemical-Physical Processes of CNR,Via G. Moruzzi 1,56124 Pisa, Italye-mail: tognoni@ipcf.cnr.itFax: +39-050-3152230

space evolution of the plasma plume produced by double-pulse irradiation are very different from those in thesingle-pulse configuration [5, 8, 11, 13]. Acquisition ofdouble-pulse spectra under conditions optimized for single-pulse spectra may therefore lead to underestimation of thepotential of the double-pulse configuration. Equally, com-parison of single and double-pulse spectra obtained underconditions optimized for the double-pulse configurationmay lead to overestimation of the benefits achievable [13].

For these reasons a systematic investigation has beendevised to determine the dependence of the signal-to-noiseratio (SNR) on acquisition conditions in single and double-pulse configurations. Measurements on certified sampleswere then performed under the conditions found to providethe best analytical performance in both single and double-pulse configurations. Finally, the results are discussed interms of the measured plasma conditions.

Experimental

Instrumentation

The Modì instrument, a mobile LIBS system developed inthe Applied Laser Spectroscopy Laboratory, Pisa, Italy,was used to perform the measurements. The schematicarrangement of the equipment is depicted in Fig. 1. Theinstrument integrates a dual-pulse Nd:YAG laser, operatingat the fundamental wavelength, in which a single flashlamp simultaneously pumps two active rods. The laser canemit two collinear pulses of variable energy between 20and 120 mJ per pulse at a maximum repetition rate of 10 Hzand with an inter-pulse delay which can be set from 0 to60 μs. The pulse width is 12–13 ns for both channels,irrespective of the applied inter-pulse delay, at least in therange explored in this work (0 to 5 μs). A 100 mm-focallength lens was used to focus the beam on the samplesurface. A translation stage enabled fine positioning of thesample relative to the focus of the lens, and a video cameraprovided a magnified image of the irradiated area. With theobjective of achieving the best possible reproducibility, the

lens-to-sample distance was tuned to approximately 95 mm[22]. The spatially integrated LIBS signal was collectedthrough an optical fiber (3 cm distance, 45°-tilted relativeto the plane of the sample surface, N.A.=0.22) and sent to acompact echelle spectrometer coupled to an intensifiedCCD camera for spectral acquisition. The Δλ/λ spectralresolution, constant over the available spectral range of2000–9000 Å, was approximately 1/5000. The ICCDadjustable settings include the integration time width andthe delay of the acquisition relative to the laser pulses. TheLIBS spectra, after acquisition and storage, were analyzedby use of proprietary software (LIBS++).

For these measurements the pulse energy was set to30 mJ per pulse with 1 Hz repetition rate. The pulsestability at the chosen energy level was approximately 5%,after discarding the first five shots of the sequence, used forpreparation and not for analytical purposes. As aconsequence of the loose focusing, irradiance on thesample was approximately 3 GW cm−2.

Samples and procedures

A set of aluminium alloy certified samples and a set of steelalloy certified samples were used to construct calibrationplots for Mg, Al, Si, Ti, Cr, Mn, Fe, Ni, and Cu. Tables 1and 2 list the certified concentrations of the elements underinvestigation in the sets of aluminium and steel samples,respectively.

Before the calibration measurements, an investigationwas performed to find the experimental conditions leadingto the best SNR (on average over the different lines ofinterest, as explained below) for both the single anddouble-pulse configurations. Certified aluminium sampleD and certified steel sample R were used in this preliminaryinvestigation. Samples with rather high analyte concentra-tions were chosen to this purpose to enable measurement ofa detectable signal even in the single-pulse configurationwith the expected less favorable timing settings (short gate,long delay). Usually a more complete and reliable proce-dure may be envisaged for choice of the optimum

Double beam Nd-YAG Laser

PC

2100 2120 2140 2160 2180 2200 2220

0

5x102

1x103

2x103

Inte

nsity

(a.

u.)

W avelength (A

Scope

Focusinglens

Sample

ICCD Echelle

Fiber holder

Translationstage

Beam splitter

Mouvable mirror

VIDEO CAMERA

Fig. 1 Schematic diagram ofthe experimental arrangementused for measurement

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experimental conditions [23]. The acquisition gate widthand delay were varied among the values 100, 500, 1000,and 5000 ns and 500, 1000, 2000, 3000, and 5000 ns,respectively; the inter-pulse separation was varied amongthe values 0 (single-pulse), 1000, 2000, 3000, and 5000 ns.Each spectrum was obtained by accumulation of ten shots(or bursts) after five cleaning shots on a fresh area of thesurface. Because of the availability of a broad-bandspectrum, as provided by the echelle-CCD detectionequipment, several spectral lines from the elements ofinterest were considered for evaluation of the SNR underthe conditions investigated. For each element, lines werechosen for which spectral interference from other alloycomponents was minimal; the lines investigated for thealuminium and steel samples are listed in Table 3; spec-troscopic data were extracted from NIST database [24].The lines used to evaluate the detection limits were thenchosen from among those listed in Table 3, on the basis oftheir signal-to-noise ratio. In general, lines among the mostintense for each element were chosen, to guaranteesatisfactory detection limits, disregarding possible non-linearity of the calibration curves at high concentrations.

To calculate the signal-to-noise ratio, the peak intensitywas determined for each line in Table 3 after a best fittingprocedure to separate the contributions from the continuumand from possible overlapping lines. The analyticalfunction used for fitting was a Voigt function. The noisewas determined as the standard deviation of the continuumbaseline calculated for a spectral interval contiguous withthe line and free from discrete emissions.

For the aluminium alloy the best SNR values werealways obtained at the longest gate of 5000 ns. Thisevidence enables the gate width to be omitted as a variable

setting; the dependence of the signal-to-noise ratio on thetwo remaining settings (acquisition delay and inter-pulsedelay) can then be easily displayed as a 2D graph. For easeof representation, the results obtained from all the lineshave been merged in two maps showing the average trendof the signal-to-noise ratio for neutral and ionized lines(Fig. 2a,b respectively). Because the SNR values of thedifferent lines spanned different ranges (for example 2–37for Cr 4254.33, 12–195 for Mn 2949.20), the two-dimensional maps corresponding to the different lineswere previously normalized and then summed; specifically,the average SNR value for each map was set to unity and allthe values of the SNR for the same line were rescaledaccordingly. The final signal-to-noise ratio values shown inFig. 2 should therefore be regarded as being given inarbitrary units.

From these two maps it is evident that the signal-to-noiseratio is greatly improved by use of double-pulse rather thansingle-pulse irradiation of the same total energy (inter-pulse delay=0 in Fig. 2). Moreover, whereas neutral linesseem to benefit from long acquisition delays, for any valueof the inter-pulse separation, for intermediate values of theacquisition delay ionic lines have the best signal-to-noiseratio.

On the basis of the results shown in Fig. 2a,b theconditions listed in Table 4 were chosen for subsequentmeasurements on the set of aluminium samples. Analysisof the steel spectra, not shown here, led to similar results(Table 4).

Table 1 Certified mass concen-tration (%) of the elementsunder investigation in the alu-minium samples used duringthis work

The aluminium concentration isalso provided for reference

Element Sample

A B C D E F

Mg – 0.1 0.31 0.329 0.05 0.016Al 92.85 97.775 86.98 84.42 87.17 98.979Ti 0.031 0.025 0.108 0.114 0.1575 –Cr 0.022 0.01 – 0.032 0.039 –Mn 0.37 1.46 0.8 0.19 0.331 0.025Fe 0.58 0.2 0.5 0.9 0.47 0.58Ni 0.0205 0.05 0.21 0.356 0.0572 –Cu 5.05 0.05 9.45 2.93 0.0256 0.017

Table 2 Certified mass concen-tration (%) of the elementsunder investigation in the steelsamples used during this work

The iron concentration is alsoprovided for reference

Element Sample

P Q R S T

Al 0.2 0.17 0.05 0.01 0.04Si 1 0.8 0.17 0.29 0.01Ti 0.34 0.11 0.04 0.01 0.01Cr 1.3 3 0.19 1.2 0.01Mn 1.5 0.4 1.2 0.44 0.14Fe 92.636 92.44 93.247 95.723 99.67Ni 1.1 0.1 3.4 1.5 0.02Cu 0.39 0.11 0.47 0.15 0.02

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Results

Measurements on the certified samples were performed inaccordance with the procedure already followed foroptimization. Each spectrum was acquired by irradiatinga new area and accumulating ten laser shots (or burst) afterfive cleaning shots. Five spectra were acquired for eachcombination of settings listed in Table 4, to evaluate thereproducibility of the measurement. The intensity of thespectral lines chosen for the construction of the calibrationplots was evaluated by means of best fitting using a Voigtfunction. Detection limits were calculated according to the3s IUPAC criterion:

LOD ¼ 3sBS

(1)

where sB is the standard deviation of the continuumbackground (called “noise”, above) and S is the slope of thecalibration plot at the lowest measured concentration.

General enhancement of the signal intensity wasobserved in double-pulse spectra compared with single-pulse spectra obtained at the same total energy. Thisresulted in steeper slopes of the calibration plots, as shownin the example in Fig. 3.

Although noise was no lower in double-pulse spectra(rather, it increased slightly compared with single-pulsespectra), the balance between strong line enhancement andthe weakly increased noise resulted in general improve-ment of the detection limits by approximately one order ofmagnitude in double-pulse experiments for the elementscontained in the aluminium alloys (Table 5). The double-pulse configuration also resulted in improved LOD inanalysis of the steel alloys, although the magnitude of theimprovement was less than for aluminium.

It should be borne in mind, however, that the resultsobtained here are specific to the experimental equipment

Table 3 Wavelength and spectral characteristics of the lines investigated in this study for both aluminium and steel alloys

Wavelength (Å) Element Ionization Ej (eV) Ei (eV) gj gi Aij(108 s−1) Al alloy Steel alloy

2524.11 Si I 0.010 4.920 3 1 1.818 X2599.40 Fe II 0.000 4.768 10 10 2.2 X2852.13 Mg I 0.000 4.346 1 3 4.95 X2881.58 Si I 0.781 5.082 5 3 1.894 X2933.06 Mn II 1.174 5.400 5 3 2 X2949.20 Mn II 1.174 5.377 5 7 1.9 X X3088.02 Ti II 0.049 4.062 10 8 1.25 X3247.54 Cu I 0.000 3.817 2 4 1.37 X X3341.88 Ti I 0.000 3.709 5 7 0.65 X3349.41 Ti II 0.049 3.749 10 12 1.33 X X3414.76 Ni I 0.025 3.655 7 9 0.55 X X3515.05 Ni I 0.109 3.635 5 7 0.42 X3719.93 Fe I 0.000 3.332 9 11 0.162 X3961.52 Al I 0.014 3.143 4 2 0.98 X4030.76 Mn I 0.000 3.075 6 8 0.174 X X4254.33 Cr I 0.000 2.913 7 9 0.315 X X4823.52 Mn I 2.319 4.889 10 8 0.499 X

a

0 1 2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s) 0.32

0.64

0.97

1.29

1.61

1.94

SNR (a.u.)

b

0 1 2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

0.2

0.5

0.8

1.1

1.4

1.7

SNR (a.u.)

Fig. 2 Two-dimensional maps showing the dependence of thesignal-to-noise ratio on inter-pulse delay (x-axis) and acquisitiondelay (y-axis), obtained by averaging the results corresponding tothe neutral (a) and ionized (b) lines investigated for the aluminium-based alloy. The acquisition gate was fixed at 5000 ns. The averagesignal-to-noise values were normalized to unity

319

and conditions adopted during our measurements. LODbetter than those in Table 5 have previously been obtainedby other groups working with multiple pulses and differentexperimental setups (for example: measurements per-formed in argon buffer gas, with a Paschen–Rungespectrometer and with pulse energies of the order of200 mJ) [25, 26]. The main objective of this work,

however, was not achievement of the best possible LODbut evaluation of the effect of delayed-pulse irradiation atthe same total energy value.

Under our conditions, double-pulse seems to be moreefficient at improving detection limits for aluminium alloysthan for steel. On the basis of the definition of detectionlimit given in Eq. 1, we can try to find the source of thedifferent improvement. One possible explanation can beidentified in the large number of weak lines emitted by themajor element (iron) in the steel matrix—in double-pulsespectra, especially, the crowding of these lines may beresponsible for overestimation of the noise and, therefore,of the detection limits.

If, on the other hand, we compare the line-intensityenhancements (and thus the slope increase) obtained by useof double-pulse irradiation for the two matrices, we obtaina factor of 4–5 for steel and a factor of 20–40 foraluminium. Therefore, not only possible overestimation ofthe noise, as mentioned above, but also less effectiveenhancement of the signal itself seem to contribute to thereduced improvement of the detection limits for steelcompared with aluminium alloys.

Close inspection of the results listed in Table 5 revealsthat for several elements (Cr, Cu, Ni) the same emissionlines gave the best LOD for both matrices whereas for otherelements different lines were chosen for the different

Table 4 Summary of the irra-diation and acquisition settingschosen for measurement afteroptimization of the SNR

Alloy Irradiation Emittingspecies

Optimum inter-pulsedelay (μs)

Optimum acquisitiondelay (ns)

Optimum gatewidth (ns)

Aluminium Single Neutral 5000 5000Ionic 3000 5000

Double Neutral 2 5000 5000Ionic 1 2000 5000

Steel Single Neutral 5000 5000Ionic 3000 5000

Double Neutral 2 5000 5000Ionic 3 2000 1000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

4000

8000

12000

16000 single pulse double pulse

Inte

nsity

(a.

u.)

Mn mass concentration (percent)

Fig. 3 Calibration plots obtained for manganese in steel, using theline Mn I at 4823 Å. Error bars represent the standard deviation ofthe intensity from five replicate measurements

Table 5 Comparison of limitsof detection obtained in thesingle and double-pulse config-urations for aluminium and steelalloys

Element Wavelength (Å) Single-pulse Double-pulse

Aluminium alloysMg 2852.13 30 ppm 4 ppmTi 3349.41 100 ppm 10 ppmCr 4254.33 100 ppm 10 ppmMn 2949.20 0.1% 90 ppmFe 3719.93 400 ppm 50 ppmNi 3414.76 600 ppm 100 ppmCu 3247.54 150 ppm 80 ppmSteel alloysAl 3961.52 30 ppm 20 ppmSi 2881.58 100 ppm 40 ppmTi 3088.02 50 ppm 25 ppmCr 4254.33 70 ppm 50 ppmMn 4823.52 300 ppm 120 ppmNi 3414.76 100 ppm 40 ppmCu 3247.54 25 ppm 5 ppm

320

matrices (Mn 2949.20, Ti 3349.41 for aluminium; Mn4823.52, Ti 3088.02 for steel). If the most importantcriteria (lack of interference, high relative intensity) arefulfilled by several lines of the same element, the choicebetween neutral or ionized lines may still be open to theanalyst. The dependence of the choice on the experimentalconditions is discussed in the next section.

Discussion

To explain the results described in the previous section, westarted with analysis of the spectra collected during theoptimization step. In particular, the plasma temperature andelectron density were calculated from these spectra. Theelectron density was evaluated from the Stark broadeningof the Hα line at 6562.85 Å [27], after subtraction of theinstrumental broadening. Even if not included in the alloycomponents, hydrogen is present in the LIBS plasmaproduced in air, because of the natural humidity. Theelectron temperature was calculated, assuming LTE (seebelow), using the Saha–Boltzmann plot method [28]. Forthe aluminium alloy the Saha–Boltzmann plot wasconstructed using several optically thin aluminium emis-sion lines: Al I at 3050.07, 3054.68, 3057.14, 3064.29,3066.14 Å; Al II at 3586.56, 4663.15 Å. For steel, thefollowing non-resonant lines from iron were used: Fe II at2598.37, 2611.87, 2613.82, 2739.55 Å, Fe I at 3815.84,3820.42, 4071.73, 4260.47, 4325.76, 4404.75, 4918.99,4920.50, 5383.37 Å. Relevant spectroscopic data wereobtained from NIST database [24]. Line intensities werecorrected for the combined spectral efficiency of thecollection–detection system.

The resulting plasma temperature and electron densitycalculated from the spectra of aluminium alloy D areshown in Fig. 4a,b respectively. The data shown in thisfigure correspond to spectra acquired with an integrationgate of 500 ns.

Inspection of Fig. 4 reveals that double-pulse irradiationleads to quite different plasma conditions than single-pulseirradiation. The plasma temperature is similar at thebeginning (taking into consideration uncertainty ofapproximately 3%) then decreases less rapidly. Theelectron density, in contrast, is lower in double-pulsespectra at the beginning but subsequent evolution leads tovalues equal to those of the single-pulse spectra. Un-certainty in the electron density may be evaluated as beingapproximately 10%. Similar behavior was observed forsteel sample R (Fig. 5). The differences between theproperties of single and double-pulse plasmas for a givendelay time are in agreement with those observed inprevious studies [5, 10] that investigated multiple pulseirradiation of steel samples. The main differences betweenFigs. 4 (aluminium alloy) and 5 (steel alloy) are the lowervalues of both temperature and electron density for thelatter.

Fulfillment of the necessary criterion for validity of theassumption of the LTE condition [29]:

ne � 1:4 � 1014T1=2ΔE3 (2)

(whereΔE, in eV, is the highest energy transition for whichthe condition holds, T, in eV, is the plasma temperature, andne, in cm−3, is the electron density) was checked aposteriori on the basis of the measured values of temper-ature and electronic density as reported in the maps. Thethreshold value for electron density was found to be1016 cm−3, therefore the LTE can be regarded as areasonable approximation under our conditions.

For a given temperature, T, and electron density, ne, theintensity of a spectral line of known spectral properties,under optically thin conditions, may be expressed as [30]:

Izij T ; neð Þ ¼ Fdetnz T ; neð ÞAij

gie�Ei=kBT

Uz Tð Þ (3)

where Izij is the measured intensity of the line (counts), nz is

the number density of the species of ionization state z in theplasma (cm−3), gi and Ei are, respectively, the statistical

a

3 4

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

0.85

0.93

1.0

1.1

1.2

1.3

T (eV)

b

0 1

0 1 2

2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s) 0.40

0.65

1.1

1.7

2.8

4.6

ne/1017 (cm )-3

5

Fig. 4 Two-dimensional maps of the dependence of plasmatemperature (a) and electron density (b) on inter-pulse delay (x-axis) and acquisition delay (y-axis) for aluminium alloy sample D

321

weight and excitation energy of the upper level of thetransition, Aij (s

−1) is the transition probability, kB is theBoltzmann constant, Uz(T) is the internal partition functionof the species at temperature T and Fdet is an experimentalfactor taking into account the detection efficiency and thegeometry of the collection.

The number density of the species considered (neutral orsingly ionized, in a typical LIBS plasma) depends on thetotal particle number density of the plasma and on theconcentration of the corresponding element. In addition tothis dependence, the number density of a species depends,in turn, on the temperature and electron density accordingto the equilibrium condition [29]:

nenII

nI¼ 2πmekBTð Þ3=2

h32UII Tð ÞUI Tð Þ e�

EionkBT (4)

where superscripts refer to the ionization state (I neutral, IIionized), me is the electron mass, h is the Planck constantand Eion is the first ionization potential of the element underinvestigation.

This discussion reveals clearly that choice of theoptimum line of a given element to be used for constructionof the calibration plot, under the constraints already stated,strongly depends on the temperature and electron density

of the laser-induced plasma under the conditions of themeasurement. In fact, assuming optically thin conditions,the ratio of the intensity of a neutral line to that of an ionicline for a given element is given by:

I Iij T ; neð ÞI IIij T ; neð Þ ¼

nI T ; neð ÞAIijgIie�EI

i=kBT

UI Tð Þ

nII T ; neð ÞAIIij

gIIie�EII

i =kBT

UII Tð Þ

(5)

where I zij is the number of photons emitted per unit volumeand the other symbols have the same meaning as definedabove. Substituting the ratio nI/nII from Eq. 4 we obtain:

I Iij T ; neð ÞI IIij T ; neð Þ

¼ ne2

AIijg

Ii

AIIij g

IIi

h3

2πmekBTð Þ3=2 e� EI

i�EIIi �Eionð Þ

�kBT (6)

where the dependence of the intensity ratio on ne and T ismade explicit. It is worth mentioning that Eq. 6 is validirrespective of the matrix, because the dependence of theintensity on the sample matrix is included in the depen-dence on the temperature and electron density. It should benoted, however, that Eq. 6 applies to the ratio of intensitiesemitted by the plasma source; the actual ratio found bydirect measurement can be different, because of thewavelength-dependence of the spectral response of thecollection/detection systems.

To evaluate the variability of the intensity ratio expectedunder a range of different experimental conditions, wecalculated, by use of Eq. 6, the ratio of intensity of the Mn Iline at 4030 Å to that of the Mn II line at 2949 Å on a gridof temperature and electron density values representing therange of conditions found during the optimization proce-

a

0 1 2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

0.70

0.78

0.87

0.95

1.0

1.1

T (eV)

b

0 1 2 3 4

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

0.20

0.70

1.2

1.7

2.2

2.7

ne/1017 (cm

-3)

5

Fig. 5 Two-dimensional maps of the dependence of plasmatemperature (a) and electron density (b) on inter-pulse delay(x-axis) and acquisition delay (y-axis) for steel alloy R

0.7 0.8 0.9 1.0 1.1 1.2

1x1017

2x1017

3x1017

4x1017

5x1017

T (eV)

Ele

ctro

n d

en

sity

(cm

-3) 5E-3

5E-2

5E-1

5E0

5E1

5E2

Ipredicted

4030/Ipredicted

2949

Fig. 6 Ratio of the predicted intensity of the neutral 4030 Mn lineto that of the ionized 2949 Mn line under different conditions oftemperature and electron density representative of LIBS spectraobtained under our experimental conditions

322

dure for both aluminium and steel. The results are shown inFig. 6.

It may be noticed that the expected intensity ratio of thetwo lines spans five orders of magnitude under theexperimental conditions found during our optimizationprocedure. In particular, under the experimental conditionschosen as the working point for the calibration plots foraluminium, the neutral line intensity is comparable withthat of the ionic line (expected ratio values are 1.7 under theconditions optimized for neutral lines and 0.5 under theconditions optimized for ionic lines). For steel, however,because of the slightly lower temperature and electrondensity conditions at the chosen working point, theequilibrium is displaced toward the neutral species.Expected ratio values are 16 under the conditionsoptimized for neutral lines and 2.3 under those optimizedfor ionic lines. It is, therefore, easy to recognize that for thesteel matrix the best LOD for Mn may be obtained by usinga neutral line whereas for the aluminium matrix the same istrue for an ionic line, as reported in Table 5.

The expression for line intensity (Eq. 3) may also beuseful in investigation of the causes of the differentenhancements obtained by using the double-pulse config-uration for aluminium and steel. Eq. 3 enables calculationof the dependence on temperature and electron densityof the expected relative intensity values for a specificspectral line, assuming constant total plasma density andvolume and element concentration (i.e. Fdet � nplasma�celement ¼ const: ). The latter hypothesis corresponds tothe assumption of stoichiometric ablation and homoge-neous distribution of the elements inside the plasma, whichare the basis of the measurement method itself. The formerhypothesis of constant plasma density and volume isformulated per absurdum, and its validity will be checkedafter comparison of predictions with experimental results.Thus, the map of expected relative intensity was calculated,under the temperature and electron density conditionsshown in Fig. 4 for aluminium alloy D, for the neutral Mn4030 line (Fig. 7a) and the ionized Mn 2949 line (Fig. 8a).The results of the prediction were then compared with themaps of experimentally measured intensity, shown inFigs. 7b and 8b, respectively.

It is evident from Figs. 7 and 8 that the predicted andexperimental behavior are different and that both changefrom the neutral line to the ionized line. This latterdifference is mainly because of changes of the relativeabundances of the neutral and ionized species, which isvery sensitive to the actual combination of temperature andelectron density in the plasma, as discussed above.

The difference between prediction and observation, onthe other hand, suggests that the assumption of constantplasma density and volume for both single and double-pulse plasmas and for the different time settings isincorrect. As already mentioned, for double-pulse irradi-ation increases of the emitting volume of a factor of threehave been reported [10, 11]. On the other hand, thecollection optics enabled spatial integration of the signalemitted from a volume of more than 100 mm3, enough togather the emission from the whole double-pulse plasma.

The difference between prediction and observation can,then, be explained by changes in the plasma density andemitting volume, i.e. in the total number of emittingparticles which is proportional in turn to the ablated mass.These changes are not surprising if we take into account thelarger mass ablated in the double-pulse configurationcompared with single-pulse mode [5, 7, 8, 14, 21]. Bydividing the experimental intensity values by the corre-sponding values calculated by use of Eq. 3 we can obtainan estimate of the relative number of emitting particles(scaled by an unknown experimental factor) under thedifferent conditions investigated. The enhancement ofablation in the double-pulse configuration compared withthe single-pulse configuration can then be found bynormalization of the result to the value obtained forsingle-pulse irradiation with the same delay time. Thesecalculations were performed for the Mn 4030 and 2949lines from the aluminium and steel samples.

Despite the large uncertainty affecting this kind ofcalculation (mainly because of errors arising from temper-ature and electron density in the predicted intensity andfrom the oversimplified assumption of plasma homogene-ity), the mass ablation enhancements estimated from theneutral and ionic Mn lines were in substantial agreementfor both aluminium and steel matrices. An average

a

0 2 3 4

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

4.8

13

21

29

36

44

Ipredicted

4030 (a.u.)

b

0 1 2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s) 50

2.7E2

4.9E2

7.1E2

9.3E2

1.2E3

Iexp

4030 (a.u.)

1 5

Fig. 7 Two-dimensional maps of predicted (a) and experimental(b) intensity of the line Mn 4030, for the aluminium matrix, showingthe dependence on the inter-pulse delay (x-axis) and the acquisitiondelay (y-axis) investigated during the optimization

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enhancement of six was obtained from the two linesobtained in aluminium. The value calculated for steel,averaged on the neutral and ionized lines, was 3.

Thus use of the double-pulse configuration with thealuminium matrix led to a larger increase of ablation thanwhen used with steel (a factor of two, approximately), inagreement with the greater enhancement of the limits ofdetection observed for use of the double-pulse configura-tion with aluminium alloys than with steel alloys. Becausethe effectiveness of double-pulse irradiation in increasingmass ablation is a result of reduction of the plasmashielding effect [12, 21], it could be hypothesized on thebasis of these experimental findings (compare Figs. 4 and 5)that reduction of plasma shielding was lower for steel becauseplasma shielding itself was lower for this matrix. Moreexperimental work is required to prove the validity of thishypothesis, however.

Conclusions

LIBS in the single and double-pulse configurations hasbeen used to determine detection limits for severalelements in aluminium and steel alloys. The measurement

conditions were chosen after optimization of the signal-to-noise ratio for the lines of interest.

General enhancement of the signal intensity wasobserved for double-pulse spectra compared with single-pulse spectra obtained with the same total laser energy.Significant and appreciable improvement of the limits ofdetection was achieved in the double-pulse experiments.The extent of the improvement was larger for aluminiumalloys than for steel. These differences for aluminium andsteel alloys may be explained in terms of greater enhance-ment of the ablated mass in the double-pulse configurationcompared with the single-pulse configuration.

References

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a

0 1 2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

1.4E2

2.3E2

3.3E2

4.2E2

5.1E2

6.1E2

Ipredicted

2949 (a.u.)

b

0 1 2 3 4 5

1000

2000

3000

4000

5000

Inter-pulse delay (µs)

Acq

uisi

tion

dela

y (n

s)

1.3E2

9.3E2

1.7E3

2.5E3

3.3E3

4.1E3

Iexp

2949 (a.u.)

Fig. 8 Two-dimensional maps of predicted (a) and experimental(b) intensity of the line Mn 2949, for the aluminium matrix, showingthe dependence on the inter-pulse delay (x-axis) and the acquisitiondelay (y-axis) investigated during the optimization

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