Vacuum ultraviolet laser-induced breakdown spectroscopy analysis of polymers

Spectrochimica Acta Part B 64 (2009) 1128–1134

Contents lists available at ScienceDirect

Spectrochimica Acta Part B 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 / s a b

Vacuum ultraviolet laser-induced breakdown spectroscopy analysis of polymers☆ Juraj Jasik a,b, Johannes Heitz a,⁎, Johannes D. Pedarnig a, Pavel Veis b a b

Christian Doppler Laboratory Laser Assisted Diagnostics, Institute of Applied Physics, Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska dolina, 842 48 Bratislava, Slovakia

a r t i c l e

i n f o

Article history: Received 27 October 2008 Accepted 23 July 2009 Available online 3 August 2009 Keywords: Laser-induced breakdown spectroscopy LIBS Vacuum ultraviolet Polymer Trace elements

a b s t r a c t Laser-induced breakdown spectroscopy (LIBS) in the vacuum ultraviolet range (VUV, λ b 200 nm) is employed for the detection of trace elements in polyethylene (PE) that are difficult to detect in the UV/VIS range. For effective laser ablation of PE, we use a F2 laser (wavelength λ = 157 nm) with a laser pulse length of 20 ns, a pulse energy up to 50 mJ, and pulse repetition rate of 10 Hz. The optical radiation of the laserinduced plasma is measured by a VUV spectrometer with detection range down to λ = 115 nm. A gated photon-counting system is used to acquire time-resolved spectra. From LIBS measurements of certified polymer reference materials, we obtained a limit of detection (LOD) of 50 µg/g for sulphur and 215 µg/g for zinc, respectively. The VUV LIBS spectra of PE are dominated by strong emission lines of neutral and ionized carbon atoms. From time-resolved measurements of the carbon line intensities, we determine the temporal evolution of the electronic plasma temperature, Te. For this, we use Saha–Boltzmann plots with the electron density in the plasma, Ne, derived from the broadening of the hydrogen H-α line. With the parameters Te and Ne, we calculate the intensity ratio of the atomic sulphur and carbon lines at 180.7 nm and at 175.2 nm, respectively. The calculated intensity ratios are in good agreement with the experimentally measured results. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In LIBS analysis of carbon based polymer materials in the UV/VIS spectral range, typically, only the two atomic C (I) lines at 193.1 nm and 247.9 nm are detected. These two neutral lines are not enough to perform a good plasma characterization, e.g., by means of a Saha– Boltzmann plot. A sufficient number of spectral lines of minor or trace elements are normally also not available. In the spectral region from about 350 to 600 nm, the LIBS spectra of polymers are additionally dominated by molecular bands, which makes the detection of atomic emission lines difficult. For these reasons, we extended the spectral region in the current study to the VUV range (wavelength λ b 200 nm). Here we see no molecular bands and find several additional neutral and ionized spectral lines of carbon, which allow to perform a plasma characterization. For the polymer materials under investigation, we observe in the spectral range below 220 nm strong emission lines besides for carbon only from the element zinc (additionally, we see quite weak signals of hydrogen and sulphur lines). Therefore, we investigate the zinc results regarding limit of detection (LOD) and temporal development of the emission line intensity. However,

☆ This paper was presented at the 5th International Conference on Laser-Induced Breakdown Spectroscopy (LIBS 2008), held in Berlin, Adlershof, Germany, 22–26 September 2008, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +43 732 24689248; fax: +43 732 24689242. E-mail address: [email protected] (J. Heitz). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.07.013

sulphur is of much more relevance (also for other materials like steel or slag from steelwork), because it is not easily accessible in the UV/VIS range. For this reason, we investigated for sulphur not only the LOD and the temporal development, but also compared the measured results with results derived in a quasi calibration-free approach using the plasma parameter obtained from the evaluation of the carbon lines. LIBS analysis of trace concentrations of sulphur using the VUV lines at 180.7 or 182.0 nm has been previously carried out for several materials (i.e., solid or liquid steel and soil) with a LOD down to about 10 µg/g [1–3]. Bengoechea and Kennedy [4] reported on LIBS investigation in the deep VUV (spectral range of 59 to 81 nm) of certified steel samples with a sulphur content of 0.38%. Here, differently ionized sulphur lines are used for plasma characterization. Another important element for our studies is zinc. Typically, the spectral lines at about 330 or 475 nm are used for LIBS analysis of zinc contents. Also the use of the 213.9 nm line is reported, which is near to the VUV spectral range and is employed in this work. With this line a LOD of 1 µg/g is claimed for the analysis of plant materials [3]. On-line detection of heavy metals in technical polymers with LIBS was successfully demonstrated by Stepputat and Noll [5]. However, some light elements are difficult to detect in the UV/VIS/NIR range, but only few articles on LIBS of polymers in VUV region were published up to now. Kaski et al. [6] examined the limits of detection (LOD) of bromine and chlorine in pure organic solids using KrF laser excitation. They used the Br(I) 157 nm and Cl(I) 135 nm lines for detection (normalized to the C(I) 143 and 156 nm lines) with a reported LOD of

J. Jasik et al. / Spectrochimica Acta Part B 64 (2009) 1128–1134

0.011 and 0.054 for Br/C and Cl/C atomic ratios, respectively. Radivojevic et al. [7] investigated VUV LIBS detection of bromine in thermoplasts from consumer electronics. With Nd:YAG laser excitation, they obtained a LOD of 0.05% (mass to mass ratio) using the Br(I) 131 nm line normalized to the C(I) 193 nm line. Fink et al. [8] analyzed recycled thermoplasts from consumer electronics with lasers emitting at 1064 and 266 nm (measurements not performed in VUV). With UV ablation, the reproducibility was improved roughly by a factor of two for the elements antimony, zinc, tin, aluminum, chromium, and lead. For nanosecond pulsed-laser ablation the VUV radiation is advantageous over UV/VIS/IR radiation as the optical penetration depth of many materials becomes small in the VUV range [9]. Therefore, VUV lasers are a good plasma excitation source even for polymer materials with low absorption in UV/VIS range as, e.g., polyethylene (PE). In this paper, LIBS by means of a F2 (157 nm) laser is employed for the characterization of PE materials. 2. Experimental 2.1. Set-up The experimental set-up is schematically depicted in Fig. 1. A 157 nm F2-laser (Lambda Physik LPF-202) delivers pulses of about 20 ns pulse length at 10 Hz repetition rate with pulse energies up to 45 mJ (after a new gas fill). The maximum laser energies steadily decrease as the gas gets older. For longer measurement series, we therefore employ only laser energies up to 35 mJ. The laser beam is guided through a nitrogen-flushed chamber and focused onto the sample with a CaF2 lens (focal length f = 300 mm) through the CaF2 entrance window of the vacuum chamber, where the sample is situated. The experiments are performed under flushing argon atmosphere (purity 5.0) at a pressure of 2 mbar. After mounting a fresh sample, the chamber was evacuated by a turbo-molecular pump below 10− 5 mbar and then the argon flow was turned on. The two argon flushing inlets in the chamber are situated close to the entrance and exit windows to prevent pollution of the optics by ablated material. The laser energy is monitored before and after each experiment by inserting a pyroelectric detector (Ophir PE-50SH) into the laser beam-path in front of the focusing lens. The measured transmittance of the focusing lens together with the chamber entrance window is about 80%. All values of the laser pulse energy, given in this paper, are corrected by the measured transmittance. The sample is mounted onto a rotation stage in the chamber. The angle of incidence of the laser beam to the sample is 45°. The dimensions of the laser spot at the sample were 0.25 × 2 mm, yielding an irradiance of about 0.4 GW/cm2 for a pulse energy of 40 mJ (corresponding to a laser fluence of about 8 J/cm2).

1129

The space-integrated emission of the laser-induced plasma is examined by an evacuated VUV spectrometer designed by the Comenius University with a spectrometer range down to 115 nm. The plasma light is collimated to the entrance slit of the spectrometer by a MgF2 lens (f = 75 mm). A toroidal aberration-corrected diffraction grating (1200 grooves/mm, f = 20 cm) is used as dispersion and focusing device in the monochromator. The VUV light passing through the exit slit of the monochromator is converted by a scintillator (sodium salicylate) into visible light, which is detected by a photomultiplier (Hamamatsu R928). The signal from the photomultiplier is amplified by a fast pre-amplifier (Stanford Research, SR-430) and recorded with a two-channel gated photon counting system (Stanford Research, SR400). The gate delay is also used for suppression of stray light from the F2 laser. The SR-400 is synchronized with the trigger output of the laser. By scanning the grating position by a personal computer (PC) controlled stepper motor, the spectra are acquired step-by-step with accumulation over several laser shots for each wavelength point. The spectral response of the spectrometer was determined using N2 and NO glow discharges and deuterium and tungsten lamps. The instrumental function of the spectrometer was measured with spectral lines of a mercury lamp in the UV range. The instrumental function of the spectrometer has a Gaussian profile with full width of half maximum (FWHM) of 0.5 nm. The VUV spectrometer is connected to the vacuum chamber by the MgF2 focusing lens as chamber exit window. The VUV spectrometer was evacuated below 10− 4 mbar using a separate turbomolecular pump. 2.2. Investigated materials We investigate the well characterized reference PE materials (ERM-EC681k and ERM-EC680k) from the Institute for Reference Materials and Measurements (Geel, Belgium) containing trace elements with different concentrations. The declared sulphur and zinc mass contents in the reference materials are shown in Table 1. The reference material was supplied in granulate form. In order to have more experimental data points for LIBS signal calibration, we mixed the two reference materials together in different mass ratios using a cryo-milling device (Spex SamplePrep 6770 Freezer/Mill) at the temperature of liquid nitrogen. We pressed the obtained powders to pellets (diameter 13 mm, thickness 2 mm) at a temperature of about 85 °C under a pressure of 4000 kg/cm2. We obtained six mixed sample types with sulphur contents of 147, 230, 310, 380, 460 and 530 µg/g and zinc contents of 280, 450, 600, 740, 900 and 1050 µg/g, respectively. 3. Results Fig. 2 displays the LIBS spectrum of the ERM-EC681k material. In the VUV and UV range, the LIBS spectrum is dominated by several intensive carbon atomic lines, which are shown in more detail in Fig. 2b. The most intensive lines are the C(I) lines at 156.1, 165.37 and 193.1 nm. In the spectrum also the atomic S(I) lines at 180.7 and 182.0 nm and Zn(I) line at 213.9 nm can be clearly detected. A summary of observed atomic lines is given in Table 2 listed in multiplet form together with the corresponding spectroscopic constants obtained from the NIST database [10]. The strongest carbon lines are also visible in the 2nd order of

Table 1 Declared sulphur and zinc mass contents and uncertainties in reference PE materials and resulting element to carbon molar ratios. Element

Fig. 1. Experimental set-up.

S Zn

Mass fraction [µg/g]

Molar ratio

ERM-EC680k

ERM-EC681k

ERM-EC680k

ERM-EC681k

76 ± 4 137 ± 20

630 ± 40 1250 ± 70

33 × 10− 6 29 × 10− 6

277 × 10− 6 269 × 10− 6

1130

J. Jasik et al. / Spectrochimica Acta Part B 64 (2009) 1128–1134

Fig. 2. (a) LIBS spectrum of ERM-EC681k reference PE material (gate delay tdel = 350 ns, gate time tg = 20 µs) in arbitrary units (A.U.); (b) zoomed part of the spectrum in the VUV and UV range. All clearly identified lines and bands are labeled.

diffraction. The strong H-α line at 656 nm results from the hydrogen in the PE chain. Another H(I) line is visible at 486.1 nm. Except of the atomic lines, which are in the focus of the current article, a strong Table 2 List of neutral (I) and single ionized (II) atomic lines observed in LIBS spectra of ERMEC681k reference PE material in the VUV and UV range and relevant spectroscopic constants [10]. Line

C(I)

C(II) H(I) S(I) Zn(I)

Wavelength

Ei

Ek

(nm)

(eV)

(eV)

132.9 135.6 143.2 146.3 148.2 154.2 156.1 160.3 165.7 175.2 176.4 193.1 199.3 247.9 133.5 121.6 180.7 182.0 213.9

0.00 1.26 4.18 1.26 1.26 2.68 0.00 2.68 0.00 2.68 2.68 1.26 1.26 2.68 0.01 0.00 0.00 0.05 0.00

9.33 10.41 12.84 9.74 9.63 10.72 7.95 10.42 7.49 9.76 9.71 7.68 7.49 7.68 9.29 10.20 6.86 6.86 5.80

gi

gk

Aki (108 s− 1)

9 5 5 5 5 1 9 1 9 1 1 5 5 1 6 2 5 3 1

9 7 15 7 5 3 15 3 9 3 3 3 9 3 10 6 3 3 3

2.39 1.04 2.08 1.88 0.39 0.22 1.18 0.45 3.40 0.91 0.04 3.51 0.00026 0.34 2.84 6.27 3.80 2.20 7.09

Ei and Ek are the energies of the lower and upper levels, respectively, gi and gk the corresponding degeneracy of the levels, and Aki the transition probabilities.

emission of the C2 Swan molecular system (d3Πg −a3Πu; Δv = −1… + 2) appears in the visible range. A detailed discussion of the molecular emission spectra will be presented in a future publication. Additionally, there is a peak at 157 nm, which we attribute to remaining stray light originating from the F2 laser. Fig. 3 shows time-resolved measurements of the S(I) and Zn(I) signal. The monochromator was set to the peak intensity of the selected lines, respectively. The signal is accumulated over 200 laser pulses for sulphur and 100 pulses for zinc with a gate time of tg = 200 ns and increasing delay times td after the laser shot in successive steps of Δtd = 200 ns at the begin and Δtd = 1000 ns at later delay times. The background signal at 179.9 nm and 213.2 nm beside the peaks of sulphur and zinc, respectively, is acquired under the same conditions. The sulphur peak signal shows a double-exponential decay: a fast exponential decay with a characteristic time of texp = 110 ns and a slow exponential decay with texp = 3100 ns for delays over 1000 ns. The background signal has a similar behavior but with faster decay times (fast decay with texp = 40 ns and slow decay with texp = 860 ns). If not stated differently, we chose the delay time of 400 ns and a gate time of 5 µs for LIBS analysis of sulphur, because with these parameters we have the best signal to background ratio. The zinc signal shows a different behavior. For times shorter than 600 ns after the laser pulse, a strong background continuum in the spectral region around 210 nm dominates the spectrum. After that time, the background intensity decays exponentially with a decay time texp = 480 ns and later, after approximately 2000 ns, the background decay is slower with texp = 5800 ns. The zinc line can be distinguished

J. Jasik et al. / Spectrochimica Acta Part B 64 (2009) 1128–1134

1131

Fig. 5. LIBS calibration curve of the Zn(I) 213.9 nm line intensity I(Zn) normalized to the C(I) 175.2 nm line intensity I(C) as function of the zinc content in PE samples with 95% prediction bands (PB).

declared uncertainties of the reference materials. The linear correlation of the both intensity ratios with corresponding mass ratios is reasonably good. From the statistical analysis of the data we derive the 95% confidence band, which is also shown in the figures. For this analysis, only the standard deviations of the signal are taken into account. The correlation of the sulphur signal is better than that of zinc. Additionally, the zinc calibration curve seems to have an off-set. The reason for this could be the continuum background observed in the region of zinc line. The precision of calibration is about 15% for the sulphur calibration curve and about 30% for zinc. The limit of detection LOD (determined from 95% prediction bands [11]) is 50 µg/g for sulphur and 215 µg/g for zinc. Fig. 3. Time behavior (a) of the S(I) 180.7 nm line intensity compared to the background signal measured at 179.9 nm and (b) of the Zn(I) 213.9 nm line with background at 213.2 nm.

4. Discussion 4.1. Calculation of sulphur signal intensity

from the background after about 600 ns. The signal of the zinc line extracted from background decays exponentially with texp = 1900 ns. For zinc signal observation in LIBS characterization, we chose a delay time of 600 ns and a gate time of 5 µs. We constructed calibration plots of quantitative LIBS analysis for the trace element concentration of sulphur and zinc in PE, which are shown in Figs. 4 and 5, respectively. For analysis, we use the S(I) line at 180.7 nm and the Zn(I) line at 213.9 nm. The intensity of both lines was normalized to the intensity of the C(I) line at 175.2 nm, which is chosen due to a low possibility of self-absorption (as is discussed in more detail in Section 4.2. below). The error bars in the plots result from the shot-to-shot standard deviations of the signal and the

In the following, we will compare the measured sulphur to carbon lines intensities ratio (of the lines at 180.7 nm and 175.2 nm) with theoretical values calculated from the declared sulphur to carbon mass ratio in the samples using the parameters Te and Ne derived from the LIBS spectra. For these considerations, we suggest that the relative element concentrations in the plasma are identical to those in the sample. The data are extracted from measurements of an ERMEC681k sample (declared sulphur content 630 µg/g). Supposing Boltzmann distribution of the excited atomic levels, the neutral line emission in an optical thin plasma is expressed as [12]: ð0Þ

εki =

ð0Þ ð0Þ

ð0Þ

ð0Þ E hc Aki gk N exp − k ð0Þ ð0Þ 4π λ kTe Q ðT Þ e ki

! ð1Þ

where Te is the electron temperature [K], A(0) ki the transition probability [s− 1], g(0) the upper-level degeneration, λ(0) k ki the wavelength (0) of the transition [m], Ek the upper level of the transition, and ! ð0Þ E ð0Þ Q ð0Þ ðTe Þ = ∑i gi exp − i the partition function and h, c, and k kTe

Fig. 4. LIBS calibration curve of the S(I) 180.7 nm line intensity I(S) normalized to the C(I) 175.2 nm line intensity I(C) as function of the sulphur content in PE samples with 95% prediction bands (PB).

are the Planck constant [m2 kg s− 1], the speed of light [m s− 1], and the Boltzmann constant [m2 kg s− 2 K− 1], respectively. The superscript (0) indicates values of neutral atoms. In local thermal equilibrium, the neutral atom number density can be directly related to the total number density. If higher degree of ionization is neglected, the ionization ratio of the element γ = N(1) / N(0) is given by the Saha equation (for details see ref. [12]). For our samples, the PE chains (C2H4)n are by far the major component of the samples with a negligible content of minor elements. Therefore, the molar ratio of sulphur to carbon atoms [S]/[C] in the

1132

J. Jasik et al. / Spectrochimica Acta Part B 64 (2009) 1128–1134

The temperature is determined from the slope of an iterative linear

sample is in good approximation given by fit:

½S MC + 2MH mS ≅ ½C MS m

ð2Þ

where mS/m is the mass ratio of the declared sulphur content in the sample and MC, MH and MS are the molar weights of carbon, hydrogen and sulphur, respectively. The relation between the ratio of sulphur to carbon emissivities for the S(I) line at 180.7 nm and the C(I) line at 175.2 nm (εkS180/εkC175) and the molar ratio ([S]/[C]) obtained from Eq. (2) can be expressed using Eq. (1) as follows:   ð0Þ εkS180 ½S AkiS180 gkS180 λkiC175 ð1 + γC ÞQC ðTe Þ E −EkC175 exp − kiS180 = ð0Þ εkC175 kTe ½C AkC175 gkC175 λkiS180 ð1 + γS ÞQ ðTe Þ S

ð3Þ (0) (0) (0) The values of spectroscopic constants A(0) are ki , gk , λki and Ek listed in Table 2. The partition functions Q(Te) are calculated using the data from the NIST database [10]. The most crucial parameter in the Eq. (3) is the electron temperature Te, while the Saha equation strongly depends on the electron density Ne of the plasma.

4.2. Determination of Te and Ne Te can be derived from the linear fit in a Saha–Boltzmann plot of the relative intensities of the carbon lines in the VUV LIBS spectra (for details see [13]). The application of the Saha–Boltzmann method in LIBS was also examined in several works of Aragon, Aguilera and other authors and reviewed in [14]. Shortly, the Saha–Boltzmann plot is constructed from the dependency of 8 > > > > > > <

ð0Þ ð0Þ

ln

λki εki

!

for neutrals ð0Þ ð0Þ   Aki gk λki εki ⁎ 2 3 ln = ! 3=  3 ð1Þ ð1Þ > Aki gk 2 > λki εki = mk T > 2 e > 5 for ions > > ln ð1Þ ð1Þ − ln42 : Ne 2πℏ2 Aki gk

  λ ε ⁎ 1 ln ki ki = B− E⁎ kTe k Aki gk

where B is the intercept of the curve with the ordinate. An example of a Saha–Boltzmann plot is given in Fig. 6. The data are obtained with a laser energy of 45 mJ/pulse and delay time tdel = 1 µs and gate time tg = 0.2 µs. The points in the figure derived from neutral carbon line intensities are rather scattered and do not exhibit a good linear correlation. However, after sorting of the lines according to the energy of the lower state in the transition (Ei), groups with consistent linearity can be found. We observe this behavior systematically for measurements with different delay times. The reason of this effect is probably selfabsorption in the plasma, which is most pronounced for lines with lower energy levels Ei of the transition. Therefore, we chose only the data derived from four neutral lines with the highest Ei = 2.68 eV together with value derived from one ionic line. The resulting five data points show a good linearity in the Saha–Boltzmann plot. We did the linear fit of the Saha–Boltzmann plot in an iterative approach, similar as in [13]. For construction of the Saha–Boltzmann plot, we have to insert the electron density Ne, which is determined by means of the H-α 656 nm line broadening. This technique is widely used in different types of plasmas. The application of the technique in LIBS is reviewed in [14]. Gigosos et al. [15] showed that values of full width at half area (FWHA) of the H-α profile are much less affected by ion dynamics effects than values of full width at half maximum (FWHM). They calculated the following formula, which we use for electron density determination:  FWHA = 0:549 nm ×

ð4Þ

ð6Þ

Ne 1023 m−3

0:67965

ð7Þ

ð5Þ

We obtain the experimental values of FWHA by deconvolution of the experimental H-α line profile with the known instrumental function of the spectrometer. The experimental values of Te and Ne are plotted in Fig. 7 as a function of the delay time. The values of Te are calculated from Saha– Boltzmann plots for carbon lines (as in Fig. 6) with different delay times. Values for delay times below 400 ns are not shown, because here the ionic line is not resolved from the background. The error bars represent the standard deviation of the Saha–Boltzmann linear fits. The profiles of the H-α line were sequentially scanned following the same timing scheme as it was in case of carbon line measurements. The electron temperature in the plasma rapidly decays from about

Fig. 6. Saha–Boltzmann plot of carbon lines in LIBS spectra of PE (F2 laser pulse energy 45 mJ, gate times tdel = 1 µs and tg = 0.2 µs).

Fig. 7. Time dependency of electron temperature Te and electron density Ne in the LIBS plasma of PE on delay time (F2 laser pulse energy 35 mJ, gate time tg = 200 ns).

on ⁎ Ek

8 < Eð0Þ for neutrals k : Eð1Þ Eð0Þ for ions ∞ k

J. Jasik et al. / Spectrochimica Acta Part B 64 (2009) 1128–1134

1 eV at 400 ns delay time to 0.7 eV at 2 µs. At delay times above 2 µs, the decay of the plasma temperature is much slower. The electron density has a very similar shape of the time behavior as the electron temperature. The density rapidly falls down from 15 × 1016 cm− 3 at 400 ns delay time to about 3 × 1016 cm− 3 after 2 µs and then decays only slowly. The beginning of the saturation of Te and Ne after about 1 µs may be explained by means of vapor plume expansion into the ambient gas according to the theoretical model of Arnold et al. [16]. The model predicts a hypersonic shock-wave expansion, in which the hot plume front rapidly expands. After the compressed ambient gas in the external shock-wave has enough mass, the reflected internal shockwave is moving inward. As a consequence, a homogenization of the plasma occurs and the energy is redistributed over the internal plasma, remaining nearly constant for a certain time. The homogenization changes the expansion dynamics, which we proofed experimentally for pulsed laser ablation of the polymer Teflon PTFE [17]. From comparison to the calculated curves obtained there, we would estimate that the homogenization with a laser pulse energy of 35 mJ in a background pressure of 2 mbar Ar occurs at a delay time of 500 to 700 ns. This is nearly consistent with the saturation of the measured values of Te and Ne. The dependency of Te and Ne on the laser energy is shown in Fig. 8. For this figure, the profiles of the selected spectral lines were sequentially scanned at a delay time of tdel = 600 ns and a gate time of tg = 500 ns. The temperature and density increase with increasing laser energy. The obtained values of Te and Ne are typical for LIBS experiments as reviewed by Aragon and Aguilera [14]. The same authors also reported recently on dynamical measurements of Te and Ne for ablation of Fe–Ni alloys in air at atmospheric pressure [18]. The reported decay times of Te and Ne were considerably longer than in our case, which may be related to the much higher values of irradiance (≫1 GW/cm2) used there. At these irradiances another dynamics of plume wave expansion may be relevant, because a so-called lasersupported radiation wave is produced.

1133

Fig. 9. Comparison of calculated and measured ratios of S(I) 180.78 nm to C(I) 175.2 nm line intensities as a function of the delay time in LIBS spectra of PE (F2 laser pulse energy 35 mJ, gate time tg = 200 ns).

The comparison of calculated and measured values of εS180/εC180 ratios is shown in Fig. 9. The calculated ratios are obtained by means of formula (3). We achieved good coincidence especially for values obtained with delay times shorter than 2 µs. The values at longer delays are probably affected by decreasing signal-to-noise ratios. For delay times shorter than about 2 µs, the ratio εS180/εC175 increases with increasing delay time. This could be a consequence of decaying electron temperature, because the upper state of the sulphur line transition has a lower energy than that of the carbon transition.

Therefore, the sulphur excited state is relatively more populated than the carbon state for lower electron temperatures. I.e., if the electron temperature decreases from Te = 1.05 eV to Te = 0.7 eV (as in Fig. 7), the relative thermal population according to the Boltzmann distribution between the upper energy levels of the S(I) 180.7 nm line (Ek = 6.86 eV) and the C(I) 175.2 nm line (Ek = 9.76 eV) increases by a factor of about four (ignoring temperature dependence of the partition function). Fig. 10 shows the energy dependency of the measured and calculated εS180/εC175 ratios. Again, good agreement is achieved. The ratio decreases with increasing laser energy probably as a consequence of increasing electron temperature, which results in a lower relative population of the excited sulphur state. Similar as for Figs. 7 and 9, we see a reasonable qualitative agreement between the results shown in Figs. 8 and 10. To our knowledge, there are no comparable studies for carbon based materials, mainly due to a lack of sufficient number of carbon lines necessary for plasma characterization. Our results may contribute to the development of calibration-free LIBS analysis of minor and trace elements in polymer materials. For this approach, the polymer matrix has to be known. In some potential practical applications (e.g., recycling), the exact polymer type is unknown and has to be analyzed. This could be achieved by an additional analysis method as for instance infrared spectroscopy or by a more detailed investigation of the molecular emission bands typically observable in LIBS spectra of polymers in the VIS range. We have performed a detailed analysis of the molecular bands visible in our LIBS measurements, which will be presented in a separate publication.

Fig. 8. Dependency of electron temperature Te and electron density Ne in LIBS spectra of PE on F2 laser pulse energy (delay time tdel = 600 ns, gate time tg = 500 ns).

Fig. 10. Comparison of calculated and measured ratios of S(I) 180.78 nm to C(I) 175.2 nm intensities as a function of laser energy in LIBS spectra of PE (delay time tdel = 600 ns, gate time tg = 500 ns).

4.3. Comparison of calculation and measurement

1134

J. Jasik et al. / Spectrochimica Acta Part B 64 (2009) 1128–1134

5. Conclusion We recorded VUV and UV/VIS spectra of the light emitted from F2 laser-induced plasmas at PE in a 2 mbar Ar atmosphere for LIBS investigations. We could demonstrate that the LIBS in the VUV range is a good method especially for the detection of the trace element sulphur in PE, which cannot be detected in the UV/VIS range. For the determination of the LOD, we constructed LIBS calibration curves for sulphur and zinc. The obtained LOD for sulphur is 50 µg/g and for zinc 215 µg/g. The use of VUV spectroscopy gave also access to several intensive neutral and ionized atomic carbon emission lines, which allow the determination of the electron plasma temperature Te by means of Saha–Boltzmann plots. Additionally, the electron density in the plasma Ne was determined by means of hydrogen H-α 656 nm line broadening. We could demonstrate that Te and Ne are highly correlated in time resolved measurements and in measurements with variable laser energies. From Te and Ne we calculated with the known spectroscopic constants the intensity ratio of the S(I) line at 180.7 nm and the C(I) line at 175.2 nm. The calculated results were in good agreement with the measured values. This opens up the possibility of application of calibration-free methods for the investigation of polymer materials. Acknowledgements We want to thank the Christian Doppler Research Society (Vienna, Austria) and the Scientific Grant Agency of the Slovak Republic (under project VEGA 1/0609/08 and VEGA 1/3044/06) for funding. Also the tight cooperation and financial support of the voestalpine Stahl GmbH (Linz, Austria) and the AVE Österreich GmbH (Hörsching, Austria) are highly acknowledged. References [1] R. Noll, H. Bette, A. Brysch, M. Kraushaar, I. Mönch, L. Peter, V. Sturm, Laser-induced breakdown spectrometry — applications for production control and quality assurance in the steel industry, Spectrochim. Acta Part B 56 (2001) 637–649.

[2] I. Radivojevic, C. Haisch, R. Niessner, S. Florek, H. Becker-Ross, U. Panne, Microanalysis by laser-induced plasma spectroscopy in the vacuum ultraviolet, Anal. Chem. 76 (2004) 1648–1656. [3] D.A. Schechter, L.J. Radziemski, Handbook of Laser-Induced Breakdown Spectrocopy, Wiley, Chichester, 2006. [4] J. Bengoechea, E.T. Kennedy, Time-integrated, spatially resolved plasma characterization of steel samples in the VUV, J. Anal. At. Spectrom. 19 (2004) 468–473. [5] M. Stepputat, R. Noll, On-line detection of heavy metals and brominated flame retardants in technical polymers with laser-induced breakdown spectrometry, Appl. Opt. 42 (2003) 6210–6220. [6] S. Kaski, H. Haakkanen, J. Korppi-Tommola, Determination of Cl/C and Br/C ratios in pure organic solids using laser-induced plasma spectroscopy in near vacuum ultraviolet, J. Anal. At. Spectrom. 19 (2004) 474–478. [7] I. Radivojevic, R. Niessner, C. Haisch, S. Florek, H. Becker-Ross, U. Panne, Detection of bromine in thermoplasts from consumer electronics by laser-induced plasma spectroscopy, Spectrochim. Acta Part B 59 (2004) 335–343. [8] H. Fink, U. Panne, R. Niessner, Analysis of recycled thermoplasts from consumer electronics by laser-induced plasma spectroscopy, Anal. Chim. Acta 440 (2001) 17–25. [9] D. Bäuerle, Laser Processing and Chemistry, 3rd ed.Springer-Verlag, New York, 2000. [10] Y. Ralchenko, A. Kramida, J. Reader, NIST ASD Team, Nist atomic spectra database (version 3.1.5), [Online]. Available: http://physics.nist.gov/asd3 [2008, July 24]. National Institute of Standards and Technology, Gaithersburg, MD. (2008). [11] J. Mermet, Limit of quantitation in atomic spectrometry: an unambiguous concept? Spectrochim. Acta Part B 63 (2008) 166–182. [12] H. Griem, Principles of Plasma Spectroscopy, First ed. Cambridge University Press, Cambridge, 2005. [13] J. Aguilera, C. Aragon, Characterization of a laser-induced plasma by spatially resolved spectroscopy of neutral atom and ion emissions: comparison of local and spatially integrated measurements, Spectrochim. Acta Part B 59 (2004) 1861–1876. [14] C. Aragon, J. Aguilera, Characterization of laser induced plasmas by optical emission spectroscopy: a review of experiments and methods, Spectrochim. Acta Part B 63 (2008) 893–916. [15] M.A. Gigosos, M.A. Gonzalez, V. Cardenoso, Computer simulated Balmer-alpha, -beta and -gamma stark line profiles for non-equilibrium plasmas diagnostics, Spectrochim. Acta Part B 58 (2003) 1489–1504. [16] N. Arnold, J. Gruber, J. Heitz, Spherical expansion of the vapor plume into ambient gas: an analytical model, Appl. Phys. A 69 (1999) S87–S93. [17] N. Huber, J. Gruber, N. Arnold, J. Heitz, D. Bäuerle, Time-resolved photography of the plasma-plume and ejected particles in laser ablation of polytetrafluoroethylene, Europhys. Lett. 51 (2000) 674–678. [18] C. Aragon, J.A. Aguilera, Spatial and temporal scaling and common apparent excitation temperature of laser-induced plasmas generated at constant irradiance with different pulse energies, J. Appl. Phys. 103 (2008) 013310–013319.