Role of Purging Gases in the Analysis of Polycarbonate With Laser-Induced Breakdown Spectroscopy

Journal of Russian Laser Research, Volume 35, Number 3, May, 2014

ROLE OF PURGING GASES

IN THE ANALYSIS OF POLYCARBONATE

WITH LASER-INDUCED BREAKDOWN SPECTROSCOPY

W. A. Farooq,1 ∗ W. Tawfik,1,4 Z. A. Alahmed,1 K. Ahmad,3 and J. P. Singh2

1Department of Physics and Astronomy

College of Science, King Saud University

Riyadh 11451, Saudi Arabia2Institute for Clean Energy Technology

Mississippi State University

Starkville 39759, USA3College of Engineering, King Saud University

Riyadh 11451, Saudi Arabia4National Institute of Laser-Enhanced Sciences

Department of Environmental Applications

Cairo University, Cairo, Egypt∗Corresponding author e-mail: [email protected]

Abstract

Polycarbonate is a strong, durable, and high-transparent material. It is being used extensively inelectronics applications, eyewear lenses, exterior automotive components, construction industry, andmanufacturing of containers. Laser-induced breakdown spectroscopy (LIBS) is a well-known andwidely-used technique in quantitative and qualitative analysis of materials. The analysis is basedon the spectra obtained in the LIBS process. We investigate the LIBS spectrum of polycarbonate(C12H18O7)n polymer in the presence of air, helium, argon, neon, and mixed-gas atmospheres usinga Spectrolaser-7000 system. We investigate the peak intensities of hydrocarbon fragments like C2,C, and CN in He, Ar, and Ne atmospheres. Moreover, we observe the CN molecular lines in theLIBS spectrum of polycarbonate in air only. We show that, in the He and Ne atmospheres, the peakintensity of the hydrogen (656 nm) line increases 1.85 and 4.5 times, and the peak intensity of thecarbon (247 nm) line increases 1.46 and 2.6 times, respectively, as compared to that in air. Theseresults are very useful and will help in the analysis and detection of polymers and bio-samples, whichare mostly comprised of carbon and hydrogen.

Keywords: quantitative, purging, polymer, LIBS, peak intensity.

1. Introduction

Laser-induced breakdown spectroscopy (LIBS) is a well-known and widely-used analytical technique

to provide rapid, remote, in-situ, and multi-elemental analysis of trace and bulk samples in any state

(solid, liquid, and gaseous) with minimal or no sample preparation [1, 2]. In this technique, an intense

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Manuscript submitted by the authors in English on December 3, 2013.

1071-2836/14/3503-0252 c©2014 Springer Science+Business Media New York

Volume 35, Number 3, May, 2014 Journal of Russian Laser Research

beam of a pulsed laser is focused on a very small volume of a target, which brought the target material

to a transient plasma state where the sample’s components are essentially reduced to individual atoms.

In such high-temperature plasma, atoms are brought to excited states and ionized. These states, after

decay, emit radiation from the IR to UV regions of the spectrum. By analyzing the recorded spectrum,

one can determine the elemental composition of the target [3–7]. In order to detect the spectral signature

of each element, very high-resolution spectral analysis is required. The identified atomic/ionic lines are

used for qualitative identification and quantification of the elements present in the plasma. Their relative

intensities can be used for the quantitative analysis of the elements present in the target material [8, 9].

Enhancement in intensities of the elements present in the target materials and stability in the LIBS

plasma can improve the detection and identification of the species in the target. Time-resolved measure-

ments of the LIBS plasma may improve the signal-to-noise ratio of the spectra [10]. In order to perform

precise elemental analysis, the characteristics of the LIBS plasma, such as the temperature, the number

densities, and the electron density of different elements present in the plasma should be optimized. These

parameters depend on the laser-pulse properties: laser irradiance, pulse width, wavelength, physical and

chemical characteristics of the sample material, atmospheric conditions, space, and time. Previously

reported studies have been carried out on each parameter to understand the complex phenomenon for

its application in different fields [1, 11].

The atmosphere around the target material is one of the most important parameters in the LIBS

experiment, which affects the rest of the LIBS plasma parameters severely. Only a few papers discuss the

effects of the atmosphere on the LIBS plasma. Sdorra and Niemax investigated plasma from a copper

target produced with a Nd:YAG laser in the environment of different ambient gases (Ne, He, N, and

air). They demonstrated that plasma in the argon atmosphere has higher temperature, higher electron

density, and higher intensity of a chosen element under the same experimental conditions as compared to

the plasma in other gases. They also observed that under reduced-pressure atmospheres of inert gases, the

self-absorption effect can be reduced, and a sharp spectral line with low background can be obtained [12].

Grant and Paul in 1990 studied the effect of different gas atmospheres (air, Ar, and He) and pressures

(0.5, 50, and 760 torr) on the plasma of a steel target using an excimer laser [13]. They achieved the

best signal-to-noise ratio and higher intensity and longer lifetime of the plasma in the presence of the

argon atmosphere. Many researchers have also observed the same behavior in the atmosphere of argon

and helium [14–16].

Another important effect from the Ar environment is protection of the excited atoms from forming

stable compounds such as oxides, which might reduce the LIBS emission from the analyte. Since polymers

differ in their stoichiometric C/H ratio, this information can be used to identify the type of polymers [16,

17]. Moreover, the C/H ratio and intensity of C&H lines may be used for diagnostics of bio-materials [18].

In this study, we investigate the LIBS of polycarbonate (C12H18O7)n polymer in the presence of air,

helium, neon, argon, and mixed-gas atmospheres using a Spectrolaser-7000 system. In doing so, we ana-

lyze the spectra of polycarbonate in different environments and study the effects of different environments

on the carbon (247.28 nm) and hydrogen (656.8 nm) lines. In addition, we also observe the CN molecular

band and C2 Swan band in the spectra.

2. Material and Method2.1. Polycarbonate

A polycarbonate molecule is composed of a bisphenol A part and a carbonate group. Bisphenol A

contains two aromatic rings, which are responsible for the polycarbonate’s inability to crystallize. This

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Journal of Russian Laser Research Volume 35, Number 3, May, 2014

amorphous structure gives the polymer its particular transparency. The molecular structure of polycar-

bonate is shown in Fig. 1 (adopted from [19]).

Fig. 1. Molecular structure of polycarbonate.

Polycarbonate was first developed in 1953 by

General Electric in the USA and Bayer in Germany

independently. The popular trade name is Lexan.

Polycarbonate is a transparent thermoplastic mate-

rial, tough, dimensionally stable, and widely used

in exterior automotive components, housing equip-

ment, brackets, and structural parts. Moreover, it

is used for medical supply components and plastic lenses for eyeglasses because of its properties, such

as good heat resistance, fair chemical resistance, excellent physical properties, environment friendliness,

and moderate to high price. Standard grade polycarbonate sheet, tube, and rod are not UV stabilized

but available in UV stabilized and mar-resistant grades [19].

2.2. Method

In this experiment, we used a “Spectrolaser 7000” LIBS system (Laser Analysis Technologies, Aus-

tralia). It is a fully software controlled integrated analysis system comprising a Nd:YAG laser, focusing

optics, sample chamber with rotating and translational stage, camera, option for the gas cylinder, gated

CCD detectors, and spectrographs. The setup is shown in the block diagram (Fig. 2).

Fig. 2. Block diagram of a “Spectrolaser 7000” LIBSsystem.

Table 1. Specification of the Nd:YAG Laser in the

“Spectrolaser 7000” System.

Description Details

Emission Line Nd:YAG 1,064, 532 or 355 nm

Laser Energy 50–350 mJ

Pulse Width 7 ns

Beam Divergence < 1 mrad

Beam Diameter 5 mm

Repetition Rate Variable,

Maximum up to 15 Hz

The specification of the Nd:YAG laser used in the “Spectrolaser 7000” LIBS system is given in Table 1.

In the present experiment, we used a 1,064 nm laser beam. The laser beam was focused on the sample

placed on a translational stage in the sample chamber with adjustable optics. We fixed the focal length

of the lens at 49 mm, so that the laser beam was focused at the sample surface. The laser energy was

varied from 50 to 325 mJ in different gas environments for optimization of signal on the polycarbonate

sample. A gate width of 1 μs was fixed during the whole study.

A polycarbonate (C12H18O7)n polymer sample was cut from the sheet to fix in the sample holder. Air,

helium, argon, neon, argon–neon mixture, and argon–helium mixture were filled in the sample chamber

at 1 atm in the present investigation. The purity level of argon, helium, and neon was 99.99 %. All data

were recorded at 10 short averaging keeping the laser beam repetition rate at 10 Hz.

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3. Results and Discussions

3.1. LIBS Spectrum

The LIBS spectrum from polycarbonate was optimized in air at different laser energies from a Nd:YAG

laser with 1,064 nm wavelength. Due to the transparent nature of the sample in the NIR and visible

ranges, good emission spectrum from the plasma was detected at energy higher than 75 mJ. The emission

spectrum of the polycarbonate plasma in air at 200 mJ laser energy is shown in Fig. 3.

Fig. 3. LIBS spectrum of polycarbonate in air at 1 atm and 200 mJ laser energy.

3.2. Role of Purging Gases

3.2.1. Effect of Purging Gases on Molecular Spectra

We have observed the molecular lines of cyanide (CN) and the C2 Swan band. The CN molecular

lines are shown in Fig. 4 a. Cyanide consists of a carbon atom triple bonded to a nitrogen atom. It is

toxic in nature. Cyanide is not present in the polycarbonate sample, but it makes a strong bond with

nitrogen molecules from air during the LIBS process. The CN molecular lines did not appear in the

other gaseous environments, as shown in Fig. 4 b. The C2 lines are present in all spectra in different gas

environments. The CN lines in polymers have been investigated in [22] where these lines were used to

identify the type of polymer. In view of the present investigations, the C and C2 lines can give a more

accurate result in the identification of polymers instead of using the CN lines only. Since the appearance

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of the CN lines depends on the concentration of nitrogen present in the atmosphere, we cannot use only

the CN lines to identify the type of polymer.

In the present study, we also investigated the effect of changing the delay time in the air, He, Ne,

Ar–Ne (mixture), and He–Ne (mixture) gas environment at 1 atm on the carbon (247.8 nm) and hydrogen

(656.28 nm) lines of polycarbonate [20–22].

a)

c)

b)

Fig. 4. Expanded view of the LIBS spectrum presentedin Fig. 1 at 370–390 nm (a) and the appearance of CNlines (b) and C2 lines in the polycarbonate sample (c),air (solid curve), Ar (dashed curve), He (dotted curve),and Ne (dash-dotted curve) atmospheres.

3.2.2. Effect of Purging Gases on the Atomic Spectra

Air Atmosphere. In the LIBS spectrum, we have observed very prominent lines of carbon, hydrogen,

and nitrogen. We selected the carbon (247.8 nm) and hydrogen (656.28 nm) lines to observe the role of

purging gases in the LIBS spectra. The behavior of the carbon (247.8 nm) and hydrogen (656.28 nm)

lines in the air atmosphere with delay times 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and 2.0 μs is shown in Fig. 5.

In Fig. 5, we show the variation in broadening and peak intensity of the carbon (247.8 nm) line

with increasing delay time. The broadening is mainly due to the Stark effect and depends on the

electron concentration Ne. For the hydrogen atom and H-like ion, the electron concentration is given as

follows [23,24]:

Ne = C(Ne, T )λ2/3FWHM, (1)

where C is a constant adopted from the literature [23]; it is responsible for the effect of the electrostatic

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a) b)

Fig. 5. Behavior of the carbon (247.8 nm) line in air with different delay times 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and2.0 μs from the top to the bottom (a), and the variation in the peak intensity of this carbon (247.8 nm) line withthe same delay times (b).

a) b)

Fig. 6. Behavior of the hydrogen (656.28 nm) line in air with different delay times 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and2.0 μs from the top to the bottom (a), and the variation in the peak intensity of this hydrogen (656.28 nm) linewith the same delay times (b).

field and weakly depends on Ne and temperature T . For a non-H-like atom, Ne reads

Ne ≈ (Δλ/2w) · 1016 cm−3, (2)

where W is the value of the Stark broadening. In both cases, Ne depends on λFWHM. We can see from

Figs. 5 and 6 that λFWHM decreases with increase in the delay time. A decrease in the line broadening

indicates that both the plasma temperature and the electron concentration decrease with increase in the

delay time.

Variations in the peak intensities of the carbon and hydrogen lines in air with increase in the delay

time is presented in Figs. 5 b and 6 b respectively. Both lines have different decreasing trends.

Helium Atmosphere. In Fig. 7, we show the behavior of peak intensities of the carbon (247.8 nm)

and hydrogen (656.28 nm) lines with different delay times in the helium atmosphere. The peak intensities

of both lines have almost similar exponential decreasing trends. The reason is quite clear, i.e., with

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increasing delay time the LIBS plasma is expanding and eventually the plasma temperature is decreasing

very rapidly, reduces the excitation mechanism of the atoms.

a) b)

Fig. 7. Variations in the peak intensity of the carbon (247.8 nm) line (a) and the hydrogen (656.28 nm) line (b)with different delay times in the helium atmosphere.

Due to the high conductivity of He gas, the plasma in the He atmosphere will cool rapidly through

collisions, which is also the cause of a rapid decrease in the peak intensities of the carbon and hydrogen

lines. We also observed that the intensity can be recorded with up to 10 μs time delay.

Neon Atmosphere. In the neon atmosphere, the behavior of the carbon (247 nm) and hydro-

gen (656 nm) lines with increasing delay time has an identical decreasing trend. This decrease is almost

linear with the delay time as shown in Fig. 8. This decrease is not rapid as compared to that in the He

atmosphere. This might be due to the low thermal conductivity of Ne as compared to He. The peak

intensity of the carbon and hydrogen lines are not observable at a time delay of 10 μs.

a) b)

Fig. 8. Variations in the peak intensity of the carbon (247.8 nm) line (a) and the hydrogen (656.28 nm) line (b)with different delay times in the neon atmosphere.

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Table 2. Relative Peak Intensities of the Carbon (247 nm) and Hydrogen (656.28 nm) Lines at Different

Purging Gases (Delay Time 200 ns, Laser Energy 200 mJ).

Element Air He Ne Ar–Ne He–Ne

Hydrogen (656 nm) 5600± 500 10385± 500 25354± 2100 18489± 1100 19204± 1450

Carbon (247 nm) 4256± 420 6249± 800 11131± 1500 7431± 850 5781± 850

The comparison of different parameters of the LIBS plasma of polycarbonate in different environments

is summarized in Tables 2–5. From Table 2, we see that the peak intensity of the hydrogen (656 nm)

line increases 1.85 and 4.5 times in the He and Ne atmospheres, respectively, as compared with air. The

peak intensity of the carbon (247 nm) line increases 1.46 and 2.6 times in the He and Ne atmospheres,

respectively, as compared with air. It is also found that the peak intensities of these lines increase in

a heavy-gas surrounding environment. At the same pressure, the confinement of plasma with heavy

surrounding gases is much stronger than with light gases. The enhancement of peak intensities of Fe and

Si in the study of the LIBS plasma of molten Aluminum alloy in the pure gas environment has also been

reported in [26].

a) b) c)

Fig. 9. Variations in the peak intensities of the carbon (247.8 nm) line with different delay times in the argon–neon atmosphere (a) and in the helium–neon atmosphere (b), and variations in the peak intensities of the hydro-gen (656.28 nm) line with different delay times in the helium–neon atmosphere (c).

Mixed Gases. In this study, we used two mixtures of gases, Ar–Ne and He–Ne. Both gases are

mixed with equal quantity by weight. The behaviors of the carbon (247 nm) and hydrogen (656 nm)

lines are shown in Fig. 9. In the case of the Ar–Ne mixture (for carbon as a relatively heavy atom), the

carbon (247 nm) line has a decreasing trend. The decrease is not linear, and it decreases sharply from

0.2 to 0.4 μs delay, and from 0.4 to 1.0 μs is nearly a plateau. After 1.0 μs, it decreases again sharply

(Fig. 9 a). In the case of helium (as a light atom), the hydrogen (656 nm) line has an oscillating behavior

— a sharp decrease followed by a zigzag shape; then, after 1.0 μs, it has an overall decreasing trend

(Fig. 9 b). From the present data, we conclude that the atomic size plays an important role in the LIBS

plasma and its environment. It is also recognized that the plasma behavior in a heavy-gas environment is

very complicated, and in the gas mixture the heavy atomic-gas behavior dominates. There is no regular

or periodic behavior with increasing delay time.

In the case of the He–Ne mixture, the carbon and hydrogen lines have a similar behavior. The

relative peak intensity decreases exponentially with increasing delay time (Fig. 9 b and c). As explained

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before for the case of He, the relatively light gas-mixture environment affects the LIBS plasma expansion,

which causes an eventual decrease of the plasma temperature. The latter, in turn, reduces the excitation

mechanism for both carbon and helium atoms.

3.3. Effect of Purging Gases on the Plasma Parameters

For comparison, the plasma temperatures in different gas environments are given in Table 3. The

following Boltzmann equation is used to calculate temperatures of plasma in different gas environ-

ments [23, 24]:

ln I(λ/Akigk) = −(1/KT )Ek + ln[CF/U(T )], (3)

Table 3. Behavior of the Plasma

Temperature in Different Atmospheres

(Carbon Lines with 1 μs Delay Time at

200 mJ Laser Energy).

Atmosphere Temperature (K)

Air 13794± 560

He 13079± 600

Ne 15985± 800

Ar–Ne 14387± 350

He–Ne 14387± 500

where I is the population of corresponding energy levels of the

element used to calculate the plasma temperature, λ is the

wavelength of the energy level, Aki is the transition probability,

gk is the statistical weight for the upper level, Ek is the excited

energy of the element, T is the temperature, U is the partition

function, F is an experimental factor, and C is the species con-

centration. In calculations of plasma temperatures, we used the

carbon lines 193.09, 247.85, 833.51, and 940.55 nm.

The measured values λ1/2 and calculated values of the

Stark broadening W for both carbon (247 nm) and hydro-

gen (656 nm) lines have an error of ± ≈ 10% in determination

of the Stark broadening. The electron concentration Ne, if the

carbon (247 nm) line is used in different environments, is given

in Table 4, where Ne is calculated using Eq. (2).

Table 4. Line Width and the Electron Concentra-

tion for the Carbon (247 nm) Line.

λ1/2 W Ne

Air 0.11 3.40E-04 1.62E+18

He 0.105 3.40E-04 1.54E+18

Ne 0.105 3.40E-04 1.54E+18

Ar–Ne 0.106 3.40E-04 1.56E+18

He–Ne 0.116 3.40E-04 1.54E+18

Table 5. Line Width and the Electron Concentra-

tion for the Hydrogen (656.28 nm) Line.

λ1/2 Ne

Air 1.505 4.41E+17

He 0.506 8.87E+16

Ar–Ne 0.631 1.23E+17

He–Ne 0.580 1.08E+17

Ne 0.630 1.22E+17

Results of calculations of the line width and electron concentration of the H (656 nm) line are given

in Table 5.

The concentration of electrons Ne was evaluated from the width of Hα using the formula [27]

Hα : FWHM = 0.549 nm

(Ne

1017 cm−3

)0.67965

.

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4. Summary

In this study, we identified hydrocarbon fragments including C2, C, CN, and H in the LIBS plasma of

polycarbonate spectra. In spite of the fact that cyanide was not present in the sample, the hydrocarbon

plasma reactions with the surrounding air produced CN. We also observed that the atomic size in the

LIBS plasma environment affects the time-dependent relaxation processes. This might be due to the

energy transfer of atoms and molecules in the LIBS plasma of different species in different environments.

We concluded that the peak intensities of the hydrogen (656 nm) and carbon (247 nm) lines increase

manyfold in pure purging gases He and Ne. Also we found that variations in peak intensities of the

hydrogen (656 nm) and carbon (247 nm) lines in polycarbonate had similar behavior in pure gases like

He and Ne.

Acknowledgments

This project was supported by the Research Center of the College of Science of the King Saud

University, Riyadh.

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Role of Purging Gases in the Analysis of Polycarbonate With Laser-Induced Breakdown Spectroscopy

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