Elemental analysis of bead samples using a laser-induced plasma at low pressure

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Spectrochimica Acta Part B

Elemental analysis of bead samples using a laser-induced

plasma at low pressure

Tjung Jie Lie a, Koo Hendrik Kurniawan a,*, Davy P. Kurniawan a, Marincan Pardede a,

Maria Margaretha Suliyanti b, Ali Khumaeni c, Shouny A. Natiq c, Syahrun Nur Abdulmadjid d,

Yong Inn Lee e, Kiichiro Kagawa f, Nasrullah Idris f, May On Tjia g

a Research Center of Maju Makmur Mandiri Foundation, 40 Srengseng Raya, Kembangan, Jakarta Barat 11630, Indonesiab Graduate Program in Opto Electrotechniques and Laser Applications, Faculty of Engineering, The University of Indonesia, 4 Salemba Raya,

Jakarta 10430, Indonesiac Department of Physics, Faculty of Mathematics and Natural Sciences, Diponegoro University, Tembalang Campus, Semarang 50275, Indonesia

d Department of Physics, Faculty of Mathematics and Natural Sciences, Syiah Kuala University, Darussalam, Banda Aceh 23116, Indonesiae Physics Department, Chonbuk National University, Chonju 561-756, South Korea

f Department of Physics, Faculty of Education and Regional Studies, Fukui University, 9-1 bunkyo 3-chome, Fukui 910-8507, Japang Department of Physics, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, 10 Ganesha, Bandung 40132, Indonesia

Received 26 September 2005; accepted 8 December 2005

Available online 19 January 2006

Abstract

AnNd:YAG laser (1064 nm, 8 ns, 30 mJ) was focused on various types of fresh, fossilized white coral and giant shell samples, including samples

of imitation shell and marble. Such samples are extremely important as material for preparing prayer beads that are extensively used in the Buddhist

faith. The aim of this research was to develop a non-destructive method to distinguish original beads from their imitations by means of spectral

measurements of the carbon, hydrogen, sodium and magnesium emission intensities and by measuring the hardness of the sample using the ratio

between Ca (II) 396.8 nm and Ca (I) 422.6 nm. Based on these measurements, original fresh coral beads can be distinguished from any imitation

made from hard wood. The same technique was also effective in distinguishing beads made of shell from its imitation. A spectral analysis of bead

was also performed on a fossilized white coral sample and the result can be used to distinguish to some extent the fossilized white coral beads from

any imitation made from marble. It was also found that the plasma plume should be generated at low ambient pressure to significantly improve the

hydrogen and carbon emission intensity and also to avoid energy loss inside the crater during laser irradiation at atmospheric pressure. The results of

this study confirm that operating the laser-induced plasma spectroscopy at reduced ambient pressure offers distinct advantage for bead analysis over

the conventional laser-induced breakdown spectroscopy (LIBS) technique operated at atmospheric pressure.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Bead analysis; Laser-induced plasma; C and H emission; Si and Mg emission; Low pressure plasma

1. Introduction

Spectrochemical methods of analysis are among the most

widely used analytical methods both for the qualitative and

quantitative analysis of elements in an analyte. Trace element

determinations are most frequently carried out by atomic

emission or atomic absorption spectroscopy. Due to the

0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.sab.2005.12.007

* Corresponding author. Tel.: +62 21 5867663, +62 21 5867660; fax: +62 21

5867670 or +62 21 5809144.

E-mail address: [email protected] (K.H. Kurniawan).

URL: http://www.mmm.or.id (K.H. Kurniawan).

undesirable requirement of a tedious sample pretreatment in this

technique, and the continued improvements in emission detec-

tion techniques, emission spectroscopy has enjoyed an increasing

popularity. This is particularly true for elemental analyses based

on atomic emission from a laser plasma, which is generated by

focusing a laser beam on the sample surface. The spectroscopic

analysis of this emitted light yields information on the elemental

composition of the sample. This technique is commonly referred

to laser atomic emission spectrochemical analysis (LAESA) and

was first proposed by Brech and Cross [1].

At the present time, one witnesses a strong revival in the

study and application of LAESA, and its recent development

61 (2006) 104 – 112

ww

Fig. 1. Diagram of the experimental setup.

Fig. 2. Photograph of the sample used in this experiment.

T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112 105

follows essentially two main streams [2–4]. One of these

employs a high pressure surrounding gas and is usually

referred to laser-induced breakdown spectroscopy (LIBS),

which was first proposed by Loree and Radziemski [5,6]. In

the case of LIBS at atmospheric pressure, a pulsed Nd:YAG

laser with a typical energy of several tens of mJ is focused

onto the surface of the sample, resulting in a plasma with a

high temperature and electron density. A gated OMA (Optical

Multichannel Analyzer) is used to remove the strong

background emission emitted from the plasma. The use of

LIBS in rapid quantitative analyses has been demonstrated in

various fields such as soil analysis [7–9], metallurgy and

mining industry [10–12], art conservation [13], ceramic

analysis [14], organic materials analysis [15–20] and phar-

maceutical products [21–23]. However, the number of reports

on the application of this technique dedicated to the analysis

of hydrogen in metal samples as well as carbon in solid

samples is limited. This is due to the severe diminution in the

emission when a plasma is produced at atmospheric pressure,

while these effects have been shown to be negligible for

heavy elements [24]. This explains why only few applications

of LIBS to solid organic samples were performed, even

though information concerning C and H emission is

extremely important.

The development of the LAESA method along the other

direction involves the use of low gas pressures and this method

is referred to as laser-induced shock wave plasma spectroscopy

(LISPS) [25,26], due to the inherent role of a shock wave in

this method. When a laser plasma is produced under a

surrounding gas at reduced pressure, the intensity of the

background emission is largely suppressed. In our previous

studies [27,28], we demonstrated the unequivocal detection of

the sharp H (I) 656.2 nm emission line from metal samples by

means of the LISPS method. This was made possible because

of the typical LISPS detection conditions employed, i.e., a low

pressure of surrounding gas which is crucial to overcome the

undesirable broadening effect and the resulting diminution in

the efficiency of hydrogen emission. The resulting calibration

curve clearly indicates the potential of this technique for

quantitative analysis.

In relation to the above studies, the authors explored the use

of laser plasma spectroscopy on the analysis of beads which are

mainly made from organic material in which carbon and

hydrogen are the major constituents. We point out that this

study of corals and shells is of particular importance in the

context of the Buddhist faith. Prayer beads are commonly used

by practitioners of the Buddhist faith as a tool of communi-

cation with the deities of the faith. The teachings of the religion

dictate that the beads must be made from either original,

fossilized white coral or giant shells that are believed to have

supernatural powers. However, there are many imitation prayer

beads in circulation, made from a mixture of glass, marble and

shell, that is melted and formed into beads. The aim of this

research was therefore to develop a non-destructive method for

distinguishing original beads from imitations.

2. Experimental procedure

The basic experimental setup used in this study was similar

to that used in our previous works [27,28]. For convenient

reference, the schematic diagram of the actual experimental

setup employed in this study is given in Fig. 1. In this

experiment, an Nd:YAG (Quanta Ray, LAB SERIES, 8 ns,

1064 nm, up to 450 mJ) laser was operated in the Q-sw mode at

a 10 Hz repetition rate with an output energy of 30 mJ, yielding

a power density onto the target surface of about 1 GW/cm2.

The laser beam was focused by a lens ( f =100 mm) through a

quartz window onto the sample surface. The shot-to-shot

fluctuation of the laser was estimated to be approximately 3%

by measuring a part of the laser light using energy meter.

The samples employed in these experiments are shown in

Fig. 2; namely, the fresh white coral (1), marble (2), beads from

shell (3), imitation beads that may be composed of a mixture of

melted plastics, shell and a large amount of magnesium oxide

(4), as well as fossilized white coral (5). In view of compositional

variations of those naturally occurring materials, a number of

samples were collected and included in the experiment. In each

experiment, the sample was placed in a small, vacuum-tight

metal chamber measuring 11 cm�11 cm�12.5 cm, which

T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112106

could be evacuated with a vacuum pump and filled with a

surrounding gas at the desired pressure. The gas flow through the

chamber was regulated by a needle valve in the air line and a

second valve in the pumping line. The chamber pressure was

monitored by means of a digital Pirani meter. The chamber

pressure was fixed at 1.3 kPa for low pressure plasma

observation and 99 kPa for atmospheric plasma observation

(LIBS).

Plasma radiation was detected by means of an optical

multichannel analyzer (OMA system, Princeton Instrument

IRY-700) attached to a monochromator with a focal length of

150 mm and connected to an optical fiber with its entrance

placed in front of the observation window of the vacuum

chamber. The detector used in this system was a gateable

intensified photodiode array with a gating width ranging from

40 ns to 80 ms. The spectral window, covered by the detector,

had a width of 80 nm at 500 nm and the spectral resolution of

the OMA system is 0.4 nm at 500 nm. The detected signals

were monitored on a screen. For all these experiments, the

OMA system was set at a gate delay and gate width of 200 ns

and 15 As, so that the S/B becomes optimum.

In this experimental work, the fixed spot irradiation is

expected to create a crater on the sample, especially a soft

sample such as fresh white coral. It is therefore important to

have an estimate of the size and depth of the crater for the

assessment of its effect. For this purpose a digital microscope

of Keyence (VHX-100 series) was used to produce the entire

image of the crater chosen for illustration. The 3-dimensional

image was constructed from the 2-dimensional images of the

crater inner wall recorded set-wise along the crater axis at a

step size of 50 Am. The 3-D display function used for the

construction is a software package based on the standard

‘‘DFD’’ (Depth From Defocus) method that came with the

microscope system.

3. Results and discussion

In the case of bead analysis, a non-destructive technique is

mandatory in order to preserve the original quality appearance

of the bead. For this purpose, the requirement of relatively

low laser fluence and fixed irradiation spot must be strictly

observed. However, for a soft sample such as the fresh white

coral, repeated irradiations on a fixed position of its surface

will inevitably create a quickly deepening crater. We have

shown previously [29] that atomic emission from plasma

generated in a deep crater exhibited rapid intensity drop with

increasing ambient pressure. The interplay between ambient

pressure and plasma temperature as well as the crater depth

(ablated mass) and hence plasma emission intensity was

discussed in details by Sdorra and Niemax [30]. Specifically,

higher pressure was shown to give rise to higher laser

shielding effect and thereby leading to higher plasma

temperature and smaller ablated mass (shallower crater),

while the reverse was true at low pressure. Somewhere in

between these two extremes, the maximum emission was

observed, corresponding to a compromise achieved between

the high plasma temperature and large ablated mass. Further,

the effect of decreasing emission intensity due to the

increasing crater depth at a fixed ambient pressure was also

discussed by Corsi et al. [31] at the operating condition of

conventional LIBS. It was shown specifically that a percep-

tible decrease in emission intensity from copper plasma

occurred as the crater depth was increased from 1.0 mm to

1.5 mm. In view of those effects, we have found it useful to

give some idea of the sizes of craters produced on the soft as

well as hard samples investigated in this experiment. Fig. 3

shows the crater produced by 50 shots of laser irradiation on

fresh white coral under 99 kPa. Specifically, Fig. 3(a) gives

the front view from the surface and (b) describe the entire

crater shape obtained by VHX-100 digital microscope as

explained at the end of the previous section. The crater

diameter and depth are 400 Am and 3.8 mm, respectively.

This result shows that even at atmospheric pressure, the

resulted crater on a soft sample can be very deep. Based on

the result of a previous study cited earlier [31], with a crater

of such a depth, the conventional LIBS is not likely to be

suitable for bead analysis. On the other hand, we have shown

in our previous study using a low pressure atmosphere, that

even if a hole is produced, the bright plasma emission

emerging from the hole is more or less the same as that in the

absence of a hole [29]. Therefore in this study, all of the

analyses were performed in a low pressure surrounding air.

Another reason for using low pressure surrounding gas is

specifically related to the plasma emissions of the light

elements of hydrogen and carbon which are extremely weak

at high pressures even though they are the major constituents in

the sample. The case of hydrogen emission was already

reported before [27,28] while the pressure-dependence of

carbon emission will be described below. Fig. 4(a) shows the

pressure dependence of the C (I) 247.8 nm emission intensity

using fresh coral samples. Each data point was obtained by an

average of 20 laser shots. It can clearly be seen that the carbon

line intensity increases steeply from 260 Pa up to 1.3 kPa and

sharply decreases from 1.3 kPa up to 6.5 kPa followed by a

slower decay thereafter. It should be noted that the line

intensity obtained at 99 kPa is practically negligible compared

to that obtained at 1.3 kPa (about a factor 30). Fig. 4(b) shows

the pressure dependence of the C (I) 247.8 nm intensity from a

marble sample. As in the case of fresh coral sample, the carbon

line intensity sharply increases from 260 Pa up to 1.3 kPa and

finally declines to about one third of its maximum at 99 kPa. It

should be noted however that while sharing similar basic

pattern of variation, the two curves do differ in quantitative

details. Although no specific explanation is offered from this

study, the differences may have their origins in the different

hardness or matrix effects as well as the crater depth effects as

evidenced by the shallow crater depth of 0.66 mm shown in

Fig. 3(c) for the marble sample, compared to that shown in Fig.

3(b) for the fresh coral sample.

As we described in the Introduction, fresh and fossilized

white corals and shells are precious materials for beads.

However, a lot of imitation beads are found in the market.

They are mainly made either from hard wood as imitations of

white coral beads, or from marble or a mixture of plastic and

0

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pressure (kPa)

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) C I 247.8 nm

(a)

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pressure (kPa)

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C I 247.8 nm(b)

Fig. 4. Pressure dependence of the C (I) 247.8 nm emission intensity in (a) fresh

white coral and (b) marble. The standard deviation of the measurement was

5.1%.

Fig. 3. Three-dimensional crater shape of (a) fresh white coral, front position;

(b) fresh white coral, crater shape and (c) marble, front position. Crater

produced by the 50 shots laser irradiation under air pressure of 99 kPa.


shell, as imitations of fossilized corals and shells. Therefore a

spectral analysis of the above material might be useful to

discriminate between them. Since both coral and shell are

organic materials produced in the sea, our main focus was

directed to the analysis of C, H and Na elements. Aside from

the above elements, ratio of ionic Ca and neutral Ca was also

examined as a marker for determining the hardness of the

samples as suggested previously [32].

Fig. 5 shows the emission spectra of fresh white coral in the

wavelength region between (a) 220 and 290 nm, (b) 370 and

440 nm, (c) 550 and 620 nm and (d) 620 and 680 nm. Fig. 5(a)

shows the C (I) 247.8 nm line and two magnesium lines, Mg (II)

279.5 nm and Mg (I) 285.2 nm which is comparable with the C

(I) 247.8 nm emission. This specific characteristic was exhibited

by all samples of fresh white coral investigated in this work. In

contrast to this, the spectra obtained from hard wood samples

invariably show extremely weak emission intensity of magne-

sium as compared to the associated carbon emission intensity.

Fig. 5(b) shows the ionic calcium emission (Ca (II) 393.3 nm

and Ca (II) 396.8 nm) and also neutral calcium emission (Ca (I)


422.6 nm). The ratio of Ca (II) 396.8 nm/Ca (I) 422.6 nm for

this sample is around 1.68, while it is around 0.80 for hard

wood. This result clearly shows that higher (lower) ratio of Ca

(II) 396.8 nm/Ca (I) 422.6 nm emission intensities corresponds

to harder (softer) material in agreement with our previous work

[32]. This ratio can therefore be used as an additional marker to

distinguish between coral and hard wood. In the longer

wavelength region, two strong sodium emission lines (Na (I)

588.9 nm and Na (I) 589.5 nm) were detected as seen in Fig.

5(c), although they are not well resolved due to the low

resolution of the spectrograph used in these experiments. In

contrast, only weak emission of sodium was observed from a

hard wood sample. It is interesting to add that in both cases, two

strong peaks occur at 560 nm and 610 nm, both of them feature

relatively broad tails as shown in Fig. 5(c). This spectral

characteristic has been observed previously and attributed to the

emission from C–Hmolecular cluster formed during the plasma

expansion [33]. Fig. 5(d) shows a sharp hydrogen emission line

(H (I) 656.2 nm), although the intensity is somewhat low

considering that hydrogen is one of the major constituents of

fresh white coral. This may well be related to the fact that in a

soft sample, ablation takes place with relatively weak repulsion

0

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210 230 250 270 290

wavelength (nm)

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C I 247.8 nm

Mg II 279.5 nm

Mg I 285.2 nm

(a)

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400

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1400

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1800

2000

540 560 580 600 620wavelength (nm)

inte

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Na I 588.9 nm

Na I 589.5 nm

(c)

Fig. 5. Emission spectra of a fresh white coral sample in a low pressure plasma of 1.3

(c) 550 and 620 nm and (d) 620 and 680 nm.

and hence relatively weak shock wave and less effective

excitation process [33]. It should be noted that other sharp

and strong emission lines appearing near the hydrogen line are

associated with the second order calcium emission.

Fig. 6 shows the emission spectra of a giant shell in the

wavelength region between (a) 220 and 290 nm, (b) 370 and

440 nm, (c) 550 and 620 nm and (d) 620 and 680 nm. Fig. 6(a)

shows a very strong carbon emission (C (I) 247.8 nm) along

with magnesium lines (Mg (II) 279.5 nm and Mg (I) 285.2 nm).

This is a typical spectrum of a shell, characterized by the

relatively large ratio of C (I) 247.8 nm/Mg (II) 279.5 nm which

was found to be around 1.67 (in the case of fresh white coral,

this ratio is slightly less than 1). Actually, we have found from

the spectra of various types of shells that the ratio of C (I)

247.8 nm/Mg (II) 279.5 nm is more or less the same. Fig. 6(b)

shows the detection of ionic calcium emission (Ca (II) 393.3 nm

and Ca (II) 396.8 nm) and neutral calcium (Ca (I) 422.6 nm).

The ratio of Ca (II) 396.8 nm/Ca (I) 422.6 nm is approximately

3.70. This result clearly shows that shell is much harder than

fresh white coral (1.68). On the other hand, the general spectral

pattern in the wavelength region of 540 nm–620 nm shown in

Fig. 6(c) is quite similar to Fig. 5(c), including the two strong

0

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3500

350 370 390 410 430 450

wavelength (nm)

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Ca II 393.3 nm

Ca II 396.8 nm

Ca I 422.6 nm

(b)

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610 630 650 670 690wavelength (nm)

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kPa in the wavelength region between (a) 220 and 290 nm, (b) 370 and 440 nm,

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C I 247.8 nm

Mg II 279.5 nm

(a)

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Ca II 393.3 nm

Ca II 396.8 nm

Ca I 422.6 nm

(b)

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Na I 589.5 nm

(c)

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(d)

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wavelength (nm)

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(e)

H I 656.2 nm

Fig. 6. Emission spectra of an original giant shell in a low pressure plasma of 1.3 kPa in the wavelength region between (a) 220 and 290 nm, (b) 370 and 440 nm,

(c) 550 and 620 nm, (d) 620 and 680 nm and (e) 620 and 680 nm in a high pressure plasma of 99 kPa.



sodium emission lines (Na (I) 588.9 nm and Na (I) 589.5 nm) as

well as the two strong emission peaks associated with the C–H

clusters in the plasma as already mentioned earlier. Unlike the

case of fresh white coral, the hydrogen emission line (H (I)

656.2 nm) presented in Fig. 6(d) is sharp and strong with an

extremely low background signal in clear contrast to that shown

in Fig. 5(d). Thus, this hydrogen emission can serve as a useful

marker to distinguish the organic sample from glass sample

which does not give rise to such a strong hydrogen emission. It

is important to point out however that this suggestion only holds

for H (I) 656.2 nm detection from low pressure plasma [27,28].

In fact the result obtained from the same sample by means of

conventional LIBS is presented in Fig. 6(e) which shows an

extremely weak H (I) 656.2 nm emission with a very broad

FWHM (full width at half maximum) which clearly indicates

the general advantage of hydrogen analyses at low pressure

surrounding gas, even for organic samples. In addition to

spectral feature related to hydrogen emission, it is also

interesting to add that the strong broad band spectra around

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C I 247.8 nm

Mg II 279.5 nm

Mg I 285.2 nm

(a) (b

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Na I 588.9 nm

(c)

620

(d

Fig. 7. Emission spectra of an imitation bead in a low pressure plasma of 1.3 kPa in t

and 620 nm and (d) 620 and 680 nm.

610–630 nm observed for fresh white coral samples (Fig. 5(d))

is significantly reduced in the present case. The reduction of

those band spectra is easily understood by the fact that shell is

much harder than fresh white coral and, in such a case, ablation

takes place more effectively due to the strong repulsion force of

the hard sample, and the ablated clusters will be dissociated into

the atomic state.

In order to compare the original beads made of shell with

any imitation, in Fig. 7 the emission spectra of imitation

beads were measured in the same wavelength regions of (a)

220–290 nm, (b) 370–440 nm, (c) 550–620 nm and (d)

620–680 nm. Fig. 7(a) shows a very weak emission line

specific to carbon (C (I) 247.8 nm) along with emissions from

magnesium (Mg (II) 279.5 nm and Mg (I) 285.2 nm). This

result is quite different from the result obtained from giant

shell (Fig. 6(a)) as distinguished by the very low ratio of C (I)

247.8 nm/Mg (II) 279.5 nm of around 0.09, compared to that

for giant shell, 1.67. It is found that measurements performed

on a variety of imitations collected from local market have

0

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350 370 390 410 430 450wavelength (nm)

inte

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Ca II 393.3 nm

Ca II 396.8 nm

Ca I 422.6 nm

)

0

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610 630 650 670 690wavelength (nm)

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H I 656.2 nm

)

he wavelength region between (a) 220 and 290 nm, (b) 370 and 440 nm, (c) 550


yielded practically the same ratio with a standard deviation of

less than 3%. Based on these results, the ratio of C (I) 247.8

nm/Mg (II) 279.5 nm can be used as one of the parameters to

distinguish between original shells and imitations. Fig. 7(b)

shows the detection of ionic calcium emission (Ca (II) 393.3

nm and Ca (II) 396.8 nm) along with neutral calcium

emission (Ca (I) 422.6 nm). The ratio of Ca (II) 396.8 nm/

Ca (I) 422.6 nm is around 3.68. This result is more or less the

same as for giant shells (3.70), which is very similar to

original beads made from shell. Likewise, Fig. 7(d) shows a

strong and sharp hydrogen emission line (H I 656.2 nm) with

an extremely low background signal, as was also found in the

case of original shell (Fig. 6(d)). On the other hand, the

sodium emission lines (Na (I) 588.9 nm and Na (I) 589.5 nm)

in Fig. 7(c) are relatively weak compared to the sodium

emission obtained from the giant shell sample (Fig. 6(c)).

This provides an evidence that the imitation beads are not

made from the shell alone. Based on this result, the imitation

bead is probably made from a mixture of plastic, shell and a

large amount of MgO.

As the material for a bead, fossilized white coral is much

more desirable than fresh white coral. Therefore a spectral

analysis of fossilized white coral was also carried out in this

study. We confirmed that carbon emission remained visible to

some extent. Measurement of the ratio of Ca (II) 396.8 nm/Ca

(I) 422.6 nm yielded a result of 2.0 which is much higher than

fresh white coral. This implies that coral becomes harder

during the fossilization process. Two strong sodium lines were

also observed in this fossilized coral, indicating that this

material has the same origin as its predecessor.

Another interesting result obtained in this fossilized coral

sample is the absence of silicon emission which was in contrary

to our expectations for a fossilized sample. A similar

phenomenon was also observed in our previous work on a

horn fossil sample [34]. In order to clarify this finding, a thin

film was prepared from the fossilized white coral sample

because atomization process is expected to proceed more easily

from a thin film than that from a hard bulk sample. For this

purpose, the substrate for the film was tilted at an angle of 45-with respect to the laser light and placed at a distance of 20 mm

from the laser focusing position on the sample surface.

Altogether 12,000 laser shots were used to deposit a thin film

of fossil above a copper substrate. A microscopic inspection of

the fossil film showed that less than micro-scale particles were

deposited on the copper substrate. The resulting film was then

irradiated by a laser light under �10 mm defocusing conditions

in order to avoid the ablation of the copper substrate while

being sufficiently strong to atomize the film sample. Visual

inspections of the fossil thin film during spectral acquisition

revealed the absence of green color associated with copper

emission. In this case specific emission lines of Si (I) 251.6 nm

and Si (I) 288.1 nm were clearly observed.

4. Conclusion

The results reported herein show that non-destructive bead

analysis can be achieved using an approach based on the

laser-induced plasma spectroscopy. For this purpose, a low

pressure plasma technique was used instead of the conven-

tional LIBS technique in air. This choice is mainly justified

by the fact that for soft sample as fresh white coral, the crater

created is very deep, resulting in an energy loss inside the

crater at atmospheric ambient pressure while the same adverse

effect was relatively negligible at low ambient pressure.

Another reason for the choice comes from the fact that the

hydrogen and carbon emissions as the major spectral

components for the analysis are seriously degraded in terms

of emission efficiency and due to a broadening effect

occurring at atmospheric pressure, a condition usually found

in the conventional LIBS technique. By comparing the

carbon, hydrogen, sodium and magnesium emission intensi-

ties and by measuring the hardness of the sample using the

ratio of Ca (II) 396.8 nm/Ca (I) 422.6 nm, original fresh coral

beads can be distinguished from imitations made from hard

wood. The same technique was also effective in distinguish-

ing between beads made from shell and imitations. A further

spectral analysis on fossilized white coral sample indicates

that the results can be used to some extent to distinguish

between the fossilized white coral beads and imitations made

from marble. It should be noted that the results presented in

this report are those representing the typical and consistent

characteristics of each sample group. In spite of the lack of

statistical justification for quantitative evaluation, the results

obtained in this study are certainly still meaningful for

qualitative analysis. As such, the results obtained by the

non-destructive method for bead analysis reported in this

paper also holds promises for extension of this method to

applications in other areas of forgery inspection.

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Elemental analysis of bead samples using a laser-induced plasma at low pressure

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