Elemental analysis of bead samples using a laser-induced plasma at low pressure
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Transcript of Elemental analysis of bead samples using a laser-induced plasma at low pressure
w.elsevier.com/locate/sab
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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0.1 1 10 100
pressure (kPa)
inte
nsi
ty (
cou
nts
) C I 247.8 nm
(a)
0
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300
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0.1 1 10 100
pressure (kPa)
inte
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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.
T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112 107
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)
T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112108
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
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1400
1600
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210 230 250 270 290
wavelength (nm)
inte
nsi
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cou
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)
C I 247.8 nm
Mg II 279.5 nm
Mg I 285.2 nm
(a)
0
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400
600
800
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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)
inte
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cou
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)
Ca II 393.3 nm
Ca II 396.8 nm
Ca I 422.6 nm
(b)
0
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800
610 630 650 670 690wavelength (nm)
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(d)
kPa in the wavelength region between (a) 220 and 290 nm, (b) 370 and 440 nm,
0
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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.
T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112 109
T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112110
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
0
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inte
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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
100
200
300
400
500
600
350 370 390 410 430 450wavelength (nm)
inte
nsi
ty (
cou
nts
)
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)
inte
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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
T.J. Lie et al. / Spectrochimica Acta Part B 61 (2006) 104–112 111
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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