NOTES
ArF Laser Induced Plasma
Spectroscopy of Lead Ions in
Aqueous Solutions: Plume Reheating
with a Second Nd : YAG Laser Pulse
X. Y. PU* and N. H. CHEUNG†
Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, People’s Republic
of China
Index Head ings: Laser-induced breakdown spectroscopy; ArF laser
ablation; Plasma reheating; Lead; Aqueous samples.
INT RODUCTIO N
Laser-induced breakdown spectroscopy (LIBS) is a
versatile technique for elemental analysis. Its sensitivity
may not be impressive, but we recently showed that the
detection limits for metal ions in aqueous solutions could
be improved if an ArF (193 nm) laser was used for ablation. 1 The plasm a temperature could be m aintained at
about 0.35 eV for several ms, which was near optimal for
‘soft’ transitions such as the Na 589 nm line. Accordingly, detection limits as low as 0.4 ppb were demonstrated.
For ‘hard’ transitions like the Pb 405.8 nm line, however,
the plasma was cooler than ideal and the detection limit
was only about 300 ppb. The detection limit would be
even poorer if the sample solution did not absorb at 193
nm. This shortcoming might be overcome if the cooling
plume was reheated by a second laser pulse. 2–5 The plasma temperature could be raised and the sensitivity enhanced. In this short note, we report on the application
of laser rekindling to the LIBS analysis of Pb in aqueous
solutions.
amounts of the stock to water blanks. Unlike our previous
study,1 no 193 nm absorbent was added. For temperature
determination, Fe was used as the reporter element and
450 mM Fe(NO 3) 3 solution was ablated.
The experimental setup, shown in Fig. 1, was similar
to that reported earlier.1,6 Only a brief account will be
given here to aid discussion. The sample solution was
pressure-fed by compressed nitrogen into a  ow cell and
ejected in the form of a stable vertical stream through
0.5-mm -i.d. glass tubing. It was ablated downstream by
a focused ArF laser pulse (193 nm, 15 ns, 5 Hz, 18 mJ,
12 J cm 2 2 ). The plume created was subsequently intercepted by a Nd : YAG laser pulse (1064 nm, 6 ns, 56 mJ,
29 J cm 2 2 ). The Nd : YAG  uence was chosen to produce
intense heating yet without breaking down air. The two
laser beams were horizontal and subtended an angle of
308 at the sample. The Nd : YAG beam was displaced
towards the plume front to just miss the liquid jet (see
Fig. 1).
Along a direction transverse to the ArF beam, the luminous plume was imaged (1:2, magniŽ ed) onto the entrance slit of a 0.5-m spectrograph equipped with 2400
lines/m m grating. The slit width was set to 100 and 290
EXPERIMENTAL
Lead nitrate Pb(NO 3 ) 2 was dissolved in double-distilled
deionized water to form the stock solution. Sam ple solutions of graded [Pb] were prepared by adding measured
Received 26 July 2002; accepted 2 January 2003.
* Present address: Department of Physics, Yunnan University, Kunming, PRC.
† Author to whom correspondence should be sent. E-m ail:
[email protected].
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Volume 57, Number 5, 2003
F IG . 1. Schematic representation of the experimental setup, as viewed
from the top. The sample jet was ablated by an ArF laser pulse. The
plume was subsequently reheated by a Nd : YAG laser pulse. Plasma
emissions were imaged onto a spectrograph and detected on an ICCD.
0003-7028 / 03 / 5705-0588$2.00 / 0
q 2003 Society for Applied Spectroscop y
APPLIED SPECTROSCOPY
F IG . 2. LIBS spectra of the plume emissions produced by the Nd :
YAG laser alone (bottom trace), the ArF laser alone (middle trace), and
the ArF pulse followed 1.5 ms later by the Nd : YA G pulse (top trace).
The aqueous sample contained 25 ppm of Pb. The ICCD gate delay td
was 7, 0.5, and 2 ms for the three respective cases. The correspondeing
gatewidth tb was 15, 5, and 5 ms. Each trace was the accum ulation of
300 events. Note that the gentle undulations in the background of the
top trace were reproducible features, probably due to molecular emissions of air, OH, and H 2 O.
mm for linewidth measurem ents and analyte sampling,
respectively, giving an overall spectral resolution of
about 0.08 and 0.24 nm for each case. Except for the
intensely bright core of the plume, the rest of the luminous plume was imaged onto the relatively wide slit. To
avoid the strong 193 nm scatter from the cylindrical jet,
the axis of the collection optics was tilted about 158 from
the horizontal. Plume emissions were detected by a gateable intensiŽ ed charge-coupled device (ICCD) mounted
at the exit plane of the spectrograph.
RESULTS AND DISCUSSION
Rekindling depended critically on the time delay Dt
between the Ž rst ArF pulse and the subsequent Nd : YAG
pulse. If the Nd : YAG laser was Ž red too early (Dt , 200
ns), the plume would not have reached the Nd : YAG
beam path and little reheating was observed. If the Nd :
YAG laser was Ž red too late (Dt . 200 ms), the vapor
F IG . 3. Time evolution of the plasma temperature determined from the
intensity ratio of the Fe(I) 372.0 and 375.8 nm lines, for the respective
cases of the ArF laser alone (open circles) and ArF pulse followed 1.5
ms later by the Nd : YAG pulse (solid circles). A 450 mM [Fe] solution
was ablated. Each data point was based on the accumulated spectral
traces of 300 events. ICCD gatewidth was 100 ns.
F IG . 4. Calibration curve for lead (Pb) analysis. Five data points (open
circles) were collected for each concentration, and every data point was
based on the accumulated spectral traces of 500 events. The non-zero
y-intercept was due to incomplete background subtraction. The slope m
of the best straight line was 1.88 3 10 4 counts ppm 2 1 . For the lowest
[Pb] sam ple analyzed, the standard deviation s of the data scatter was
8.5 3 10 2. The detection limit (3s/m) was 136 ppb.
would have propagated too far and the rekindled plume
would appear thin and elongated. A delay of 1–2 ms maximized the coupling between the 1064 nm photons and
the material vapor to produce the brightest plume with
the loudest crack. The plume enlarged visibly from about
1 mm to about 3 mm in diameter upon reheating. It was
found that a Dt of 1.5 ms gave the best Pb 405.8 nm line
spectrum, which is shown in Fig. 2 (top trace). An ICCD
gate delay t d of 2 ms (since the Nd : YAG pulse) and gatewidth t b $ 5 ms was employed. That combination maximized the ratio of the Pb signal to the background noise.
Also shown in Fig. 2 is the LIBS spectra produced by
the ArF laser alone (m iddle trace) and the Nd : YAG laser
alone with the laser beam redirected towards the sample
jet (bottom trace), with t d and tb properly optimized in all
cases. As can be seen, the detection sensitivity was signiŽ cantly improved by reheating.
The signal enhancement could be attributed to the larger luminous volume as well as the sustained high temperature of the plasm a. The latter is depicted in Fig. 3.
The prolonged heating produced by the Nd : YAG pulse,
in contrast to the m ore transient heat burst generated by
the ArF pulse, contributed to the brighter and longer-lived
Pb 405.8 nm emissions.
In order to quantify the analytical sensitivity, we deŽ ned the gross signal as the average intensity (CCD
counts) in the spectral region from 405.48 through 405.94
nm, bracketing the Pb 405.78 nm line. Background was
deŽ ned as the average intensity in a neighboring region
from 402 through 404 nm . The net signal was deŽ ned as
the gross signal minus the background. Because the continuum background slanted up towards the long wavelength end (see top trace of Fig. 2), the net signal was
not zero even for blank samples. Figure 4 plots the net
signal against Pb concentration. At each concentration,
Ž ve data points were taken; and every data point was the
accum ulation of 500 events. The best straight line was
also drawn. The slope m was 1.88 3 10 4 counts ppm 2 1 .
For the lowest [Pb] of 250 ppb that was analyzed, the
average standard deviation s of the data scatter was 8.5
APPLIED SPECTROSCOPY
589
3 10 2 counts. The detection limit (3s/m) was therefore
estimated to be 136 ppb.
Calibration curves equivalent to Fig. 4 were also obtained for the m ore conventional case of single laser ablation without reheating. The corresponding detection
limits were found to be 2.02 ppm and 12.9 ppm for 193
nm and 1064 nm ablation, respectively.‡ Rekindling improved the detection limit by one to two orders of m agnitude.
Reversal of the laser pulses, with the Nd : YAG laser
Ž red before the ArF laser, was also investigated. However, the analytical performance was poorer because of
two reasons: Nd : YAG laser ablation of the liquid jet was
more chaotic and the continuum emissions generated
were more persistent.
In summ ary, we demonstrated that the LIBS analysis
‡ The detection limit of 2.02 ppm was higher than the 300 ppb reported
in Ref. 1. This is because the present samp le did not absorb at 193
nm. The detection limit of 12.9 ppm was consistent with that reported
in R. Knopp et al.7
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of Pb ions in water could be made sensitive if the plume
created by an ArF laser pulse was reheated by a subsequent Nd : YAG laser pulse. The detection limit for lead
was 136 ppb.
ACK NOW LEDGM ENT
We thank K. M. Lo for his assistance during the initial stages of this
study. This work was supported by the Research Grant Council of the
University Grants Committee of Hong Kong and a Faculty Research
Grant from Honk Kong Baptist University.
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