June 10, 2009 / Vol. 7, No. 6 / CHINESE OPTICS LETTERS 545
Correction and analysis of lead content in soil by
laser-induced breakdown spectroscopy
Chengli Xie (
«««
|||
)
1∗
, Jidong Lu (
ººº
UUU
ÀÀÀ
)
1∗∗
, Pengyan Li (
ooo
+++
ýýý
)
1
,
Jie Li (
ooo
$$$
)
1
, and Zhaoxiang Lin (
îîî
)
2
1
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
2
College of Electrics and Information Engineering, South-Central University for Nationalities, Wuhan 430074, China
∗
E-mail: chenry xie@sina.com.cn; **e-mail: jdlu@mail.hust.edu.cn
Received October 7, 2008
The laser-induced breakdown spectroscopy is used to analyze the lead content in soils. The analyzed
spectral line profile is fitted by Lorentzian function for determining the background and the full-width
at half-maximum (FWHM) intensity of spectral line. A self-absorption correction model based on the
information of spectral broadening is introduced to calculate the true value of spectral line intensity, which
refers to the elemental concentration. The results show that the background intensity obtained by spectral
profile fitting is very effective and important due to removing the interference of spectral broadening, and
a better precision of calibration analysis is acquired by correcting the self-absorption effect.
OCIS codes: 300.0300, 330.1880.
doi: 10.3788/COL20090706.0545.
Laser-induced breakdown spectroscopy (LIBS) has been
researched as a diagnostic tool of c omposition analysis
since the early 1980s, and has developed rapidly for many
purp oses over the past two decades, such as identifying el-
ements, material identification, process monitoring, ma-
terial sorting, and site scr e e ning
[1]
. The LIBS technique
offers the prospect of non-destructive, in situ, rapid, and
highly selective and sensitive detection and analysis of
both natur al and man-made materials. Especially, LIBS
combining with fiber optic technology brings the poten-
tial for designing a portable and s tandoff distance LI BS
analyzer, which is re quired highly in environmental pol-
lution monito ring.
The early resear ch institutes in the environmental ap-
plication of LIBS included Diagnostics Instrumentation
and Analysis Laboratory (DIAL) at Mississippi State
University and US Army Research Labo ratory (ARL),
which respectively analyzed the contaminated soil fr om
army site and detected the resources conservation and
toxic heavy metals in the off-gas of industrial plants
and in liquids
[2,3]
. The application of LIBS on the
analysis of polluted soils, especially on the detection of
noxious heavy metals, was investigated by some other
researchers
[4]
. Recently, LIBS was researched on the
detection of lunar soil for moon exploration
[5]
. Much
work was addressed to the important issues of sensitiv-
ity and analytic precision, and the quantitative analy sis
was verified universally to be influenced remarkably by
the variation of matrix component and self-absorption
phenomenon in optical emission spectrosc opy
[6,7]
. The
well-known curve of growth (COG) method was used by
Bulajic et al.
[8]
for correcting the self-abso rption effect in
calibration-free LIBS. A self-a bs orption model in LIBS
measurements on soils and sediments was constructed
for correcting the calibration curves of intensity by Lazic
et al.
[9]
. These mo dels are generally based on detailed
plasma parameters such as size, temperature, electron
density, and so on, some of which are hard to be ob-
tained or involve complicated calcula tion. I n this letter,
the c ontaminated soil with wide range co nce ntration of
lead element is analyzed by LIBS. The spec tral profile of
Pb 405 .78-nm atomic emission line is fitted in Lorentzian
curve according to the main broadening mechanism of
LIBS. Also a co rrection model is introduced to calculate
the influence degree of self-absorption on the net line in-
tensity, and the effectiveness of the model is verified ex-
perimentally.
The degre e of spe ctral line self-absorption is parame-
terized by the self-absorption coefficient (SAC), which is
defined as the ratio of the measured spectral line peak
intensity to the spectral line peak intensity without self-
absorption. With this definition, SAC is equal to 1 if
the line is not self-absorbed and decreases towards 0 if
the self-absorption increases. According to the classic
sp e ctroscopy theory
[10,11]
, the emission spectral line peak
intensity (W/cm
3
) corresponding to the transition from
the upper level j to the lower level i of a n excited atom
or ion is determined by
I(λ) = F
8πhc
2
λ
3
0
n
j
n
i
g
j
g
i
(1 − e
−k(λ)l
). (1)
If the plasma is assumed optically thin, i.e., the self-
absorption is absent and k(λ)l≪1, the spec tral line pea k
intensity I
0
(λ) can be approximated as
I
0
(λ) ≈ F
8πhc
2
λ
3
0
n
j
n
i
g
j
g
i
k(λ)l. (2)
In Eqs. (1) and (2), F is a constant factor depending
on the mea suring instrument, h is the Planck constant
(J·s), c is the speed of light (m/s) λ
0
is the transition
wavelength (m), n
j
, n
i
, g
j
, and g
i
are the number densi-
ties (cm
−3
) and the degeneracies (dimensionless) of the
upper and the ground atomic levels (for the resonance
transition), respectively, k(λ) is the frequency-dependent
absorption co e fficient (cm
−1
), and l is the absorption
path length (cm).
1671-7694/2009/060545-04
c
2009 Chinese Opt ics Letters