Comparison of Calibration Curve Method and Partial Least Square Method
in the Laser Induced Breakdown Spectroscopy Quantitative Analysis
Open Access JCC
Figure 5. Predicted VS. Reference concentration (wt%) of
Pb.
3.3. The Advantage of PLS Method
In PLS method, the regression coefficient means the re-
lationship between all independent variable X a nd de-
pendent variable Y, it can be calculated in the corres-
ponding number of principal components. In Section 3.2,
independent variable X (LIBS spectra data) and depen-
dent variable Y (element concentration) are used to es-
tablish the PLS calibration model. The regression coeffi-
cient between spectra data and element concen tration of
Al is shown in the Figure 6. The da rk line of Figure 6
means the regression coefficient with ordinate on the
right side, the light line means the spectra intensity with
ordinate on the left side. The spectral wavelength range
is 350 - 400 nm in the Figure 6. It can be observed that
at wavelength 394.25 nm and 396.06 nm, the regress ion
coefficient is 1.82 × 10−4 and 2.99 × 10−4, that is a larger
value. It means at this wavelength, the element concen-
tration and the spectra data have great correlation in PLS
model. Access to NIST, line 394.4 nm and 396.15 nm is
the Al characteristic spectral line. So in PLS model, in-
dependent variables and dependent variables have great
correlation at element characteristic spectral line (small
differences between the above wavelengths are caused by
error of spectrometer). But the intensity of spectra at the
same wavelength is weak and unstable. So the CC me-
thod calibration model using single spectral line intensity
is not good enough .
At wavelength 352.29 nm there is a peak but not Al
characteristic spectral line, it does not work in the CC
method Al calibration model. The regression coefficient
at the same wavelength is −0.92 × 10−4, it means the
spectral intensity and Al concentration are negative cor-
relation at this wavelength in PLS method calibration
model.
Unlike CC method to select the individual characteris-
tic peak, PLS method is to establish the relationship be-
Figure 6. The regression coefficient and spectrum of Al.
tween the intensity of full spectrum and element concen-
tration. By changing the independent variable space to
build a new principal components coordinate syste m, PLS
method establish positive or negative correlation rela-
tionship between all the spectral line intensity and ele-
ment concentrations. Then by selecting the number of
principal components to reduce the dimension of raw
data, PLS method establish a more stable quantitative
calibration model. We believe that this is the reason why
PLS quantitative calibration model is more effective than
CC method.
4. Conclusion
In this paper, both partial least squares and the calibra-
tion curve method were used to make the quantitative
analysis of LIBS spectral data of lead brass. After estab-
lishing the PLS calibration model, we can quickly get the
results of all the elements concentration. Compared with
the CC method, PLS method is more suitable for the
complicated matrix, element content different alloy.
5. Acknowledgements
This work has been sup po rted by the Equipment Devel-
opment Programs of the Chinese Academy of Sciences
(Grant No. YZ201247), the National High-Tech Research
and Development Program of China (863 Program) (Grant
No. 2012AA04060 8) and the National Natural Science
Fund (Grant No. 61004131).
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