3.3. Calibration
Fig. 4 shows the calibration curves produced by LIBS on the
filter paper standards for Ca (a), Mg (b), and Zn (c). The error bars
correspond to the standard deviations of the signal acquired as
five pulse averages in five different locations on the paper.
Here we note that although resonance lines of Ca (Ca II 396.8 nm)
and Mg (Mg I 285.2 nm) are used for calibration, there is no evidence
of self-absorption even at 5000 ppm Ca. The peaks exhibit no “flattops”
or depressions at their maxima and the calibration curves are
described nicely by linear fits – R2¼0.9987 for Ca and R2¼0.9904 for
Mg. In fact, the only calibration data that suggest the possibility of
self-absorption is for the Zn calibration curve which appears to
decrease in slope as concentration increases. However, the line used
for this calibration (Zn I 481.0 nm) is a non-resonant line and the
calibration curve extends only to 1000 ppm. Under these conditions,
self-absorption is not likely to be an issue [17].
3.4. Determination of Ca, Mg, and Zn
Table 2 shows the concentration of Ca, Mg, and Zn determined
by LIBS using the filter paper calibration technique and determined
by acid digestion and analysis by AA. For LIBS, the numbers
in parentheses are the standard deviation of the concentration