The line C(I) 247.856 nm was also evaluated
and the emission signal profiles are presented in Fig. 1. The
effect of the delay time was investigated using a spot size of
50 lm, a 10 Hz laser repetition rate and 75 accumulated laser
pulses at 25 mJ/pulse. The delay time was evaluated as being in
the range of 0–0.4 ls. Fig. 2 shows the effect of delay time on the
emission signal profile, emission signal intensity and SBR for K
(Fig. 2a and c) and Mg (Fig. 2b and d) measurement. The increase
of delay time decreases the integrated area of the emission signal.
However, the emission continuum also decreases with the delay
time and, consequently, SBR increases. For Mg, a delay time higher
than 0.3 ls drastically increases the relative standard deviation
(RSD) since the intensity of the emission signal becomes weak.
Despite the higher delay time being the best alternative, 0.25 ls
was selected because a higher RSD was observed for Mg when
the delay time was higher than 0.3 ls.
The same investigation was carried out for C measurement,
which was used as the internal standard for K and Mg measurement.
The use of internal standard calibration in LIBS is a good
alternative to minimizing the standard deviation of the results,
caused especially by the variations of the laser energy and by the
heterogeneity in the density of material [13], which changes
the amount of ablated material and consequently the precision of
the results. The use of C as an internal standard in wood analysis