The number and initial positions of the bands
were determined by calculating the second derivative of the massif for
each spectrum (Fig. 3). Each minimum indicates the position of aband
(Fig. 3 a, b). The positions are manually read at the minimum of each
band. It can be noticed that for the archeological copal, the internal
and external sampling CH stretching regions present the same number
and band positions. The different parameters are introduced to the
ASREL software: the bands positions P are entered according to the
second derivative minima's values; W has an arbitrary initial value of
10 cm−1 (viable value for a molecular vibrator) and is constrained to
positive values, K has an initial value of 0.5 and is confined between 0
(Gaussian profile) and 1(Lorentzian profile), and finally A is just
limited to positive values. Concerning the fitting procedure, it is based
on a least squares method which adjusts the band parameters to fit
the experimental spectrum.