All measurements were carried out using an ANALYTIK Jena AG
(Jena, Germany) model novAA 300 atomic absorption spectrometer
equipped with SpectrAA (Varian, Australia) hollow cathode lamps
as the radiation source. An acetylene-air or acetylene-nitrous oxide
flame was used; the gas flow rates and the burner height were adjusted
in order to obtain the maximum absorbance signal for each
element. Other instrumental parameters were set to the values
shown in Table 1