As indicated in Table 1, C, N and H contents were determined as using an elemental analyzer (Vario EL, Hanau, Germany). Other elements were measured by inductively coupled plasma optical emission spectrometry (ICP-OES)
using a PS1000 system (Teledyne Leeman Labs, Hudson, NH USA) or inductively coupled plasma mass spectrometry (ICP-MS) using an Elan 6000 system (Perkin Elmer-Sciex, Concord, Canada) or atomic absorption spectrometry (AAS) using a Unicam 939 system (Offenbach, Germany)