Ethylenediaminetetraacetic acid (EDTA), ninhydrine, norleucine and amino acid
standards were purchased from Sigma (St Louis, MO, USA). Chloroform, methanol and
hydrochloric acid were supplied from Panreac (Barcelona, Spain). The Milli-Q water
used was produced using an Elix3 water purification system coupled to an Advantage10
Milli-Q module (Millipore, Molsheim, France). All other chemicals and reagents were
of analytical grade.