A stock solution comprising a compound of formula (I) Wherein R1 together with the carbonyl group to which it is attached is a residue of a carboxylic acid, and NR2R3, in which R3 is H or together with R2 and the N-atom to which they are attached, a 5-membered ring, is a residue of an amino acid, in particular a proteinogenic amino acid, ornithine, gamma-aminobutyric acid or beta alanine, or a 1-amino cycloalkyl carboxylic acid.