Filter packages for the NH4+ diffusion technique were prepared as follows: KHSO4 (10 μL, 2.5 M) was pipetted onto a strip of glass fibre filter (APFE, 25 mm, Millipore), and the filter was
then wrapped and enclosed with a hydrophobic filter (‘Mitex’, PTFE, 47 mm diameter, Millipore) to form the filter package