@Nick: a quick comment - when you measure the voltage drop on a 1Meg resistor pls do consider a standard meter has ~10Meg internal resistance. So, 1Meg || 10Meg = 0.909Meg, thus your nA values will be 9% off.. 
@Nick: a quick comment - when you measure the voltage drop on a 1Meg resistor pls do consider a standard meter has ~10Meg internal resistance. So, 1Meg || 10Meg = 0.909Meg, thus your nA values will be 9% off..