Tubes were placed into reaction vial and vacuumed to 100 millitorr before
adding 20 μl of fresh redrying solution (consisting of methanol: water: ethylamine at a ratio of 2:2:1 v/v) and was then mixed thoroughly. Samples were vacuumed to 70 millitorr before adding fresh 20 μl of derivation reagent (consisting of methanol: water: triethylamine: PITC at a ratio of 7:1:1:1), mixed and tubes returned to reaction vial and valve closed.