Effect of added organic solvent
To improve the extraction efficiency, the kind of added organic
solvents was investigated. The results indicated that the recoveries
obtained by adding methanol were higher than those obtained by
others. Afterwards, the amount of added methanol was investigated
from 0 to 700 lL, the results were shown in Fig. 2a. It was
indicated that 500 lL were adequate for extraction, adding more
methanol did not improve the extraction efficiency but increased
the organic solvent consumption.