A variation of the signal exceeding ±5.0% in the determination of
furosemide, due to the presence of other ions or compounds in urine,
was considered to be indicative of interference. Given this criterion,
only urea caused interference in the analysis.
Urea was therefore eliminated using solid-phase extraction, since
it (and all the other ions) could be easily eluted with water, while
furosemide (which is insoluble in water) was subsequently eluted
using ethanol, during the pre-concentration step.