主题：【讨论】瘦肉精比较奇怪的萃取方法

下面是一段测定瘦肉精在人尿液中的样品前处理方法。
作者在尿液中加了C18 吸附剂，然后用乙酸乙酯萃取。最后，弃取C18吸附剂，收集乙酸乙酯。
非常不解，这个C18吸附剂有什么用？
To validate the activity of the β-glucuronidase solution, 100 μL of a 31.7 ng/mL ractopamineglucuronide solution was added to 2 mL of control urine containingenzyme, buffer, and ractopamine internal standard as described above.Enzyme activity was validated with each sample set run. After the 16 hhydrolysis period, 160 μL of 2 M sodium carbonate, 100 mg of sodiumchloride, and 100 mg of C18 sorbent were added to each sample.Samples were subsequently extracted with ethyl acetate (1 mL × 3),followed by centrifuging at 3000g for 10 min. The supernatant wasseparated, placed in 7 mL tubes, and evaporated under a stream ofnitrogen. The residue was reconstituted in 200 μL of 20% aqueousacetonitrile containing 0.1% formic acid and passed through a 0.45 μmPTFE filter. The solution was stored at -20 °C in a [url=https://insevent.instrument.com.cn/t/Yp]LC-MS vial withina silanized vial insert until analyzed
原文连接：http://pubs.acs.org/doi/abs/10.1021/acs.jafc.6b02723