How do I know whether my miRNA has been successfully purified and how much is expected?

The amount of miRNA varies a lot from different tissues and cells. Generally, the amount of miRNA present is less than 1% of total RNA, but in cell-free RNA (e.g. RNA from serum or plasma), the miRNA content can be much higher. The amount of miRNA is not measurable by UV-absorption such as Nanodrop. miRNA quantification is usually carried out by qPCR.  For cell-free samples such as serum and plasma, we advise the user to add a spike-in control such as C. elegans miR-39 miRNA mimic (miRNeasy Serum/Plasma Spike-In Control) during the extraction, and measure the presence of the miRNA mimic at the end of purification by qPCR to help determine whether the extraction is successful.