Peaks from 150 – 160 bp (see the graph below on the left) are residues of digested non-specific amplification products. They come from incomplete removal of small-molecular-weight materials during magnetic bead purification after digestion. The digestion reagent cuts non-specific amplification products into small pieces, which are then removed during magnetic bead purification. Peaks from150 – 160 bp are usually caused by inaccurate pipetting of magnetic beads when making a large number of libraries in one day, OR insufficient removal of ethanol during washing steps. To remove any possible small-molecular-weight materials, it is wise to pool all indexed libraries (that will be sequenced in the same lane) and do one additional round of 1.2X magnetic bead purification, before the quantification step prior to sequencing.
Significant peaks from 150 – 160 bp may be caused by under-amplification of the library (see the graph below on the right). Certain panels may tend to produce more non-specific amplification products. In this case, two rounds of magnetic bead purification after the digestion step solves the problem.