Illumina TruSeq Small RNA library preparation

[Ilena Li Mura]

Hi, I'm some problems with micro RNA library preparation using Illumina TruSeq Small RNA protocol.
I attach the images of bioanalyzer before gel purification of Ambion Human Brain Total RNA that I use as positive control.
One images is about the control after 11 cycles of PCR Amplification (that is the cycles number that I usually use), but as you can see there is an evident decrease of material greater than approximately 100bp. We observe this kind of profile in all our samples and, in our experience, these are not normal patterns.
The other image is the same control with 15 cycles of PCR amplification and is quite different.

Are these patterns normal or there is any trouble?

Have you any suggestion to solve the problems?

Thanks in advance

[psreddy]

At 15 cycles, 147bp and 198bp bands reduced and 137bp and 184bp bands emerged. How did the gel purification go? On the gel also did you see similar pattern? Were you able to cutout the band excluding adapter dimer. However, i dont see adpter dimer band in this case. Thanks.