I performed Tricine SDS PAGE according to the Nature protocol. Included in the gel is 4% stacking, 10% spacer, and 16% resolving. I intended to separate peptides lower than 10kDa. Therefore, I used the Vivaspin 10kDa membrane filter. I loaded around 30 ug of protein. However, there were smaller than 10kDa bands appear within the retentate of the 10kDa filter. How is this possible and how to resolve this? Attached is the photo. Thank you in advance everyone.