I want to separate 1600 Da peptide on SDS PAGE, on 20% GEL I didn't get any band, How can I separate on PAGE? Can I use 25% SDS PAGE?
Image is of an 18% Tris-tricine small-peptide SDS-polyacrylamide gel stained with coomassie blue dye [source].
Your peptide is 1.6 kDa; it would not run off in 20% gel if you don't run it for a long time. Also, try to prevent heating up of the gel (you can run it in a cold room or in front of an A/C vent). However, it is better to use other techniques such as chromatography. You can also try dialysis if other unwanted peptides are much larger.
If band is below the dye front it might not stain very well. Tricine gels are best for such small proteins, and you might might just run until dye front is only half way down the gel. It is also possible that your protein does not stain with conventional coomassie-based stains. Maybe try a silver stain.