Difficulty pushing peptide solution through filter into pen cartridge

This happens to me with those 4 mm filters pretty often. Grab a fresh one. Some of them just seem stubborn. I wet mine first with bac water and that sometimes makes a difference, though not every time.

One suggestion is to use PBS as the buffer for this peptide, while others recommend a small amount of sodium bicarb instead because it makes filtering easier.

Not sure about this one, but with reta and mots I had similar issues. Adding an extra needle into the cartridge for venting helped. Did you try that?

No experience with this peptide, but I tried filtering some other compound vials with a 4 mm filter and it wouldn't move either. Ended up switching to a 13 mm filter instead.

13 mm looks like the smallest size people usually recommend, but I need to check further. A quick search on syringe filter types and loss rate turned up several useful links. I might go with a good 13 mm once I find one, but the whole point was filtering for endotoxins and it seems they can't handle that.

I checked with an AI and it looks like the 4 mm size is actually the right choice for peptides. The membrane material matters a lot for keeping loss low, and PES or PVDF are preferred because they bind less. Filter diameter also changes how much volume stays behind, with smaller ones losing far less than bigger ones. For volumes up to a few milliliters the 4 mm is best, while 13 mm suits larger amounts.