@pep_researcher yeah - all respect to that dude. Honestly, he's the GOAT and so many other vendors and researchers owe alot to him, his testing rigor, and even his pen education. Wish him the best. A+ guy and I have had nothing but great success ordering there.

People mix up three different questions:

Purity - what fraction of the UV-absorbing stuff is the main peak
Identity - is that peak the peptide on the label
Amount - how many mg of actual peptide are in the vial

A used Agilent 1100/1200 + C18 column + 0.1 mg balance will do #1 and a version of #3. It will not do #2. FOr #2 you need a Mass Spec machine.
HPLC separates by hydrophobicity and measures peak area at 214/220 nm. Purity is just:
main peak area / all peak areas

Practical cheap setup for HPLC: complete used 1100/1200 gradient HPLC with UV/DAD, one new C18, HPLC ACN/water/TFA, 0.1 mg balance. Ballpark $8–15k could be less on used lab equipment sites but need to make sure it has all the modules. Not easy to keep these machines working well without knowhow.

For identity of what is in the vial you need Mass Spec
Best cheap option: ESI single-quad LC-MS

Agilent 6120/6130/6140, Shimadzu LCMS-2020, Waters QDa/SQD.

Used working ones usually ask $35k–$55k. “Powers on” eBay units at $8k–$20k are often dead. Also really tough to run and maintain - need nitrogen tank at home etc.