Price per vial is the easiest thing to compare and the least informative. A structured set of questions separates suppliers far more reliably, and most of them can be answered before you spend anything.
The five questions
1. Can I see the certificate of analysis for the specific lot you would ship? Not a sample certificate, not one for a previous lot. This single question separates operations that hold documentation from ones that produce it on request. If the answer is a generic specification sheet, that is the answer.
2. Does the COA include the chromatogram? A purity figure with no trace behind it cannot be checked. The trace is where you see whether the remainder is one related impurity or general degradation.
3. What is the peptide content? The figure that determines how much peptide you actually receive. Its absence means your mass arithmetic carries an unknown error, typically 10 to 25 percent.
4. What is the salt form? TFA or acetate. Immaterial for most applications, decisive for some, and a supplier who knows the answer is a supplier who has read their own documentation.
5. How is it stored and shipped? Lyophilised material tolerates ambient shipping, so the absence of cold chain is not a fault. The question is whether they can describe their conditions at all.
Reading the answers
Speed and specificity matter more than the content. A supplier who answers all five within a day, with figures rather than reassurance, is demonstrating that the information exists and is organised. Vagueness on documentation is rarely vagueness about documentation alone.
Why the cheapest listing often is not
Compare on cost per milligram of peptide, not per vial. A $100 vial at 90% peptide content delivers more than a $90 vial at 76%, and the sticker comparison points the wrong way. Add shipping, which is flat rather than proportional and therefore dominates small orders.
Then add the cost of material you cannot use. A cheap lot that arrives without documentation, or behaves inconsistently, costs a repeat experiment — which is almost always more than the saving.
A practical first order
Buy one single vial before buying a pack. It answers the documentation questions, the packaging questions and the communication questions for the price of finding out cheaply. The cost-per-milligram premium on a single vial is the price of that information, and it is usually worth paying once.