Why a 5 mg Vial Costs More Than Another 5 mg Vial

Buyers reasonably assume that 5 mg of one peptide should cost roughly what 5 mg of another does. It does not, and the gap can be tenfold. The reason is visible in the molecular formula.

Peptides are built one residue at a time

Solid-phase peptide synthesis assembles the chain on a resin support. Each amino acid is coupled, washed and deprotected before the next is added. A five-residue peptide is five cycles. A forty-residue peptide is forty.

Each cycle is efficient but not perfect. Suppose a coupling step succeeds 99.5% of the time, which is good. Over five steps you keep about 97.5% of chains intact. Over forty steps you keep about 82%. The remainder are deletion sequences: chains missing a residue, chemically almost identical to the target and therefore difficult and expensive to separate out.

That is the whole cost curve.

Worked from this catalogue

Tesamorelin is nearly thirteen times the mass of GHK-Cu. It is not thirteen times harder to make, it is considerably more than that, because difficulty rises faster than length.

What this means when comparing suppliers

A high purity figure on a short peptide is weak evidence. Almost anyone can make a tripeptide at 99%. It says little about whether the same supplier can handle a 40-residue chain.

A high purity figure on a long peptide is strong evidence, but only with a chromatogram attached. The impurities that matter on long chains co-elute close to the main peak, and only the trace shows whether they were actually resolved rather than merged into it.

So when two suppliers both claim 99% on retatrutide and one is half the price, the question is not who is cheaper. It is which one can show the trace.

The counterion nobody quotes

Synthetic peptides are isolated as salts, usually trifluoroacetate or acetate. That counterion is real mass in the vial. A vial can be 99.9% pure and still contain appreciably less peptide than the label mass suggests, because purity describes what the material is, not how much of it is peptide.

Net peptide content is the measurement that closes the gap, and it is rarely reported because quoting it makes a product look worse beside a competitor who does not. Covered in why purity figures differ between suppliers.

Related

• What HPLC purity percent actually means
• How to read a certificate of analysis
• Peptide comparison index
• Peptide glossary