peptide purity levels FAQ

Contact the supplier immediately and request a replacement or refund—batch-to-batch variation should not drop below the advertised specification by more than 0.5%. Verify the COA lists batch number, synthesis date, and analytical method (RP-HPLC with gradient details). Legitimate suppliers like Real Peptides provide batch-specific documentation with HPLC chromatograms showing the target peak and impurity profile. If the purity drop is consistent across multiple batches, the supplier's synthesis or purification process has degraded—source from a provider with documented quality control like our full peptide collection where every batch ships with third-party verification.

Perform a secondary purification step in-house using preparative HPLC with a different column chemistry than the vendor used for initial purification. If the vendor used reverse-phase C18, switch to a C4 column with a shallower acetonitrile gradient. This often resolves deletion peptides that co-elute on C18. Collect the main peak fraction, lyophilize, and re-analyze by HPLC to confirm purity improvement. This approach typically increases purity by 2–3 percentage points but requires access to HPLC equipment and extends peptide preparation time by 3–5 days. For peptides where 98%+ purity is non-negotiable. Such as Sermorelin used in growth hormone receptor binding studies. The investment in secondary purification eliminates the dominant source of experimental error.

The deletion peptides in the remaining 5% are likely competing for receptor binding without full activation, shifting your EC50 and preventing saturation. Switch to a 98% pure batch of the same peptide and re-run the assay—if the curve now plateaus cleanly, impurity interference was the variable. This is common with peptides like Ipamorelin or Hexarelin where truncated sequences missing the C-terminal residues bind growth hormone secretagogue receptors but fail to activate the Gq signaling cascade. You're not seeing bad technique—you're seeing pharmacological interference from synthesis by-products.

Not necessarily—HPLC chromatograms always show multiple peaks because no synthesis is 100% pure. The target peptide appears as the dominant peak, with smaller peaks representing deletion sequences, salts, and residual solvents. Purity percentage is the area under the target peak divided by total peak area. Examine the chromatogram: if the target peak accounts for 98% of the total area and the remaining peaks are small (each <1%), the peptide meets research-grade standards. If you see a second large peak representing 5–10% of the total area, that's a deletion peptide—the batch doesn't meet spec and should be rejected. Real Peptides provides annotated chromatograms that label the target peak and identify major impurities so you're not interpreting raw HPLC output.

Request the raw HPLC chromatogram and verify the integration method used to calculate purity. Vendors sometimes use manual baseline correction or peak integration settings that overestimate the target peptide peak area by 2–5%. If the chromatogram shows poorly resolved peaks or a rising baseline, the reported purity may reflect operator judgment rather than objective measurement. Re-analyze the peptide using an independent HPLC method with a shallower gradient to improve peak separation. If additional impurity peaks emerge, the true purity is lower than stated. For dose-response studies already underway, calculate a correction factor based on the true purity and adjust all stock concentrations retroactively to maintain internal consistency across your dataset.

Multiple MS peaks usually indicate oxidation of methionine or cysteine residues, deamidation of asparagine or glutamine, or residual TFA adducts that don't significantly alter retention time on HPLC. Check the mass difference between peaks: a +16 Da shift from the expected mass indicates methionine oxidation, while a +1 Da shift suggests deamidation. Both modifications occur during storage if the peptide is exposed to air or stored in solution rather than lyophilized. If MS shows peaks at +96 Da or +196 Da intervals, the peptide contains TFA adducts from incomplete removal during lyophilization. These don't affect biological activity but skew concentration measurements if you're quantifying by weight rather than UV absorbance. For oxidation-sensitive peptides, store under argon or nitrogen atmosphere at −20°C and reconstitute in degassed buffer immediately before use.

Poor solubility usually indicates lyophilization technique or counterion selection, not purity. Peptides synthesized as trifluoroacetate (TFA) salts dissolve faster than acetate salts but are more hygroscopic and harder to dose accurately. If a peptide won't dissolve in bacteriostatic water at the expected concentration, try adjusting pH slightly with dilute acetic acid (lower pH for basic peptides) or ammonium hydroxide (raise pH for acidic peptides). If the peptide still clumps, aggregation may have occurred during storage due to temperature excursions above 8°C—aggregated peptides are functionally inactive even if HPLC purity was correct at synthesis. Request a replacement and verify cold chain integrity during shipping.

Normalize all peptide concentrations to the batch with the lowest purity to maintain internal consistency. If Batch A is 98.2% pure and Batch B is 96.8% pure, calculate the ratio (96.8 / 98.2 = 0.986) and multiply all concentrations prepared from Batch A by 0.986 before analyzing dose-response data. This correction assumes the impurities are biologically inactive. A reasonable assumption for deletion peptides in most cases but not for peptides where truncated sequences retain partial activity. For studies spanning multiple years where batch-to-batch variability could introduce artifacts, purchase a single large batch at the highest available purity and aliquot it for long-term storage at −80°C to eliminate purity drift as a variable. At Real Peptides, we've seen multi-year receptor pharmacology studies succeed or fail based entirely on whether the lab used a single batch or mixed batches without purity correction.