Amino acid, peptide, sequence
Amino acids are the building blocks. Link several into a chain and you have a peptide; make the chain long enough and it is called a protein. The sequence is the order of amino acids in the chain, written with letter codes, for example GEPPPGKPADDAGLV for BPC-157. The sequence defines the identity of a peptide: two vials with the same sequence contain the same molecule whatever the label says. Short peptides have a few residues (GHK-Cu three, KPV three), long ones have dozens (retatrutide 39). Length and any additions, such as a lipid chain, determine how demanding the synthesis is and why prices differ so much between peptides.
BAC water (bacteriostatic water)
Bacteriostatic water is sterile water with 0.9% benzyl alcohol, which inhibits bacterial growth. In the laboratory it serves as a solvent for reconstituting lyophilised peptides when smaller amounts have to be drawn from the same vial several times for analysis. The preservative suppresses microbial contamination across repeated punctures of the stopper; it does not replace aseptic technique. A review of preservatives for protein and peptide formulations gives exactly this as the main reason for using preserved water in multi-use vials. Slovenian searches spell it bakteriostatična, bakteriostatska, bakteriostatik or BAC voda; all mean the same thing.
CAS number
A CAS number is the unique identifier of a chemical substance, assigned by the Chemical Abstracts Service. Think of it as the tax number of a molecule: names vary by language and trade name, the CAS number does not. A serious product sheet and COA list it because it allows verification in independent databases. Be careful: the free peptide and its salt or complex can carry different CAS numbers, so when comparing suppliers make sure you are comparing the same form.
COA (certificate of analysis)
A certificate of analysis is the document in which a laboratory summarises what it measured for one specific batch. It must state the identity of the peptide, the measured purity, the methods used, the batch number and the date. A COA without a batch number or date has little evidential value, because quality varies between batches. A practice article on certificates of analysis in pharmacy describes which data must be present and why. On peptid.si the COA for every batch is public in the COA Vault; the batch number on the vial label must match the document.
HPLC and purity
HPLC (high-performance liquid chromatography) separates the components of a sample and measures what share of the signal belongs to the main peptide. The result is a purity figure in percent, for example at least 98%. The remainder is impurities: truncated or misassembled chains, oxidised forms and leftovers from synthesis. HPLC tells you how pure a sample is, not which molecule the main peak is, which is why it is always paired with mass spectrometry. The methods review on HPLC analysis and purification of peptides is the standard reference for how a chromatogram is read.
Lyophilised (freeze-dried) powder
Lyophilisation is freeze-drying: a peptide solution is frozen and the water removed under vacuum, leaving a dry powder or a compact cake. Without water, the chemical breakdown driven by hydrolysis slows dramatically, which is why a lyophilised peptide stays stable for months in a refrigerator while a solution lasts only a short time. A review of instability in pharmaceutical proteins lists lyophilisation among the basic stabilisation strategies. On arrival the powder is white to slightly yellowish; GHK-Cu is blue because of the copper. A colour that does not fit the compound is a warning sign.
MS (mass spectrometry)
Mass spectrometry measures the mass-to-charge ratio of the molecules in a sample and so confirms that the main HPLC peak really belongs to a peptide of the expected mass. If the COA states a measured mass, you can compare it with the theoretical mass of the compound (about 1419 Da for BPC-157). Analytical studies show that liquid chromatography coupled with high-resolution mass spectrometry detects impurities below 0.1%, which UV detection alone cannot. HPLC and MS are a pair: the first measures how much, the second confirms what.
Reta, BPC, GHK: the nicknames
Reta is the shorthand for retatrutide (LY-3437943) that took hold in online communities. In the same way BPC stands for BPC-157, GHK for GHK-Cu, TB for TB-500 (thymosin beta-4), and semaglutide often becomes "sema". Nicknames are convenient but misleading in a research setting, because they do not distinguish forms of the same substance (free peptide, salt, complex) and because sellers use them inconsistently. On a COA and a label always look for the full name, the sequence or the CAS number. A seller who uses only nicknames in a description and adds promises about effects is showing a warning sign we covered in the article on spotting counterfeit peptides.
RUO (Research Use Only) and SPPS
RUO means the substance is intended exclusively for research: in vitro experiments and model studies, not use in humans or animals, diagnosis or treatment. The label does not mean lower quality; it means a different purpose and a different legal framework. In April 2026 JAZMP warned that injectable peptides sold as products for people hold no marketing authorisation. SPPS (solid-phase peptide synthesis) is the method by which almost every synthetic peptide is made today: amino acids are attached one by one to a chain anchored on a resin, in the procedure Bruce Merrifield introduced in 1963. Understanding these two abbreviations explains why the same molecule can exist as a laboratory reagent and, at the same time, as a drug candidate in clinical trials.
References / Links
- Kent SBH (2025). Fundamental Aspects of SPPS and Green Chemical Peptide Synthesis. Journal of Peptide Science. PubMed
- Mant CT, Chen Y, Yan Z, Popa TV, et al. (2007). HPLC analysis and purification of peptides. Methods in Molecular Biology. PubMed
- Zeng K, Geerlof-Vidavisky I, Gucinski A, et al. (2015). Liquid Chromatography-High Resolution Mass Spectrometry for Peptide Drug Quality Control. The AAPS Journal. PubMed
- Zolner WJ (2012). Why do I need a Certificate of Analysis? International Journal of Pharmaceutical Compounding. PubMed
- Stroppel L, Schultz-Fademrecht T, Cebulla M, Blech M (2023). Antimicrobial Preservatives for Protein and Peptide Formulations: An Overview. Pharmaceutics. PubMed
- Akbarian M, Chen SH, Ràfols C, et al. (2022). Instability Challenges and Stabilization Strategies of Pharmaceutical Proteins. Pharmaceutics. PubMed
