[{"data":1,"prerenderedAt":379},["ShallowReactive",2],{"article-\u002Fresearch\u002Fnet-peptide-content-vs-purity":3,"article-compounds-\u002Fresearch\u002Fnet-peptide-content-vs-purity":342,"article-more-\u002Fresearch\u002Fnet-peptide-content-vs-purity":364},{"id":4,"title":5,"body":6,"description":309,"extension":310,"faq":311,"meta":324,"navigation":325,"path":326,"publishedAt":327,"readingMinutes":328,"relatedCompounds":329,"seo":333,"stem":334,"tags":335,"updatedAt":340,"__hash__":341},"research\u002Fresearch\u002Fnet-peptide-content-vs-purity.md","Net Peptide Content vs Purity: The Two Numbers Everyone Confuses",{"type":7,"value":8,"toc":294},"minimark",[9,22,25,30,33,39,46,54,60,64,67,72,79,82,89,92,96,105,109,114,157,164,167,171,178,181,209,212,216,219,239,242,246,256,262,268,274,278,281],[10,11,12,13,17,18,21],"p",{},"Here is a result that surprises people the first time they meet it: a vial of\npeptide can be ",[14,15,16],"strong",{},"99% pure"," and contain ",[14,19,20],{},"25% less peptide than the label says",",\nwith no dishonesty anywhere in the chain.",[10,23,24],{},"Both figures are correct. They measure different things. Confusing them is the\nmost common source of systematic concentration error in peptide work, and it is\ninvisible — nothing about the experiment tells you it happened.",[26,27,29],"h2",{"id":28},"purity-is-a-chromatographic-ratio","Purity is a chromatographic ratio",[10,31,32],{},"When a certificate reports 99.2% purity, it means this:",[34,35,36],"blockquote",{},[10,37,38],{},"Of the total peak area recorded by the detector during the HPLC run, 99.2%\nbelongs to the main peak.",[10,40,41,42,45],{},"This is a statement about the ",[14,43,44],{},"peptide-related material"," in the sample. The\nchromatogram is comparing the target peptide against truncated sequences,\ndeletion products, oxidised species and other synthesis by-products.",[10,47,48,49,53],{},"Critically, it is a comparison ",[50,51,52],"em",{},"within"," what the detector can see. UV detection\nfor peptides is typically at around 214 nm, where the peptide bond absorbs.\nSodium chloride does not absorb there. Water does not absorb there.\nTrifluoroacetate barely does.",[10,55,56,59],{},[14,57,58],{},"Everything in the vial that is not peptide is largely invisible to the purity\nmeasurement."," Purity is not describing the vial; it is describing the peptide\nfraction of the vial.",[26,61,63],{"id":62},"where-the-non-peptide-mass-comes-from","Where the non-peptide mass comes from",[10,65,66],{},"Two sources, both unavoidable consequences of how peptides are made.",[68,69,71],"h3",{"id":70},"counter-ion-salt","Counter-ion salt",[10,73,74,75,78],{},"Synthetic peptides are purified by reversed-phase HPLC using mobile phases\ncontaining an acid — most commonly ",[14,76,77],{},"trifluoroacetic acid (TFA)",", which gives\nexcellent peak shape and is the reason it remains standard.",[10,80,81],{},"Basic residues in the peptide — lysine, arginine, histidine, and the N-terminus —\nget protonated in that acidic mobile phase, and each protonated site pairs with a\ntrifluoroacetate counter-ion. When the material is lyophilised, the salt\nlyophilises with it.",[10,83,84,85,88],{},"TFA has a formula mass of about 114 Da. A peptide with several basic residues can\ncarry several TFA molecules. For a small peptide, that mass adds up quickly: a\n1,400 Da peptide carrying three TFA counter-ions is roughly ",[14,86,87],{},"20% TFA by mass","\nbefore anything else is accounted for.",[10,90,91],{},"Peptides rich in basic residues — BPC-157 has a lysine, ipamorelin has a lysine\nand a histidine, MOTS-c has three arginines and a lysine — carry proportionally\nmore.",[68,93,95],{"id":94},"residual-water","Residual water",[10,97,98,99,104],{},"Lyophilisation removes water; it does not remove all of it. Peptides are\nhygroscopic and a freeze-dried cake typically retains a few percent water by\nmass, more if the vial has been opened in a humid environment or the\nlyophilisation cycle was cut short. See\n",[100,101,103],"a",{"href":102},"\u002Fresearch\u002Fwater-content-and-lyophilisation-quality","water content and lyophilisation quality",".",[26,106,108],{"id":107},"net-peptide-content","Net peptide content",[10,110,111,113],{},[14,112,108],{}," is the figure that accounts for all of it: the proportion\nof gross vial mass that is genuinely peptide.",[115,116,117,132],"table",{},[118,119,120],"thead",{},[121,122,123,126,129],"tr",{},[124,125],"th",{},[124,127,128],{},"What it measures",[124,130,131],{},"Typical range",[133,134,135,147],"tbody",{},[121,136,137,141,144],{},[138,139,140],"td",{},"Purity (HPLC)",[138,142,143],{},"Main peak as % of total peak area",[138,145,146],{},"95–99%+",[121,148,149,151,154],{},[138,150,108],{},[138,152,153],{},"Peptide as % of gross mass",[138,155,156],{},"70–90%",[10,158,159,160,163],{},"A vial labelled 10 mg at 99% purity and 78% net peptide content contains\n",[14,161,162],{},"7.8 mg of peptide",". The 99% describes how good that 7.8 mg is. The 78%\ndescribes how much of it there is.",[10,165,166],{},"Both numbers are honest. Only one of them belongs in your concentration\ncalculation.",[26,168,170],{"id":169},"the-error-this-produces","The error this produces",[10,172,173,174,177],{},"Reconstitute a \"10 mg\" vial with 2 mL of diluent and the arithmetic says\n5 mg\u002FmL. If net peptide content is 78%, the actual peptide concentration is\n",[14,175,176],{},"3.9 mg\u002FmL"," — a 22% overestimate.",[10,179,180],{},"The properties of this error make it particularly awkward:",[182,183,184,191,197,203],"ul",{},[185,186,187,190],"li",{},[14,188,189],{},"It is systematic, not random."," It biases every measurement in the same\ndirection, so it does not average out across replicates.",[185,192,193,196],{},[14,194,195],{},"It is invisible."," Nothing about the experiment flags it.",[185,198,199,202],{},[14,200,201],{},"It varies between batches."," Salt load depends on purification conditions, so\ntwo batches of the same peptide can differ. Results computed from gross mass\nare not strictly comparable across batches.",[185,204,205,208],{},[14,206,207],{},"It varies between suppliers."," A supplier who salt-exchanges to acetate\nproduces a different net content than one who ships as TFA salt.",[10,210,211],{},"For qualitative work, a 20% concentration error is often tolerable. For\ndose-response curves, IC₅₀ or EC₅₀ values, binding constants, or anything\nreported in molar terms, it is a systematic error embedded in the result.",[26,213,215],{"id":214},"how-net-peptide-content-is-determined","How net peptide content is determined",[10,217,218],{},"It requires a separate analysis from the purity run:",[182,220,221,227,233],{},[185,222,223,226],{},[14,224,225],{},"Amino acid analysis"," — the peptide is hydrolysed to free amino acids, which\nare derivatised and quantified. The reference method, and the most reliable.",[185,228,229,232],{},[14,230,231],{},"Elemental nitrogen analysis"," — total nitrogen measured and back-calculated\nagainst the sequence's theoretical nitrogen content.",[185,234,235,238],{},[14,236,237],{},"Quantitative UV"," — absorbance against a known extinction coefficient.\nRequires aromatic residues, so it does not work for every peptide.",[10,240,241],{},"All are additional cost on top of the HPLC and MS that constitute a standard\nrelease package. That is the honest reason most research peptide certificates\nomit the figure — not concealment so much as it not having been paid for.",[26,243,245],{"id":244},"what-to-do-about-it","What to do about it",[10,247,248,251,252,104],{},[14,249,250],{},"Ask whether a net peptide content figure exists"," for the batch. If it does,\nuse it. Multiply gross vial mass by net content and use the result as your vial\nmass in the ",[100,253,255],{"href":254},"\u002Ftools\u002Freconstitution-calculator","reconstitution calculator",[10,257,258,261],{},[14,259,260],{},"If it does not exist, decide whether your work needs it."," Qualitative\nscreening: probably not. Anything quantitative and reported: probably yes, and it\nis worth commissioning amino acid analysis on the batch.",[10,263,264,267],{},[14,265,266],{},"Check the salt form."," A certificate stating \"acetate salt\" versus\n\"trifluoroacetate salt\" changes the expected non-peptide mass considerably —\nacetate has a formula mass of about 60 Da against TFA's 114 Da, so acetate salts\ncarry roughly half the counter-ion mass for the same number of basic sites.",[10,269,270,273],{},[14,271,272],{},"Keep the batch number with the data."," Because net content varies between\nbatches, a result is only comparable to another result from the same batch unless\nboth were corrected.",[26,275,277],{"id":276},"the-short-version","The short version",[10,279,280],{},"Purity tells you how good the peptide is. Net peptide content tells you how much\npeptide there is. A certificate that reports only the first has answered the\nquestion you were less likely to get wrong on your own.",[10,282,283,284,288,289,293],{},"Related: ",[100,285,287],{"href":286},"\u002Fresearch\u002Fhow-to-read-a-peptide-coa","how to read a peptide COA"," walks\nthrough the rest of the document, and\n",[100,290,292],{"href":291},"\u002Fresearch\u002Finterpreting-hplc-chromatograms","reading an HPLC chromatogram"," covers\nwhat the trace shows that the percentage does not.",{"title":295,"searchDepth":296,"depth":296,"links":297},"",2,[298,299,304,305,306,307,308],{"id":28,"depth":296,"text":29},{"id":62,"depth":296,"text":63,"children":300},[301,303],{"id":70,"depth":302,"text":71},3,{"id":94,"depth":302,"text":95},{"id":107,"depth":296,"text":108},{"id":169,"depth":296,"text":170},{"id":214,"depth":296,"text":215},{"id":244,"depth":296,"text":245},{"id":276,"depth":296,"text":277},"A vial can be 99% pure and still contain 25% less peptide than the label says. Why purity and net peptide content are different measurements, and which one your concentration calculation actually needs.","md",[312,315,318,321],{"question":313,"answer":314},"What is net peptide content?","The proportion of a vial's gross mass that is actually peptide, as opposed to counter-ion salt, residual water and other non-peptide mass. A 10 mg vial at 80% net peptide content contains 8 mg of peptide and 2 mg of everything else.",{"question":316,"answer":317},"Can a peptide be 99% pure and still have low net peptide content?","Yes, and this is the crux of the confusion. Purity is chromatographic — the main peak's share of total peak area, describing the peptide fraction only. Salt and water do not absorb UV at peptide detection wavelengths, so they are invisible to the purity measurement entirely. The two numbers describe different things and a high value in one implies nothing about the other.",{"question":319,"answer":320},"Does net peptide content matter for in-vitro work?","It matters wherever molar accuracy matters. For a qualitative experiment, a 20% error in concentration may be tolerable. For dose-response curves, binding constants or anything reported as a molar figure, working from gross vial mass introduces a systematic error in a direction you cannot see.",{"question":322,"answer":323},"How is net peptide content measured?","Most commonly by amino acid analysis, in which the peptide is hydrolysed into its constituent amino acids and those are quantified. Elemental nitrogen analysis and quantitative UV against a known extinction coefficient are also used. All are separate analyses from the HPLC purity run and cost extra, which is why most certificates omit the figure.",{},true,"\u002Fresearch\u002Fnet-peptide-content-vs-purity","2026-08-19",8,[330,331,332],"bpc-157","retatrutide","tirzepatide",{"title":5,"description":309},"research\u002Fnet-peptide-content-vs-purity",[336,337,338,339],"COA","net peptide content","TFA","concentration",null,"jXDGxHiX6Oy1GdGE5UyQvEqoamJzFZukh9SPdv2QJiE",[343,351,358],{"path":344,"name":345,"alsoKnownAs":346,"summary":347,"category":348,"molecularWeight":349,"purity":350},"\u002Fcompounds\u002Fbpc-157","BPC-157","Body Protection Compound 157 · PL 14736","A synthetic 15-amino-acid sequence derived from human gastric juice protein BPC, studied extensively in tissue repair and angiogenesis models.","Repair & Recovery","~1,419.5 Da","≥99%",{"path":352,"name":353,"alsoKnownAs":354,"summary":355,"category":356,"molecularWeight":357,"purity":350},"\u002Fcompounds\u002Fretatrutide","Retatrutide","LY3437943","A single-molecule GLP-1, GIP and glucagon receptor triple agonist, the third generation of incretin-based metabolic research peptides.","Metabolic","~4,731 Da",{"path":359,"name":360,"alsoKnownAs":361,"summary":362,"category":356,"molecularWeight":363,"purity":350},"\u002Fcompounds\u002Ftirzepatide","Tirzepatide","LY3298176 · GIP\u002FGLP-1 dual agonist","A dual GIP and GLP-1 receptor agonist, the second-generation incretin analogue and the reference compound for comparative metabolic work.","~4,813 Da",[365,370,375],{"path":366,"title":367,"description":368,"publishedAt":369},"\u002Fresearch\u002Fchoosing-a-peptide-supplier-australia","Choosing a Research Peptide Supplier in Australia: A Checklist","Fourteen questions that separate suppliers with a real quality process from suppliers with a good website — covering documentation, testing claims, provenance, handling and the language they use.","2026-08-24",{"path":371,"title":372,"description":373,"publishedAt":374},"\u002Fresearch\u002Fcounterfeit-peptides-australia","Counterfeit and Substituted Research Peptides: What Actually Gets Faked","Substitution, dilution, wrong-form supply and underfilled vials — the four ways research peptides are misrepresented, why each is hard to detect, and which analytical test catches which.","2026-08-23",{"path":376,"title":377,"description":378,"publishedAt":374},"\u002Fresearch\u002Fresearch-peptides-australia-legal-status","Research Peptides in Australia: The Regulatory Position","How the TGA, the Poisons Standard and import rules apply to peptides supplied for laboratory research in Australia — what research-use-only means legally, and where the line sits.",1787473952615]