Almost every data sheet for a research peptide carries a percentage figure. It is usually read as the quality statement. In fact it answers only one of three questions that can be asked of a research sample. This page separates the three dimensions of purity, identity and content from one another.<\/p>\n
Purity is a ratio, identity is an assignment, content is a quantity. None of the figures replaces another.<\/p>\n
A chromatographic purity figure is an area ratio: the area of the main peak divided by the sum of all integrated peak areas. The value therefore describes the proportion of the main component in what the method made visible under the chosen conditions.<\/p>\n
Two limitations are associated with this. First, the result depends on the method: gradient, column, detection wavelength and run time all help determine which minor components are separated and recorded at all. Second, everything that does not absorb at the chosen wavelength remains invisible. Water and inorganic salts typically do not appear in a UV chromatogram. In detail in HPLC in peptide analysis<\/a>.<\/p>\n A high purity proves only that a substance is uniform, not which substance it is. A cleanly synthesised but incorrect product can look impeccable chromatographically. The question of identity is therefore answered with an independent method, in practice usually by mass spectrometry. For questions of sequence the mass alone is not sufficient, because sequence variants with the same composition can have the same mass. Details in LC-MS in peptide analysis<\/a>.<\/p>\n A lyophilised peptide is rarely pure peptide substance. The mass is typically distributed across four constituents, only one of which is the peptide:<\/p>\n Precisely those constituents that dilute the peptide fraction are therefore the ones a UV-based chromatography does not record. The net peptide content describes the mass fraction that actually falls to the peptide substance: peptide mass = weighed portion multiplied by the net peptide content.<\/p>\n Peptides with many basic side chains, for example arginine or lysine, bind a correspondingly large number of counter-ions. The counter-ion content is therefore not a question of diligence but a consequence of the sequence and the purification system. A low net peptide content is therefore not automatically a quality defect; what matters is that the value is known and documented.<\/p>\n Purity and content vary independently of one another. Two batches with identical purity figures can contain different amounts of peptide per milligram if their proportion of counter-ions and residual moisture differs. Conversely, a batch with more minor components in the chromatogram can show the same peptide fraction as a chromatographically more uniform one. Both constellations are internally consistent. A comparison between batches is only meaningful if both figures are available and use the same reference basis.<\/p>\n Pay attention to the reference basis: a figure given as is or as supplied refers to the substance in the condition delivered, including counter-ions and residual moisture. A figure given as peptide or net peptide refers to the pure peptide mass. The two must not be mixed.<\/p>\n A certificate of analysis is the place where the three dimensions are documented on a batch-specific basis. What matters is that every figure carries a method and a batch identifier; without this assignment a number is not verifiable. How such a document is read is described in Reading a COA correctly<\/a>; the link between document and sample is covered by Batch testing and traceability<\/a>. Batch-related documents are published under Analytics & Certificates<\/a> insofar as corresponding real documents are available.<\/p>\n For laboratory research only. This text serves as technical information and contains no medical, diagnostic or therapeutic statements, no dosages and no application instructions for humans.<\/em><\/p>\n Back to overview: Research & Analytical Resources<\/a> \u00b7 Research Compound Library<\/a><\/p>","protected":false},"excerpt":{"rendered":" Auf fast jedem Datenblatt zu einem Forschungspeptid steht eine Prozentzahl. Sie wird meist als die Qualit\u00e4tsangabe gelesen. Tats\u00e4chlich beantwortet sie nur eine von drei Fragen, die man an eine Forschungsprobe stellen kann. Diese Seite trennt die drei Dimensionen Reinheit, Identit\u00e4t und Gehalt voneinander. Drei Dimensionen, drei Verfahren Reinheit \u2013 Wie gro\u00df ist der Anteil der […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":455,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-460","page","type-page","status-publish","hentry"],"yoast_head":"\nIdentity: is it the expected molecule?<\/h2>\n
Content: what the weighed portion really consists of<\/h2>\n
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Why the values cannot be converted into one another<\/h2>\n
Common misinterpretations<\/h2>\n
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Documentation context<\/h2>\n