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Glossary

Terms beginning with S

21 of 238 glossary terms.

Shoulder: An unresolved peak appearing as an asymmetric bulge on the leading or trailing edge of a larger peak. The main source of integration ambiguity.
Solvent front: The disturbance at the start of a chromatogram caused by the injection solvent. Compounds eluting within it cannot be integrated.
System suitability: Injections demonstrating the chromatographic system was performing acceptably when the sample was run. Required by every pharmacopoeia, documented on almost no research-peptide certificate.
Space charge: Mutual repulsion between ions at high density, shifting measured m/z. Why an overloaded injection can read a different mass from an on-scale one.
Salt-free basis: A content figure recalculated as if the sample contained no counter-ion. Like the anhydrous basis, must be stated explicitly.
Salt form: Which counter-ion the compound was isolated as. Trifluoroacetate is 114 Da per site; acetate is 60. At equal purity, acetate vials contain more peptide.
Sulfated ash: The inorganic residue remaining after ignition with sulfuric acid. A measure of inorganic contamination and part of the mass balance.
Sulfoxide: The +15.995 Da methionine oxidation product. Elutes earlier than the parent on reverse phase. A handling finding, not a synthesis finding.
Shared bulk: One physical batch of material sold by two or more suppliers under different lot numbers. Normal in a contract-synthesis market; means two results are one measurement.
Stability-indicating method: A method demonstrated to separate the analyte from all its degradation products. A stability claim from a non-stability-indicating method means nothing.
Solid-phase peptide synthesis (SPPS): Stepwise chain assembly on an insoluble support, with excess reagent driving each coupling. The origin of almost every peptide in this index.
Salt exchange: Converting a peptide from one counter-ion to another, typically trifluoroacetate to acetate, by repeated lyophilisation from the new acid or by ion exchange. Rarely complete.
Sterility test: Incubation of the article or its filtrate in growth media for at least 14 days. Destructive, low-powered, and not a statement about the lot.
Selection bias: Distortion arising because the sample was not drawn at random. The index's largest unmeasured confounder: lots that get tested are lots someone chose to test.
Survivorship: A related bias: the material that reaches a laboratory is not representative of material that does not. Compounded here by who chooses to submit and when.
Standard uncertainty (u): The uncertainty of a measurement expressed as a standard deviation. Combined across sources in quadrature.
Supplier-supplied: Provided by the supplier for testing. The supplier chose the vial. Reads a mean 0.31 pp higher than buyer-sourced across this index.
Specificity: Demonstration that the response is due to the analyte and nothing else. The hardest characteristic to establish for peptides because of mass-identical isomers.
Specification: A decision about what result is acceptable. Not a measurement. Conflating the two is how "≥ 99.0 %" becomes "99.0 %" on a certificate.
Submission: A report offered to the index by a buyer or laboratory. Becomes a record after normalisation and provenance classification.
Suppressed statistic: A statistic the index declines to publish because n < 3. The individual values are shown instead.