A certificate is a short document full of abbreviations, and most of them are names of methods, not jargon. Once you know which question each one answers, the page reads quickly. These are grouped by what they tell you, not alphabetically, because that is how you read a certificate.

Terms about what the material is

ESI-MS

Electrospray ionisation mass spectrometry. The sample is sprayed from a solution through a charged needle, which produces ions without breaking the molecule, and the instrument measures their mass-to-charge ratio. (Reference: Fenn et al., 1989) Peptides usually appear carrying several charges at once, so one compound gives a family of related signals that the software converts back to a single mass.

MALDI-TOF

Matrix-assisted laser desorption ionisation, with a time-of-flight analyser. An alternative way of making ions: the sample is mixed with a matrix that absorbs laser light, and the flight time to the detector gives the mass. (Reference: Karas & Hillenkamp, 1988) It tends to produce singly charged ions, which makes the spectrum simpler to read.

Observed mass and theoretical mass

The theoretical mass is calculated from the intended sequence, atom by atom. The observed mass is what the instrument measured. An identity check is the comparison of the two, and the certificate should show both, not a bare "conforms". Our note on mass spectrometry explains why the two figures rarely match to the last decimal.

Monoisotopic and average mass

Two legitimate ways of calculating the same molecule's mass. Monoisotopic uses the lightest isotope of each element; average uses the natural isotopic mix. They differ by a few units on a peptide, which is enough to look like an error if you compare a monoisotopic measurement against an average calculation.

Terms about how much of it there is

RP-HPLC

Reversed-phase high performance liquid chromatography. The separation behind almost every purity figure: the sample travels through a column packed with a nonpolar stationary phase, and components leave at different times according to how strongly they stick. (Reference: Dong, 2006) "Reversed-phase" is historical: the stationary phase is the nonpolar one, the reverse of the earlier arrangement.

C18

The most common stationary phase, an eighteen-carbon chain bonded to silica particles. Shorter chains such as C8 and C4 retain less strongly and are often chosen for larger peptides.

Gradient

The changing solvent mixture pumped through the column during a run, typically increasing acetonitrile over a set time. The gradient shape changes how well two close peaks separate, which is why it belongs next to a purity figure.

Area normalization

How a purity percentage is usually calculated: the target peak's area divided by the total area of all detected peaks. The phrase matters because it defines what the number does not include. Anything the method did not detect is absent from both the numerator and the denominator, so salts and water do not appear at all. Our note on purity by HPLC covers this in detail.

Detection wavelength

Usually 214 nm or 220 nm for peptides, where the peptide bond itself absorbs, so every peptide in the sample is visible. At 280 nm only aromatic residues absorb, so a peptide without tryptophan, tyrosine or phenylalanine would barely register.

Retention time

How long a component takes to leave the column, given in minutes. It is a property of the method, not of the molecule, so it only means something alongside the conditions that produced it.

Terms about mass versus purity

Net peptide content

The share of the weighed powder that is peptide, as opposed to counterion, absorbed water and any excipient. Distinct from purity, which describes the detected peaks. A batch can be 99 percent pure by area and well below that by mass, without either figure being wrong.

TFA

Trifluoroacetic acid, the additive used in most reversed-phase purifications. It leaves the peptide as a TFA salt, so part of the powder's weight is counterion. TFA content is sometimes reported for that reason. Our note on what else is in the vial covers the rest.

Counterion

The ion paired with the charged groups on the peptide. Usually trifluoroacetate, sometimes acetate or chloride. It is a real part of the solid's mass and is why net content and purity diverge.

Loss on drying and water content

Two ways of measuring the water in a solid. Loss on drying weighs the sample before and after heating; Karl Fischer titration measures water specifically by a chemical reaction. Lyophilised material is hygroscopic and picks up water from the air, so this figure moves with handling.

Terms about contamination

LAL and EU/mg

The Limulus amebocyte lysate assay, which detects bacterial endotoxin, reported in endotoxin units per milligram. It matters for work with cultured cells, where endotoxin activates immune signalling at very low concentrations, and is irrelevant to many chemistry experiments. (Reference: Raetz & Whitfield, 2002) Our note on endotoxins explains why it is not removed by ordinary sterilisation.

Residual solvents

Traces of the solvents used in synthesis and purification, most often acetonitrile. Reported in parts per million when measured at all.

Deletion sequence

A peptide missing one residue, produced when a coupling step does not go to completion during synthesis. It is the commonest reason for a small peak sitting close to the main one, because it differs from the target by a single amino acid.

Terms about the document itself

Specification

The threshold a batch was required to meet, written as "≥98%" or "not more than 1%". A specification is a rule, not a measurement. A certificate that lists only specifications tells you what was required, not what was found.

Result

The measured value for this batch on this day: "99.2%". This is the figure that describes your material.

Batch or lot number

The identifier linking the document to a physical quantity of material. Without it, the certificate cannot be attached to anything you can hold, and it is what you use to look a batch up.

Method validation

The evidence that an analytical procedure does what it claims, assessed through specificity, accuracy, precision and other defined characteristics. (Reference: Swartz & Krull, 2012) It is why "by RP-HPLC" carries meaning instead of being decoration.

Research use only

All products sold by Codex Research are strictly for laboratory research and development. They are not for human or animal consumption and are not intended to diagnose, treat, cure, or prevent any disease.

Frequently asked questions

What does RP-HPLC mean on a certificate of analysis?

Reversed-phase high performance liquid chromatography, the separation technique behind almost every purity figure. The sample passes through a column with a nonpolar stationary phase and its components leave at different times according to how strongly they are retained.

What is net peptide content?

The share of the weighed powder that is peptide, once counterion, absorbed water and excipient are set aside. It answers a different question from purity, which describes the detected peaks and not the mass.

Why does TFA appear on a peptide certificate?

Trifluoroacetic acid is the usual additive in reversed-phase purification, so the peptide is isolated as a TFA salt and part of the powder weight is trifluoroacetate counterion.

What is the difference between a specification and a result?

A specification is the threshold a batch had to meet, such as ≥98%. A result is the value measured for that batch on a given day. Only the result describes the material you are buying.

What does EU/mg mean?

Endotoxin units per milligram, from the LAL assay. It quantifies bacterial endotoxin, which matters for work with cultured cells and is often irrelevant to chemistry experiments.

What is a deletion sequence?

A peptide missing one residue because a coupling step during synthesis did not finish. Differing from the target by a single amino acid, it typically shows up as a small peak close to the main one.

References

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