Reading a certificate

What a certificate of analysis contains

This page sets out what a certificate of analysis contains, what each figure means and what it cannot tell you, so that the right thing can be asked for from any supplier.

What the numbers mean

Purity by HPLC

The sample is dissolved and carried through a chromatography column. Substances leave the column at different times, and the detector records each one as a peak. Purity is the area of the main peak divided by the total area of every peak. It compares what is in the sample with the rest of what is in the sample. The chromatogram on the certificate is drawn from those same peak areas, so the trace and the figure are one measurement rather than two.

Identity by mass spectrometry

Mass spectrometry weighs the molecule. A certificate gives two masses: the theoretical mass, calculated from the sequence, and the measured mass, reported by the instrument. When the two agree, the material is the compound named on the label. Purity cannot show this. A sample can be almost entirely one substance while that substance is the wrong one, such as a peptide missing a single residue. Where one trade name covers more than one molecule - and several do - the measured mass settles which molecule is in the vial.

Peptide content

A lyophilised vial does not hold peptide alone. The solid also holds water and residual salts left by purification, most often acetate or trifluoroacetate. Peptide content is the share of the mass in the vial that is peptide. It is a separate determination from purity, and it is the figure that states how much peptide the vial holds.

Water, by Karl Fischer

Karl Fischer titration measures water through a reaction that responds to water and not to the rest of the sample. A lyophilised powder takes up water from the air, so some is always present. The figure is on the certificate because water is part of the mass in the vial, and it is one of the parts that is not peptide.

Purity is a ratio. Peptide content is a quantity.

The two figures answer different questions, and only one of them is about quantity. Purity compares the peaks of a chromatogram with each other, so the main peak can dominate the trace whatever the vial contains. Peptide content states how much of the mass in the vial is peptide at all. Two vials can carry the same purity figure and hold different amounts of peptide. A seller who publishes purity and nothing else has answered the first question and left the second one open - and the second one is what decides how much material is in the vial.

What a certificate covers

A certificate belongs to one batch

Peptides are made in batches, and each batch is analysed on its own. A certificate describes the batch it names. It does not describe the compound in general, and it does not describe the next batch of the same compound. That is why the question to ask a supplier is for the certificate of a batch number rather than for the compound, and why the number is printed on the vial itself rather than only on the outer box.

Where the result comes from

A certificate is worth what its source is worth. It should name the laboratory that ran the analysis, the date of the test and that laboratory's own report reference, and the laboratory should be independent of the seller. A certificate published only by the seller it vouches for proves little, because a document is editable: the question worth asking is whether the same result can be found anywhere the seller does not control.

What can be checked, and what cannot

A shop page that says “high purity” cannot be checked by anyone. A whole report tied to a batch number can: it names a laboratory, a method and a date, and the result can be put back to the laboratory that produced it. That is the difference worth insisting on from any supplier.