Almost every product sheet states a purity percentage. The number is useful, provided you know what was measured.
How purity is measured
The standard method is HPLC. A dissolved sample passes through a column that separates its components. A detector measures how much light each component absorbs, usually at a wavelength of around 214 to 220 nanometers, where the bond between amino acids absorbs.
The result is a chromatogram: a line with peaks. Purity is the area of the main peak divided by the area of all the peaks combined. At 99%, then, 1% of the signal comes from something else.
What that “something else” is
Impurities in a synthetic peptide often closely resemble the product itself:
- chains missing one amino acid;
- truncated chains;
- oxidized or deamidated variants;
- remnants of protecting groups from the synthesis.
Precisely because they resemble the product, they can also be active in an experiment. Lower purity means more uncertainty about what causes a measured effect.
What the number does not tell you
Not how much peptide is in the vial
A lyophilized powder does not consist of peptide alone. It also contains counterions, such as acetate, and a little bound water. The share of actual peptide is called the peptide content, and it is typically between 70 and 90 percent by weight. A powder can therefore be 99% pure and contain 80% peptide. One number describes the composition of the peptide, the other the composition of the powder.
Not which compound it is
HPLC shows that there is one main component, not what that component is. That takes mass spectrometry. This is why purity and identity belong together on a certificate of analysis.
Nothing about sterility or endotoxin
A chromatogram does not detect bacteria or endotoxins. Those are separate tests.
Comparing numbers
The outcome depends on the method. A short run with a steep gradient separates less well than a long one, and can hide an impurity under the main peak. So only compare percentages when you know the measurement conditions, and look at the chromatogram itself.
When 99% matters
For a qualitative experiment, 95% is often enough. For quantitative work, such as a dose-response curve in a cell model or a binding assay, you want as few confounding factors as possible. In that case, high purity is not a luxury.
At PulseChem, a batch is released at an HPLC purity of ≥99%. The measured value for your batch is on its certificate, which you can find on the Lab results page. See also the laboratory standard.
This article is background information on laboratory research. It is not medical advice or instructions for use. All products are for research use only.