Purity starts with you

People will spend an evening comparing purity figures on peptide certificates, and never once check their own bloodwork. That is the wrong order.

The results you get from anything depend first on the system it goes into: how you train, what you eat, whether the nutrients you need are actually reaching the places you are short, and whether you carry a genetic trait that changes how your body handles them. A low ferritin, a vitamin D level in your boots or an untreated thyroid will blunt your recovery far more than a percentage point on a certificate ever could.

So the first lab test should be yours. A basic panel covering vitamin D, B12, folate, ferritin and iron, magnesium, thyroid function, blood sugar and lipids tells you where you are genuinely deficient, so you can fix it with food, training and targeted supplementation. And where there is a reason to look, such as family history or a result that does not make sense, clinically validated genetic testing through your doctor can pick up inherited conditions that change what your body needs. Hereditary haemochromatosis, which makes the body store too much iron, is a good example: the fix is the opposite of taking more iron. A word of caution here: consumer DNA "diet and fitness" reports are a different thing, and most of their claims rest on thin evidence. Stick to tests a clinician would order.

Once your foundations are dialled in, you are in a position to judge anything you add. And that includes learning what a purity number really means, because, as this manual shows, "99% pure" on its own tells you much less than it seems. None of this is hidden. It is all in the book, written by chemists for chemists. It just rarely makes it onto a product page.

Problem 1: the invisible mirror image

Most amino acids come in two mirror-image forms, left-handed (L) and right-handed (D). Your body uses the L forms. During synthesis, an amino acid can sometimes flip to the wrong form. Chemists call this epimerisation or racemisation.

The manual discusses this repeatedly. The introductory chapter notes that incorporating cysteine is likely to cause some level of racemisation, that histidine can epimerise too, and that the entire design of coupling additives like HOBt exists partly to prevent it. The microwave chapters add that cysteine and histidine may need to be coupled at lower temperature to avoid it.

Here is the catch: a mirror-image amino acid has exactly the same mass. Mass spectrometry cannot see it. Look back at the table of mass shifts in Part 6. Racemisation is not on it, because there is nothing to detect. On a standard HPLC run it may also co-elute with the correct peptide.

So a peptide can pass a mass check and look pure on HPLC while containing a portion of molecules with one amino acid flipped. Those molecules might be less active, inactive, or behave differently. Detecting them requires additional, specialised analysis that is not part of a routine certificate.

Problem 2: near-twins hiding in the peak

Part 4 explained deletion sequences: chains missing a single amino acid. Some separate cleanly on HPLC. Some do not. Aspartimide-related by-products, one of the most notorious side reactions in peptide chemistry, can rearrange into forms that have the same mass as the correct peptide and elute very close to it.

HPLC purity is calculated from peak areas. If an impurity hides under the main peak, it is counted as product.

Problem 3: purity is not content

This is the one that surprises most people. "98% pure by HPLC" means that of the substances detected by UV at that wavelength, 98% of the signal came from the main peptide peak.

It does not mean that 98% of the powder in the vial is peptide.

Freeze-dried peptide powder also contains water, and a counterion: a charged partner left over from the final steps. For peptides made by Fmoc chemistry and purified in TFA, that is usually trifluoroacetate. The Boc chapter even points out that HF-cleaved peptides end up as fluoride salts, which behave differently from TFA salts. Salts and water do not show up on the UV trace, so they do not lower the "purity" number, but they do take up weight.

The actual proportion of peptide by weight, called net peptide content, is often substantially lower than the purity figure. A vial labelled "5 mg" might contain 5 mg of powder, of which a meaningful fraction is not peptide at all. Unless the supplier reports net peptide content separately, you cannot tell.

Problem 4: the leftover acid

The manual's storage section adds another wrinkle. It warns that peptides can contain residual solvents such as TFA, which can cause unwanted reactions, including breaking acid-sensitive bonds (aspartate-proline) and deamidation of glutamine and asparagine. It recommends freeze-drying before storage for exactly this reason.

So TFA is not just an inert passenger. If enough is left over, it can slowly damage the peptide itself.

Problem 5: what is not measured at all

Here is the gap that matters most for anything injected. This manual is about making and characterising peptides for research: biology, biochemistry, drug discovery. It does not cover, and does not claim to cover:

  • Sterility: whether the product contains living microbes.
  • Endotoxins: fragments of bacterial cell walls that can cause fever and serious inflammatory reactions when injected, even if the bacteria are dead.
  • Residual solvents and reagents at the levels regulators require for medicines.
  • Stability over time under real storage conditions.

Pharmaceutical manufacturing tests for all of these under strict rules. A research-grade "99% pure" certificate typically tests for none of them.

Problem 6: the book assumes an expert

To be fair to the manual, it never pretends to be anything but a lab book. The preface says the protocols are for specialists and non-specialists in a research setting. But that is exactly the point. The knowledge needed to judge a peptide's quality properly, understanding racemisation, counterions, net content, aspartimide chemistry, is specialist knowledge. The typical buyer of a research peptide has none of it, and the typical product page does not provide it.

The steelman

For research chemistry, these limitations are well understood and managed. A trained chemist ordering a peptide for a cell experiment knows what an HPLC purity figure means and does not assume it means more. High HPLC purity with a correct mass is genuinely good evidence the synthesis worked. Many suppliers do excellent work.

The takeaway

"99% pure" is a real measurement of one thing. It is not a guarantee of correct stereochemistry, not a measure of how much peptide is in the vial, and not a statement about sterility or endotoxins.

So check in the right order. First, your own numbers: training consistent, diet built around enough protein and whole foods, deficiencies found by a blood test and corrected, and any inherited condition identified through proper clinical testing. That is where most of the result comes from.

Then, if you ever evaluate a peptide product, ask the better questions: is there both HPLC and mass data for this batch? Is net peptide content reported? What is the counterion? And, for anything that goes into a body, has it been tested for sterility and endotoxins? A supplier who can answer all four is telling you far more than a big number ever could. A person who knows their own bloodwork is in a far better position to judge any of it.

This series explains laboratory chemistry for general readers. It is not a guide to making or using any compound. Nothing here is medical advice.