What "99% purity" actually measures.
A 99% purity figure on a peptide certificate is a chromatographic peak-area ratio, not a statement of how much peptide is in the vial by weight. Read through one real published AKH batch to see what the number measures, what it leaves out, and how it differs from net peptide content.
"99% purity" is the single most quoted figure on a peptide product page, and it is also the most over-read. The number comes from high-performance liquid chromatography (HPLC), and what it states precisely is a proportion of detected peak area — how much of what the detector saw was the target compound relative to everything else that eluted. It does not, by itself, say what fraction of the vial's powder is peptide by weight.
This post is written for researchers and laboratory personnel who read a certificate of analysis and want to know what the headline percentage is actually claiming. It walks through one certificate of analysis AKH BioLabs has published — a real batch, not a specimen — to show what a 99%-class HPLC figure measures, what a separate measurement called net peptide content measures instead, and why the two numbers can differ meaningfully on the same vial.
Key takeaways
- HPLC purity is the target compound's chromatographic peak area divided by the total detected peak area — a proportion of what the detector responded to, not a proportion of the powder's mass.
- Net peptide content is the separate measurement that states what fraction of the powder is actually peptide, after residual water and counterion are accounted for; a sample can read 99% pure by peak area and still be well under that by mass.
- Purity and quantity are independent claims: a purity percentage says nothing about how much material is in the vial, and a label mass says nothing about how pure it is.
- A real AKH batch — Thymosin Alpha-1, batch 26172, tested by the Brown Institute of Biomolecular Research on 2026-07-27 — reports "Thymosin Alpha-1: 10mg | Purity: 99.79%", and neither figure on that line is the other.
- The counterion carried over from synthesis (commonly trifluoroacetate, sometimes exchanged for acetate) is part of the powder's mass and largely invisible to the HPLC measurement — a 2025 peer-reviewed study found it is also not inert in biological assays, which is why its quantification is treated as a distinct question from purity.
What the number is actually a ratio of
High-performance liquid chromatography separates the dissolved components of a sample as they travel through a column at different rates, then detects them, typically by ultraviolet absorbance, as each one elutes. The output is a trace of peaks, and the purity figure quoted on a certificate is calculated by area normalization: the target peak's area divided by the sum of all detected peak areas, expressed as a percentage.
That is the plain meaning of an HPLC purity figure, and it carries a consequence certificates rarely spell out: the ratio is a comparison between what the detector saw, not a measurement of the vial's total mass. Anything that does not absorb strongly at the detection wavelength — residual water, the counterion left over from purification — is part of the powder but contributes little or nothing to either side of that ratio. A sample can post a 99%-class HPLC purity figure while containing meaningfully less peptide by weight than the label mass implies.
None of this makes the HPLC figure less useful for what it does measure: among the components the run detected, how much of the peak area belongs to the target sequence rather than a synthesis by-product, a truncated chain, or a degradation product. It is a purity measurement in the chromatographic sense. It was never a quantity measurement, and reading it as one is the error this post exists to correct.
Reading one actual AKH batch
The distinction is easiest to see against a genuine certificate rather than a description of one. This is a real batch AKH BioLabs has published, not a specimen: Thymosin Alpha-1, batch 26172, tested by the Brown Institute of Biomolecular Research on 2026-07-27. Every field below is taken from that report.
| Field | Value on this batch |
|---|---|
| Product | Thymosin Alpha-1 |
| Batch | 26172 |
| Test date | 2026-07-27 |
| Laboratory | Brown Institute of Biomolecular Research |
| Report reference | #1812468 |
| Result | Thymosin Alpha-1: 10mg | Purity: 99.79% |
"10mg" is a quantity claim
This is the labelled fill mass — a statement about how much powder the vial is claimed to hold. It is measured and stated independently of the purity figure on the same line.
"99.79%" is a purity claim
This is the HPLC peak-area ratio described above: the target peak's share of everything the chromatography run detected on this batch, on this test date, in this laboratory.
The two numbers do not check each other
A batch could in principle report the same 99.79% purity figure at a different fill mass, or the same fill mass at a different purity. Nothing about reading one figure tells a reader the other.
Neither figure is net peptide content
This certificate reports HPLC purity and fill mass. It does not report what fraction of the 10mg is peptide once residual water and counterion are subtracted — that would be a separate, less commonly ordered measurement.
Purity vs quantity: two claims, not one
Purity and quantity answer different questions, and treating one as a proxy for the other is the most common way a certificate gets over-read. Purity is a ratio — what fraction of the detected material is the intended compound. Quantity is a mass claim — how much material is in the vial. A powder can be extremely pure in a small quantity, or plentiful and comparatively impure; a certificate that states only purity has said nothing about fill mass, and a label that states only fill mass has said nothing about purity.
The gap widens further because the fill mass on a label is the gross powder weight, not the peptide weight. Residual water and the counterion described below count toward the grams on the label but not toward the peptide itself, so even an honestly stated label mass overstates peptide content unless net peptide content was separately measured and reported.
Where the label mass and the peptide mass diverge
Two things sit in a lyophilised peptide's powder besides the peptide: residual moisture left after freeze-drying, and the counterion carried over from synthesis and purification. Solid-phase peptide synthesis relies on trifluoroacetic acid, and the peptide is typically obtained as a trifluoroacetate (TFA) salt unless an additional ion-exchange step swaps the counterion for acetate. Either way, the counterion is part of the powder's weight and is not part of the peptide.
A 2025 peer-reviewed study from ETH Zurich, published in Pharmaceuticals, examined exactly this gap: it confirms that peptides made by solid-phase synthesis are obtained as TFA salts, that residual TFA is quantifiable by methods including HPLC, FT-IR and NMR, and that the counterion is not a neutral bystander — the same paper reports measurable effects of counterion identity on membrane permeability in a biological assay, which is why the authors describe counterion quantification and specification as important wherever a peptide's mass or activity is being compared across reports. None of that shows up in an HPLC purity figure, because the measurement is blind to species that barely absorb at the detection wavelength.
Net peptide content is the measurement that closes this gap: the fraction of the powder that is actually peptide once water and counterion are subtracted, typically determined by amino-acid analysis. It is reported far less often than HPLC purity, which is exactly why a 99%-class purity figure and a peptide-by-weight figure are not interchangeable, and why a certificate silent on net content has not addressed the mass question at all.
What a purity figure leaves unanswered
An HPLC purity figure answers one question about one sample: among what the run detected, how much was the target peak. It does not confirm identity — that the peak is the intended sequence rather than a different compound of similar retention time — which is why a complete certificate pairs the chromatography with a mass-spectrometric identity check. It says nothing about microbial contamination or bacterial endotoxin, both of which require separate, specifically commissioned testing. And it describes one sample from one batch on one date; it does not establish that a different vial, or the same vial after storage, still matches the figure printed on the report.
- 01
Read the purity figure as a ratio, not a mass
It states a proportion of detected peak area, not what fraction of the powder is peptide.
- 02
Check whether net peptide content is reported separately
Where it is, the mass question is answered directly; where it is not, the label's fill mass remains a gross-weight figure.
- 03
Look for an identity check alongside the purity figure
Chromatographic purity and mass-spectrometric identity answer different questions, and a complete certificate carries both.
- 04
Treat the batch number as the anchor
A purity or quantity figure is a statement about one tested batch — match the batch on the certificate to the batch on the vial before relying on either number.
Common questions.
- What does 99% purity mean for a peptide?
It means that, in the HPLC run used to test the batch, the target compound's peak accounted for 99% of the total detected peak area — a chromatographic ratio, not a statement that 99% of the vial's powder by weight is peptide. Residual water and the counterion left over from synthesis are part of the powder's mass and are largely invisible to that ratio, which is why a separate measurement, net peptide content, exists to answer the mass question directly.
- Does a 99% purity figure mean the peptide is 99% of the vial by weight?
No. HPLC purity is calculated from peak area among the components a chromatography run detected; it is not a mass balance of the powder. A sample can post a 99%-class purity figure while the peptide itself accounts for a meaningfully smaller fraction of the total powder weight once residual water and counterion are subtracted — the fraction that net peptide content, not HPLC purity, is designed to measure.
- Is peptide purity the same thing as peptide quantity?
No, and the two are routinely conflated. Purity is a ratio describing how much of the detected material is the intended compound. Quantity is a mass claim about how much material the vial contains. A certificate can report a high purity figure and say nothing about fill mass, or state a fill mass and say nothing about purity — reading either number as evidence of the other is not supported by what each one measures.
- Why doesn't HPLC purity account for the peptide's counterion?
HPLC purity is calculated from the peak area detected during a chromatography run, and counterions such as trifluoroacetate or acetate typically do not absorb strongly at the wavelength used to detect the peptide, so they contribute little to the ratio in either direction. They are still part of the powder's mass. A 2025 peer-reviewed study on TFA-salt synthetic peptides found the counterion is quantifiable by separate methods and is not inert in biological assays, which is why counterion content is treated as a measurement distinct from HPLC purity rather than folded into it.
Sources
- 01Erckes V, Streuli A, Chamera Rendueles L, Krämer SD, Steuer C. "Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides and Its Influence on Membrane Permeation." Pharmaceuticals (Basel) 2025;18(8):1163.
- 02AKH BioLabs certificate of analysis — Thymosin Alpha-1, batch 26172, Brown Institute of Biomolecular Research, 2026-07-27