What does ≥99% HPLC purity actually mean?
"99% purity" looks straightforward. What the number measures, what it does not — and why it must be read alongside mass spectrometry data.
The number and what it measures
When a supplier states "≥ 99% HPLC purity", they are reporting a result from reversed-phase high-performance liquid chromatography (RP-HPLC). The value is expressed as an area percentage: the area of the main peak divided by the total area of all detected peaks in the chromatogram, multiplied by 100.
This is a measure of chromatographic purity — how much of the UV-absorbing material elutes at the expected retention time relative to everything else. It is not a direct mass measurement, and it is not an identity measurement.
Purity (area%) = (Main peak area / Sum of all peak areas) × 100
Section 1
How RP-HPLC for peptides works
Reversed-phase HPLC separates compounds based on hydrophobicity. The stationary phase is a non-polar C18 (octadecyl) or C8 silica column. The mobile phase begins aqueous (typically 0.1% TFA or 0.1% formic acid in water) and increases in organic modifier (usually acetonitrile) over the gradient.
Peptides elute in order of increasing hydrophobicity. Detection is most commonly UV absorbance at 214 nm, which captures the absorbance of the peptide bond (amide n→π* transition). This wavelength is preferred over 254 nm because it detects all peptides regardless of aromatic residue content, making it more universally sensitive.
Standard conditions for peptide purity analysis
- Column: C18, 150–250 mm × 4.6 mm, 3–5 μm particle size
- Mobile phase A: 0.1% TFA in water; Mobile phase B: 0.1% TFA in acetonitrile
- Gradient: typically 5–95% B over 20–30 minutes
- Detection: UV at 214 nm (primary) or 220 nm
- Flow rate: 1.0 mL/min
This method is standardised per USP ⟨621⟩ (Chromatography) and is widely used in both quality control and research settings. Compliant methods must include system suitability testing — verification that the column resolves reference compounds adequately before samples are run.
Section 2
What 99% purity does not tell you
Understanding the limits of area% purity is essential for interpreting COA data correctly. There are several things that a 99% HPLC purity result does not confirm:
It does not confirm identity
A sample could be 99.5% pure by HPLC and still be a completely different compound. If the synthesis produced the wrong peptide sequence at high purity, the HPLC result would be excellent. Identity requires mass spectrometry or amino acid analysis — HPLC cannot distinguish between compounds that co-elute or have similar hydrophobicity.
It does not detect endotoxins
Bacterial endotoxins (LPS) do not absorb at 214 nm at the concentrations typically found in peptide samples. A sample that is 99% chromatographically pure can carry sufficient endotoxin to activate TLR4 signalling in cell assays. Endotoxin testing requires the LAL assay, not HPLC.
Area% is not mass%
Different compounds have different UV absorptivities (extinction coefficients) at 214 nm. A minor impurity with a high extinction coefficient will contribute more to the total peak area than its actual mass proportion suggests. Conversely, a compound with low UV absorptivity at 214 nm could be present at a significant mass fraction without contributing proportionately to the peak area. Accurate mass purity requires quantification against a characterised standard.
It does not detect residual solvents or counter-ions
TFA (trifluoroacetic acid), DMF, and acetonitrile do not absorb strongly at 214 nm. A lyophilised peptide prepared using TFA in the cleavage step will contain TFA counter-ions (as trifluoroacetate salt) that are invisible to standard HPLC analysis. These can affect in-vitro assay results if not removed or accounted for.
Section 3
The purity threshold — why ≥ 95% or ≥ 99%?
The threshold cited in a specification reflects the acceptable level of co-synthesised impurities — principally deletion sequences, truncated peptides, and incompletely deprotected side chains from the synthesis process.
≥ 95% (standard purity)
Adequate for many in-vitro biochemical assays where compound-specific activity is being assessed at concentrations well above background. The 5% impurity fraction may consist of closely related peptide fragments with minimal independent biological activity.
≥ 99% (high purity)
Required for quantitative binding studies, receptor pharmacology, and any assay where the impurity profile could contribute to the measured signal. Also appropriate where the total compound loading is low and the impurity fraction would represent a significant proportion of any effect observed.
All FAB Research peptides are tested to a minimum ≥ 99.0% area purity specification by reversed-phase HPLC at 214 nm, by an ISO 17025 accredited laboratory, alongside ESI-MS identity confirmation.
Reading an HPLC result correctly
- Confirm the detection wavelength is 214 nm (peptide bond) or 220 nm, not 254 nm (aromatic only)
- Check the gradient run time — very short runs (< 10 min) may not resolve all impurities
- Read the chromatogram, not just the number — a sharp, well-resolved main peak is the ideal
- Always pair HPLC data with ESI-MS or MALDI-ToF identity confirmation
- For cell-based work, also check the endotoxin result — HPLC cannot detect LPS