HPLC analysis of MOTS-c: how purity is measured

MOTS-c Labs uses high-performance liquid chromatography with UV detection (HPLC-UV) as the purity test for every lot of MOTS-c research material. Each published lot record states the lot, the test date, the method and the reported result, and links to the full certificate. This article explains how the method works and how to interpret those results.

What HPLC measures

HPLC separates the components of a sample so that each can be detected on its own. It answers one question well: of the material the detector can see, how much is the main compound? It does not, on its own, say what the main compound is.^1,2

Reversed-phase HPLC

Peptides are analyzed by reversed-phase HPLC. The column is packed with particles carrying hydrophobic chains, most commonly C18. The sample is introduced in a mostly aqueous solvent, and the proportion of organic solvent (typically acetonitrile with a small amount of acid) rises over the run. Each component stays bound until the solvent is strong enough to release it, so more hydrophobic molecules elute later.^1,2

Closely related peptide impurities, such as sequences missing one residue, sequences with a protecting group left on, or oxidized forms, usually differ slightly in hydrophobicity. That is why reversed-phase HPLC is the standard purity method for synthetic peptides.¹

UV detection

As components leave the column they pass through a UV detector. Peptide bonds absorb strongly at low UV wavelengths (about 210–220 nm), so nearly every peptide-related component produces a signal, regardless of sequence. The detector output over time is the chromatogram, with absorbance (mAU) on the vertical axis and time on the horizontal axis.

Retention time

Retention time is when a peak elutes. It depends on the column, gradient, flow rate and temperature, so it is characteristic of a compound only under one specific method. A matching retention time supports identity but does not prove it.

The main peak and secondary peaks

• Main peak: the dominant peak, labeled as the product on the certificate. On MOTS-c certificates it is a single sharp, symmetrical peak.
• Secondary peaks: any other integrated peaks. Related-peptide impurities usually elute close to the main peak.
• Early signals: small signals near the start of the run elute close to the column's void time. These are typically very polar, unretained species rather than related peptides.
• Baseline: a flat, low baseline means little detected material outside the main peak. A slow upward drift across the run is normal as the solvent composition changes.

Area normalization: how the purity number is calculated

The software integrates the area under each peak. Purity by area normalization is the main peak's area divided by the total area of all integrated peaks, expressed as a percentage:

Purity (%) = main peak area ÷ total integrated peak area × 100

A result of 99.98% means that, under this method, 0.02% of the UV-detected peak area belongs to something other than the main peak.

MOTS-c HPLC results by lot

Results from the published certificates. "Other detected area" is simply 100% minus the reported purity.

Interpreting a purity result

• Compare against the specification. MOTS-c Labs’ stated release specification is ≥95% purity by HPLC-UV. Current lots exceed it by more than four percentage points.
• Treat small lot-to-lot differences with care. A result printed to two decimals is not necessarily reliable to two decimals. Every validated method has a characterized precision.³
• Compare like with like. Purity values are directly comparable only when measured by the same method, ideally in the same laboratory.
• Read it alongside identity. A high purity figure means the sample is homogeneous; the separate identity test (HPLC retention-time match) is what confirms the main component is MOTS-c.

Method limitations

• Identity: a clean peak shows homogeneity, not which compound it is.
• Co-elution: an impurity with nearly identical hydrophobicity can hide inside the main peak.
• Non-absorbing material: water, salts and counterions produce little or no UV signal and are not counted.
• Quantity: purity is a ratio. The amount of peptide in the vial is measured separately as net content. See What a purity percentage does and doesn't tell you.
• Method dependence: results reflect one column, gradient and wavelength.

FDA's adoption of ICH Q2(R2) describes the characteristics a purity method should be validated for, including specificity, precision and range.³