Does a TFA mobile phase change the purity number I get for orforglipron on a C4 column?

What I have: a TFA mobile phase · orforglipron · a C4 column.

I can predict the outcome but I cannot explain it, which means I will get the next case wrong.

I would like to know how confident the field actually is about this.

What is actually going on here, physically?

The method is the measurement in reverse-phase chromatography, more so than in almost any other analytical domain, and two methods that look identical can easily produce different results.

Sample preparation is almost always under-appreciated — a reconstituted peptide in strong solvent will distort its own peak on the gradient.

Concretely, reverse-phase stationary phases use C18 or C8 chains bonded to silica, and the pore size of the silica matters more for a peptide of this chain length than the ligand length does.

Worth noting that the achievable resolution depends on the chemistry of the molecule — some peptide sequences separate easily while others are notoriously difficult regardless of method.

If two labs give different numbers, the method difference is the first hypothesis, not lab quality.

edited 7 Feb 2025 by eoin_mcgarry — clarified the distinction between purity and content

Reverse-phase HPLC is the workhorse for peptide purity work, but it is almost universally run under conditions that are not optimal for a peptide of this chain length.

The 214 nanometre wavelength is chosen because it corresponds to the amide backbone absorption, making response roughly proportional to the number of peptide bonds.

Temperature affects both the viscosity of the mobile phase and the dynamics of molecular interactions, and a method developed at 25 degrees and run at 40 degrees will not behave identically.

The resolving power of a separation is quantified by the resolution parameter R, defined from the heights and widths of adjacent peaks, and pharmacopoeial methods typically demand R greater than 1.5 for a method to be considered validated.

The limitation is that co-elution is invisible — if two species happen to have the same retention time, they will report as a single peak at their combined area.

Ask for the chromatogram and the system suitability data, not just the number.

The limit of this technique for these molecules is reaching a point where small improvements require proportionally large investments in method development.

Wider-pore phases — 300 Angstrom rather than 100 Angstrom — have faster mass transfer and narrower peaks for peptides above three kilodaltons, which is almost every peptide you will see.

The caveat is that HPLC is a purity technique and says almost nothing about whether the main peak is actually your target compound — that is why identity confirmation from mass spectrometry or peptide mapping matters.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

Gradient design is usually described as though it is a minor technical detail rather than the primary determinant of what the method measures.

Detector linearity is why a heavily loaded peak can give false purity by compressing the main peak height while leaving impurity shoulders unchanged.

Peptide impurities from solid-phase synthesis include deletion sequences, truncations from premature cleavage, racemised residues from epimerisation and oxidised variants, each of which may have different chromatographic behaviour.

One qualification: the limit of detection on a reversed-phase HPLC is set by the noise on the baseline, which for these molecules is usually in the range of a tenth of one per cent or less, and anything smaller is not reproducibly detectable.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.