HPLC vs mass spec: what each test proves

What does HPLC with UV detection measure?

HPLC with UV detection measures chromatographic purity: the main peak's area as a percentage of all the peak area the detector saw. Reversed-phase HPLC pushes a dissolved sample through a column packed with a water-repellent stationary phase, usually C18, while the solvent shifts gradually from water towards an organic solvent. Peptides leave the column at different times depending on how strongly they cling to it. A UV detector at the outlet, most often set near 214 nm where the peptide bond absorbs, draws each one as a peak.

Chromatographic purity is the main peak's area as a percentage of all the peak area the detector saw. That is a useful number, and it is the one most certificates lead with, but it has three built-in limits. First, it says nothing about what the main peak is: a wrong peptide made cleanly gives the same single tall peak as the right one. Second, an impurity that leaves the column at the same time as the main compound sits under its peak and is counted as product. Closely related impurities, such as a sequence missing one amino acid or one with a single residue flipped to its mirror form, are exactly the ones most likely to do this. Third, anything that absorbs little or no UV at that wavelength, such as water and inorganic salts, does not register at all, so it cannot count against the figure.

Area percent also assumes that each impurity absorbs UV about as strongly as the main compound. For impurities that are close relatives of the peptide, that is usually a reasonable approximation; for unrelated contaminants it may not be. Reviews of reversed-phase methods for peptides cover how column choice and conditions change what a run can separate (Fekete et al., 2012).

What does mass spectrometry measure in a peptide sample?

Mass spectrometry measures molecular mass, which confirms a peptide's identity but not its purity. A mass spectrometer turns molecules into charged ions, most often by electrospray ionisation (ESI) or MALDI, and measures their mass-to-charge ratio. For a peptide, the useful output is an observed molecular mass that can be compared with the mass calculated from its sequence. A match within the instrument's tolerance is strong evidence that the compound is the one on the label.

On its own, mass spectrometry is a poor measure of purity. Different molecules ionise with very different efficiency, and one compound can suppress the signal of another, so the size of a peak in a mass spectrum does not track how much of that compound is present. It also cannot separate molecules that share a mass. A peptide with one amino acid in its mirror-image form, a known by-product of synthesis (D'Hondt et al., 2014), has the same mass as the intended one.

Why does HPLC plus mass spec prove more than either alone?

Run together, the two give a purity figure and an identity check from the same sample. Coupling the two as LC-MS puts the mass spectrometer behind the HPLC column, so each peak gets a mass as it elutes. The UV trace gives the purity figure; the mass of the main peak confirms what it is; and the masses of the smaller peaks often show what the impurities are, such as a missing residue or an oxidised side chain. One laboratory study reported that adding a simple mass detector to routine peptide quality control identified the main compounds and small impurities directly, without reference standards (D'Hondt et al., 2016).

The case for doing both is practical, not theoretical. When researchers re-tested a set of synthetic peptides bought for laboratory work, only 44% met the purity they had ordered, the figures differed widely from the suppliers' certificates, and in one sample the main compound had a different structure from the peptide requested (Verbeke et al., 2015). A purity figure without an identity check would not have caught that last case.

How does a reference standard (HPLC-RTM) confirm identity?

Mass is not the only route to identity. In HPLC-RTM, a retention-time match, the laboratory runs an authenticated reference standard of the compound under the same conditions as the sample and checks that the main peaks come off the column at the same time. Pharmacopoeial peptide monographs specify identification tests separately from purity and assay, and reviews comparing those monographs set the three side by side (Vergote et al., 2009).

A retention-time match is only as good as the reference standard behind it, and two different molecules can occasionally elute together. It answers a slightly different question from mass: it shows the sample behaves like a known authentic sample, whereas mass shows the molecule has the expected composition. The two approaches are complementary, and neither is a purity test.

What does neither HPLC nor mass spec show?

Neither test measures net peptide content, counter-ions, water or endotoxin. A vial of lyophilised peptide is never 100% peptide by weight, even at 99% chromatographic purity. Net peptide content, the share of the powder's weight that is the peptide itself, is usually measured by amino acid analysis or nitrogen determination, not by HPLC area or by mass.

The rest of the weight is mostly counter-ions and water. Synthetic peptides are commonly isolated as trifluoroacetate (TFA) or acetate salts, and residual trifluoroacetate from synthesis and purification is a recognised impurity class (D'Hondt et al., 2014). Counter-ions are not peptides, so a peptide purity figure does not count them. Water is measured separately, typically by Karl Fischer titration. Bacterial endotoxin is measured by a separate assay such as LAL. A certificate that reports only HPLC and mass says nothing about any of these, and the reader should not infer them.

Which tests do Aion's peptide certificates report?

Six of our 14 certificates include an identity test, five by mass and one against a reference standard; eight do not. The certificates do not all report the same tests. This is what each one lists, read from the document itself; the testing page groups them by laboratory, and every certificate is in the COA library.

Five add identity by mass. The Freedom Diagnostics certificates for BPC-157, TB-500, GHK-Cu, Tesamorelin and the GLOW Blend list purity by HPLC-UV and identity by LC-MS.

One uses a reference standard: an ILS Laboratories certificate lists purity by HPLC and identity by HPLC-RTM, plus a fentanyl screen.

Seven report purity with no identity test. MOTS-c reports purity by HPLC only. The Vanguard Laboratory certificates for Selank and IGF-1 LR3, and the ILS certificate for NAD+, add heavy metals and endotoxin, and the Vanguard pair also microbial limits, but none of the three confirms identity. The North South Precision Testing certificates for Semax, Epitalon and KPV report purity and impurities without naming the method, along with moisture, pH, solubility and heavy metals. For the remaining certificate, RTT-3 20mg, we have not recorded a test list.

So six of our 14 certificates include an identity test, and eight do not. None of the test lists includes net peptide content or counter-ion analysis. Moisture appears only on the three North South certificates, and endotoxin only on the Vanguard pair and NAD+.

What does the TB-500 certificate show about LC-MS identity claims?

The TB-500 certificate reports purity and an LC-MS identity test but prints no observed mass, so a reader cannot check the identity claim. The TB-500 certificate shows both the strength and the limit of a combined claim. Its method line reads "HPLC-UV, LC-MS". The HPLC details are specific: a C18 peptide column, a formic acid mobile phase, which suits mass spectrometry better than TFA because TFA suppresses electrospray signal, and detection at 214 nm. Chromatographic purity is 99.35%, and the chromatogram is printed.

The certificate prints a molecular weight of 4963.44 g/mol under "Sample Information", next to the formula. That is the weight calculated from the formula, not a measurement. Nowhere does the certificate give an observed mass, a spectrum or a mass error. So the LC-MS identity claim rests on the laboratory's statement that the test was run, not on a figure a reader can check. The other four Freedom Diagnostics certificates use the same layout and also print no observed mass; on the GLOW blend, the weight shown is the sum of its three components, a figure no single molecule has.

That does not mean the identity test was not done. It means the document does not show it. When you compare certificates, from us or anyone else, look for the observed mass beside the expected one. Our guides on reading a certificate, what a purity percentage means and certificate red flags go further.

References

1. D'Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B (2014). Related impurities in peptide medicines. J Pharm Biomed Anal. [REVIEW] PubMed record for reference 1, D'Hondt 2014
2. Fekete S, Veuthey JL, Guillarme D (2012). New trends in reversed-phase liquid chromatographic separations of therapeutic peptides and proteins: theory and applications. J Pharm Biomed Anal. [REVIEW] PubMed record for reference 2, Fekete 2012
3. D'Hondt M, Gevaert B, Wynendaele E, De Spiegeleer B (2016). Implementation of a single quad MS detector in routine QC analysis of peptide drugs. J Pharm Anal. [IN VITRO] PubMed record for reference 3, D'Hondt 2016
4. Verbeke F, Wynendaele E, Braet S, D'Hondt M, De Spiegeleer B (2015). Quality evaluation of synthetic quorum sensing peptides used in R&D. J Pharm Anal. [IN VITRO] PubMed record for reference 4, Verbeke 2015
5. Vergote V, Burvenich C, Van de Wiele C, De Spiegeleer B (2009). Quality specifications for peptide drugs: a regulatory-pharmaceutical approach. J Pept Sci. [REVIEW] PubMed record for reference 5, Vergote 2009