LC-MS analysis of GLP-3: how identity is confirmed

GLP-3 Research Supply uses liquid chromatography–mass spectrometry (LC-MS) as the identity test for every lot of GLP-3 research material. Each lot record lists the ions the laboratory observed, and GLP-3 Research Supply publishes the values calculated from GLP-3 RT's molecular weight beside them so the comparison can be checked. This article walks through that calculation.

What a mass spectrometer measures

A mass spectrometer measures the mass-to-charge ratio (m/z) of ions, not mass directly. In LC-MS, the sample is first separated by liquid chromatography, and the eluting components are ionized, most often by electrospray ionization (ESI), before entering the mass analyzer.

ESI is a "soft" technique: it moves large molecules from solution into the gas phase as intact ions, typically by adding protons.¹ A peptide the size of GLP-3 RT picks up several protons, so one molecule appears as a series of ions at different charge states.

Charge states

An ion carrying three extra protons is written [M+3H]³⁺; four protons, [M+4H]⁴⁺. The more charges, the lower the m/z. The charge of an ion can be read from its isotope pattern: every molecule contains a natural mixture of isotopes (mainly carbon-12 and carbon-13), so each ion appears as a cluster of peaks spaced 1 ÷ charge apart. For a 3+ ion the spacing is about 0.33 m/z. That is why the certificates show small clusters, for example 1577.2 to 1578.3, rather than single lines.

Calculating the expected ions

For an ion carrying n protons:

m/z = (M + n × 1.0073) ÷ n

M is the molecular weight and 1.0073 is the mass of a proton. For an adduct, the adduct's mass is added to the numerator. Using GLP-3 RT's average molecular weight of 4731.33 g/mol:

Expected values are calculated by GLP-3 Research Supply from the average molecular weight (4731.33 g/mol; H⁺ 1.0073, Na⁺ 22.99, K⁺ 39.10). Observed values are as printed on each lot's mass confirmation panel.

Expected versus observed, lot by lot

Working backward from each observed ion recovers the neutral mass of the molecule that produced it: M = (m/z × charge) − (protons × 1.0073) − adduct mass. If the material is GLP-3 RT, every back-calculated mass should land close to 4731.33.

Calculated by GLP-3 Research Supply from the observed values printed on each certificate, using the average molecular weight 4731.33 g/mol, H⁺ 1.00728, Na⁺ 22.9892 and K⁺ 39.0978.

Mass error

Across all assigned ions on the three current certificates, back-calculated masses fall within about 1.4 Da of 4731.33, a deviation of roughly 0.03%. Observed m/z values sit within about 0.4 m/z of prediction. Two things account for most of the spread: the labeled value is the laboratory's peak pick from an isotope cluster, and errors in m/z are multiplied by the charge when converted back to mass. A peptide differing by a single residue would shift the mass by roughly 57 to 186 Da, far outside this range.

Adducts

Certificates also show a potassium adduct, [M+3H+K]⁴⁺, near m/z 1193. Trace potassium in solvents or glassware attaches to the peptide in place of one proton. This is routine in electrospray.

Identity confirmation

The laboratory reports identity as "Confirmed" on the certificate, with the matching ions labeled on the mass spectrum. The back-calculation above lets anyone check that result independently: for all three current lots, the labeled ions point back to GLP-3 RT's molecular weight.

Analytical limitations

• Molecular weight, not sequence order. Two peptides with the same amino acids in a different order have the same mass. Distinguishing them requires fragmentation (tandem MS) or other methods.
• Not quantitative here. Ion intensity depends on how readily each species ionizes, so purity is measured by HPLC-UV instead. See HPLC analysis of GLP-3.
• Average versus monoisotopic mass. Calculations here use the average molecular weight. High-resolution instruments can resolve individual isotope peaks and are compared against the monoisotopic mass instead.

Pairing a separation (HPLC) with an orthogonal identity method (MS) is a standard way to establish the specificity of an analytical procedure.^2,3