Peptidetesting

Understand testing before you trust it

GC-MS vs. LC-MS: Which method when?

For peptides, LC-MS is the standard approach: liquid chromatography separates, mass spectrometry weighs. GC-MS needs volatile substances - intact peptides aren't. A COA tells you what was measured - but only for that exact sample.

What is the difference between GC-MS and LC-MS?

The main difference between GC-MS and LC-MS is the state of matter used for separation: GC-MS separates substances in the gas phase, while LC-MS uses the liquid phase. For peptides, LC-MS is the standard method, because these chains of amino acids stay stable in liquids and can be analyzed precisely there.

A mass spectrometer measures the mass-to-charge ratio (m/z) of ions, which is simply like weighing the molecules. While gas chromatography (GC) vaporizes the sample, liquid chromatography (LC) dissolves it in a solvent. The subsequent mass determination works the same way after both separation methods.

Why are intact peptides not suitable for GC-MS?

Intact peptides are not suitable for GC, because gas chromatography (GC) requires volatile substances that do not decompose when vaporized. Since peptides are complex, non-polar molecules, they would need to be chemically converted (derivatized) or broken down into amino acids for GC analysis, which prevents examining the intact molecule.

Even individual amino acids must be made volatile through derivatization for GC-MS analysis. In the scientific literature, there is no method for intact peptides using GC-MS. Instead, samples are often hydrolyzed, meaning broken down into amino acids, or directly analyzed with the more suitable LC-MS method.

How does LC-MS analysis work for peptides?

LC-MS analysis first separates peptides in a liquid column and then determines their exact weight in the mass spectrometer. Liquid chromatography (LC) uses the interaction of substances with the column for separation, while mass spectrometry (MS) provides the mass-to-charge ratio (m/z) of the molecules as a signal.

For peptides, electrospray ionization (ESI) is usually used to convert dissolved substances into ions. To avoid disturbing the ionization, interfering salts or detergents must be removed beforehand. A single LC-MS run can quantify thousands of different peptides simultaneously.

What advantages does mass spectrometry offer over UV detection?

Mass spectrometry offers additional certainty compared to pure UV detection (HPLC-UV), because it measures the exact molecular mass instead of just the retention time. While UV detection only recognizes when a substance leaves the column, MS analysis confirms the identity of the peptide through its specific weight.

This is comparable to a friend you recognize at the train station not only by arrival time, but also by weighing them. LC-MS provides both the time and the mass. With tandem-MS (MS/MS), the molecule can also be broken down into fragments, which further secures the identity, while UV-HPLC cannot distinguish co-eluting substances.

What do HPLC, LC-MS, and GC-MS mean on a COA?

The method name on a COA (Certificate of Analysis) describes the depth of identity testing: HPLC usually stands for a UV measurement of time, LC-MS for an additional mass determination. The designation LC-MS/MS refers to an extended form with fragmentation, which allows even more specific identification of peptides.

GC-MS on a peptide COA is unusual, because intact peptides cannot be analyzed by gas chromatography. If this method is nevertheless listed, the laboratory would have to explain the sample pretreatment. This is usually done by hydrolyzing the peptide into its individual amino acids.

Why does a COA result only refer to the tested sample?

A COA result only applies to the submitted sample, because each analytical method is validated specifically for a certain matrix - meaning the environment of excipients and impurities. According to ICH guidelines, the laboratory documents specificity and purity only for the tested individual piece, not automatically for an entire batch.

The principle of quality control requires that each sample is tested individually. Since the quantification limit and linearity also refer to specific concentration ranges and matrices, transferring results to other samples without re-testing is not permissible.

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