HPLC and LC-MS in peptide analysis

HPLC shows separation under specified conditions

HPLC stands for high-performance liquid chromatography. The sample's components travel through a column with a flowing liquid. Differences in their interaction with the column and liquid produce different retention times: the time until a substance is recorded by the detector.

A chromatogram shows the detector's signal over time. Peptide analysis often uses UV detection. A main peak and smaller peaks can provide information about composition, but two substances may also elute at the same time. One peak therefore does not necessarily represent a single substance.

Retention time depends on the analytical conditions. A time from one method cannot be used as a universal identifier in another. The report must provide enough information for the laboratory to explain what the peaks and area calculation mean.

LC-MS links mass information to the peaks

A mass spectrometer records ions by their mass-to-charge ratio, m/z. Peptides can produce ions with multiple charges. Interpreting these signals can yield a calculated molecular mass for comparison with the expected mass.

Zeng and colleagues investigated LC-HRMS, liquid chromatography with high-resolution mass spectrometry, for quality control of three peptide medicines in 2015. Their work shows how mass information can help investigate impurities and differences between samples that chromatography alone does not explain sufficiently.

A mass that matches expectations is useful information. Structures with the same mass may still require further investigation. LC-MS/MS analyses fragments of selected ions and can provide more information about the sequence. The interpretation itself must state what has been confirmed.

Choose the measurement to fit the question

Purity profile, identity and quantitative content require different assessments. Content determination needs a defined relationship between quantity and measurement signal. This may include calibration and a suitable reference material. The area of a chromatographic peak is not automatically a complete measure of quantity.

Mass spectrometric signal intensity is also affected by ionisation and instrument conditions. Two substances can produce different signals at the same quantity. Dividing one MS signal by the sum of all signals therefore does not automatically yield a reliable mass percentage.

A method must suit the sample in question

Selectivity describes how well the method distinguishes the analyte from other components. Precision concerns variation between measurements; accuracy concerns how closely results agree with an accepted reference. These characteristics are assessed for the particular analytical task, rather than simply for the instrument type.

Studies of LC-MS/MS systems also use measures of chromatographic performance, mass accuracy and signal intensity, among other parameters. Such checks can reveal changes in a system's performance between runs. They give a reason to ask about system suitability when comparing reports.

A method that works well for one peptide is not necessarily as selective for another. Labels such as HPLC-tested or MS-confirmed should therefore be accompanied by a specific explanation of what the test covers.

Other properties need separate investigations

Sterility, endotoxins and residual solvents are separate areas of analysis. A standard chromatogram or mass spectrum does not provide an overall conclusion about them. Any results must appear as separate, identified tests with their own criteria.

EMA's guideline for synthetic peptides discusses several such quality parameters in drug development. It is used here as a scientific reference for distinguishing the measurements. It does not turn a raw material into an approved medicine or document safety in humans.

When reading a COA, write down which question each result answers. If a necessary measurement is missing, record it as missing documentation rather than filling the gap with an assumption about what another test means.