When documentation reports that a research material has been characterized, two distinct questions are usually being answered: what is it? and how pure is the tested sample? These are not the same question, and they are not answered by the same analytical method. Understanding which technique addresses which question is central to reading a Certificate of Analysis with precision.

This guide explains the two methods most commonly cited for research-material characterization — reverse-phase high-performance liquid chromatography (RP-HPLC) and liquid chromatography–mass spectrometry (LC-MS) — what each one measures, why they are used together, and how to interpret the method information recorded on a COA. The aim is documentation literacy, not laboratory instruction.

Identity and Purity Are Separate Questions

It is tempting to treat a single high number as proof of everything, but purity and identity are independent properties. A sample can be highly pure and still not be the intended material; conversely, the correct material can be present alongside measurable impurities. Because the two questions are distinct, credible characterization addresses them with methods suited to each, and a COA typically records both separately.

Keeping the two apart is the first habit of careful interpretation: read a purity result as a statement about composition under a method, and an identity result as a statement about what the measured species is.

Reverse-Phase HPLC and Purity

Reverse-phase HPLC separates the components of a sample as they travel through a column at different rates, producing a chromatogram in which each component appears as a peak. The relative size of the target peak, under the stated method and detector, is what underlies a reported purity value.

What an RP-HPLC purity result describes

A purity figure derived from RP-HPLC describes the proportion of the tested sample attributable to the target peak under the specific conditions used — column, mobile phase, gradient, detector, and reporting limit. It is a property of the sample as measured, not an intrinsic, method-independent constant.

Why conditions matter

The same material can yield different chromatograms under different conditions, because separation depends on the method. This is why a purity value is only fully interpretable alongside the method that produced it. A figure presented without its method is difficult to compare or rely upon.

LC-MS and Identity

Liquid chromatography–mass spectrometry couples chromatographic separation with mass spectrometry, which measures mass-related information about the species present. Where RP-HPLC reports how much of the sample is the target peak, LC-MS contributes evidence about what that material is, by relating the measured mass information to the expected species.

Identity confirmation is therefore typically grounded in a method like LC-MS rather than in a purity number alone. The mass-related evidence supports the identification, and the method and conditions used are recorded so the basis for the conclusion is transparent rather than asserted.

Why the Two Methods Are Used Together

RP-HPLC and LC-MS are complementary precisely because they answer different questions:

  • RP-HPLC separates and quantifies, supporting a purity result.
  • LC-MS adds mass-related information, supporting an identity confirmation.

Used together, they allow documentation to address both composition and identity for the tested sample, each with an appropriate technique. This pairing is a common reason a COA cites more than one method, and why reviewing both is more informative than focusing on a single figure. How these results sit within the wider document is covered in our guide to Certificates of Analysis.

Reading Method Information on a COA

Because results depend on methods, the method fields are not boilerplate — they are part of the result. When reviewing analytical data on a COA, laboratories commonly look for:

  • The method named for each result, so the technique behind the value is explicit.
  • The conditions sufficient to place the result in context.
  • Reporting or detection limits, which bound statements such as "not detected" — a substance not detected at or above a limit is not the same as proven absent.
  • Consistency between the methods cited and the results reported.

Confirming that method information is present and coherent is a standard part of a documentation review before a material enters a workflow — see our documentation review checklist.

What These Methods Do Not Establish

Analytical identity and purity results are evidence about a tested sample under stated conditions. On their own they do not establish:

  • Suitability for a specific research workflow — a determination made by the laboratory against its own SOPs.
  • That every unit in a shipment is identical to the tested sample; testing is performed on a sample.
  • Stability after the test date, particularly outside documented storage and handling conditions.
  • Any property a method does not measure, or any conclusion about human or veterinary use.

Key Takeaways

  • Purity and identity are separate questions answered by different methods.
  • RP-HPLC separates and quantifies, supporting a purity result that depends on the stated method.
  • LC-MS adds mass-related information that supports identity confirmation.
  • The two are complementary, which is why a COA often cites both.
  • A result is only interpretable alongside its method, conditions, and reporting limits.