HPLC and LC-MS: Purity and Identity Are Different Questions

Analytical reports are often read as answering a single question — "is this good material?" — when they answer several narrower ones. Two of those narrower questions are separable and are measured differently. How much of the detected material is the thing of interest, and what is the thing of interest. Chromatography is well suited to the first. Answering the second normally requires measuring something structural.

Chromatography separates. Its output is a set of resolved bands and their relative detected response, which supports statements about how many components a method could distinguish and in what proportion. Under a fixed method, retention time is reproducible enough to be compared against a reference standard, and that comparison is genuine evidence — but it is evidence about behaviour under conditions, not about molecular structure.

• Separates components by their interaction with a stationary phase
• Quantifies relative response by integrated peak area
• Supports comparison against a reference standard run under identical conditions
• Blind to anything the detector does not respond to
• Cannot distinguish two substances that co-elute

Coupling a mass spectrometer to the chromatograph adds a second, different measurement to each separated band. The eluent is ionised and ions are sorted by mass-to-charge ratio, so alongside "something eluted at this time" there is now "and its ions appeared at these m/z values". Because mass-to-charge relates to molecular composition, this is structural information in a way retention time is not. For peptides, which commonly carry several charges, the observed series of multiply-charged ions is itself informative.

A measured mass consistent with an expected molecular formula is strong evidence and weak proof. Different substances can share a nominal mass, and for peptides the same amino acid composition in a different sequence has an identical mass — so a matching mass cannot by itself distinguish a sequence from its rearrangement. Higher-resolution instruments narrow the field considerably, and fragmentation experiments narrow it much further, but the general point holds: a mass constrains identity rather than determining it.

• Isomers share a molecular formula and therefore a mass
• Sequence variants of a peptide can be mass-identical
• Resolution determines how finely masses can be distinguished
• Fragmentation data, where reported, adds structural constraint

ICH Q2(R2) describes specificity and selectivity as demonstrable through absence of interference or by comparison against an orthogonal procedure — a second, well characterised procedure that ideally applies a different measurement principle. Chromatographic retention and mass detection are different measurement principles, which is precisely why they are used together. The guideline is explicit that where one procedure does not provide sufficient discrimination, a combination of two or more is recommended.

When a certificate reports both, the useful questions are whether they describe the same batch and the same sample, whether the method conditions are stated, and whether identity and purity are reported as separate results rather than merged into one number. A single figure labelled "purity/identity" is less informative than two figures with their methods named.

• Do the batch and sample references match across both results?
• Are the method conditions stated, or only the outcome?
• Are identity and purity reported separately?
• Is the expected mass given alongside the observed one?
• Is the analysis date present, and how long ago was it?