Identity asks which substance is present. Purity asks what proportion of the material consists of the desired component according to a particular method. The questions are related but not identical.
This topic is closely related to What is HPLC? and What is a Certificate of Analysis (CoA)?, which provide additional context for the interpretation and characterization of chemical materials.
A chromatogram with one dominant peak shows that one component predominates, but without additional data it does not automatically establish which component it is.
A spectrum can strongly support the presence of the intended compound while other components may still be present.
HPLC can provide a purity profile, LC-MS mass information, NMR structural information and FTIR spectral features. Together they often provide a stronger picture.
A sound assessment treats identity and purity separately and then combines both types of evidence.
Chemical identity concerns whether the material corresponds to the stated compound and chemical form. That can include molecular structure, salt or free form, hydration state and, where relevant, stereochemistry. A name or label is therefore a claim that analytical data should support.
Purity is always linked to what was measured and how it was measured. Chromatographic purity, assay, water content and inorganic residue describe different aspects of a material. A single percentage should therefore be read together with the method behind it.
Suppose an HPLC chromatogram contains one peak representing 99.5% of integrated UV area. This is strong evidence that one UV-detectable chromatographic component dominates under that method. It does not by itself prove that the peak is the intended molecule or that 99.5% of the total sample mass is that molecule.
Water, residual inorganic salts, counterions or compounds with poor detector response may not be represented proportionally in a particular chromatogram. Conversely, a strongly responding trace impurity can look larger than its mass fraction. Detector response and sample chemistry therefore matter.
Identity is usually strengthened by orthogonal information: expected molecular ions by MS, structural resonances by NMR, characteristic infrared bands by FTIR or comparison with an authenticated reference. The best method depends on the question and the chemical substance.
A purity assessment may combine chromatographic profile with assay, water determination, residual solvents or other tests when relevant. No universal set of tests applies to every compound. The reported specification should make clear what the percentage represents.
On a Certificate of Analysis, the words identity, purity and assay should not automatically be treated as interchangeable. Check the test name, method, specification and result. A CoA becomes more informative when these fields are internally consistent and linked to a specific batch.
Independent techniques fail in different ways. HPLC separates, MS measures ions by mass-to-charge ratio, NMR probes structural environments and FTIR measures vibrational absorption. Agreement across different principles reduces the chance that one ambiguous observation is being overinterpreted.
First verify the stated chemical identity and exact form. Then determine which tests support that identity. Next examine what the reported purity or assay actually measures, and finally check whether other relevant components such as water, solvents or counterions are addressed.
Identity answers 'what is it?' and purity answers 'how much of the measured material meets a defined criterion?'. Keeping those questions separate prevents one of the most common errors in reading analytical documentation: treating a high percentage as automatic proof that the substance itself has been correctly identified.
Analytical data cannot be separated completely from the conditions under which they were produced. Instrument settings, sample preparation, reference materials, calibration and data processing can all influence the final result. Two values that look identical at first glance may therefore not carry exactly the same analytical meaning. When documents are compared, the method and the definition of the reported value matter just as much as the final number.
A reliable reference material can help compare analytical observations with a known substance. Its value depends on the question being asked: retention behaviour, a spectrum or another characteristic can provide strong support under controlled conditions. Reference comparison does not replace critical review of the complete dataset. The identity of the reference, the method used and the similarity of measurement conditions determine how strong the comparison really is.
A technically convincing result consists of more than a percentage or one prominent signal. Clear sample identification, interpretable raw or derived data, logical specifications and a suitable analytical method make a result easier to assess. Internal consistency also matters: chemical name, formula, molar mass, batch information and analytical conclusion should not contradict one another. Where different elements do not align, further verification is preferable to treating one favourable value as decisive.
Information has different functions in an SDS, Technical Data Sheet and Certificate of Analysis. An SDS primarily addresses safety and hazard information, a TDS describes technical characteristics, and a CoA generally reports results for a specific batch. Analytical information has the right meaning only when it is clear which document contains it, which material or batch it concerns, and whether it is a specification, typical value or actual measured result.
