Impurities are unwanted chemicals that may be present in drug substances or finished products as a result of manufacturing, storage, or handling.

Types of Impurities
Process-related → unreacted starting materials, by-products, catalysts.
Degradation-related → formed through heat, oxidation, hydrolysis, or light.
Residual solvents → examples include methanol and dichloromethane.
Enantiomeric impurities → incorrect stereoisomers in chiral drugs.
According to ICH Q3A/Q3B, any impurity at or above 0.1% must be identified, with stricter thresholds for genotoxic impurities.

Identifying Impurities Using HPLC
Detection
The main API peak appears alongside smaller impurity peaks.
Characterization
Compare retention times with reference standards.
Use PDA detectors to check peak purity.
Confirm identity by co-injection with standards.
Structural Identification
LC-MS provides molecular weight and fragmentation patterns.
LC-NMR or LC-FTIR supplies structural fingerprints.
Together, these techniques give the complete chemical identity.

Examples of Well-Known Impurities

Nitrosamines (e.g., NDMA, NDEA)
Discovered in valsartan, ranitidine, and metformin between 2018–2019.
Classified as probable human carcinogens.
Regulatory limits are extremely low (nanogram levels).
Identified using HPLC-MS/MS.
p-Aminophenol (Paracetamol)
A toxic degradation product affecting liver and kidney.
Strictly limited to ≤0.1%.
Formaldehyde / Acetaldehyde
Residual solvent-related impurities found in excipients.
Detected using derivatization followed by HPLC.
Epimer impurities in chiral drugs
Wrong stereoisomers can be inactive or harmful.
The thalidomide case highlighted the risks.
Controlled and identified using chiral HPLC columns.

Why Impurity Profiling is Essential
Patient safety → prevents toxic exposure.
Regulatory compliance → required by ICH and FDA.
Process understanding → reveals weaknesses in synthesis or storage.
Shelf-life assurance → guarantees product safety until expiry.

Key Takeaway

Finding an extra peak in an HPLC chromatogram is only the first step. Comprehensive quality control requires:

Detection – observing the impurity peak.
Identification – determining chemical structure with advanced techniques.
Control – ensuring levels remain below regulatory limits.