Impurities in Pharmaceutical Substances: Sources and Types of Impurities, and Limit Tests for Chloride, Sulphate, Iron, Arsenic, Lead, Heavy Metals, Including Modified Limit Tests
Impurities
Impurities are unwanted chemical substances that are present in a pharmaceutical substance or product along with the active pharmaceutical ingredient (API) or excipients. These substances have no therapeutic benefit and may affect the quality, safety, efficacy, and stability of a drug. Impurities may arise during manufacture, storage, or distribution of pharmaceutical products and therefore must be identified, quantified, and controlled as per pharmacopoeial and regulatory guidelines.
In pharmacy, an impurity is defined as any component of the drug substance that is not the chemical entity defined as the API or an excipient. Even very small amounts of impurities can be significant because they may cause toxicity, adverse reactions, reduced potency, or instability of the medicine.
Impurities can originate from raw materials, manufacturing processes, degradation of drugs, contamination, or interactions with packaging materials. Their presence is strictly regulated by standards such as Indian Pharmacopoeia (IP), British Pharmacopoeia (BP), United States Pharmacopeia (USP), and ICH guidelines.
Types of impurities in pharmacy with examples
1. Organic impurities
These impurities arise from the manufacturing process or during storage of the drug substance. They include starting materials, intermediates, by-products, and degradation products.
Example:
Paracetamol may contain p-aminophenol as an impurity, which is a toxic degradation product formed during improper storage or incomplete synthesis.
2. Inorganic impurities
These are impurities resulting from reagents, catalysts, heavy metals, or inorganic salts used during synthesis.
Example:
Presence of lead, arsenic, iron, or mercury as impurities in pharmaceutical substances due to contaminated raw materials or processing equipment.
3. Residual solvents
Residual solvents are organic volatile chemicals used in the manufacturing process that are not completely removed.
Example:
Residual methanol or acetone found in tablets prepared using solvent-based granulation methods.
4. Degradation impurities
These impurities form due to chemical decomposition of the drug caused by heat, light, moisture, or oxygen.
Example:
Aspirin undergoes hydrolysis to form salicylic acid and acetic acid, which are considered degradation impurities.
5. Microbial impurities
These include bacteria, fungi, or their toxins that contaminate pharmaceutical products, especially liquid and semi-solid preparations.
Example:
Microbial contamination in syrups due to inadequate preservatives or poor storage conditions.
6. Impurities from packaging materials
Chemical substances may leach from containers, closures, or packaging materials into the drug product.
Example:
Plasticizers leaching from plastic containers into ophthalmic solutions.
Importance of controlling impurities
Impurities must be controlled because they can:
- Cause toxic or allergic reactions
- Reduce therapeutic effectiveness
- Affect stability and shelf life
- Lead to regulatory rejection of pharmaceutical products