Loss on drying (LOD) is a fundamental gravimetric analytical technique used to determine the moisture and volatile matter content in pharmaceuticals, foods, chemicals, and polymers. This article explains the core principle of the loss on drying experiment, the major testing methods, applicable international standards, and its practical applications in quality control laboratories.
Technical Overview
Loss on drying (LOD) is a gravimetric determination of the amount of water and other volatile substances lost from a sample when it is heated under precisely controlled conditions. The underlying principle is straightforward: a pre-weighed sample is dried at a specified temperature to constant weight, and the percentage of mass lost during drying is calculated as the loss on drying value. This weight loss typically represents free moisture, adsorbed water, water of crystallization released at the drying temperature, and any other volatile components such as residual solvents or essential oils.
Unlike Karl Fischer titration, which is specific to water molecules, loss on drying measures total volatile matter and therefore reports a combined result. The method is governed by well-established pharmacopoeial and international standards, including USP General Chapter <731>, European Pharmacopoeia 2.2.32, the Chinese Pharmacopoeia, GB/T 5009.3 for foodstuffs, and ASTM D6980. Because the equipment required is simple—a precision balance, a drying oven or moisture analyzer, and a desiccator—the technique remains one of the most widely used moisture determination methods in industrial quality control.
Testing Samples
Pharmaceutical Raw Materials and Excipients
Active pharmaceutical ingredients (APIs), excipients such as lactose, microcrystalline cellulose, starch, and finished dosage forms like tablets and capsules are routinely tested for loss on drying. Moisture content directly affects product stability, flowability, compaction behavior, and shelf life, making LOD a batch-release test in most pharmaceutical manufacturing environments.
Food and Agricultural Products
Flours, milk powders, grains, spices, dehydrated vegetables, confectionery, and feed materials are common test samples. In the food sector, moisture content correlates with texture, microbial stability, processing behavior, and compliance with compositional standards.
Chemicals, Polymers, and Coatings
Inorganic salts, detergents, pigments, plastic resins, adhesives, and coating powders are tested to verify dryness after synthesis or processing, to calculate assay results on a dry basis, and to control volatile organic content.
Cosmetics and Personal Care Ingredients
Botanical extracts, surfactants, and powder ingredients used in cosmetic formulations are analyzed for water and volatile content to ensure formulation consistency and preservative efficacy.
Testing Items
Typical parameters measured in a loss on drying experiment include:
- Moisture content (percentage by mass of the original sample)
- Total volatile matter, including residual solvents released at the drying temperature
- Dry matter or total solids content (100% minus the LOD value)
- Water of crystallization for hydrated crystalline substances, where specified
- Constancy of mass verification, confirming the drying endpoint has been reached
Testing Methods
Conventional Drying Oven Method
The reference procedure described in pharmacopoeial monographs involves accurately weighing 1–2 g of sample into a pre-dried, tared weighing bottle, drying it in a gravity or forced-convection oven at 100–105 °C or at the temperature specified in the individual monograph, cooling in a desiccator over silica gel, and reweighing. The process is repeated until two consecutive weighings differ by no more than 0.3 mg, a criterion known as constant weight. Loss on drying is calculated as the mass loss divided by the original sample mass, multiplied by 100%.
Vacuum Oven Drying
For thermolabile substances that decompose, melt, or undergo chemical change at atmospheric boiling conditions, drying is performed under reduced pressure, typically 2.7 kPa (20 mmHg) or lower, often at 60–70 °C. Vacuum lowers the effective boiling point of water, allowing gentle removal of moisture from sugars, gelatin, vitamin preparations, and similar sensitive matrices.
Rapid Infrared and Halogen Moisture Analyzers
Moisture balances integrate a halogen or infrared heating source with an integrated precision balance, enabling LOD results in 5–15 minutes. These instruments are widely used for in-process control in production environments and are calibrated against the reference oven method to ensure equivalent results.
Thermogravimetric Analysis (TGA)
TGA continuously records sample mass as temperature is ramped in a controlled atmosphere, producing a mass-loss curve that distinguishes moisture, solvent, and decomposition stages. It is particularly useful for research, method development, and polymers.
Specialized Procedures
For substances containing volatile components that oxidize at elevated temperatures, pharmacopoeias allow drying in a desiccator over phosphorus pentoxide at atmospheric pressure or at reduced pressure, or using azeotropic distillation where applicable. Melted samples may be dried at a temperature above their melting point to expose the full surface for volatilization.
Applications
- Pharmaceutical batch release, stability studies, and compliance with individual monograph LOD limits
- Food labeling, nutritional calculation on a dry basis, and shelf-life evaluation
- Chemical and polymer process control, verifying drying efficiency after synthesis, crystallization, or spray drying
- Assay correction, converting analytical results from an as-received basis to a dry-weight basis
- Incoming material inspection and supplier qualification in accredited third-party laboratories and in-house QC facilities
- Packaging and storage studies, since excessive moisture accelerates hydrolysis, caking, and microbial growth
FAQ
What is the basic principle of the loss on drying experiment?
The principle is gravimetric comparison: the sample is weighed before and after drying under specified temperature and pressure conditions. The percentage difference between the two masses represents the water and volatile matter content of the sample.
How does loss on drying differ from Karl Fischer titration?
Loss on drying measures all volatile substances lost upon heating—water plus residual solvents and other volatiles—whereas Karl Fischer titration is a chemical method selective for water only. For hydrated or solvent-retaining materials, the two methods can give different results.
What does "drying to constant weight" mean?
It means repeating the drying, cooling, and weighing cycle until the difference between two consecutive weighings does not exceed 0.3 mg (or another limit specified by the standard), confirming that volatiles have been essentially completely removed.
Why must samples be cooled in a desiccator before weighing?
Hot samples would create convection currents that falsify the weighing, and exposure to ambient air would allow reabsorption of moisture. A desiccator provides a dry atmosphere for cooling, both protecting accuracy and preventing moisture pickup.
Which drying temperature should be selected?
The temperature is specified by the relevant monograph or standard, commonly 100–105 °C for most substances. Heat-sensitive materials are dried at lower temperatures under vacuum, while samples that melt may be dried slightly above their melting point, following the directions of the applicable standard.
How long does a typical loss on drying test take?
The classic oven method usually requires 2–6 hours including cooling and constant-weight verification, while rapid halogen moisture analyzers deliver results within 5–15 minutes, making them suitable for high-throughput in-process control.