Loss on Drying Test Procedure: Standard Operating Protocol and Quality Inspection Guide

Loss on drying (LOD) testing is a fundamental gravimetric method used in pharmaceutical, food, and chemical laboratories to determine the moisture and volatile matter content of solid and semi-solid samples. This article presents a complete standard operating procedure for loss on drying determination, covering sample preparation, drying conditions, constant-weight criteria, calculation, and inspection checkpoints in accordance with recognized international standards. Whether performed in-house or by accredited third-party laboratories, a validated LOD procedure is essential for product release, stability evaluation, and regulatory compliance.

Technical Overview

Loss on drying (LOD) is a gravimetric determination of the volatile matter—primarily moisture—that a sample loses when heated under precisely controlled conditions. The test is performed by accurately weighing a sample before and after drying and expressing the mass difference as a percentage of the original sample weight. Because the measurement captures all evaporable components, including free water, absorbed moisture, and residual volatile solvents, loss on drying is one of the most widely applied release tests in pharmaceutical, food, and chemical quality control.

The operating procedure described in this guide is consistent with the general requirements of recognized national and international standards, including GB/T 6284 for chemical products, GB/T 5009.3 for foodstuffs, ISO methods for feed and agricultural materials, and applicable pharmacopoeial general chapters. Reliable results depend on a calibrated analytical balance, a qualified drying oven with verified temperature uniformity, active desiccant, and strict adherence to constant-weight criteria. Proper documentation of every weighing, drying cycle, and calculation is required to satisfy good laboratory practice and good manufacturing practice data-integrity expectations.

Testing Samples

Pharmaceutical Active Ingredients and Excipients

Active pharmaceutical ingredients, intermediates, and excipients such as starches, lactose, cellulose derivatives, and powdered granules are the most common sample category. Many pharmacopoeial monographs specify a loss-on-drying limit, typically in the range of 0.5 percent to 10.0 percent, which must be verified before batch release.

Foods, Agricultural Products, and Feed Materials

Flours, milk powders, dehydrated vegetables, spices, plant extracts, and animal feed materials are routinely tested for moisture to predict shelf life, control microbial growth, and confirm dry-matter content for nutritional labeling.

Industrial Chemicals and Fine Powders

Inorganic salts, polymer powders, catalysts, pigments, and surfactants are tested to verify drying completeness after synthesis, ensure flowability, and confirm compliance with contractual moisture specifications.

Cosmetic and Specialty Raw Materials

Talc, kaolin, botanical powders, and other cosmetic raw materials may carry moisture limits defined by product standards, making loss on drying part of routine incoming-material inspection.

Testing Items

The core scope of a loss on drying inspection typically includes the following determinations:

• Loss on drying expressed as mass percentage of the original sample
• Moisture and total volatile matter content under a specified drying condition
• Constant-weight verification of the dried residue
• Residual volatile content of dried or lyophilized products
• Verification of sample compliance against monograph or contractual limits

Testing Methods

Oven Drying Method (Reference Procedure)

Clean, dry, flat weighing bottles with lids are dried at the specified temperature to constant weight, cooled in a desiccator for approximately 30 minutes, and accurately weighed. A sample of about 1.0 g to 2.0 g, or the mass specified in the individual monograph, is placed in the bottle and spread to a uniform layer not exceeding 5 mm in thickness, then weighed precisely. The bottle, with the lid displaced, is dried in a thermostatically controlled oven at the specified temperature until constant weight is achieved. After each drying cycle, the lid is replaced, the bottle is cooled in a desiccator, and the weight is recorded. Drying cycles are repeated until the difference between two consecutive weighings does not exceed 0.3 mg.

The result is calculated as: Loss on drying (%) = (m1 − m2) / m1 × 100, where m1 is the mass of the sample before drying and m2 is the mass after drying to constant weight. Unless otherwise specified, the default drying temperature is 105 °C ± 2 °C.

Vacuum Drying and Desiccator Drying

For heat-sensitive or easily oxidized substances, drying is performed in a vacuum oven at a reduced temperature, commonly around 60 °C, under reduced pressure until constant weight is reached. For materials that decompose even under mild heating, drying over an effective desiccant such as phosphorus pentoxide or freshly activated silica gel at room temperature is permitted. Substances that melt below the standard drying temperature are dried at 5 °C to 10 °C below their melting point to avoid melting and encapsulation of residual moisture.

Rapid Loss on Drying Using Moisture Analyzers

Infrared and halogen moisture analyzers provide rapid results, often within 5 to 20 minutes, and are widely used for in-process control. The instrument heating program must be qualified against the reference oven method for each specific matrix, and the correlation must be documented before the rapid method is adopted for release testing.

Instrument Qualification and Inspection Checkpoints

A compliant inspection verifies that the analytical balance is calibrated and traceable, the oven temperature has been mapped for uniformity and accuracy, the desiccant remains effective, and the balance room humidity is controlled. All raw data, drying curves, replicate weighings, and calculations must be recorded in a traceable manner with second-person review, and results should be reported to the precision required by the specification.

Applications

Loss on drying testing supports quality decisions across the full product life cycle in regulated industries:

• Batch release testing of pharmaceutical ingredients, excipients, and finished dosage forms
• Incoming raw material inspection and supplier qualification
• In-process moisture control during granulation, drying, and blending operations
• Long-term stability and accelerated storage studies
• Shelf-life estimation and packaging suitability evaluation for foods and feeds
• Verification of drying completeness in chemical manufacturing

FAQ

What is the difference between loss on drying and water content?

Loss on drying measures all volatile matter driven off under the drying condition, including water and residual solvents. Water content determined by Karl Fischer titration is specific to water molecules only. For hydrates or samples containing volatile solvents, the two values can differ significantly, and the correct method should be selected according to the applicable monograph.

What does constant weight mean?

Constant weight is achieved when the difference between two consecutive weighings, after further drying under the same conditions, does not exceed 0.3 mg or the limit defined by the method. This criterion confirms that volatilization is complete and the result is stable.

How much sample is required?

Typically 1.0 g to 2.0 g of a homogeneous sample is used, unless a monograph specifies otherwise. Larger crystalline particles should be rapidly crushed to below approximately 2 mm before weighing to ensure uniform drying while avoiding moisture exchange with the environment.

How is the drying temperature selected?

Unless otherwise specified, 105 °C is the default condition. Lower temperatures with vacuum, or ambient desiccator drying, are used for thermolabile materials, while materials with low melting points are dried safely below their melting point.

Can a halogen moisture analyzer replace the oven method?

It can be used for rapid in-process checks and routine control once it has been qualified against the reference oven method for the specific product. For regulatory release testing, the validated reference gravimetric procedure generally remains the primary method unless equivalence has been formally demonstrated.

How should results be reported and interpreted?

Results are reported as mass percentage loss on drying, compared directly against the acceptance limit in the specification. Any out-of-limit result should trigger investigation of sample handling, instrument status, and drying conditions before a final disposition is made.