Loss on Drying Sample Quantity Determination: Principles, Standards, and Best Practices

Loss on drying (LOD) is one of the most widely performed tests for quantifying moisture and volatile content in pharmaceuticals, chemicals, and food products, and the accuracy of the result depends directly on how the sample quantity is determined. This technical article explains the principles behind loss on drying sample quantity determination, including pharmacopoeial requirements, the relationship between sample mass and balance sensitivity, and recommended test portions for different material types. The guidance draws on recognized standards such as USP General Chapter <731>, Ph. Eur. 2.2.32, ChP 0831, and GB/T 6284 to support reproducible laboratory practice.

Technical Overview

Loss on drying (LOD) determines the amount of volatile matter—primarily water, together with residual solvents—that is released when a test portion is heated under specified conditions. The result is expressed as a percentage of the initial sample mass and is calculated from the weight difference before and after drying. Although the procedure appears simple, the sample quantity taken for the test has a decisive influence on accuracy, precision, and representativeness.

Harmonized pharmacopoeial guidance establishes a default rule: unless otherwise specified in the individual monograph, the determination is performed on 1 to 2 g of the test substance, accurately weighed. This requirement appears in USP General Chapter <731> (Loss on Drying), Ph. Eur. 2.2.32, JP 2.41, and ChP 0831, while GB/T 6284 provides the corresponding framework for industrial chemical products. When a monograph specifies a different quantity—for example 0.5 g for highly volatile or hygroscopic substances—that value takes precedence.

The default range of 1 to 2 g balances two competing needs: the portion must be large enough that the weight loss is comfortably above the uncertainty of a 0.1 mg analytical balance and representative of a heterogeneous powder, yet small enough to dry quickly in a thin, uniform layer without splattering. When establishing or justifying a sample quantity, the following factors must be evaluated:

• Expected loss on drying of the material (the lower the expected LOD, the larger the sample required).
• Readability and calibration status of the analytical balance (typically 0.1 mg).
• Physical form, homogeneity, and particle size of the sample (crystals larger than 2 mm should be rapidly crushed before the test portion is taken, per ChP 0831).
• Risk of splattering, caking, or crust formation as moisture is released.
• Surface area of the weighing bottle or dish, since the sample layer should not exceed approximately 5 mm (10 mm for loose, bulky materials).
• Drying mode: atmospheric oven, vacuum oven, or desiccant-based drying.

Testing Samples

Different material classes require adapted sample quantities and handling precautions. The subsections below summarize commonly accepted starting amounts; a validated method or product specification always takes precedence over generic guidance.

Pharmaceutical APIs and Excipients

For typical crystalline or amorphous powders with an expected LOD between 0.5% and 5%, the standard quantity of 1 to 2 g is appropriate. The sample is placed in a flat weighing bottle previously dried to constant weight, distributed in a uniform layer no thicker than 5 mm, and dried at 105°C for the period stated in the monograph, commonly about 3 hours.

Chemicals and Inorganic Salts

Inorganic salts, catalysts, and industrial chemicals covered by GB/T 6284 are usually sampled at 1 to 2 g or the mass stated in the product standard. For substances with a very low expected moisture content (below about 0.5%), the larger end of the range—2 g or the maximum the dish allows while keeping the layer within 5 mm—is preferred so that the measured loss remains well above the weighing uncertainty.

Hygroscopic and Air-Sensitive Materials

Hygroscopic samples rapidly gain weight during handling, so the weighing bottle is fitted with a tight cover and the test portion is transferred and weighed as quickly as possible. A quantity near 1 g shortens exposure time; when heating would cause decomposition, drying may be performed at room temperature under vacuum over a desiccant such as phosphorus pentoxide.

High-Moisture, Liquid, and Semi-Solid Samples

For materials expected to lose more than about 10–20% of their mass—syrups, gels, plant extracts, and many food matrices—a reduced quantity of 0.3 to 0.5 g minimizes splattering and shortens drying time. An inert drying aid such as pre-dried sand, or a short glass rod, may be added to increase the evaporating surface and prevent skin formation that traps moisture. Conversely, highly heterogeneous matrices such as soils, addressed in ASTM D2216, conventionally use much larger portions (around 20 g), illustrating that sample size must always be matched to expected loss and homogeneity.

Thermosensitive and Low-Melting-Point Substances

When a substance melts below the specified drying temperature, melting would occlude residual moisture and bias the result low. In such cases the sample of about 1 g is dried at a mildly elevated temperature such as 60°C under reduced pressure, or at room temperature in a vacuum desiccator, with the layer kept as thin as practical.

Testing Items

A complete loss on drying determination with a defined sample quantity comprises the following checks and measurements:

• Verification of balance calibration and of the minimum sample weight at the test location.
• Pre-drying of the weighing bottle or dish to constant weight (two consecutive weighings differing by no more than about 0.3 mg).
• Accurate weighing of the chosen test portion into the tared container.
• Drying under the specified conditions (temperature, pressure, duration) to constant weight.
• Cooling in a desiccator over silica gel or phosphorus pentoxide before each weighing.
• Calculation of the percentage loss on drying and comparison with the acceptance criterion.
• Replicate determinations to confirm repeatability, commonly two portions agreeing within a defined absolute tolerance such as 0.1 to 0.2 percentage points.

Testing Methods

Direct Oven Drying at Atmospheric Pressure

Unless otherwise specified, the test portion (1–2 g, layer no thicker than 5 mm) is heated at 105°C for the time stated in the monograph, cooled in a desiccator, and reweighed. Drying is repeated for shorter intervals until two consecutive weighings agree within the constant-weight criterion.

Vacuum Drying

Thermosensitive materials are dried at a lower temperature, for example 60°C, under reduced pressure—commonly not exceeding about 2.7 kPa (20 mm Hg) or as specified in the monograph—until constant weight. Reduced sample quantities and thin layers accelerate moisture removal under vacuum.

Desiccant Drying

Samples that cannot tolerate heat are dried at room temperature in a vacuum desiccator over freshly activated phosphorus pentoxide or silica gel until constant weight is reached. Because this mode is slow, the sample quantity is kept near 1 g with the maximum practical exposed surface.

Calculation and Evaluation of Sample Quantity Suitability

Loss on drying is calculated as: LOD (%) = (m1 − m2) / (m1 − m0) × 100, where m0 is the mass of the dried empty container, m1 is the mass of the container plus sample before drying, and m2 is the mass after drying to constant weight.

The sample quantity is judged adequate by a simple uncertainty check. With a 0.1 mg-readability balance, the cumulative weighing error is roughly ±0.3 mg across the three weighings. For a 1 g sample at 1.0% LOD (10 mg lost), this corresponds to about 3% relative error; at 0.1% LOD (1 mg lost), it approaches 30%, which is unacceptable. A practical rule is that the anticipated weight loss should be at least 50 to 100 times the balance readability. Typical starting quantities are:

• Expected LOD below 0.5%: use 2 g or the largest portion the dish allows with a 5 mm layer.
• Expected LOD between 0.5% and 5%: use 1–2 g (the pharmacopoeial default).
• Expected LOD between 5% and 20%: use 0.5–1 g, with a drying aid if needed.
• Expected LOD above 20%: use 0.3–0.5 g, or a rapid moisture analyzer validated against oven LOD.

Applications

Sample quantity determination for loss on drying is applied wherever moisture or volatile content must be controlled for quality, stability, or regulatory compliance:

• Release testing of active pharmaceutical ingredients and excipients against pharmacopoeial monographs.
• In-process control of granulations and dried intermediates in GMP manufacturing environments.
• Long-term and accelerated stability studies, where a consistent sample size supports meaningful trend analysis.
• Industrial chemicals, fertilizers, and polymer powders tested under GB/T 6284 and related national standards.
• Food, feed, and agricultural matrices where moisture or solids specifications are contractually defined.
• Method development, transfer, and validation in accredited third-party laboratories, where the sample size rationale must be documented to demonstrate accuracy and precision.

FAQ

Why is 1 to 2 g the default sample quantity for loss on drying?

This range balances representativeness and analytical sensitivity: the portion is large enough that the weight loss is comfortably above the uncertainty of a 0.1 mg analytical balance and representative of a heterogeneous powder, yet small enough to dry quickly in a thin layer without splattering. USP General Chapter <731>, Ph. Eur. 2.2.32, JP 2.41, and ChP 0831 therefore adopt 1–2 g unless a monograph states otherwise.

Can a smaller or larger sample be used than the monograph specifies?

For compendial testing, the monograph quantity must be followed. During development of non-compendial methods, deviations are acceptable if scientifically justified—documenting balance precision, expected LOD, homogeneity data, and replicate repeatability—and approved under the laboratory's quality system.

How does balance sensitivity affect the choice of sample quantity?

The test portion must be large enough that the measured weight loss is many times the balance's uncertainty. As a practical rule, the anticipated loss should be at least 50 to 100 times the balance readability. If this cannot be achieved, increase the sample mass, use a balance with finer readability, or select a more sensitive technique such as Karl Fischer titration for water.

Why must the sample layer not exceed about 5 mm?

Thick layers slow the escape of moisture from the lower regions and can form a dried crust that traps volatiles, causing falsely low results and unnecessarily long drying times. ChP 0831 limits the layer to about 5 mm (10 mm for loose, bulky material), which is achieved by matching the sample quantity to the dish diameter.

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

Loss on drying captures all mass lost on heating—water plus volatile solvents and, in some materials, minor decomposition products—whereas Karl Fischer titration is specific to water. The LOD sample quantity is therefore chosen with the total volatile burden in mind, not moisture alone.

How is constant weight confirmed?

After the initial drying period, the sample is cooled in a desiccator, weighed, dried again for a shorter interval, and reweighed. Constant weight is achieved when two consecutive weighings differ by no more than about 0.3 mg, or by the limit stated in the individual monograph.

Should large crystals be crushed before the test portion is taken?

Yes. ChP 0831 directs that large crystals be rapidly crushed to particles below about 2 mm before sampling, ensuring the portion is homogeneous and dries uniformly. The crushing step must be performed quickly to avoid moisture exchange with the ambient air.