Loss on Drying (LOD) Constant Weight Testing: Principles, Methods, and Applications

Loss on drying (LOD) constant weight testing is a gravimetric analytical technique used to determine the moisture and volatile matter content of pharmaceuticals, foods, chemicals, and polymers by drying a sample until it reaches a stable, constant mass. The method is recognized by major standards organizations such as ASTM, ISO, and GB and serves as a fundamental quality control parameter for stability, shelf life, and regulatory compliance. This article explains the testing principle, applicable sample types, procedures, calculation methods, and practical applications of constant weight loss on drying analysis.

Technical Overview

Loss on drying (LOD) constant weight testing measures the amount of mass a sample loses when it is heated or dried under controlled conditions until no further measurable weight change occurs. The weight loss reflects the total content of water and other volatile substances—including residual solvents, low-boiling additives, and adsorbed moisture—that evaporate during the drying process. Unlike instrument-specific water determination techniques such as Karl Fischer titration, loss on drying is a non-selective gravimetric method: it captures all evaporable matter, not only water.

The defining feature of this technique is the "constant weight" (also called constant mass) criterion. A sample is considered to have reached constant weight when the difference between two consecutive weighings, after additional drying and cooling cycles, falls below a specified tolerance—commonly 0.3 mg or a defined percentage of the sample mass, depending on the applicable standard. This criterion ensures that the drying process is complete and that the reported result represents the full volatile fraction rather than an intermediate value.

The test is typically performed using a calibrated analytical balance with 0.1 mg readability, a laboratory drying oven or vacuum oven, a desiccator with fresh desiccant, and pre-cleaned weighing vessels such as glass dishes, evaporating dishes, or covered crucibles. Common reference standards include GB/T 6284 for chemical products, GB 5009.3 for food moisture determination, ASTM D2216 for soil and aggregate water content, ASTM D6980 for moisture in plastics, and ISO 6496 for animal feeding stuffs.

Testing Samples

Pharmaceutical Samples

Active pharmaceutical ingredients (APIs), excipients, intermediates, granules, and finished dosage forms such as tablets and capsules are routinely tested for loss on drying. Pharmacopoeial monographs frequently specify LOD limits—for example, a maximum of 0.5% or 1.0%—because excessive moisture can trigger hydrolysis, microbial growth, reduced shelf life, and compromised tablet hardness or flowability.

Food and Agricultural Products

Flours, starches, dairy powders, dehydrated vegetables, spices, tea, feed ingredients, and grain products are tested to determine moisture and total solids. Moisture content directly affects texture, microbial stability, processing behavior, nutritional labeling (dry basis values), and compliance with commodity trading specifications.

Chemical Raw Materials

Inorganic salts, organic intermediates, catalysts, dyes, fertilizers, and surfactants are evaluated for volatile content to verify purity grades, monitor drying operations, and control water of crystallization. Certain hydrate compounds require carefully selected drying temperatures so that only surface and adsorbed moisture, not constitutional water, is driven off.

Polymers and Plastics

Resin pellets, polymer powders, and compounded materials are tested for moisture because residual water causes hydrolytic degradation during melt processing (such as injection molding or extrusion of polyesters and polyamides) and creates voids or splay marks in finished parts.

Minerals, Soils, and Inorganic Materials

Soils, sediments, mineral fillers, pigments, and ceramic powders are oven-dried to constant mass to determine water content, which is essential for expressing analytical results on a dry-mass basis and for geotechnical classification.

Testing Items

Constant weight loss on drying testing typically covers the following measurable parameters:

• Moisture and volatile matter content (LOD, expressed as weight percentage of the original sample)
• Total solids and dry matter content (the complementary value to LOD)
• Residual volatile substances evaporable at the specified drying temperature
• Constant mass verification of empty weighing vessels used in subsequent ash or residue determinations
• Dry basis conversion data for reporting other analytical results

The primary calculated result is expressed 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. Some specifications instead require the result on a dry basis, in which case the calculation is adjusted accordingly.

Testing Methods

Atmospheric Oven Drying

The reference procedure requires drying a pre-weighed sample in a forced-convection oven, typically at 105°C ± 2°C, until constant weight is achieved. The sample, generally 1–10 g depending on the material and standard, is spread in a thin layer (usually no more than 5 mm thick, or 10 mm for porous or bulky materials) in a shallow vessel to maximize surface exposure. After each drying interval—commonly 1 to 4 hours—the vessel is transferred to a desiccator, cooled to room temperature, and weighed. The cycle of drying, cooling, and weighing is repeated until the mass difference between two consecutive weighings meets the constant weight tolerance.

Vacuum Oven Drying

Heat-sensitive samples such as certain APIs, sugars, proteinaceous materials, and polymers with low softening points are dried under reduced pressure (often 2.7 kPa or below) at a lower temperature, commonly 60–80°C. Vacuum lowers the boiling point of water and volatiles, allowing effective moisture removal without thermal decomposition or melting.

Desiccator Drying at Ambient Temperature

For extremely thermolabile substances that degrade even under mild heating, the sample is placed in a desiccator over a strong desiccant such as silica gel or phosphorus pentoxide and dried at room temperature until constant weight. This method is slower but avoids any thermal stress.

Rapid Balance and Infrared Moisture Analysis

Halogen lamp or infrared heated moisture analyzers provide fast results (typically 5–20 minutes) using a built-in balance and an automatic shut-off criterion. These instruments are widely used for in-process control; however, for release testing and dispute resolution, the oven-based constant weight method described in pharmacopoeial or national standards remains the referee method.

Procedure Essentials and Quality Control

• Pre-dry and pre-weigh the empty vessel to constant weight before adding the sample
• Use a calibrated analytical balance with 0.1 mg readability and calibrated ovens with verified temperature uniformity
• Use fresh or regenerated desiccant; cool vessels fully in the desiccator before each weighing to prevent moisture uptake
• Weigh hygroscopic samples rapidly using covered vessels to minimize atmospheric moisture absorption
• Perform duplicate determinations and check reproducibility against the acceptance criteria in the applicable standard
• Confirm the result differs between consecutive weighings by no more than the specified tolerance (commonly 0.3 mg for pharmacopoeial work) before reporting

Applications

Constant weight loss on drying testing is applied across many industries and decision contexts:

• In pharmaceutical manufacturing, verifying that APIs and excipients meet pharmacopoeial LOD limits supports stability assessments, packaging selection, and expiry dating
• In food production, moisture data governs drying endpoint control, water activity correlation, preservative dosing, and dry-basis nutritional labeling
• In chemical processing, LOD monitoring optimizes crystallization, filtration, and drying unit operations and confirms product grade conformity
• In polymer processing, moisture verification prevents processing defects and molecular weight degradation in moisture-sensitive resins
• In environmental and geotechnical laboratories, constant mass oven drying establishes soil water content for engineering design and contamination assessments
• In incoming inspection and supplier qualification at accredited third-party laboratories, LOD serves as a straightforward identity and purity screening indicator

FAQ

What does "constant weight" mean in a loss on drying test?

Constant weight means that two consecutive weighings, separated by a further drying and cooling cycle, differ by no more than the tolerance specified in the method—commonly 0.3 mg or less for small pharmaceutical samples, or a defined percentage of sample mass for larger industrial samples. Reaching this threshold confirms that drying is essentially complete.

How does loss on drying differ from Karl Fischer moisture determination?

Loss on drying measures all substances that evaporate under the test conditions—water plus volatile solvents and other volatiles—whereas Karl Fischer titration is chemically specific to water. LOD is simpler and cheaper but non-selective; Karl Fischer is preferred when only true water content matters or when the sample is heat-sensitive.

What drying temperature should be used?

The temperature is specified by the relevant standard or product monograph. A general default is 105°C for stable materials; lower temperatures (60–80°C under vacuum) are used for heat-sensitive substances, while some methods specify 120–130°C for materials with tightly bound moisture. Using an overly high temperature risks thermal decomposition and falsely high results.

How much sample is required?

Typical sample sizes range from 1 to 10 g. Pharmacopoeial methods often use about 1 g, while food, soil, and polymer methods may require 3–10 g for representative results. The sample should be spread thinly to expose maximum surface area and shorten drying time.

Why must the empty vessel be dried to constant weight first?

Pre-drying the vessel removes residual moisture and residues on the glassware so that any subsequent mass change is attributable solely to the sample. It also thermally stabilizes the vessel, improving the accuracy and repeatability of the measurement.

What are the main sources of error in this test?

Common errors include moisture reabsorption during weighing or cooling, incomplete drying, thermal decomposition of the sample, loss of fine particles during handling, inadequate desiccant performance, and uncalibrated balances or ovens. Careful technique, covered vessels, calibrated equipment, and duplicate determinations minimize these risks.