Loss on drying (LOD) is one of the most widely used gravimetric tests for determining moisture and volatile content in pharmaceuticals, foods, chemicals, and polymeric materials. Accurate loss on drying result calculation and analysis is essential for quality control, process optimization, and regulatory compliance. This article provides a comprehensive guide to LOD testing principles, calculation formulas, precision criteria, error sources, and the correct interpretation of analytical data.
Technical Overview
Loss on drying (LOD) is a gravimetric analytical procedure that measures the mass a sample loses when it is dried under precisely controlled conditions of temperature, pressure, and time. The mass loss typically corresponds to moisture (free and, in some cases, part of the bound water) plus any volatile substances that evaporate at the selected drying temperature, such as residual solvents or low-boiling organic compounds. Because the measurement is mass-based rather than water-specific, LOD is broader in scope than coulometric or volumetric water determination and must be interpreted with the sample chemistry in mind.
The test is performed according to well-established compendial and national standards, including USP General Chapter <731> Loss on Drying, European Pharmacopoeia method 2.2.32, GB/T 6284 for chemical products, GB 5009.3 for foods, ASTM D2216 for soil and rock, ASTM D3173 for coal and coke, ISO 287 for paper and board, and ISO 665 for oilseeds. Accredited third-party laboratories routinely apply these methods for lot release, incoming material inspection, and stability evaluation, making a clear understanding of the underlying calculation and data analysis indispensable for laboratories and manufacturers alike.
In practice, a weighed sample is dried in an oven, vacuum oven, or rapid moisture analyzer until a constant mass is reached or a validated drying program is complete. The difference between the initial and final mass, expressed as a percentage of the original sample mass, constitutes the loss on drying result. Correct calculation, duplicate assessment, rounding, and comparison against specification limits form the core of LOD result analysis.
Testing Samples
Pharmaceutical and Nutraceutical Materials
Active pharmaceutical ingredients (APIs), excipients such as lactose, starch, microcrystalline cellulose and povidone, wet granulations, capsule and tablet blends, herbal powders, and finished dosage forms are routinely tested. For hygroscopic or thermolabile actives, vacuum drying or reduced-temperature programs are specified to avoid thermal decomposition while still removing surface moisture.
Food and Agricultural Products
Flour, milk powder, infant formula, tea, spices, dehydrated vegetables, grains, oilseeds, feed ingredients, and sugar-containing products are common sample categories. Food matrices high in sugars or fats may require reduced drying temperatures, sand dispersants, or vacuum conditions to prevent decomposition and spattering that would bias the result.
Chemicals, Catalysts, and Inorganic Materials
Inorganic salts, reagent-grade chemicals, catalyst powders, dyes, surfactants, ceramic precursors, and mineral fillers are tested to verify purity grades, assess hydration state, and confirm suitability for moisture-sensitive reactions or sintering processes.
Polymer, Coating, and Adhesive Materials
Resin powders, engineering plastics, polymer pellets, emulsions, latexes, coatings, and adhesives are analyzed because residual moisture causes hydrolysis, bubbles, voids, and dimensional defects during extrusion, injection molding, or curing operations.
Cosmetic and Personal Care Products
Face powders, dry masks, talc-based preparations, and botanical extracts are tested to control microbial growth risk, texture stability, and label claims related to solids content.
Testing Items
Typical testing items associated with loss on drying determination include:
- Loss on drying (LOD) at a specified temperature, pressure, and duration
- Moisture content, reported on a wet-mass basis or dry-mass basis
- Volatile matter content (moisture plus volatiles released under defined conditions)
- Total solids or dry substance content (100% minus LOD)
- Constant-weight verification of the dried residue
- Duplicate determination precision and mean-value reporting
- Comparison of measured values against product specification limits
Testing Methods
Hot Air Oven Drying (Constant-Weight Gravimetric Method)
A pre-dried, desiccator-cooled weighing vessel is accurately weighed, charged with typically 1 to 2 g of test sample (unless the applicable monograph specifies otherwise), and dried in a forced-convection oven, commonly at 105 °C ± 2 °C, until constant weight is achieved. The vessel is cooled to room temperature in a desiccator before each weighing to prevent moisture uptake from ambient air. This approach, described in USP <731>, GB/T 6284, GB 5009.3, and ISO 287, remains the reference technique for most solid materials.
Vacuum Oven Drying
Heat-sensitive substances such as certain sugars, gelatin, vitamin-rich products, and oilseed materials are dried at reduced temperatures (for example 60 to 80 °C) under vacuum, typically 5 kPa or lower, following approaches aligned with ISO 665. Vacuum lowers the boiling point of water, allowing efficient moisture removal without thermal degradation of the matrix.
Rapid Infrared and Halogen Moisture Analyzers
These instruments integrate a halogen or infrared heating source with a precision balance, delivering results in 5 to 15 minutes. They are widely used for in-process control after the drying program has been correlated with the reference oven method on the specific matrix. Method validation should document the equivalence between the rapid and reference procedures.
Thermogravimetric Analysis (TGA)
TGA records mass loss continuously as temperature is ramped at a controlled rate, producing a mass-loss curve that distinguishes overlapping events such as surface moisture release, crystal-water dehydration, and thermal decomposition. It is a powerful tool for method development and for understanding which mass-loss fraction the LOD test actually measures.
Result Calculation and Analysis
Standard Calculation Formula
The fundamental loss on drying calculation is expressed as:
LOD (%) = (W2 − W3) / (W2 − W1) × 100
- W1 = constant mass of the clean, dried, empty weighing vessel (g)
- W2 = mass of the vessel plus sample before drying (g)
- W3 = constant mass of the vessel plus sample after drying (g)
The numerator (W2 − W3) represents the total mass lost, and the denominator (W2 − W1) represents the original sample mass. For example, if W1 = 20.0132 g, W2 = 21.0185 g, and W3 = 20.9754 g, the mass loss is 0.0431 g and the sample mass is 1.0053 g, giving LOD = 0.0431 / 1.0053 × 100 = 4.29%.
Dry Basis and Wet Basis Conversions
Wet-basis moisture Mw = mass of water / total wet mass × 100, while dry-basis moisture Md = mass of water / dry mass × 100. The two are interconverted using Md = 100 × Mw / (100 − Mw) and Mw = 100 × Md / (100 + Md). Confusing the two bases is a frequent source of specification disputes, so the reporting basis must always be stated explicitly.
Constant Weight Criterion
A sample is judged to be at constant weight when the difference between two successive weighings, taken after an additional drying interval (commonly one hour), falls below the limit defined by the method, for example 0.3 mg or another mass criterion scaled to sample size. Premature termination of drying underestimates LOD, while unnecessarily prolonged drying at high temperature risks decomposition and overestimation.
Precision Assessment and Reporting
Duplicate determinations should be performed on independently weighed portions. The absolute difference between duplicates, or the relative deviation from the mean, is compared with the repeatability limit stated in the method (for example, an absolute difference not exceeding 0.05% for well-behaved powders). Acceptable results are averaged and reported to the number of decimal places required by the specification, commonly 0.01% for pharmaceuticals and 0.1% for foods, along with the drying temperature, duration, and basis.
Sources of Error and Data Interpretation
Key factors that must be evaluated when analyzing LOD data include balance calibration and readability, ambient humidity and sample exposure during weighing, sample heterogeneity and particle size, layer thickness in the vessel, moisture re-adsorption during cooling, loss by spattering, and mass loss unrelated to water such as thermal decomposition, crystal-water release, or solvent evaporation. When a result is out of specification, a structured investigation should verify weighing records, drying conditions, constant-weight data, and analyst technique before any retesting decision is made. Because LOD is not water-specific, significant divergence between LOD and Karl Fischer water content is often analytically meaningful and may indicate volatiles or bound water in the matrix.
Applications
Loss on drying data support a broad range of industrial and regulatory activities:
- Pharmaceutical quality control: lot release of APIs and excipients against moisture limits, blend and granulation control, and long-term and accelerated stability studies under ICH guidance
- Food industry: verification of total solids, moisture specifications for powders and bakery ingredients, shelf-life assessment, and label-declaration support
- Chemical manufacturing: purity grading of reagents, confirmation of hydration state, and control of moisture-sensitive synthesis intermediates
- Polymer processing: incoming inspection of resins and pellets to prevent hydrolysis and void defects in extrusion and injection molding
- Agriculture and feed: monitoring of grains and feedstuffs to inhibit mold growth and ensure safe storage
- Coal, mineral, and paper trade: determination of moisture for net-mass settlement, calorific-value correction, and compliance with ISO and ASTM contractual methods
FAQ
What is the difference between loss on drying and water content?
Loss on drying measures all mass lost under the drying conditions, including water and any other volatiles, whereas specific water-content methods such as Karl Fischer titration quantify water alone. For samples containing residual solvents or decomposable components, LOD values can exceed true water content, and the two parameters should not be used interchangeably.
What does "constant weight" mean in LOD testing?
Constant weight is reached when repeated drying and weighing cycles produce a mass change smaller than the acceptance threshold defined in the method, commonly 0.3 mg between successive determinations. Reaching constant weight confirms that the drying process is complete and the result is stable.
How much sample is typically used for a determination?
Pharmacopoeial practice generally calls for 1 to 2 g of sample unless the monograph directs otherwise, while food and industrial methods may specify from 2 to 10 g depending on expected moisture level and balance readability. Larger sample masses improve weighing robustness but lengthen drying time.
Why do duplicate LOD results sometimes disagree?
Discrepancies usually arise from sample heterogeneity, inconsistent particle size, uneven distribution in the vessel, variable exposure of hygroscopic material to room air during weighing, or differences in cooling time in the desiccator. Standardized handling, adequate mixing, and rapid, consistent weighing minimize variability.
When should vacuum drying be used instead of oven drying?
Vacuum drying is preferred when the material melts, decomposes, caramelizes, or oxidizes at 105 °C. Reduced pressure allows moisture removal at 60 to 80 °C, protecting heat-sensitive matrices while still achieving complete drying, as applied in ISO 665 for oilseeds and in pharmacopoeial monographs for gelatin and sugar-rich products.
How should LOD results be reported and rounded?
Reports should state the mean of acceptable duplicates, the drying temperature, pressure, duration or constant-weight criterion, the reporting basis (wet or dry mass), and the applicable specification. Rounding follows the conventions of the governing standard, for example two decimal places for pharmaceutical assays and one decimal place for many food specifications.
Can a rapid moisture analyzer replace the oven method?
A halogen or infrared moisture analyzer may be used for routine in-process control provided its drying program has been validated against the reference oven method on the same matrix, with documented correlation and acceptable bias. Reference-method verification remains necessary for release testing and dispute resolution.