0.48
g
4.8
%
95.2
%
0.952
0.48
g
4.8
%
95.2
%
0.952
The Loss on Drying (LOD) Calculator determines the percentage of volatile substances, primarily moisture and solvents, that are lost when a sample is dried under specified conditions. LOD is a critical quality control parameter in the pharmaceutical industry, where it is used to ensure that drug substances and excipients meet compendial specifications defined by pharmacopeias (USP, EP, BP). Unlike true moisture content analysis, LOD measures all volatile matter lost during drying — including water, residual solvents, and any thermally labile components. This distinction is important because a sample may contain volatile organic solvents in addition to water. The test is straightforward, requiring only an analytical balance and a drying apparatus, yet it provides essential information about product stability, potency calculations, and storage conditions.
Loss on drying is calculated from the difference between initial and final masses:
$$LOD = \frac{m_{initial} - m_{final}}{m_{initial}} \times 100$$
Where:
The mass lost represents all volatile matter:
$$m_{lost} = m_{initial} - m_{final}$$
The residual (non-volatile) mass percentage is:
$$\text{Residual} \% = \frac{m_{final}}{m_{initial}} \times 100 = 100 - LOD$$
Pharmacopeial LOD tests specify exact conditions: temperature (typically 105°C), duration (2-4 hours or to constant weight), sample size (1-2 g), and container type (shallow weighing bottle). Some compounds require vacuum drying at reduced temperatures (60°C under vacuum) to prevent decomposition. The method assumes that only volatile materials are lost and that no chemical decomposition occurs under the test conditions.
LOD specifications vary by substance. Pharmaceutical excipients like lactose typically require LOD ≤ 0.5%, while active ingredients may specify tighter limits (≤ 0.1%). An LOD above specification indicates excess moisture or solvent, which can affect drug potency, promote degradation, cause caking, or harbor microbial growth. An unexpectedly low LOD might suggest the material was over-dried, potentially altering its crystalline form. The residual mass percentage is useful for potency calculations, as drug assay results are often adjusted to a "dried basis" using the LOD value. It is important to note that LOD is not identical to water content — Karl Fischer titration should be used when water-specific determination is required.
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Results
LOD of 0.31% is within the typical specification of ≤0.5% for pharmaceutical-grade lactose monohydrate.
Inputs
Results
An LOD of 4.91% is unusually high for most APIs. This may indicate residual solvent from crystallization or inadequate initial drying. Investigation and possible re-drying is warranted.
LOD measures all volatile matter lost during drying, including water, residual solvents, and any thermally labile components. Moisture content specifically refers to water. A sample with 2% LOD might contain 1.5% water and 0.5% residual ethanol. Karl Fischer titration measures water specifically, while LOD captures all volatiles.
The temperature depends on the substance and is specified in the individual monograph. Common temperatures are 105°C for general materials, 80°C for heat-sensitive compounds, and 60°C under vacuum for thermally labile substances. The USP General Chapter <731> provides procedural guidelines.
Constant weight means that two consecutive weighings, separated by an additional drying period, differ by no more than 0.5 mg or 0.05% (per USP). This ensures all volatile matter has been completely removed. Typically achieved after 2-4 hours of drying, depending on the material.
Drug assay results are often adjusted to an anhydrous or dried basis to account for variable moisture. The formula is: Potency (dried basis) = Potency (as-is) × 100 / (100 - LOD). This standardizes results regardless of moisture variation between batches and ensures accurate dosing.
Yes, infrared (IR) moisture analyzers can perform rapid LOD determinations in minutes rather than hours. However, official pharmacopeial methods typically specify oven drying. IR methods must be validated against the compendial method and may not be suitable for regulatory submissions without proper correlation studies.
Shallow, flat-bottomed weighing bottles or dishes are used to maximize surface area and promote even drying. The container should be pre-dried and weighed with its lid. Glass or aluminum dishes are common. The sample should be spread in a thin layer (typically 5 mm or less) for uniform drying.
Vacuum drying reduces atmospheric pressure, lowering the boiling point of water and solvents. This allows drying at lower temperatures (40-80°C), preserving heat-sensitive compounds. Vacuum drying is specified for substances that decompose, melt, or undergo polymorphic changes at higher temperatures.
A negative LOD (final mass greater than initial) indicates an error or that the sample absorbed moisture or gases during the test. Possible causes include an improperly sealed desiccator, hygroscopic sample absorbing oven moisture, chemical reactions generating mass gain, or balance malfunction. Re-check the procedure and equipment.
Smaller particle sizes dry faster due to increased surface area-to-volume ratio. If a compendial method specifies grinding or sieving, this must be followed. Large particles or lumps may retain trapped moisture that isn't fully released during the standard drying period, giving falsely low LOD values.
No, LOD and LOI are different tests. LOD uses moderate temperatures (60-105°C) to remove volatiles. LOI uses high temperatures (500-1000°C) to combust organic matter, similar to ash content determination. LOI measures total organic and volatile inorganic matter, while LOD measures only easily volatile components.
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