There are many situations in which the moisture content of a substance must be accurately determined in order to establish the quality for standard specifications. This is often necessary for a range of products and raw materials, from chemicals through pharmaceutical agents to foodstuffs. The moisture content is crucial in the shelf life of many raw materials and products.
A method commonly used for moisture content determination is the loss-on-drying method, or LOD. It is used to specify many major quality specifications. This is based on the thermogravimetric principle, in which a substance is heated until no more weight is lost, that is, it is completely dry. At the beginning and after dryness is achieved, the weight of the substance is measured. The final weight loss is calculated, and represents the moisture content of the sample.
It is important to understand the meaning of the term “moisture” in this context. Moisture refers to all matter within a sample which can be vaporized, and thus includes not just water but fats, volatile solvents, and alcohols.
Methods
A variety of techniques is used to bring about vaporization of moisture, including a drying oven heated by gas, which works on convection, and ovens which work by infrared or microwave radiation.
The instrument associated with conventional LOD is the drying oven. The use of convection is being replaced by the use of infrared to speed up the drying time, using a moisture analyzer. This is also called a moisture meter, or a moisture balance.
The drying chamber may be heated by gas, oil, microwave, infrared or simply consist of a climate-controlled room.
Drying Oven
Once placed in the room or convection oven, the sample is weighed repeatedly following a preset schedule, and the weight loss is recorded. Once the weight remains unchanged, the difference between the initial weight and the final weight is calculated, which is equivalent to the moisture vaporized. This is used to calculate the percentage of moisture.
The advantages of the drying oven technique include high reproducibility and reliable results, and as a result it is the standard in many manufacturing procedures. It has the serious disadvantage of requiring a long time for completion.
Microwave Drying
This is another gravimetric method which uses several steps to dry the sample while taking repeated weight measurements. The microwaves heat and vaporize the moisture in the sample extremely fast, taking minutes rather than hours or days. Moreover, the sample size does not have to be very small. The disadvantage is the inability to use it on samples which undergo degradation at high temperatures, since the sample may heat up to 140°C to 750°C with microwave heating. The occurrence of structural change or alteration in composition may lead to false readings due to the production of water during degradation, or may introduce errors due to mass loss other than due to moisture vaporization.
Infrared Drying
Infrared radiation, or heat, may also be used to find the moisture content of a sample, but is suitable for only small samples weighing from 0.1 to 120 grams. For small samples, the determination takes little time, but it shares the disadvantage of the microwave method in that the sample is exposed to high temperatures. While the weight loss determination can be used to automatically calculate the moisture lost from the sample, the chances of false readings increase when the sample size is small.
Moisture Analyzer
A moisture analyzer comprises two elements - a heating unit and a weight balance. The sample is first weighed accurately, before being heated. The temperature is typically kept below the melting point and below the point of degradation. The heating is accomplished using a halogen lamp which emits infrared radiation to dry the sample very quickly. The wavelength of the radiation is such that much of it is absorbed by the sample, and thus the temperature of the material may be above that which is displayed, which is that expected from surface heating by infrared radiation. The importance of this is that the drying time is much lower compared to the conventional convection drying oven. Once the process begins, the sample is automatically weighed and the weights recorded repeatedly. On achieving constant state, the balance shuts down. The device then finds the weight loss at the end, and uses it to find the moisture content of the sample.
The results of moisture determination using the moisture analyzer should be compared to those obtained using the drying oven, which is the official standard. If such a value is not available for comparison, the experiment should be re-run using exactly the same material under the same conditions, several times to make sure that the results can be reproduced without losing the rapidity of determination. The sample in such cases requires proper preparation, because it requires to be homogeneous, fine enough to expose the maximum surface area to drying, and capable of being uniformly spread over the pan.
The advantages of an integrated heating-balance setup include the ability to generate real-time moisture curves and graphs of the drying rate much faster than is achievable using conventional methods. Software accompanying the physical device also makes it possible to store the data for comparison with other studies and samples, as well as for more detailed analysis if required.
Further Reading