Loss on Drying (LOD) - Moisture Method Development
Know How

Loss on Drying - Moisture Method Development

Know How

Learn How to Develop a Reliable Moisture Method

USP <731> ("Loss on Drying"), AOAC and ISO describe the thermogravimetric method using a drying oven as a standard procedure for determining moisture content.

A Valid Alternative to the Drying Oven

An alternative thermogravimetric procedure that provides comparable results to the drying oven in a considerably faster time is conducting loss on drying (LOD) with a moisture analyzer. Results are delivered in 5-15 minutes, in contrast to the drying oven, which typically takes 2-3 hours. The procedure is also far simpler and doesn't require specialist skills. By validating the results of halogen moisture analysis against those from the drying oven, companies can benefit from a convenient and compliant alternative loss on drying solution. The European Pharmacopeia has recently updated Chapter 2.2.32, "Loss on Drying", to include halogen moisture analyzers as an acceptable alternative to the drying oven reference method.

How to Ensure Reliable Moisture Analysis Results

As with the drying oven, the key to achieving accurate results lies in the development of the drying method. Different samples require different drying methods according to the individual sample properties. A robust drying method includes sample preparation, a suitable drying temperature, the appropriate drying program, and the right switch-off criterion.

Take advantage of METTLER TOLEDO's moisture analysis expertise to improve your moisture analysis processes. Explore the links below to see our wide range of tools and our application method library. Take a look at our range of moisture analysis instruments to discover which one best suits your company's needs.

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Moisture Method Development

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Moisture Technology Comparison

Drying Oven

Principle:
Thermogravimetry


Measuring Method

Heating of sample by convection.Sample is dried in the oven for a defined period of time at constant temperature. Mass is determined before and after drying. The moisture content percentage is determined from the difference in weight before and after drying.

Advantages
  • Often reference procedure (for historical reasons this procedure often forms part of legislation)
  • Several samples can be determined at the same time
  • Large sample volumes possible
Disadvantages
  • Very long determination period (hours)
  • Substances other than water may evaporate
  • Prone to errors because of the high level of manual handling and calculations involved
  • Unsuitable for at.line use - requires analytical balance and dessicator

Halogen Moisture Analyzer

Principle:
Thermogravimetry


Measuring Method

Heating of sample through absorption of IR radiation from a halogen radiator. Continual determination of mass during drying process. The moisture content percentage is determined from the difference in weight before and after drying.

Advantages
  • Quick measurement (typically 5-15 min.)
  • Simple handling, no manual calculations
  • Compact instrument. No balance or desiccator required
  • Suitable for at-Iine use
Disadvantages
  • Substances other than water may evaporate

Karl Fischer Titration

Principle:
Thermogravimetry


Measuring Method

The chemical reaction of water with a Karl Fischer reagent (containing Iodine and Sulphur Dioxide) is monitored with a polarized sensor to determine the water content. This is either perfomed volumetrically, with the additon of the reagent measured, or coulometrically, where the reaction is driven in-situ and is proportional to electrical current applied.

Advantages
  • Water specific, no other solvent or "moisture" determined
  • Fast, typically 305 to 3 mins per sample
  • Measure from 2ppm to 100% water
  • Compact instruments, automatic calculations, connect Mettler Toledo balances for complete result calculations
Disadvantages
  • Requires dedicated chemicals and chemical know-how
  • Sample preparation can becritical for complete water extraction (homogenizer, long extraction times)

FAQ - How to Develop a Moisture Method

  • We have to use the standard method with the drying oven but using a moisture analyzer would really speed up our processes. Is it possible to switch to a moisture analyzer?
    Many industry regulations require manufacturers to follow a standard method. However, it is acceptable to use a moisture analyzer if you can show its results are similar (accuracy and repeatability) to those from the drying oven procedure. This requires developing a drying method for the moisture analyzer and cross-validating it against the drying oven through a series of analyses.
  • How do the results on the moisture analyzer compare to the standard method?
    In most cases, the moisture analyzer can deliver an equivalent loss on drying moisture result, and typically better repeatability, in much less time, making it a fast alternative to USP <731> ("Loss on Drying") and ASTM D6980 – Standard Test Method for Determination of Moisture in Plastics by Loss in Weight – and other industry standards.  METTLER TOLEDO's application database contains over 150 free ready-to-use cross-validated drying methods. Contact us if you don't find what you need.
  • Why do I need to do method development?
    A good moisture result heavily depends on a good and robust moisture method. There is no one-size-fits-all method for moisture content determination as the samples vary greatly in properties. To develop a reliable moisture method the sample at hand therefore should be understood and a loss on drying method, including sample preparation and presentation, that fits the sample should be developed.
  • How do I develop a loss on drying method on my moisture analyzer for my substance?
    In general there are two ways to find a suitable LOD method for your sample. If you already have a reference value (e.g. from the drying oven or other moisture/water determination procedure) you can orient your method development to target this reference value. If no reference value exists, try finding a method that dries your sample in a repeatable and fast manner. Consult our reference paper, which guides you through the setup of a robust moisture method for your sample, including the right sample preparation and sample application. If you own a METTLER TOLEDO moisture analyzer (HX204, HS153 or HC103) use the integrated Method Wizard functionality that guides you step-by-step to a reliable moisture method with minimum effort.
  • What is the drying oven method?
    The drying oven method is a thermogravimetric method (loss on drying) in which the sample is dried for a defined period of time at constant temperature. The moisture content is determined by weighing the sample before and after drying and determining the difference.
  • How are samples heated in the drying oven and in the moisture analyzer?
    In the drying oven, samples are heated by convection. This means the samples are at the same temperature as the drying oven. When using a moisture analyzer, the sample heats up and dries by absorbing infrared radiation from the heating element. The sample's temperature and drying time depends on its absorption characteristics.
  • I have many different samples (liquid, paste-like, solid, powder). Which technique should I use for moisture content determination?
    In general, all kinds of samples are suitable for both the drying oven and the halogen moisture analyzer. Which method is best for you depends on how fast you need the result or if you need the result using an official method. Sometimes, for example, at goods-in or during final product quality control, moisture analyzers are used in parallel with a drying oven. Fast analysis with the moisture analyzer is done to optimize processes and the drying oven is used for official documentation purposes.

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