Guide to Moisture Analysis

Moisture determinations need to be carried out reliably and at speed so that any interventions in the production process can be made quickly to avoid interruptions. One quick and accurate way of determining moisture is thermogravimetric measurement using a halogen moisture analyzer: the sample is weighed and heated with an infrared radiator (halogen lamp). The loss in weight is continuously recorded and drying ends once a defined criterion is reached. The moisture content is automatically calculated from the difference in weight.

The halogen moisture analyzer works along the lines of the thermogravimetric principle, i.e. the sample's start weight is recorded, then a halogen radiator dries it while an integrated balance continually records the sample weight. The total loss in weight is interpreted as the moisture content. Drying with the halogen radiator is a further development of the infrared drying method. The heating element consists of a glass pipe filled with halogen gas. As the mass of the halogen radiator is very low compared with that of a conventional infrared radiator, the maximum heating output can be reached quickly and outstanding controllability is achieved. Combined with even distribution of the thermal radiation over the entire sample surface, this is indispensable to achieve repeatable results.

In contrast to the traditional oven in which the sample is heated using convection and dried over a long period of time, the sample in the halogen moisture analyzer absorbs the infrared radiation (thermal radiation) from the halogen lamp, and as a result, heats up very quickly.

Since moisture measurement using the moisture analyzer is based on a high-precision weighing procedure, accuracy and repeatability are closely linked to the instrument's location. To ensure that your moisture analyzer works under the best conditions, please observe the guidelines mentioned in the above document.

To ensure precise measurement results, the following information about care, calibration intervals, and maintenance should be observed:

• By regularly calibrating (testing) and, if necessary, adjusting the heating module, you will ensure consistent and reproducible heat output for the entire lifetime of your instrument. We therefore recommend that you define test intervals for testing the weighing unit and heating module (depending on risk).
• To ensure quality moisture results at all times, METTLER TOLEDO offers SmartCal™. This unique temperature-sensitive substance with a known moisture content is used in one single test to quickly and easily verify the instrument’s overall functionality. The SmartCal test is based on a regular measurement with a moisture analyzer. Refer to the SmartCal User Guide for more information: SmartCal Moisture Analyzer Test Substance User Guide
  
• Annual maintenance performed by the METTLER TOLEDO service team will guarantee the quality, measurement accuracy, and value retention of your halogen moisture analyzer.

The correct preparation of samples once they have been taken is also key to repeatable and reliable results.

• Ensure even granulation (particle size).
• If necessary, increase the sample surface area by breaking up the sample.
• This will ensure a better and faster release of moisture during drying (faster diffusion of moisture to the surface).
• The sample should not be heated at this stage as this would cause moisture to be lost during preparations.
• Mechanical crushing can be carried out e.g. using a mortar, grinder (water-cooled), or simply by cutting.

There are often statutory requirements, standards customarily used in the trade or internal corporate instructions for substances, which define the method of moisture content determination. The oven method (loss on drying) or Karl Fischer titration is usually used as the reference procedure. In such cases, the aim is to obtain the same results with the halogen moisture analyzer as with the reference procedure (or for the deviation from the reference value to be known and reproducible). In order to achieve this, adjustments must be made to the setting parameters, such as drying temperature, drying program, and sample weight, as well as the handling of the sample. This is known as method development where the aforementioned parameters describe a method.

Certain samples need a special procedure for quickly and correctly determining moisture. This section contains information about how you can work with such samples to optimize moisture determination.

Liquid and very moist samples:

• Use the glass-fiber filter.
• Tare the filter with the sample pan and then place the sample on it.
• Rapid drying is suited to samples with a very high moisture content (> 30%).
• In this process, the target temperature is exceeded by 40% for 3 minutes to accelerate the measuring process.
• Step drying can be used as an alternative to rapid drying. Here the duration of the temperature increase and the temperature are freely selectable.

Various measurement processes have been developed for determining moisture content. The table below shows a selection of typical measurement technologies, and describes the advantages, and disadvantages of these procedures.

Moisture (moisture content): In thermogravimetric processes, the moisture of a material includes all substances which volatilize during warming and therefore contribute to the material's loss of mass. Alongside water, this may also include alcohol or decomposition products. When using thermogravimetric measurement methods (drying using infrared, halogen, microwaves, or ovens) no distinction is made between water and highly volatile components.

Drying oven procedure: Thermogravimetric method for determining the moisture content of a sample. This sample is dried in the oven for a defined period of time at constant temperature. The moisture content percentage is determined from the difference in weight before and after drying. For historical reasons this procedure often forms part of legislation (food regulations, USP etc.).