Moisture content is an important quality control criterion across the board and relates to establishing or conforming to specified levels of moisture in a product. Correct moisture levels are established by government, trade and professional organizations and relate to the percentage of water and other liquids in a product. Therefore, moisture analysis, can be viewed as first establishing recommended moisture content then checking to confirm that products meet the criterion during and after processing prior to packaging.
All of us experience the impact of incorrect moisture content. Salt and sugar, for example, absorb moisture from the air and cake up. Crispy potato chips quickly get gummy on humid picnic days. Portland cement will turn into a block if left unsealed in the garage.
On the other side, when you buy a product by weight you don’t want to pay for excess water. If water is intentionally added to a product (pre-processed chicken for example) the label should reflect that.
Another way to look at this is if the product’s moisture content is lower than specified the vendor packages more to make the package weight; if higher than specified the buyer pays for water rather than product.
Taking the topic beyond the supermarket and home improvement depot, pharmaceutical excipients perform many functions including flavoring, preserving, coloring, filling and lubricating. While they are not the active pharmaceutical ingredient (API) excipients are subject to strict approval processes. Moisture content of excipients is among the quality control measurements, and moisture analysis is a fast, accurate way to confirm moisture content at bulk processors of excipients.
Enough of this for the moment. Now, just what is moisture?
Types of Moisture for Analysis
Moisture is water, right? Yes, but there is more!
Type moisture in the Google search bar for this definition: water or other liquid diffused in a small quantity as vapor, within a solid, or condensed on a surface.
A key point here is “other liquid.” This broadens the definition to anything that can be evaporated thereby reducing sample weight. Examples: grease, oils and solvents in addition to water.
Moisture can lurk in places many folks would not suspect. Three examples of where moisture can exist include:
- On the surface of a sample. This can be defined as bulk water attracted to other substances. Even though you might not see it, it can be there and its presence affects the weight of the substance. Surface moisture is most easily removed in a moisture analyzer.
- In pores or capillaries, where it more difficult to remove. Absorbed moisture can be inside the substance particle (think wet soil) or in tiny capillaries or crevices of roughly-shaped substances. Generally evaporating this type of moisture takes more time than required to remove surface moisture.
- Water molecules that are chemically bonded with other molecules as water of crystallization or as hydrates or adhesively bound water chemically bonded to a material at the molecular level. An example is gypsum. Evaporating water of crystallization is more complex than surface and absorbed moisture because the substance may also have surface moisture as well as chemically bonded water. Removing surface moisture is one part of the process; removing water of crystallization is another and more difficult part. This is because the heat required may in some instances result in product decomposition.
Moisture Analysis Measurement Methods
Following is a brief description of three basic methods to measure moisture content. For more detail on this topic see our post on moisture measurement techniques.
- Chemical Methods Karl Fischer titration is an accurate reference method for water detection vs. any volatile substance that is identified by thermogravimetric methods (see below). It is based on a reagent that reacts with water and converts it into a non-conductive chemical. Disadvantages of this method are that it requires the use of dangerous chemicals along with skilled technicians to conduct the analysis.The calcium carbide method of moisture determination, while attractively priced also calls for trained personnel because it can form explosive materials through a chemical reaction.
- Spectroscopic Moisture Analysis Spectroscopic methods include infrared (surface moisture), microwave (total moisture) and nuclear magnetic resonance (NMR) spectroscopy. These indirect measurement methods can be quite complex and/or time consuming because they require multiple samples for calibration. For that reason they are not widely used for moisture content quality control checks along packaging lines.
- Thermogravimetric methods This process calculates moisture content based on loss of weight on drying. Several methods are employed.Drying ovens use circulating hot air, sometimes under vacuum, to establish reference moisture content. They can process large samples and multiple samples at a time but can take hours to produce the data and are labor intensive. This method is frequently cited in laws governing determining moisture content in food. Professional microwave ovens can accommodate smaller samples than drying ovens but do not offer a high degree of temperature control. Microwave drying is not suitable for samples with water content less than 2%. And finally as the focus of this post are moisture analyzers (also called moisture balances). These are configured as halogen or infrared heaters positioned over a precision analytical balance. These units provide accurate moisture measurement of random small samples under programmed conditions including time, temperature and how heat is applied. Benchtop moisture analyzers such as offered by Tovatech are widely used for quality control checks as product moves through or exits processing lines before it is packaged for shipment. This type of moisture analysis is quick, enabling operators to adjust processing procedures if required.
How a Benchtop Moisture Balance Operates
Drying process parameters for a particular product are required at the outset. These may be developed with experimentation then programmed into the moisture analyzer using its keypad. Once established the moisture analysis program can be called up as needed; some moisture balances can store multiple drying programs in memory for quick call-up.
Samples, usually in grams, are placed in a tared weighing pan and placed in the drying chamber. Analysis begins with the lid is closed (or otherwise activated). The instrument calculates moisture by comparing the initial weight with the weight when drying is complete. This is called loss on drying (LOD) with results and test parameters displayed on the unit and transferred to a printer or PC.
Sample Prep for Moisture Analysis
Sample prep is a crucial part of conducting accurate moisture analysis. For example, high-resolution halogen moisture analyzers such as the EM 120-HR offered by Tovatech can detect moisture content to readout of 0.0001 gram or 0.001 percent moisture.
- To assure moisture measurement accuracy samples must be representative of the total quantity. Sample drawing procedures vary between products being analyzed and usually are provided by industry standards. In terms of representation more is better, because the more samples analyzed the greater the reliability of the results.
- Sample prep may require grinding or cutting to yield a suitable sample size for the moisture balance and improve measurement accuracy. This can affect the moisture content of the sample due to heat or the giving off or taking on moisture from ambient conditions. Take steps to minimize this change. As well, ambient conditions will affect the moisture content of samples. This calls for speed, but not at the expense of accuracy. Protect samples in airtight containers if there is a time lapse between sample taking and moisture analysis.
- The accuracy of a moisture analysis exercise is improved with maximum sample exposure to the drying process. If your samples develop crusts on exposure to heat or are pasty in nature mixing them with quartz sand will improve results. This might require using larger sample pans.
- Glass fiber filters are generally recommended when performing analyses on samples that are liquid, pasty, contain fats, and that tend to melt, form drops, form skins or are temperature sensitive. In most cases samples are placed on the filters and carefully spread. If temperature-sensitive samples or where skins are formed filters may also be used to cover samples. In all cases remember to tare the filter(s) along with the pan, and in cases where extreme accuracy is required filters should be pre-dried and stored in a desiccator before use.
Moisture Balance Drying Profiles
Drying profiles, also termed drying modes, vary depending on samples being analyzed because not all substances react equally to moisture analysis.
Moisture analyzers offered by Tovatech allow users to set drying profiles to accommodate the types of moisture in substances being analyzed. We go into more detail on this in our post on setting up a moisture analyzer. Here is a quick summary of four drying profiles:
- Standard drying is suitable for most substances.
- Soft drying is recommended for samples that do not tolerate the full power of the halogen heater. This helps protect sensitive samples from decomposing or forming a protective “skin.”
- Fast drying is selected for samples with moisture content between 5% and 15%. With this setting the halogen heater quickly raises the temperature to approximately 40% above the set temperature (called preheat) to rapidly drive off the bulk of the moisture and then returns it to the set temperature to approach the end point slowly.
- Step drying can be used for samples with more than 15% moisture. One example of step drying is for measuring two different types of moisture in the same sample – see water of crystallization above. The first step is a low temperature to remove surface moisture then it ramps to a high temperature to remove water of crystallization. Some moisture analyzers display both moisture content counts.
Establishing Moisture Analyzer Switch-Off Criteria
As part of programming moisture analysis technicians also establish what is called the switch-off criterion – when the analysis ceases and results are displayed. Three methods are usually available. The first is manually. The second is automatic, activated when the drying weight is stable – that is at a specified weight loss per unit of time. A third method is time controlled.
As a Final Word
As a precision instrument a moisture analyzer requires careful maintenance including recalibration at regular intervals or whenever it is moved. Our post on maintaining moisture analyzers offers excellent suggestions. As well, moisture analyzers are shipped with detailed operating and maintenance instructions.
Please call the scientists at Tovatech for professional, unbiased help in selecting the correct moisture balance for your operations.