Laboratory Tips and Tricks

Drying/Moisture Determination

Author's Note: This article discusses one of the most common laboratory techniques. Yet, this is still a method that is often performed poorly, resulting in unreliable data. The following discussion should provide some hints to help you generate better results.

Most samples are actually analyzed on an “as is” basis. Since we live in a water-based world, this means that most samples are “wet” to varying degrees. While a plastic sample or pharmaceutical tablet may contain only trace quantities of water, many foods and environmental samples are more water than solids. For this reason, many methods require reporting on a “dry weight” basis in addition to a “wet weight” (as is) basis.

The equation for determination of moisture is:

To convert wet-weight concentration to a dry-weight basis:

Drying only requires that all water be removed from the sample. The simplest, and most common method, involves evaporation. This process is a simple phase change from liquid water to the gas phase, which migrates away from the sample solids. There are several ways to dry samples:

• Oven heating - either convection or forced air ovens are available, at relatively modest cost, to dry samples that contain large quantities of water.
• Vacuum evaporation - by reducing the pressure, the boiling point of water can be reduced, which means a lower evaporation temperature. This can be important for thermally labile samples. “Lyophilization” is an example of such a technique.
• Heated evaporation- these systems use an external heat source (e.g. infrared lamp or heated gas) to warm the sample. They are much faster than other methods, but some are also more expensive.
• Heated/Vacuum evaporation - this method uses both a reduced pressure and heat source to rapidly remove moisture. Sophisticated instruments are now available that automate this process for batch evaporation of many samples.
• Moisture balance - this equipment is essentially an analytical balance with a heat source. The system heats the sample, using a predetermined program, until constant weight is achieved. This method is designed to determine moisture content only, and generally does not allow easy recovery of the residue.
• Chemical drying - reagents are available that will react with all the water present, usually generating a lower boiling organic solvent that can be removed by simple evaporation. These reagents generally work best with smaller quantities of water (i.e., solvent drying). Dimethoxypropane is one example.

You should consider the following issues when performing these tests:

• The drying temperature must be above the boiling point of water (100 °C at room pressure) at a minimum; 103 - 105 °C is common. This may seem obvious, but there are often considerable variations in temperature within an oven. If this is a concern, use a calibrated thermometer and make sure it is as close to the sample as possible. It also may be necessary to “map” the temperature profile in your oven, to determine if there are “hot” or “cold” spots.
• Drying at 105 °C will remove bulk water, but not waters of crystallization or mechanically occluded water.
• Drying at 180 °C will remove almost all occluded moisture, as well as waters of crystallization, unless sulfates are present. However, some organic components may evaporate or decompose at this temperature, resulting in a high bias for moisture content.
• If using reduced pressure techniques, such as a vacuum desiccator or evaporator, make sure your vacuum gauge is working properly and that you know the minimum temperature required (there are tables available with this information).
• Faster drying, by using a higher temperature, is not always better, particularly for high moisture samples. Rapid evaporation may result in loss of solids from bumping, giving you a higher moisture reading that isn't real.
• Some high moisture samples will “skim” over, trapping the water underneath. Once this happens, the evaporation rate becomes much slower and bumping more likely. This often happens near the end of the evaporation process for simple solutions, but can also occur near the beginning of the evaporation, especially if the sample contains high quantities of a less dense (than water) polymer, or high quantities of dissolved solids.
• The rate of mass loss slows dramatically near the end of the drying step. Be patient.
• You can only tell that your sample is dry if you obtain at least two weighings on the dried sample. Only then can you be sure that mass loss has stopped. Your method should specify a minimum difference between dry weighings that indicates the sample is “dry.” This difference is usually just slightly larger than the minimum stability of the balance (e.g., 0.2 - 0.5 mg for a four-place balance). (Note: many “validated” methods will require only a single dry measurement. This procedure is acceptable only if the sample characteristics don't change after the method is validated. Even then, multiple dry weighings should be performed on a regular basis to check the method.)
• Use enough sample so that the dried residue is not affected by uncertainties in the balance.
• Check your calculations. Reporting concentrations on a dry weight basis will always give a larger value than on a wet weight basis.

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