Previously, we have discussed several approaches to determining water content, including gravimetric methods, electrical impedance techniques, and Karl Fischer titration. We conclude this series with an overview of major spectroscopic methods.
All spectroscopic methods rely on exposing the sample to electromagnetic radiation at frequencies corresponding to characteristic absorptions of an unbound water molecule and measurement of either the absorbed or reflected radiation. The techniques are thus highly specific for water.
Spectroscopic techniques are indirect and must be periodically calibrated against a direct technique such as gravimetric determination or Karl Fischer titration, or may be accomplished relative to calibrated controls. Once calibrated, spectroscopic methods offer a number of advantages in that they are non-destructive and can be performed rapidly. Many can be used in a flow-through or flow-by process such as a conveyor belt assembly line. Instruments tend to be relatively simple to operate, can be located at the appropriate place in the process flow, and, with data processing software, allow cost-effective process control with statistical recording.
Spectroscopic techniques for moisture detection in solids and liquids use radiation ranging from microwave (MW) frequencies through near infrared (NIR). In general, the higher the frequency, the less the depth to which waves penetrate. Therefore, MW tends to be used more for bulk measurements, while NIR would be used for small samples or for near-surface (<1 mm) moisture determination. Because of the large differences in wavelengths, even though all the instruments are measuring the same thing (moisture content), the mechanics of wave generation and detection differ.
MW spectrometers use millimeter-wavelength radiation. In one configuration (noncontact), the transmitter and receiver are separate; the beam passes through the sample. In a second approach (contact), the transmitter and receiver are combined and embedded in the sample. Because spectrometers use low power levels (<10 mW), far lower than for microwave heating ovens, there is no appreciable heating of the sample.
Some MW moisture meters operate at higher power. These, however, are really gravimetric rather than spectroscopic devices; the microwaves heat and evaporate the water. Moisture content is measured by weight change. These latter techniques tend to be destructive and time consuming.
NIR techniques operate at wavelengths near 1 µm. The technology is similar to that used in Fourier Transform Infrared (FTIR) spectrometers. One adaptation of this technique uses an acoustic optical tunable filter (AOTF) to discriminate the frequency being detected, providing high sensitivity and specificity.
Spectroscopic moisture detectors are commonly used for such applications as moisture determination in grain processing and in concrete. Stringent requirements for quality control mandate continuous monitoring of moisture in powders, drugs, and lyophilized biological materials, and other critical applications in the biomedical and pharmaceutical areas. A small, portable, battery operated version of an AOTF-NIR detector is available and can even be placed on stirrers in a drying blender, making it well suited to pharmaceutical or other high-value applications. MW and NIR spectroscopic moisture detectors cover a wide range of moisture detectability, from 0.01% to over 99%. For many applications, the technologies overlap, and the choice of detector will depend on the size of the sample, process flow, degree of precision, and cost.
In contrast with the above techniques that are used in solids and liquids, Tunable Diode Laser Absorption Spectroscopy (TDLAS) is an extremely sensitive technique used for measuring very low levels—as small as 100 parts per trillion (ppt)—of moisture, or other species, in gases. In a TDLAS moisture analyzer, an infrared laser beam traverses the gas of interest and changes in the intensity due to water absorption are measured. Path lengths are increased by a mirrored cell to bounce the laser light back and forth, thus improving the sensitivity. The gas of interest can be flowing through the chamber, giving real-time monitoring capability.
Next month: a sonoluminescence cavitation probe that can be used as a metric for ultrasonic and megasonic processes.
Thanks to Mantosh Chawla for his collaboration in the preparation of these columns over the past two years. As of next month, Mantosh will be pursuing other interests and will no longer continue as a regular co-author.