Moisture Measurement, Part 4
Barbara Kanegsberg, Mantosh Chawla with Ed Kanegsberg, November 2002
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 (<10mW), 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.
Contact Barbara Kanegsberg and Ed Kanegsberg at BFK Solutions;16924 Livorno Dr., Pacific Palisades, CA 90272,310-459-3614;barbara@bfksolutions.com;contact Mantosh Chawla at Photoemission Tech., (PET), 3255 Grade Vista Dr., Newbury Park, CA, 91320;805-499-7667
