228
A. Ikehata
(a)
(b)
Spring plunger
Quartz window
Sample liquid
Reflection mirror
Spacer
Cap
Fig. 9.14 a Transflection cell and b schematic
mirror and hence termed as “transflection”. A transflection cell can be used as a DRtype instrument wherein the path length of the light is double the cell thickness. For
practical purposes, transflectance is suitable for transmittance measurement because
the cell is capable of being overhauled and cleaned, as the materials for the reflection
plate employed are mostly ceramic, aluminum, stainless steel, and gold. This type
of cell can also be used as a flow cell.
In the cases of liquid and solution samples, one must be careful about the effect
of temperature. As already explained in Sect. 4.2 (Fig. 4.3), the NIR spectrum of
water is sensitive to temperature; hence, the cell is often inserted into a thermostatted
cell holder. For clear liquids and solutions, the sample pretreatment is, in general,
not necessary; however, it is better to remove light scattering components from the
solution as per the situation.
9.2.1.2 Suspensions
For suspensions, the measurement type should be selected based on the turbidity of
a sample. A highly turbid sample such as milk scatters most of the incident light and
transmits a small part. In such a case, the DR method is effective. In contrast, if the
turbidity of a sample is low, transmittance or transflectance measurement is better
for obtaining efficient absorption spectra. In the NIR measurement of suspensions,
it is recommended to stir the sample well and measure its homogeneous part.
9.2.1.3 Solids
There are numerous types of solid samples. Typically, the DR method is employed
for opaque solid samples. For thin transparent films such as thin polymer films,
the transmittance methods are useful. In DR measurement of solid samples, it is
necessary to avoid regular reflection from the surface as much as possible, because
the intensity is very high, but the information obtained from it is relatively low.
To solve this problem, interactance method is often used for NIR measurements
A. Ikehata
(a)
(b)
Spring plunger
Quartz window
Sample liquid
Reflection mirror
Spacer
Cap
Fig. 9.14 a Transflection cell and b schematic
mirror and hence termed as “transflection”. A transflection cell can be used as a DRtype instrument wherein the path length of the light is double the cell thickness. For
practical purposes, transflectance is suitable for transmittance measurement because
the cell is capable of being overhauled and cleaned, as the materials for the reflection
plate employed are mostly ceramic, aluminum, stainless steel, and gold. This type
of cell can also be used as a flow cell.
In the cases of liquid and solution samples, one must be careful about the effect
of temperature. As already explained in Sect. 4.2 (Fig. 4.3), the NIR spectrum of
water is sensitive to temperature; hence, the cell is often inserted into a thermostatted
cell holder. For clear liquids and solutions, the sample pretreatment is, in general,
not necessary; however, it is better to remove light scattering components from the
solution as per the situation.
9.2.1.2 Suspensions
For suspensions, the measurement type should be selected based on the turbidity of
a sample. A highly turbid sample such as milk scatters most of the incident light and
transmits a small part. In such a case, the DR method is effective. In contrast, if the
turbidity of a sample is low, transmittance or transflectance measurement is better
for obtaining efficient absorption spectra. In the NIR measurement of suspensions,
it is recommended to stir the sample well and measure its homogeneous part.
9.2.1.3 Solids
There are numerous types of solid samples. Typically, the DR method is employed
for opaque solid samples. For thin transparent films such as thin polymer films,
the transmittance methods are useful. In DR measurement of solid samples, it is
necessary to avoid regular reflection from the surface as much as possible, because
the intensity is very high, but the information obtained from it is relatively low.
To solve this problem, interactance method is often used for NIR measurements