Introduction to Soil Chemistry

infrared spectroscopy 163
8.10. RELATIONSHIP TO ORIGINAL SAMPLE
At this point the original sample has been extracted, and the amount of component
of interest has been determined. But it has been determined for only
a portion of the extract and sample or area sampled. It is essential to relate
this amount back to the amount originally present in the sample taken in the
field and ultimately the field itself. This is then a process of working backward.
Thus, one must first find the total amount present in the whole extract and
then determine how this relates to the amount of sample originally taken and
then to the field sampled.
If a 1-g soil sample is extracted with 10mL of extractant, then the component
extracted is evenly distributed throughout the 10mL. This means that the
final result will need to be multiplied by 10 because the component was diluted
to 1:10. This then is related back to the volume or mass of soil in the original
sample. It may also be necessary to apply other conversion or correction
factors, such as the percent water present, depending on the procedure used.
8.11. INFRARED SPECTROSCOPY
The next portion of the electromagnetic spectrum most often used for analysis
is the midinfrared region. In the infrared spectroscopy region, changes in
the vibration, bending, and rotation of bonds in molecules results in the
absorption of radiation. These changes from one mode to another higherenergy
mode are caused by the absorption of infrared light.
In infrared spectroscopy the wavelengths become long, and so the spectrum
is usually reported in reciprocal centimeters, which is a measure of frequency
rather than wavelength. In the older literature this region will be from 4000
to 250cm -1 (2.5–50mm).Today the infrared region is divided into three distinct
regions 3 ; the near infrared (NIR), 1000–4000nm; the midinfrared (MIR), 4000–
250cm -1 ; and the far infrared (FIR), less than 250cm -1 . Instrumentation for
both the NIR and FIR regions is not as readily available as that for the MIR,
and so these regions are not as commonly used. However, there has been some
substantial development of NIR for use in the determination of components
in soil, food, and feed such as oils and protein [20,21].
Generally, infrared spectroscopy, infrared spectrometers, accessories, and
water do not mix, although there are techniques for sampling aqueous systems
for IR analysis. Salts of alkali and alkaline-earth metals and halogens, such as
sodium chloride and potassium bromide, are transparent to MIR and are used
in the optical systems of spectrometers. Large amounts of water will dissolve
parts of the optical system, rendering it inoperable. Small amounts, such as in
high-humidity air, will lead to optical components being fogged and thus less
transparent or opaque. These same salts are used for sample cells, sample
preparation, and analysis, and thus samples containing water will be deleterious
or destroy cells.Water has many strong broad adsorptions in the MIR, and
3 These are typical instrument ranges.