Introduction to Soil Chemistry

10
x-ray diffraction 149
Figure 8.1. Baseline noise from a gas chromatograph.
8.2. NOISE
Every measurement has noise—random changes in the results measured; that
is, if an instrument is left to make measurement without any sample, the baseline
will not be a straight line but will be a random recording of instrument
output. Figure 8.1 shows the noise in the baseline of a gas chromatograph at
maximum sensitivity. When an absorption or peak is vastly greater than the
noise, there is little question of its authenticity. When it is not much greater
than the noise, the issue becomes whether it is real or is noise.
There are two ways to approach this issue, and both should be investigated
in any questionable measurement. First, it is often assumed that any absorption
or other measurement that is 3 or 4 times larger than the noise is real.
This is a good start; however, there are other more scientific approaches to
this problem. If repeated measurements on different subsamples or aliquots
produce the same absorption or measurement, then it is probably a real result
and not noise. On the other hand, if on repeated measurement adsorptions in
a spectrum occur in exactly the same location and have exactly the same characteristics
such as shape and area under the peak, then they are probably not
due to the sample because some variation in measurement always occurs. This
type of problem is usually a result of instrument malfunction, which must be
investigated.
8.3. X-RAY DIFFRACTION
X-ray diffraction is a very powerful tool used extensively to identify the crystalline
clay minerals in soil. It is also used to study the characteristics of the
clay minerals in terms of shrink–swell characteristics and occluded components.
It is not, however, applicable to amorphous clays found in some tropical
soils and common in Andisols. It is carried out by irradiating, at various
1