Introduction to Soil Chemistry

166 spectroscopy
Table 8.1. Important Diagnostic Absorptions of Major Organic Functionalities in
Mid-Infrared, 1 H NMR, and 13 C Spectra
Functionality Mid-Infrared (cm -1 )
CH3 CH2 H C C H
C C H
C OH
C O
C O
H
C O
OH
C NH
H
1 H ppm
13 C ppm
3000–2900 and 2925–2825 S–N 0–1 S 0–40 S
2950–2850 and 2875–2800 S–N 1–2 S 10–50 S
3100–3000 W–N 5–7 W–N 100–170 S
2250–2100 W–N 2.5–3.1 W–N 60–90 S
3500–3100 B–S 0.5–6.0 S 50–90 S
2700–2900 (H) W–N 9–10 W 190–220 S
1650–1750 (C=O) S–N
1650–1750 (C=O) S–N NA* 190–220 S
3300–2900 (—OH) B–N 10–13 W 160–185 S
1675–1775 (C=O) S–N
3500–3200 W–N 1.0–5.0 W–N 35–50 S
3100–3000 W–N 6.5–8.5 S–N–SS 100–170 S
* NA not applicable, S strong, N narrow, B broad, W weak, SS split.
importance (see discussion of petroleum hydrocarbon analysis above). Neither
carbon tetrachloride nor carbon disulfide is used today because of toxicity,
difficulty in handling, and detrimental environmental effects.
Methods are available to remove the interferences due to solvents, such as
subtraction and compensation. These may be accomplished manually or electronically
depending on the instrumentation and how they are configured.
However, the preference is always to obtain a spectrum without the interference
of a solvent!
Perhaps the most widely and commonly used method for liquid sampling,
used with FT-IR spectrophotometers, is attenuated total reflection, the cell for
which is shown in B in Figure 8.7. A liquid sample is placed in the trough, the
bottom of which is a crystal situated such that the incident IR beam is totally
reflected inside the crystal and exits into the instrument. When reflected, the
beam appears to slightly pass into the sample in contact with the crystal, allowing
the sample to absorb energy, producing a spectrum. Another common
method is to place the pure liquid between two salt windows. A drop may be
placed between two salt windows or in a sealed cell, where the space between
the windows is predetermined and held constant. These types of cells can be
used where quantitation of the component is desired or where a volatile
solvent or solution of a compound is being analyzed.
Solid samples or solid extracts can be mixed and ground with potassium
bromide (KBr), pressed to form a transparent pellet and a spectrum obtained