Vibrations that appear in the near infrared (NIR) region are the overtones
and combination vibrations of those in the mid-IR. They are generally less
intense, and are useful in quantitative measurements. For example, water has
fundamental vibrations at 3700, 3600 and 1600 cm-^1. In the NIR of water, combi-
nation bands occur at 5300, and overtones around 7000, 11 000 and 13 500 cm-^1.
For compounds with an –OH group, overtones at 7000, 10 500 and 13 600
occur, while for aliphatic hydrocarbons, there are absorptions at 4200-4700,
5700 - 6300, 7000-7500 and so on.Instrumentation For mid-IR, NIR and Raman spectrometry, the instrumentation is different, but
the main components of spectrometers are all required.
The sources used for mid-IR are heated rods, such as a nichrome ribbon or a
‘globar’, which is a rod of silicon carbide. The Opperman source is a rhodium
heater in an alumina tube packed with alumina and zirconium silicate. When
heated to above 1000∞C, these sources emit energy over a wide range, resem-
bling a black-body radiator with a maximum intensity at about 1000 cm-^1. For
NIR, tungsten or tungsten halogen lamps are used.
In Raman spectrometry, a high intensity source is required, since Raman scat-
tering yields low intensity lines. Laser sources such as the Ar+laser give strong,
sharp lines at 488.0 nm and 514.5 nm. One disadvantage is that these wave-
lengths may cause fluorescence. This is avoided by using NIR laser sources. A
schematic of an FT-Raman spectrometer is shown in Figure 4.
Sampling for vibrational spectroscopy is an extensive subject. Only the basic
methods will be discussed here. Since glass and polymers absorb strongly in the
IR region, it is necessary to use ionic materials to contain samples. Typical
examples are listed in Table 4.
The types of samples that may be analyzed by IR are very varied and will be
considered in turn.
For gases, because they are present at much lower concentrations than pure
liquids or solids (e.g., 0.04 M for nitrogen in air, 17.4 M for liquid ethanol),
longer path lengths are required. The gas-phase spectrum of HCl at 0.2 atm may
be studied in a 10 cm glass cell with NaCl windows. Low concentrations of
238 Section E – Spectrometric techniques
Table 3. Selected group frequencies in the NIR region
Frequency Group
A Combination bands 5000 to 4000 cm-^1
5300, 4500 Water
4700 - 4600 RNH 2
4650 - 4150 CH
4700 - 4600 CC
B First overtone region 7500-5000 cm-^1
7300 Water, ROH
5500 - 5100 RCOOR, RCOOH
5400 - 4800 CONHR, CONH 2
7500 - 5700 CH, CH 2 , CH 3
C Higher overtone region 14 000 to 8000 cm-^1
13 600, 10 700 ROH, ArOH
14 000 ArCH
14 000-11 000 CH, CH 2 , CH 3