Instant Notes: Analytical Chemistry

exhaust gases may need a 10 m cell, which reflects the IR beam to achieve the

long pathlength.

Two points should be stressed if gas phase spectra are to be studied.

(i) The fine structure of the IR stretching band of HCl shows many lines. These

are due to rotational transitions superimposed onto the vibrational one. The

vibration-rotation spectrum of water vapor shows a complex pattern of lines

across much of the IR region. With heavier molecules, the moments of

inertia of the molecule are larger, and therefore the rotational lines are closer

together. For gaseous CO 2 , one vibration-rotation band occurs at about

2300 cm-^1 and, at low resolution, shows two lobes only. It is therefore impor-

tant to exclude water vapor and carbon dioxide from the spectrometer as far

as possible, and this is usually done by purging with dry nitrogen.

(ii) The second cautionary point is that, with gas-phase spectra, hydrogen

bonding is much less important. For the –OH stretch in ethanol, therefore,

instead of the broad band between 3500 and 3000 cm-^1 seen in the spectrum

of a liquid, a single sharp peak occurs at 3600 cm-^1 when the gaseous

spectrum is recorded.

Liquidsor melts are more concentrated, and may be studied directly as a thin

film between NaCl plates. For more quantitative work, accurately prepared

E10 – Infrared and Raman spectrometry: principles and instrumentation 239

Michelson

interferometer

Alignment

He/Ne laser

J. stop

F

Visible source

Sample

Detector

Nd/YAG

laser

F

J

F 2

F 1

Fig. 4. Schematic of an FT-laser Raman spectrometer.

Table 4. Window materials for infrared spectrometry

Material Range (cm-^1 ) Comment

NaCl 40 000- 625 Soluble in water

KBr 40 000- 385 Soluble in water

TlBr/I (KRS-5) 16 600–250 Slightly soluble

ZnSe (Irtran-4) 10 000–515 Insoluble