Instant Notes: Analytical Chemistry

Applications A gas chromatogram obtained from a few microliters of a 12-component
mixture is shown in Figure 2.
As with infrared spectra obtained by conventional sampling methods, the IR
absorption bands correspond to characteristic vibrational frequencies of partic-
ular functional groups. Vapor-phase spectra do not show the effects of strong
hydrogen bonding, such as the broadening of the –OH stretching peak around
3600 cm−^1.
Peak 1, representing less than 1% of the total sample (Fig. 3), shows the
characteristic group frequency bands for

(i) free hydroxyl stretch at 3600 cm−^1 , indicating an alcohol;

(ii) aliphatic CH stretch at 2900−3000 cm−^1 , suggesting an aliphatic compound;

(iii) C−O stretch at around 1050 cm−^1 confirming the alcohol.

Computer matching shows that this compound is 2-methyl propanol (iso-

butanol).

A mixture of esters might be characterized by monitoring the absorbance due

to the -CO- peak around 1700 cm−^1 , and a mixture of aromatic compounds by

selecting the aromatic CH- stretch peaks just above 3000 cm−^1. By plotting the

300 Section F – Combined techniques

Intensity

0 5 10 15 20

Time (min)

Peak 1

Fig. 2. Gas chromatogram of an unknown mixture.

4000 3000 2000 1500 1000 500

%T

cm–1

Fig. 3. IR spectrum of peak 1.