Introduction to Pharmaceutical Analysis Lecture 2: Infrared Spectroscopy

Infrared spectroscopy

The study of the scattering, reflection, absorption or

transmission of EM radiation in the 800 nm – 1 nm range

The mid infrared region is commonly used for structural confirmation

Near Infrared

(800 to 2,500 nm)

(12,500 – 4,000 cm-1

Mid Infrared

(2,500 to

50,000 nm)

(4,000 to 200 cm-1)

Far Infrared

(50,000 to

1,000,000 nm)

(200 – 10 cm-1)

Typical range of an infrared spectrum is 4,000 - 625 cm-1

Hooke’s Law

The energy of each vibration is similar to the force needed to stretch a spring

Double bonds are stronger than single bonds (higher k) ➔they vibrate at higher frequency

Triple bonds vibrate at even higher frequency

For single bonds, the reduced mass plays a major role in determining the vibration frequency

This means O-H bonds vibrate at higher frequency than C-O bonds

Functional groups have characteristic IR absorptions

They do not change from one compound to another

There are two types of instrument:

• DISPERSIVE (Monochromator)

• FOURIER TRANSFORM (Interferometer) MUCH FASTER

Advantages & limitations

Strengths

Provides a complex fingerprint which is unique the molecule/drug

Matching of a spectrum of a compound to its standard finger-print can be carried out readily by computers

Limitations

Rarely used as quantitative technique due to complexity of spectra

Only detects gross impurities in sample