intoroduction to pharmaceutical analysis lecture 1 Ultraviolet/Visible Spectroscopy

Spectroscopy

The study of matter and its properties by investigating light, sound

or particles that are

• emitted,

• absorbed, or

• scattered

by the matter under investigation

Most drugs absorb light in the ultraviolet (UV) region of the electromagnetic spectrum (190-390 nm

Spectral regions are characterised by frequency and/or wavelength

High frequency = low wavelength ➔ high energy

High wavelength = low frequency ➔ low energy

UV-Vis spectroscopy

Study of the absorption of EM radiation in the ranges

ultraviolet: 100 - 380 nm

visible: 380 - 740 nm

UV region has shorter wavelength and higher frequency than

visible region ➔ UV has higher energy

For practical reasons, the UV-Vis spectrum is divided into three regions

Far-UV: 100-200 nm

Middle/Near-UV: 200-380 nm

Visible: 380-740 nm

Difficult to measure below 185 nm – oxygen and other simple molecules absorb radiation in this region

Compounds which absorb in the visible region are coloured

These are termed chromophores

Wavelength Colour

• 620–740 nm Red R

• 590–620 nm Orange O

• 570–590 nm Yellow Y

• 495–570 nm Green G

• 450–495 nm Blue B

• 380–450 nm Violet

Bathochromic & Hyperchromic shifts

A

What causes bathochromic shifts?

• Decreased symmetry, e.g. due to substitution patterns

• Changing solvents

• Intermolecular interactions

• Autochromes - any atom or group which, when added to a chromophore, causes a bathochromic shift

• pH change

Advantages & shortcomings

Strengths

Easy-to-use, cheap and robust method

Can be used as a routine method

Limitations

Only moderately selective

Not readily applicable to the analysis of mixtures

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Simple and convenient quantitative technique

But it does not tell us which molecule is absorbing the light