Page 67 - Tandem Techniques

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passing, in which case the substance is said to be transparent to radiation of that wavelength.

As already stated, the frequency (v) and the wavelength (l) are related to the velocity of light (c) in the
following way:-

Wave frequencies are measured in Herz (cycles/second), kHz (103 cycles/second), MHz (106
cycles/second), or GHz (109 cycles/second). As seen in Figure 2.1, wavelength can be measured in a
number of different units depending on the magnitude. The units commonly employed are Angstroms
(10-10 m), milli-micron or nanometers (10- m), microns (10-6 m), centimeters or meters.
9

Spectroscopy Involving the Absorption and Emission of Electromagnetic Waves

The manner in which electromagnetic radiation interacts with a substance, and the type of information
that is provided to help determine its chemical structure, will be briefly discussed. The basic principles
involved in spectroscopic measurement will be considered with particular reference to tandem systems.
In particular the sample requirements (sample size, sample volume etc.) will be considered to evaluate
their effect on any associated separation apparatus.

UV and Visible Spectroscopy

The practical wavelength range for UV/visible spectroscopy, useful in tandem techniques, is between
about 180 nm and 700 nm, the wavelength range between 180 nm and 400 nm, being part of the UV
spectrum and 400 nm to 700 nm the whole of the visible spectrum. In general it is difficult to operate at
wavelengths below 180 nm as most substances adsorb in this region. This means that, in tandem
systems, the carrier fluid from the separation technique employed will dominate the measurement and
the solute properties will not be discernible from those of the carrier.