Forensic Applications of Gas Chromatography - Forensic Applications of Gas Chromatography - page 125

describes it is not necessarily how another person would describe it; there-
fore, we need to be able to 'standardise' colour.
There is a scientific way of describing colour: giving a colour three val-
ues to represent the proportions of red, blue and yellow. The basis of all
colour measurement is based on the CIE (International Commission on
Illumination) colour system. Work was agreed in 1931 and remains the same
today, with a few modifications (see also the Further Reading section).
Attributes of colour are
• Lightness (or brightness): The degree of lightness refers to the level
of grey of an achromatic colour.
• Hue: This is determined by the wavelengths of light that are reflected
and absorbed by an object.
• Chroma (or saturation): This is the attribute of a visual sensation
according to which an area appears to exhibit more or less of its hue.
There are two types of colourants: dyes and pigments. Both of these
colourant types tend to be supplied by a manufacturer in powder form, but
the main difference between the two is in terms of solubility. Dyes are soluble
and pigments are insoluble in the liquid medium.
As previously mentioned, we will be focussing on pigments only.
8.5.3 Why Are Pigment Molecules Coloured?
A coloured molecule will contain a chromophore and an auxochrome. A
chromophore is principally responsible for the colour, for example, an azo
group (-N=N-), the nitro (-NO 2 ) or carbonyl (C=O) functional groups.
Auxochromes, on the other hand, will 'enhance' the colour properties
of the chromophore and will be either electron withdrawing or electron
releasing. Examples include the hydroxyl (-OH) and amine (-NH 2 ) func-
tional groups. Essentially, the colour arises from electronic transitions
from a ground state to an excited state causing absorption of visible light.
Paint has been used for many years as protection and also for decoration. It
is typically composed of a pigment and extender, binder or carrier, solvent,
and additives with the proportions of each varying depending upon the
manufacturer. When explaining paint we shall use the term coating and
paint interchangeably.
The functions of paint components can be identified as follows:
Pigment: This not only provides the colour of the paint but also can add
other optical properties such as opacity and gloss reduction. The pig-
ment may also be designed to provide protective properties.