4.2 Luminescence measurements
Luminescence refers to the production of light at low temperature; if emission is the result of a chemical reaction then the term chemiluminescence is used. An example is shown in Video 4 Chemiluminescence, a demonstration.
Why is the production of light by chemiluminescence more efficient than that produced by a standard incandescent bulb?
No heat (or, strictly speaking, very little heat) is generated, in fact you'll sometimes see the phrase 'cold light emission' used to describe this process.
If chemiluminescence occurs naturally in a living organism, the term bioluminescence is used. One example of analytical importance is the glow of the firefly. This insect produces a pigment called luciferin, which reacts with oxygen to produce light. At dawn or dusk it will emit yellow, yellow-green and red luminescence with the aim of attracting a mate or prey.
The reaction may be summarised as:
This reaction is catalysed by the enzyme luciferase.
You'll note the abbreviations ATP, AMP and PPi.
ATP is adenosine triphosphate (Structure 3), a substance used to drive many biochemical reactions and cellular processes that require input of energy. In this reaction ATP forms a derivative, adenosine monophosphate (AMP), Structure 4, and the pyrophosphate anion, P2O74− (abbreviated to PPi), Structure 5.
But what use is this reaction to the analyst?
The answer lies with the ATP molecule.
ATP is an excellent indicator of biological activity, being found in all animal and vegetable matter, bacteria, yeasts and moulds. Its presence is a tell-tale sign of microbial activity, and so Equation 16 and in particular the associated bioluminescence, may be used as an indicator of microbial contamination. In other words, a measure of cleanliness.
Test kit manufacturers have produced portable apparatus incorporating inventive methods of mixing reagents, extracting ATP and portable (hand held) luminometers for measuring the emission.
One of the most common uses of these kits is monitoring hygiene in food preparation areas and hence reducing the likelihood of food poisoning. Harmful microbes can grow on food residues on surfaces and utensils which in turn can contaminate subsequent food preparation. Also, the cleanliness of workers' hands can be checked, which could lead to modifications to washing routines.