Control observations

The successful validation of any autoradiographic experiment requires a proper set of control observations for determination of background and presence of chemography.

Background

Silver grains deposited due to radioactivity of the tracer molecules incorporated into the sample constitute the signal. However, some additional grains can also develop for nonspecific reasons. These constitute the noise or background of the system. Generally the background should not exceed more than 5% of the signal grain density. Background can be estimated by taking the ratio of the grains present over the target tissue to the grains present over the same area of empty section (only embedding material coated with emulsion). The factors contributing to background are:

• Exposure to stray light or radiation.

• Pressure during drying of emulsion film.

• Static discharges in the vicinity (common in low-humidity workplaces).

• Overdevelopment (excessive time and/or temperature).

Chemography

Induction of grain formation by the chemical action of sample components on silver halide crystals is known to occur and is called positive chemography. The reverse process, of eradication of latent images by chemicals in the sample, is known as negative chemography. The former causes an increase in the background, while the latter lowers the efficiency. Interposing a thin inert layer of carbon between the sample and the emulsion usually minimizes the problem. A control for positive chemography is a non-radioactively-labeled, identically-treated sample. For negative chemography a control would be a light-fogged, identically-treated sample.

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