radioisotopes?

Question

what problems might result from using a tracer the contained a radioisotope that was either too stongly or too weakly radioactive?
also, scientict have been inserting tracer molecules containing phosphouse-32 into a population of bacteria to see where they store phosphorus. Which method might the scientist use to moniter the movement of tracers within the bacteria?

Answer 1

I assume your question is related to the usage of radioisotope as a tracing agent in live organisms.

Weak radioisotopes are hard to be detected and thus traced by common laboratory instruments (e.g. like Geiger counter can only detect beta radiation).
Strong radioisotopes are easy to be traced but might cause toxic effect to the host organism (either cytotoxic or genotoxic).

Tracing P32 within single bacterial cell is not practically possible. But scientists can apply biochemical methods to fractionate different sub-cellular organelles (e.g. nucleus, ER, Golgi complex, cell membrane etc.) or isolate different protein complexes. Then, incorporated P32 can be monitored by a scintillator or can be further separated by gel electrophoresis and then visualized simply by exposing to a X-ray film.

Answer 2

Too weakly radioactive: the air between the sample and the detector would absorb the radioactivity (e.g. alpha emitter)

Too strongly: hazard to the user (e.g. gamma emitter)

Detection method: tough call. Normally to detect radioactivity one would use a phosphorescent material that emits visible-wavelength photons when exposed to radiation, and then the visible-wavelength photons are detected by a CCD camera--just a regular plain camera.

But bacteria are only twice the size of visible wavelengths anyways, meaning you'd have to connect a microscope somehow, and that's completely impractical, and in addition if you're trying to localize something within the bactera you would absolutely have to use a technique like electron microscopy, but that is not really compatible with radioactivity. Unless you're looking at large colonies of bacteria, meaning millions or billions of cells, and trying to tell whether the radioactivity is at the edges, inside, etc. Then the CCD thing would work, and you might not necessarily even need a microscope depending on how big the bacterial colonies are.