Electron probe analysis of microdroplets. Effect of low temperatures on the stability of characteristic signals.

The effect of low temperatures on the stability of characteristic signals during electron probe analysis of microdroplets has been investigated. The kinetics of the characteristic Na, K and Cl signals from lyophilized droplets of either KCl or NaCl salts were observed for 1200 s within an electron microprobe whose specimen stage was maintained at -140 degrees C. Samples were irradiated with 15 keV electrons, at beam current densities ranging from 7 x 10(-4) to 7 x 10(-3) A . cm-2. The signals measured increased and then, in some cases, decreased. The duration and amplitude of the increases varied depending on beam current density, mass thickness of the sample and on the element concerned. The increase sometimes reached 6 times the initial value, a level never observed at room temperature. No proportionality was observed between X-ray intensity and concentration for measurements after the first 10 s irradiation. Proportionality was only restored when droplets had been irradiated for some time. In no case did the stability of characteristic signals improve. Analysis of droplets at -140 degrees C suggests that under electron beam irradiation they undergo a change not observable at room temperature.