While lifting, a small sample of unsaturated air is cooled adiabatically by expansion at the rate of about 5°F. per thousand feet. The cooling steadily decreases the small sample's capacity for water vapor so that if the lifting is sufficient the initially unsaturated small sample finally becomes saturated. Further lifting beyond this condensation level of the small sample produces additional cooling with resultant condensation of the amount of moisture in excess of that which the temperature permits to remain in the gaseous state.
The small sample chamber of the microscope is evacuated of air and filled with low-pressure helium, and a 30,000-v electric charge is then applied to the tip of the small sample. Helium atoms, colliding with the charged atoms of the small sample tip, become ionized, and because of their charge, they move along the lines of force to the viewing screen where they produce glowing spots. In this way an array of spots is produced that represents a "magnified" image of the atoms at the needle tip.
From the ratio of the activity produced and the amount of the element in the known small sample, the amount of the unknown can be computed. A third method known as isotopic dilution analysis involves adding a radio-isotope of known specific activity to the unknown small sample. The radioisotope then is recovered and its specific activity is redetermined. The amount of the element in the unknown small sample can be computed from the change in specific activity. Another application of radiochemistry is in the study of exchange reactions. It was noted earlier that carriers and radioisotopes sometimes remain in different chemical forms.
While lifting, a small sample of unsaturated air is cooled adiabatically by expansion at the rate of about 5°F. per thousand feet. The cooling steadily decreases the small sample's capacity for water vapor so that if the lifting is sufficient the initially unsaturated small sample finally becomes saturated. Further lifting beyond this condensation level of the small sample produces additional cooling with resultant condensation of the amount of moisture in excess of that which the temperature permits to remain in the gaseous state.