We consider here the measurement of substances in the gaseous state--gases and vapors. By gases we mean those compounds that at room temperature have a vapor pressure higher than one atmosphere. Examples are CO2 , CO, NO2 , NO, methane, dimethyl ether and arsine. By vapors we mean the gaseous phases of compounds that normally are liquids or solids (room temperature vapor pressure less than one atmosphere). Examples here are water, methanol, pentane, diethyl ether and naphthalene. At room temperature there are many compounds that may be measured as vapors, and when the temperature is raised, the measureable group of compounds becomes even larger. When the detection limit is extended down to 10-9 atmospheres, a large percentage of monomeric materials are detectable as gases. However, most polymers, metals, salts, sugars and other crystalline materials do not have sufficient vapor pressure for detection as gases. Most organic molecules that are part of living systems will never be detectable as gases because they will decompose before they are vaporized.

Return to Table of Contents