Mass Spectrometry is a technique that determines the molecular weight (mass) and elemental composition of a compound accurately. It records relative abundance (number of different kinds of ions) and mass/charge (m/e) ratio. An advantage of Mass Spectrometry is its ability to identify the unknown compounds using only pictogram (10-12) amounts. Because ions are very reactive and short-lived, their formation and manipulation must be conducted in a vacuum. Atmospheric pressure is around 760 torr (mm of mercury). The pressure under which ions may be handled is roughly 10-5 to 10-8 torr (less than a billionth of an atmosphere).