A mass spectrometer produces charged particles (ions) from the chemical substances that are to be analyzed. The mass spectrometer then uses electric and magnetic fields to measure the mass (weight) of the charged particles. There are many different kinds of mass spectrometers, but all use magnetic and/or electric fields to exert forces on the charged particles produced from the chemicals to be analyzed. A basic mass spectrometer consists of three parts:
1. A source in which ions are produced from the chemical substances to be analyzed.
2. An analyzer in which ions are separated according to mass.
3. A detector, which produces a signal from the separated ions.
The information that a mass spectrometer gives out is as follows:
Exact mass: Elemental composition
Fragmentation pattern: Structure & “fingerprint ”
Isotope abundances: Presence & number of certain elements
Say that some water vapor was put into the mass spectrometer. A very small amount of water is all that is needed. The water is pulled into a vacuum chamber (the “ion source”) of the mass spectrometer. If a beam of electrons is shot through the water vapor, some of the electrons will hit water molecules and knock off an electron. If we lose an (negatively charged) electron from the (neutral) water molecule, the water will be left with a net positive charge. In other words, we have produced charged particles, or “ions” from the water:
H2O + 1 (fast) electron –> [H2O]+ + 2 electrons
Some of the contacts between the water molecules and the electrons will be so hard that the water molecules will be broken into smaller pieces. For water, the only possible fragments will be [OH]+, O+, and H+.
The mass spectrum of water will show peaks that can be assigned to masses of 1, 16, 17, and 18, or:
1 = H+
16 = O+
17 = [OH]+
18 = [H2O]+
Only some combinations of elements can produce ions that have these masses. For example, the ammonium ion [NH4]+ also has an approximate mass of 18 atomic mass units, but there would be peaks at mass 14 and 15 in the mass spectrum of ammonia corresponding to a N+ and [NH]+ (nitrogen is atomic mass 14).
A high-resolution mass spectrometer can determine the mass of an ion very precisely. If we knew that the mass of our hypothetical ion at mass 18 was actually mass 18.010, we could easily distinguish it from an ammonium ion, which would have an exact mass of 18.035 (we wouldn’t have to look for mass 14 and 15…).
Here is a picture of how the mass spectrometer works. It is a diagram of how the masses are separated in a certain particle. (picture is from the internet)