Department of Chemistry

Introduction to Mass Spectrometry

Mass spectrometry is an analytical technique in chemistry. This technique studies two properties of a substance, the mass and the charge.

A charged species is called an ion. Charge is measured in units of coulombs (C). The charge of an electron is 1.60218*10-19 C. The charge of an ion can also be measured in terms of the number of excess protons or electrons that are present. If there is an excess of protons, the ion is positively charged. For each excess of one proton, the ion is assigned a charge of 1+. If there is an excess of electrons, the ion is negatively charged. For each excess of one electron, the ion is assigned a charge of 1–. Therefore, if a molecule loses two electrons, the resulting ion has a charge of 2+.

Sample Problem

Count the number of protons, neutrons, and electrons, contained in the following ions. Assume that the isotopes are all the most abundant in nature. (a) H2O+ (b) N3- (c) Au3+


(a) 10 protons, 9 electrons, 8 neutrons

(b) 7 protons, 10 electrons, 7 neutrons

(c) 237 protons, 234 electrons, 354 neutrons.

We must have a charged species in order to study it with mass spectrometry. Therefore, there must be a means to produce an ion from a neutral compound or atom. The ion source of a mass spectrometer ionizes a neutral substance. There are many different types of ion sources, such as electron impact, chemical ionization, electrospray, and laser desorption.

Many times there is a mix if ions of different sizes and charges. Once the ions have been produced, they travel through an electric field and are accelerated. Heavier ions are accelerated faster than lighter ions in an electric field. This allows for the ions to separate. The ions then leave the influence of the electric field, and travel through what is called the drift region. In the drift region, the ions travel with nearly constant speed. They then strike a detector, and a mass spectrum is produced. A mass spectrum records peaks that refer to the mass to charge ratio of the ions. The following is an example of a mass spectrum.

mass spectrum

The mass to charge ration, or m/z, is measured on the x-axis. The y-axis records the relative abundance of the ions. The most abundant type of ions is scale to 100%, and the remaining ions are scaled accordingly.