Electron Impact (EI)
Definition and meaning of Electron Impact (EI) in chemistry.
Electron impact (EI) is a common method used to turn neutral molecules into charged ions. It is a vital technique used inside mass spectrometers to identify unknown chemicals. A beam of high-energy electrons crashes into a gas of sample molecules. This violent crash knocks an electron loose and creates a positively charged radical cation.
In more detail
The process begins by heating the chemical sample until it turns into a gas. This gas then flows directly into the path of a powerful electron beam. The beam usually operates at a standard energy level of 70 electron volts.
This amount of energy is far more than a molecule actually needs to lose an electron. The sheer force of the collision leaves the newly formed ion with extra internal energy. Because of this excess energy, the ion quickly breaks apart into smaller chemical pieces.
This shattering process is known as fragmentation. The specific way a molecule shatters is highly predictable and repeats exactly every time. This creates a unique fingerprint of fragment sizes for every distinct chemical.
Chemists call this a hard ionization method because it violently rips the molecule apart. Soft methods try to keep the original molecule mostly in one piece. Because the sample must be vaporized first, this method only works for chemicals that boil easily without burning. It is usually paired with a gas chromatograph to separate complex mixtures before ionization.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Standard electron energy | 70 eV |
| Primary ion produced | Radical cation (M+•) |
| Ionization type | Hard ionization |
| Common pairing | Gas chromatography (GC-MS) |
| Sample requirement | Must be volatile and thermally stable |
A chemist wants to test a sample of liquid toluene, which has the chemical formula C7H8. They inject the toluene into a gas chromatograph linked to a mass spectrometer. Inside, a 70 eV electron beam blasts the toluene vapor. The blast knocks one electron away to create a whole-molecule radical cation (M+•) at m/z 92. This highly energetic ion quickly loses a single hydrogen atom. It forms a very stable ring-shaped tropylium cation at m/z 91. This strong 91 peak acts as a clear signature that proves the sample contains toluene.
Frequently asked questions
Why is 70 eV used as the standard electron energy?
At 70 electron volts, the shattering pattern is extremely consistent between different machines. This allows chemists to compare their results against massive global databases to identify unknown chemicals.
What is a major limitation of electron impact?
The sample must be turned into a gas before testing. This means it cannot be used on large biological molecules or chemicals that burn up when heated.
Why is it called a hard ionization method?
It transfers a massive amount of excess energy into the molecule. This extra energy forcefully breaks the chemical bonds and shatters the molecule into many smaller pieces.