What are the differences between chemical ionization and electron impact ionization method used in mass spectrometry?
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Mass Spectrometry Ionization Methods
There are many types of ionization methods are used in mass spectrometry methods. The classic methods that most chemists are familiar with are electron impact (EI) and Fast Atom Bombardment (FAB). These techniques are not used much with modern mass spectrometry except EI for environmental work using GC-MS. More modern techniques of atmospheric pressure chemical Ionization (APCI) , electrospray ionization (ESI), matrix assisted laser desorption ionization (MALDI) and other derivative methods have taken their place in the mass spectrometry laboratory. The reason for APCI over EI is that APCI forms a protonated molecule and is completely compatible with Liquid Chromatography (LC), while EI is more likely to fragment the ion leading to a possible more ambiguous identification of the molecule weight and is incompatible with LC. ESI along with Matrix assisted laser desorption ionization (MALDI) has basically eliminated the use of FAB, because it produces the protonated molecules, which made FAB so popular, but is much more sensitive. In addition, MALDI along with ESI allowed for ionization and measurement of large molecular weight that could not be done before, even by FAB. ESI has an advantage in its easy compatibility with LC. While MALDI has advantages for imaging mass spectrometry.
Electrospray ionization (ESI) - ESI is the ionization technique that has become the most popular ionization technique. The electrospray is created by putting a high voltage on a flow of liquid at atmospheric pressure, sometimes this is assisted by a concurrent flow of gas. The created spray is directed to an opening in thESI spraye vacuum system of the mass spectrometer, where the droplets are de-solvated by a combination of heat, vacuum and acceleration into gas by voltages. Eventually the ions are ejected from the droplets and accelerated into the mass analyzer by voltages. For larger molecules, the ions may contain multiple charges, allowing the detection of very large molecules on analyzers that have limited mass to charge (m/Z)) ratio ranges. Because of the natural use of a flowing liquid, it is easily adapted to liquid chromatography (LC).
There are many other techniques that are variations of electrospray, for example, nanospray is a derivative of ESI that basically is a low flow version and is highly sensitive because ESI sensitivity is dependent on concentration not the amount of sample used. Static nanospray or picospray is a similar derivative, but flow is created by only the electrostatic spray or a small gas pressure, this allows a few micro liters to last over one hour. In addition, static nanospray has the advantage of less carryover, because the needle is disposed of after each sample. Another similar ionization technique is to spray a surface with the ESI spray and have the spray extract ions from the surface into the mass spectrometer. This technique is called desorption electrospray ionization (DESI) and produces spectra similar to electrospray, but is analyzing the sample on a surface.
Atmospheric Pressure Chemical Ionization (APCI)- APCI is a method that is typically done using a similar source as ESI, but instead of putting a voltage on the spray itself, the voltage is placed on a needle that creates a corona discharge at atmospheric pressures. This discharge creates ions, in theory mostly H3O+ or water clusters. The sample is injected into the discharge by a spray created by a flow of liquid combined with a heated gas that APCI sourcevolatilizes the sample. The ions are formed by proton transfer from the H3O+ or the water clusters to the sample. These ions are then extracted into the same opening vacuum that is used for electrospray. Another variation of this technique is atmospheric pressure photo ionization (APPI), where the initial ionization is performed by photo ionization, usually of a dopant that absorbs the light and is added to the sample flow.
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