Posts Tagged “Analytical Chemistry”

When It Comes to Mass Spectrometry

Mass Spectrometry

As a vital part of analytical chemistry, Mass Spectrometry (MS) is a modernly developed method that is able to rapidly, delicately and accurately identify the molecular weight.

In 1898, when W. Wayne used electric field and magnetic field to deflect the beam of positive ions, he found if the charge is the same, ion with smaller mass will display greater deflection, while massive ion displayed less deflection. In 1913 JJ Thomson and FW Aston confirmed that neon has two isotopes—Ne20 and Ne22 with magnetic deflection device. And then in 1919, Aston made a mass spectrometer whose resolution can reach hundredth. The mass spectrometer was used to measure the relative abundance of isotopes and successfully identified a number of isotopes. But until the time 1940, the mass spectrometer was only used for gas analysis and determination of stable isotopes of chemical elements. In following days, mass spectrometry is used to analyze the petroleum fraction of a complex hydrocarbon mixture. With the confirmation of mass spectrum generation ability for complex molecules, it is used for the determination of the structure of the organic compound. This has opened up a new field for organic mass spectrum.

The principle of mass spectrometry is:

The components are ionized to generate sample ions of different charge to mass ratio, after accelerating electric field, the ion beam is formed and go into the mass analyzer, and then use electric field and magnetic field to get velocity dispersion – the slower ion beam presents greater ion deflection when go through an electric field, vise visa for high speed ion beam; in the magnetic field, the ion occurs opposite angular velocity vector deflection, which is to say that slower ion beam still presents greater ion deflection; when the deflected two fields compensate each other, their orbits will intersect at a point. At the same time, the mass of the separation in a magnetic field can occur, thus ions having the same mass to charge ratio but different speeds will focus on the same point, and vise visa. Then a mass spectrum is obtained to determine their mass.

The mass spectrometer

Mass spectrometer is based on the use of deflection of mobile ions in electric and magnetic fields. The former refers to electronically detect ions, while the latter refers to the ions are focused on a photographic plate for testing. There are many types of mass spectrometry instruments. According to the scope of use, they can be classified as inorganic and organic mass spectrometry. Commonly used organic mass spectrometers include a single focusing mass spectrometer, double focusing mass spectrometer and a quadrupole mass spectrometer. At present, the latter two are used frequently with regards to gas chromatograph.