中研院地球科學研究所 - 質譜儀實驗室
IES, Academia Sinica - Mass Spectrometer Lab


Introduction

Mass Spectrometer

What is Mass Spectrometer ?

As spectrum is the intensity of light vs. wavelength of light (Fig. 1). The mass spectrum is the intensity of isotope vs. the mass of the isotope (Fig. 2). Mass Spectrometer is the instrument we use to get the mass spectrum.

Fig. 1 (Enlarge) Fig. 2 (Enlarge)

Structure and Working Principle

The Structure of Mass Spectrometer can separate into three parts:

  1. Ion Source : To get the mass spectrum, we first need to get the ion mode of isotope. We use methods like high temperature, plasma etc. to ionize the isotope. After the ions are got, those ions will be accelerated by 10 thousands high voltage and fly into the flying tube of the mass spectrometer. (Animated demo)

  2. Mass Analyzer : When ions fly into the magnet field, ions will sense a force vertical to their velocity, this force will let those ions begin to make a turn. The radius of flying path of those ions is related to mass and electric charge of the ions. When the electric charge of ions is the same, the radius of flying path is determined by the mass of ion only. When ions fly into the magnet field, the heavier ions will have larger radius of flying path and the lighter ions will have smaller radius of flying path, so when ions fly into the magnetic field, isotope ions of different mass will be separated, each mass will follow their only flying path.

    r :
    V :
    B :
    m :
    q :

    Radius of Flying Path
    Accelerate High Voltage
    Intensity of Magnetic Field
    Mass of Ion
    Electric Charge of Ion


    In the formula we can see the radius of flying path is direct proportion to the square root of the mass – electric charge ratio, so if the electric charge of each ion is the same, in the same accelerate high voltage and magnetic field, the radius of flying path is determined by the mass of ion only. (Animated demo)

  3. Detector : Detector is the last stop of the isotope ions. When ions fly into detector, detector will get an electron from the wire connected to it to neutralize the ion. Because each ion need an electron to neutralize, so in the same time the more ions need more electrons, this means the current in the wire is higher, so we can know the higher current the detector need means the higher intensity of the isotope ions the detector got.

    Inside detector  

The working process of mass spectrometer can be summarized as : (Animated demo)

Application of Mass Spectrometer

We use mass spectrometer to measure the isotope ratios, our studies are listed here.