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  • Mass spectrometry allows chemists to weigh atoms and molecules, not directly on a balance,

  • but by measuring how ions formed from them are accelerated and deflected by electric

  • and sometimes magnetic fields. There are many types of mass spectrometer,

  • but they all work on this basic principle. This instrument is called a magnetic sector

  • instrument. The sample is introduced here and the positive

  • ions are formed here by bombardment of the sample with a beam of electrons.

  • The ions are then accelerated by an electric field and then deflected in a curve by a further

  • electric field. They then bend through a magnetic field and

  • are further deflected by another electric field here.

  • The ions then enter a detector here. By varying the electric and magnetic fields,

  • ions of different mass can be made to enter the detector.

  • These pumps keep the interior of the instrument under a high vacuum.

  • This is to allow the ions which are moving at speeds of many kilometres per second a

  • free path without collisions with air molecules. As the ions are formed from molecules of the

  • sample, they may fall apart or fragment. The ion of largest mass is often the parent

  • or molecular ion, an ionised molecule that has not fragmented.

  • Other peaks of smaller mass may represent charged fragments of this ion.

  • The masses of these fragments may give clues as to the structure of the original molecule.

  • Liquid samples are placed in the tiny sample tube, as shown.

  • Here, we're running the mass spectrum of liminine, an alkine that can be obtained from the peel

  • of citrus fruits. Only a little is required.

  • Mass spectrometry can detect down to as few as 10~12 moules.

  • Volatile samples can be run neat; less volatile ones are dissolved in a solvent such as dichloromethane

  • which then evaporates. The sample tube is placed in this probe, which

  • is then inserted into the instrument via an airlock so that the vacuum in the instrument

  • is maintained. Volatile samples will vaporise at the low

  • pressure inside the instrument but the probe can be heated to vaporise less volatile ones.

  • The electrons stream out from a heated cathode towards an anode which is held at a potential

  • of around +70 volts relative to the cathode. This is the ionisation chamber where the positive

  • ions are formed. These are formed when high energy electrons

  • strike the sample molecules. You can think of this as the electron beam

  • knocking an electron beam from the sample molecule.

  • The sample fits here and ions pass out through this slit.

  • The heater itself, is here. The electrons are accelerating in this directions

  • and the ions stream in this direction through the slit.

  • The positive ion beam passes between two charged curved plates which deflect it in a curve

  • so it enters the area here where an electromagnet produces a vertical magnetic field.

  • This further deflects the beam in an arc of a circle. On emerging from the magnetic field

  • the beam is deflected again by another set of charged plates which direct it into the

  • detector. The amount of deflection by the magnetic field

  • depends on the mass of the ion. Strictly, it's mass to charge ratio, but the

  • vast majority of ions have only a single charge. The deflection also depends on the strength

  • of the field. Heavier ions will not be deflected sufficiently

  • in the magnetic field to reach the detector. Lighter ions will be deflected too much.

  • During a run, the magnetic field is gradually increased so that ions of successively greater

  • mass enter the detector. The mass spectrum of liminine looks like this.

  • Each vertical line, called a peak, represents a different ion and it's mass, in relative

  • atomic mass units, is shown on the horizontal axis.

  • The height of a peak represents the abundance that ion.

  • The peak at 136 units represents the parent ion, the whole molecule that has lost an electron.

  • The peak at mass 121 units (136-15) is the parent ion from which one of the CH3 groups,

  • relative molecular mass 15, has broken off in the ionisation chamber.

  • The peak at mass 107 (121-14) could be formed by further loss of a CH2 group.

  • Other peaks are caused by more complex breakdowns and rearrangements.

  • Simple mass spectra show the masses of ions to the nearest whole number which is fine

  • for many purposes, however, this instrument can display masses to three decimal places

  • of an atomic mass unit. This is called a high resolution spectrum.

  • It enables us to distinguish between different molecular formulae which have the same relative

  • molecular mass to the nearest whole number. Here, we see that the relative molecular mass

  • of limine is 136.125. This means that we can distinguish C10H16 from say C9H12O which would

  • have a relative molecular mass of 136.194 to three decimal places.

  • Many mass spectrometers are of desktop size. This one takes in samples from a gas chromatography

  • instrument. As each component comes off the gas chromatography

  • column it is fed directly into the mass spectrometer. The combined technique is called gas chromatography

  • mass spectrometry or GCMS. This is the column from the gas chromatograph.

  • This particular mass spectrometer does not use magnetic deflection of the ions, it uses

  • a complex electric field. However, the principle of forming ions from

  • the sample and separating them according to mass underlies all types of mass spectrometer.

  • This is the gas chromatogram of an impure extract of orange zest. The large peak is

  • liminine and the others represent impurities. Clicking on the peak shows the mass spectrum

  • of liminine.

Mass spectrometry allows chemists to weigh atoms and molecules, not directly on a balance,

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B2 中上級

質量分析装置 MS (Mass Spectrometry MS)

  • 63 5
    Cheng-Hong Liu に公開 2021 年 01 月 14 日
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