Ionization and detection methods

The major practical challenge with mass spectrometry is the conversion of a sample into gaseous ions. Typically, less than a milligram of compound is used. Many different experi mental arrangements have been devised to produce gas-phase ions but all suffer from a tendency to fragment the compound of interest. Electron impact ionization (EI) relies on bombarding a sample with high-energy electrons to cause both vaporization and ionization. A disadvantage is that EI tends to induce considerable decomposition in larger mole cules. Fast atom bombardment (FAB) is similar to EI, but bombardment of the sample with 

Figure 8.34 A magnetic sector mass spectrometer. The molecular fragments are deflected according to their mass-to-charge ratio, allowing for separation at the detector.

Figure 8.35 A time-of-flight (TOF) mass spectrometer. The molecular fragments are accelerated to different speeds by the potential difference and arrive at different times at the detector.

fast neutral atoms is used to vaporize and ionize the sample; it induces less fragmentation than EI. Matrix-assisted laser desorption/ionization (MALDI) is similar to EI, but a short laser pulse is used to the same effect; this technique is particularly effective with polymeric samples. In electrospray ionization (ESI), charged droplets of solution are sprayed into a vacuum chamber where solvent evaporation results in generation of individually charged ions; ESI mass spectrometry is becoming more widely used and is often the method of choice for ionic compounds in solution. The traditional method of ion separation relies on the acceleration of ions with an electric field and then using a magnetic field to deflect the moving ions: ions with a lower mass-to-charge ratio are deflected more than heavier ions. As the magnetic field is changed, ions with different mass-to-charge ratio are directed on to the detector (Fig. 8.34). In a time-of-flight (TOF) mass spectrometer, the ions from a sample are accelerated by an electric field for a fixed time and then allowed to fly freely (Fig. 8.35). Because the force on all the ions of the same charge is the same, the lighter ions are accelerated to higher speeds than the heavier ions and strike a detector sooner. In an ion cyclotron resonance (ICR) mass spectrometer (often denoted FTICR, for Fourier transform-ICR) ions are collected in a small cyclotron cell inside a strong magnetic field. The ions circle round in the magnetic field, effectively behaving as an electric current. Because an accelerated current generates electromagnetic radiation, the signal generated by the ions can be detected and used to establish their mass-to-charge ratio. Mass spectrometry is most widely used in organic chemistry but is also very useful for the analysis of inorganic compounds. However, many inorganic compounds, such as those with ionic structures or covalently bonded networks (for example SiO₂), are not volatile and do not fragment into molecular ion units (even with the MALDI technique) so cannot be analysed by this method. Conversely, the weaker bonding in some inorganic coordination compounds means that they fragment much more easily than organic compounds in the mass spectrometer.

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مواضيع ذات صلة

Thermal analysis

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CHN analysis

Atomic absorption spectroscopy

X-ray absorption spectroscopy

Photoelectron spectroscopy

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Absorption spectroscopy

Neutron diffractionNeutron diffraction

Single-crystal X-ray diffraction

Powder X-ray diffraction

X-ray diffraction