Mass Spectrometry
A mass spectrometer analyses the mass-to-charge ratio (m/z) of ions in the gas phase (Griffiths et al. 2009). The mass-to-charge ratio is a physical quantity that relates the mass to the electrical charge of the particles involved, being therefore characteristic of the elements analysed. An MS instrument consists of an ion source, a mass analyser, a detector and an evaluation unit.
Ion sources are available in very different designs, depending on the type of analyte to be analysed or whether there is a coupling with a chromatographic method. Nevertheless, they all have the task of ionising the analytes. Mass spectrometric analysers are used to separate ions according to their mass-to-charge ratios (m/z ratio) and ideally have high resolution and mass accuracy over a wide mass range. As a result, a mass spectrum is obtained in which the ion abundance is plotted against the m/z (Wang et al. 2008, Griffiths et al. 2009).
Since the mass analyser and detector require a vacuum for operation, the mass spectrometer relies on a vacuum pumping system (Kraj et al., 2008). This prevents non-reproducible fragmentation reactions, due to the precipitation of unwanted fragments (de Hoffmann et al., 2007). Currently, six main mass analysers are used: quadrupoles (Q), ion traps (IT), time-of-flight mass spectrometers (ToF), Fourier transform ion cyclotron resonance (FT-ICR), sector fields, and Orbitraps. The above analyzers differ in price, size, resolution, and mass range (El-Aneed et al. 2009). The ions are detected directly (Faraday cup) via so-called secondary electrons (electron multiplier, electro-optical ion detectors), microchannel plates (MCP) or based on the trajectories (Orbitrap, Fourier-transform ion cyclotron resonance) (de Hoffmann et al., 2007). The otherwise low electrical signal is amplified using a secondary electron multiplier (SEV). An electron cascade achieves amplification (Goodrich et al., 1962).
There is a variety of ionisation techniques used in mass spectrometry. Ions are generated in the ion source by neutral molecules donating or capturing electrons in the gas phase, being protonated or deprotonated, or forming charged adducts. Ionisation is also possible during the transfer of charged particles from a liquid medium to the gas phase. Some ionisation techniques are highly energetic and lead to strong analyte fragmentation (de Hoffmann et al. 2007). Hard ionisation methods include electron impact ionisation (EI), chemical ionisation (CI) and inductively coupled plasma (ICP). In contrast, so-called soft ionisation methods can generate intact ions from fragile molecules (Desiderio et al. 2008). Examples of soft ionisation methods include electrospray ionisation (ESI), matrix-assisted laser-desorption/ionisation (MALDI), atmospheric pressure chemical ionisation (APCI), and atmospheric pressure laser ionisation (APPI) (Hommerson et al. 2011).
Mass analyzer | Acronym | Principle |
Time of flight | ToF | Time dispersion due to an electrical pulse |
fourier-transform ion cyclotron reso-nance | FT-ICR | Axial oscillation in a homogeneous magnetic field |
quadrupol | Q | Superposition of temporally constant, high-frequency electric fields, separation due to unstable trajectories (ion trajectories) |
triple quadrupole | QQQ | tandem MS method in which the first and third quadrupoles are used as mass filters and the second quadrupol is used for fragmentation of the analytes |
Orbitrap | Orbitrap | Axial oscillation in an inhomogeneous electric field |
quadrupol ion trap | QIT | Superposition of time-constant, high-frequency electric fields in conjunction with a constant magnetic field |
on trap | IT | T ions are held in the trap with electric and magnetic fields and ccan be selectively guided to the detector |
sector field: magnetic field | B | Deflection due to a magnetic pulse |
sector field: electric field | E | Focusing by means of electrostatic field |