Welcome to the mass spectrometry community

mass spectrometry

Mass spectrometry is a powerful analytical technique used to identify chemical substances within very few sample. Thank to tandem mass spectrometry, It is easily used to elucidate the structure and chemical properties of molecules and complex. Due to its sensitivity, its selectivity and its ability to perform rapid analyzes, mass spectrometry plays an important role in diverse fields, including forensic toxicology, metabolomics, proteomics, pharma/biopharma, and clinical research. Specific applications of mass spectrometry include drug testing and discovery, food contamination detection, pesticide residue analysis, isotope ratio determination.

The basic principle of mass spectrometry is to measure the mass to charge ratio (m/z) of ions. The first step in the mass spectrometric analysis of compounds is the production of gas phase ions of the compound in the ion source. Then in the analyzer these ions will be manipulated and separated according to their mass to charge ratio. Finally, the detector detects the m/z and the abundance of each ion population.

Brief history of mass spectrometry

The first mass spectrometer, J.J Phy. Mag 1897

The first mass spectrometer, Thomson J.J Phy. Mag. 1897

1897: The history of mass spectrometry began with Sir JJ Thomson, his hypothesis that channel rays are beams of charged particles with the lighter ones being more deflected than the heavier ones, this is the principle of magnetic sector analyzer. In the first decade of the 20th century, Thomson built the first mass spectrometer (then called a parabola spectrograph) to determine the ratio of mass to electron (m/e). In this instrument, the ions that are generated in discharge tubes pass through electric and magnetic fields (single focus magnetic sector analyzer). The trajectory of the ions is then curved more or less according to the mass to electron of each ion. The ions are  finally detected on a fluorescent screen or a photographic plate. Thomson received 1906 Nobel Prize in Physics for his work. “in recognition of the great merits of his theoretical and experimental research on the conductivity of gas electricity.”

1918 : Electron ionization ion source (IE) was first invented by Sir Dempster, a few years later it was perfected by Bleakney (1928). It’s the first ion source used for mass spectrometry..

1930 : Double focusing magnetic sector analyzer was developed, in this instrument the dispersion of space and kinetic energy is reduced, giving better resolution of the mass spectra and also the possibility of tandem mass spectrometry.

1946 : The concept of Time Of Flight (TOF) was proposed by William E. Stephens. In a TOF analyzer, ions are separated by their velocities. The TOF/MS is fast and high resolution so that it can be used to chromatographic detection. The mass spectrometer with analyzer TOF is nowadays much evolved and used in many fields.

1949: First on-line coupling of gas chromatography to a mass spectrometer (GC-TOF/MS).

1955 : Paul introduced the quadrupole ion trap, in this instrument the ions can be trapped by dynamic electric fields and then they can be ejected according to the voltage of the electric fields. For his invention, Paul shared a Nobel Prize in physics in 1989. Since then, the ion trap has been widely used in mass spectrometry, it can be used as analyzer or in combination with others analyzers such as, Orbitrap, ICR (Ion Cyclotron Resonance)…

1966 : Munson and Field introduced the chemical ionization (CI), this ion source require a lower amount of energy compared to electron ionization (EI). The lower energy yields less or sometimes no fragmentation, and usually a simpler spectrum, so that the application of CI extends easily to biomolecules. The chemical ionization was observed by Thomson, but at this time he didn’t understand the phenomena.

1968 : Introduction of tandem mass spectrometry (MS/MS) by Keith R. Jennings and McLafferty. The fragmentation of the precursor ions is carried out by collisions with neutral gases (Collision Induced Dissociation (CID)).

1974 : Melvin B. Comisarow and Alan G. Marshall introduced the operation of the Fourier transform to the processing of electrical signals, which are generated by ions in the ICR cell. This operation gives rise to ultrahigh resolution mass spectra. Because of its ultrahigh resolving power, FTICR MS is today one of the most valuable techniques for analyzing complex mixtures.

1985 : Franz Hillenkamp, Michael Karas and co-workers developed the Matrix-Assisted Laser Desorption Ionization (MALDI). This ion source is a soft ionization technique which preserves the molecule intact while it is ionized, so that MALDI extends easily to large molecule such as biomolecule.

1988 : John Fenn presented electrospray ionization at the ASMS meeting, for the development of this ion source he was awarded a Nobel Prize in 2002. This ion source ionizes molecules from liquid state to gas ion phase, so that it is easily combined with chromatography liquid (LC), it is soft ionization technique and which preserves the molecule intact allowing analysis of large molecules in many fields such as biology, pharmaceutical, biochemical, medicine, environmental… The development of ESI and MALDI really opened up the mass spectrometry field to a whole new group of researchers.

2005 : The first commercial Orbitrap technology implementation was a hybrid instrument (LTQ Orbitrap XL) featuring a linear ion trap front-end. Since then a lot of Orbitrap-based instruments were produced and became a common sight in analytical laboratories and facilities worldwide.

Mass spectrometer

A mass spectrometer has essential three components

  • An ion source where the ions are produced from gas, solid or liquid state.
  • One or more analyzer where the ions are manipulated (transported, turned around, fragmented, sorted, selected…).
  • A detector to count the ions and amplify their signals, or to register the image current which is induced from the movement of ions.

Finally, a computer to collect the data from these components to generate a mass spectrum.

Diagram of a Mass Spectrometer

Diagram of a mass spectrometer

Détecteur

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Détecteur destructif

Détecteur FT

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Characteristics of a mass spectrometer

The resolution

The definition of resolution in mass spectrometry expresses this value as m/Δm, where m is the mass of the ion of interest and Δm is the peak width (peak width definition) or the spacing between two equal intensity peaks with a valley between them at 0.5%, 5%, 10% or 50% (full width at half maximum (FWHM)) of their height. Resolving power in mass spectrometry is defined as the ability of a mass spectrometer or measurement procedure to distinguish between two peaks at m/z values differing by a small amount and expressed as the peak width in mass units. The resolving power of a mass spectrometer diminish when the m/z increases, Depending on the type of analyzer, the resolution is some for low resolution mass spectrometers (ion trap, quadrupole) to a several hundred thousand for very high resolution mass spectrometers (Orbitrap, FT-ICT).

the resolution of mass spectrometryFig. 1 : The resolution for an ion within 4 protons. At low resolution, the peaks overlapped.

Scan rate

The scan rate of a mass spectrometer refers to how fast it scans a mass spectrum, it is expressed in Hz that is to say the number of mass spectrum can be performed in 1 second. This parameter is important for chromatography applications where the mass spectrum must be scanned faster than the elution time of the chromatographic peak. At least 3 mass spectra have been performed to define a chromatographic peak. Generally, the scan rate is inversely proportional to the resolving power of mass spectrometer with exception of TOF-MS.

Mass accuracy

The mass of a molecule or the m/z of an ion is generally expressed as a monoisotopic mass (molecular mass) or m/z. Mass accuracy is expressed by ppm (parts per million), it is an important parameter for determining and identifying molecules. In HRMS of small molecules, the error in m/z determination will typically be in the fourth decimal place (accurate mass determination). From the accurate m/z of an ion, one can use software tools to calculate its possible elemental compositions. The number of hits from such a calculation depends on the m/z value, the number of elements considered, and the mass accuracy achieved (fig 2). High resolution is an important criterion for mass accuracy but it is not the only parameter, the mass accuracy depends on many factors such as the stability of mass spectrometer, temperature, internal and external calibration conditions, mathematical equations.

elemental composition according to mass error1Fig. 2: Number of elemental composition according to mass error, the smaller error in m/z the fewer elemental composition.

Mass range

The mass range refers to the range of m/z from the lowest m/z to the highest m/z. It is worthy to note that in a range of m/z, the ions with different m/z are not transmitted at the same performance, generally the ions at the beginning and at the end of the mass range are poorly transmitted.

Applications