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| Fig. 1: Mass spectrometers such as these were used for the Manhattan project. (Source: Wikimedia Commons) |
Mass spectrometry (MS) is an analytical method that separates an ionized mixture on the basis of mass-to-charge ratio. MS may also provide absolute and relative measurement of ion abundance. Although the development of MS began nearly a century ago with the contributions of many notable physicists, discoveries still contribute to the advancement of this analytical technique today. MS is widely used in various scientific and engineering disciplines and has many applications within nuclear physics in particular. [1-3]
In 1906, the Nobel Prize in physics was awarded to J. J. Thomson "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." [4] More specifically, he was interested in the determining if cathode rays were made of particles or waves. To this end, Thomson worked with F. W. Aston to construct an instrument that could measure the charge-to-mass ratio and mass of an electron. [5] This instrument is a primitive ancestor to the modern day mass spectrometer. Today, A. J. Dempster and Aston are recognized for inventing the first mass spectrometers in the late 1910s. [1,2]
Early MS of the early 20th century offered excellent resolution, enough to ascertain the masses of isotopes. As such, MS became an analytical method of central importance for the scientists participating in the Manhattan project during World War II, as shown in Fig. 1. [1,3,5] However, MS did not become popular until Nier recognized its potential. Trained in electrical engineering and physics, Nier spend many years developing various variations of the mass spectrometer and developing the technique into its own field. According to Dennis Schlutter, one of Nier's colleagues, "He sort of commercialized the instrument - not in the sense that he was trying to sell them, but [he] made them more useful and usable." [5]
Today, MS is still an evolving field, with major developments still occurring. In 2002, the Nobel Prize in chemistry was awarded for John B. Fenn and Koichi Tanaka for the development of electrospray ionization (ESI), a novel technique for ionizing analytes. [6]
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| Fig. 2: Schematic showing the basic operation of a mass spectrometer. (Source: Wikimedia Commons) |
There are many variations of MS instruments in use today, but there are three basic parts ubiquitous among all instruments: an ionizer, a mass analyzer, and a detector. Fig. 2 is a simple schematic of a basic mass spectrometer showing all three components. [7]
The ionizer is responsible for converting the provided analyte into a mixture of gaseous ions. Different ionization techniques may be more appropriate for different types of samples, depending on their chemical and physical properties. Further complicating the choice of an ionizer is the fact that MS instruments are often coupled to other analytical methods such as ultra-high liquid chromatography (UHPLC) instruments. [7] The ions produced by the ionizer must be compatible with the coupled instrument. [8]
A magnetic field or electric field then moves the gaseous ions to the mass analyzer, which is where the separation by mass-to-charge ratios occurs. Finally, the ions move to the detector, which measures the charge or current produced by an ion entering the detector. Ultimately, a MS instrument produces graphs called mass spectrums, which plots the abundance of the ion against its mass-to-charge ratio. [7]
F. W. Aston is credited for engineering MS instruments capable of determining if an element was polyisotopic or monoisotopic. Using his instruments, Aston was also able to quantify the amounts of isotopic species present for polyisotopic elements. His work provided important insights into nuclear stability and the nature of atomic masses that have contributed to our fundamental understanding of chemistry today. [1,5]
However, Aston's instruments could only provide relative measurements of mass-to-charge ratios. In 1950, Nier developed a way to calibrate mass spectrometers so that MS could provide absolute measurements of abundance. [1,5]
Various methods exist for determining the half-lives of double beta decay processes, including measurement of the radioactivity. However, the incredibly long half-lives complicate radioactive counting by increasing experimental noise and limiting accuracy. In this regard, MS is a superior method, as it directly quantifies the product of double beta decay. Though not without its disadvantages, measurement by MS may provide a more accurate estimation of half-life. MS can also determine the half-lives of other long processes, such as the decay of geochronometers. [1]
With its excellent sensitivity and ability to provide absolute measurements of abundance, MS has many different applications in nuclear physics beyond the two described above. [3] These include: [1,2]
Measuring fission products.
Studying neutron-capture processes.
Comparing isotope samples to a reference standard.
Identifying an upper limit for the maximum concentration of superheavy elements.
Searching for strange matter.
© Victoria Nguyen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. De Laeter, "Nuclear Applications of Inorganic Mass Spectrometry," Mass Spectrom. Rev. 29, 845 (2010).
[2] W. Kutschera, "Applications of Accelerator Mass Spectrometry," Int. J. Mass. Spectrom. 349-350, 203 (2013).
[3] H. W. Wilson et al., "A Two Stage Mass Spectrometer for Nuclear Physics Applications," Nucl. Instrum. Methods 3, 269 (1961).
[4] Nobel Lectures, Physics, 1901-1921 (Elsevier, 1967).
[5] J. Griffiths, "A Brief History of Mass Spectrometry," Anal. Chem. 80, 5678 (2008).
[6] P. Ahlberg, Ed., Nobel Lectures in Chemistry, 2001-2005 (World Scientific, 2008).
[7] E. de Hoffmann, "Mass Spectrometry," Kirk-Othmer Encyclopedia of Chemical Technology, 15 Oct 05.
[8] J. L. Tymoczko, J. M. Berg, and L. Stryer, Biochemistry: A Short Course, 3rd Ed. (W. H. Freeman, 2015).
