Thermal ionization mass spectrometry
Thermal ionization mass spectrometry, TIMS is a highly sensitive isotope mass spectrometry characterization technique. The isotopic ratios of radionuclides are used to get an accurate measurement for the elemental analysis of a sample [1]. Singly charged ions of the sample are formed by the thermal ionization effect. A chemically purified liquid sample is placed on a metal filament which is then heated to evaporate the solvent. The removal of an electron from the purified sample is consequently achieved by heating the filament enough to release an electron, which consequently ionizes the atoms of the sample [2]. TIMS utilizes a magnetic sector mass analyzer to separate the ions based on their mass to charge ratio. The ions gain velocity by an electrical potential gradient and are focused into a beam by electrostatic lenses. The ion beam then passes through the magnetic field of the electromagnet where it is partitioned into separate ion beams based on the ion's mass/charge ratio. These mass-resolved beams are directed into a detector where it is converted into voltage. The voltage detected is then used to calculate the isotopic ratio.[3]
Ionization source
The filaments used are made from tantalum (Ta), tungsten (W), platinum (Pt) or Rhenium (Re). Conventionally, there are two filaments used in TIMS. One filament is for the sample and is called the sample filament. The liquid sample is placed on the sample filament which is then evaporated to create ions. Subsequently, these ions land on the other filament, also known as the ionization filament. Here, the ion looses an electron by ionization.[4]
The single filament method is also possible. Once the sample evaporates, the ions can settle back down onto the same filament to get ionized.[4] The use of a triple filament or multifilament set-up improves ionization efficiency and provides the rate of evaporation and ionization to be controlled separately. [4]
Thermal ionization mechanism
When the hot filament heats the liquid sample, the fermi levels within the sample reaches parity with that of the metal. In turn, this allows for an electron to tunnel from the sample to the metal filament. As a result, positive ions are formed from the sample that lost an electron. This transferring of electrons also result in the formation of negative ions. The production of ions is parameterized by the Saha ionization equation or the Saha-Langmuir equation. [4]
Isotope ratio measurement
The relative abundances of different isotopes are then used to describe the chemical fractionation of different isotopes, travel in different reservoirs of non-radiogenic isotopes, and age or origins of solar system objects by the presence of radiogenic daughter isotopes.[5][6]
Elemental analysis is a predominant application of TIMS as it gives reliable isotopic ratios. Following the trend of decreasing ionization energy, elements located towards the bottom left of the periodic table are viable for TIMS. In addition, the high electron affinity seen towards the upper right of the periodic table makes these nonmetals excellent candidates.[4] The technique is used extensively in isotope geochemistry, geochronology, and in cosmochemistry.[5][6]
quantitative isotope ratio techniques include;
isotope dilution thermal ionization mass spectrometry (ID-TIMS) [7]
chemical abrasion thermal ionization mass spectrometry (CA-TIMS) [8]
References
- ^ "ScienceDirect". www.sciencedirect.com.
- ^ A Handbook of Silicate Rock Analysis. ISBN 978-94-015-3990-6.
- ^ Constantinos A. Georgiou; Georgios P. Danezis. "chapter 3- Elemental and Isotopic Mass Spectrometry". In Pico, Yolanda (ed.). Advanced mass spectrometry for food safety and quality (68 ed.). p. 131-243. ISBN 978-0-444-63340-8.
- ^ a b c d e Dass, Chhabil. Fundamentals of contemporary mass spectrometry. Wiley-Interscience. pp. 264–265. ISBN 978-0-471-68229-5.
- ^ a b Lehto, J., X. Hou, 2011. Chemistry and Analysis of Radionuclides. Wiley-VCH.
- ^ a b Dickin, A.P., 2005. Radiogenic Isotope Geology 2nd ed. Cambridge: Cambridge University Press. pp. 21-22
- ^ Duan, Yixiang; Danen, Ray E.; Yan, Xiaomei; Steiner, Robert; Cuadrado, Juan; Wayne, David; Majidi, Vahid; Olivares, José A. (October 1999). "Characterization of an improved thermal ionization cavity source for mass spectrometry". Journal of the American Society for Mass Spectrometry. 10 (10): 1008–1015. doi:10.1016/S1044-0305(99)00065-3.
- ^ Mattinson, James M. (July 2005). "Zircon U–Pb chemical abrasion ("CA-TIMS") method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages". Chemical Geology. 220 (1–2): 47–66. doi:10.1016/j.chemgeo.2005.03.011.
| Ion source | |
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| Mass analyzer | |
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| MS combination | |
| Fragmentation | |
