- Thermal ionization
In thermal ionization, also referred to as surface ionization, chemically-purified material loaded onto a filament which is then heated to cause some of the material to be
ionized as it boils off the hot filament. Filaments are generally flat pieces of metal around 1-2mm wide, 0.1mm thick, bent into an upside-down U shape and welded to steel posts that supply a current.
The likelihood of ionisation is a function of the filament temperature, the
work functionof the filament substrate and the ionization energyof the element.
This is summarised in the Saha-Langmuir equation: cite journal|title=The Saha-Langmuir Equation and its Application|journal=Journal of Applied Physics|date= January 1968|first=M. J.|last=Dresser|coauthors=|volume=39|issue=1|pages=338–339 |doi= 10.1063/1.1655755|url=http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JAPIAU000039000001000338000001&idtype=cvips&prog=normal|format=PDF|accessdate=2007-10-11 ]
: :: = ion to neutral ratio
:: = statistical weights of ion and neutral states
:: = surface
::IP = element
::T = surface temperature
Thermal ionization mass spectrometry
One application of thermal ionization is thermal ionization mass spectrometry (TIMS). This method is widely used in
radiometric dating, where the sample is ionized under vacuum. The ions being produced at the filament are focussed into an ion beam and then passed through a magnetic field to separate them by mass. The relative abundances of different isotopes can then be measured, yielding isotope ratios.When these isotope ratios are measured by TIMS, mass-dependent fractionation occurs as species are emitted by the hot filament. Fractionation occurs due to the excitation of the sample and therefore must be corrected for accurate measurement of the isotope ratio. Dickin, A.P., 2005. Radiogenic Isotope Geology 2nd ed. Cambridge: Cambridge University Press. pp. 21-22 ]
There are several advantages of the TIMS method. It has a simple design, is less expensive than other mass spectrometres, and produces stable ion emissions. It requires a stable power supply, and is suitable for species with a low ionization potential, such as
Strontium(Sr), and Lead(Pb).
The disadvantages of this method stem from the maximum temperature achieved in thermal ioization. The hot filament reaches a temperature of less than 2500 degrees Celsius, leding to the inability to create atomic ions of species with a high ionization potential, such as
Osmium(Os), and Tungsten(Hf-W). Although the TIMS method can create molecular ions instead in this case, species with high ionization potential can be analyzed more effectively with MC-ICP-MS.
Megh Nad Saha
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