High Efficiency Cavity Source (HECS) To Improve Sensitivies For Thermal Ionization Mass Spectrometry

Researchers
Lee Riciputi (PI)
Peter Todd (PI)
Eddy McBay

Introduction
Due to its sensitivity, precision, and accuracy, thermal ionization mass spectrometry (TIMS) has been the mainstay of isotopic measurement of uranium and plutonium almost since the inception of the nuclear age 50 years ago, as well as being widely applied for geological age-dating, environmental studies, and even nutritional fields. However, there are increasing needs for precise and accurate analysis of trace and ultra-trace quantities of materials in a variety of disciplines. Because the amount of material to be analyzed is so small, the efficiency at which ions are formed, transported and detected in the mass spectrometer are critical. Unfortunately, TIMS ionization efficiencies for many elements are quite low, limiting the ability to analyze small samples. For uranium, with an ionization efficiency of at best around 0.2%, an astounding 98.8% or more of the available sample is never detected!

Analytical Instrumentation
To improve TIMS measurements, the ionization efficiency of the source must be enhanced. The typical thermal ionization source consists of a filament, not unlike that found in a light bulb. Like a light bulb, when a current is passed through the filament, it heats up. As the filament heats white hot (1600 °C and above), the sample begins to evaporate, and some of the atoms in contact with the surface are ionized. Since an atom will only ionize if it is in contact with a hot surface, two of the important factors in the efficiency of ionization are a high temperature and a degree of surface contact. Unfortunately at high temperatures the sample begins to evaporate more quickly than it ionizes, and thus many evaporated atoms are lost.

ORNL is replacing the filament with a cavity, a metal rod with a narrow channel bored into one end.  The cavity is much more robust than the thin metal ribbon of a normal TIMS filament, and can be heated to much higher temperatures.  Compared with a filament, the cavity also has a restricted volume and a dramatically increased surface-to-volume ratio.  A sample placed at the bottom of the cavity can be heated to high temperatures, and evaporated atoms have the potential to collide with the hot walls of the cavity and be ionized several times before exiting, providing for significantly improved ionization efficiencies.  Conceptually, this should lead to much higher ionization efficiencies – the trick is to actually make it work.

We have modified our ThermoElectron Triton TIMS instrument
for use with the HECS source. Our testing indicates that the cavity provides ionization efficiencies in excess of 10% achieved for several elements, with good precision and accuracy for isotopic analyses. We are continuing to refine the HECS source and characterize performance characteristics for a variety of samples.

TIMS Source Geometry

TIMS Source

The high efficiency cavity ion source during operation. Note the decreasing temperature gradient from the white-hot tip at left to cavity holder at right (dark cylindrical mass).