RIMS method

Principle

For examination of the rarest isotopes, so called ultra-trace analysis, the wanted isotopic species has to be separated from the up to 20 orders of magnitude more abundant isotopes and other elements occurring in a sample (see introduction). Therefore we apply the technique of Resonance Ionization Mass Spectrometry, short RIMS. This technique can be divided into three steps: the atomization of the sample, the selective resonant ionization and the following mass separation of the produced ions.

RIMS scheme - click for big version

1. step: atomization

Atomization means evaporation of the sample using an electrothermally heated oven in which the sample has been inserted. At typical temperatures of up to 2000 K the sample is chemically atomized. Because of the design of the oven, the atoms evaporate in a well collimated beam.

2. step: resonance ionization

The most typical distinguishing atomic physical criterion of an element are its energy levels. If only one elemental species has to be excited selectively, only photons of this wavelength shall interact with the atom, whose energy transfer match the energetic gap between two characteristic atomic levels. The narrower the energy distribution, or in other words: the narrower the spectral distribution of the light source, the more selective the excitation of the atoms. Furthermore the selectivity of the excitation of atoms can be increased by not only resonant excitation in one step but by applying this distinguishing criterion several times: two-step, three-step etc. resonant excitation. If the spectral distribution in all of the excitation steps is even narrower than the isotopic shift, one can even separate not only elements but also isotopes. Only these resonantly excited atoms can be ionized by electric fields or with far infrared photons. By applying this technique one generates selectively one ion species, the so called resonance ions or laser ions.

3. step: mass separation

However, for an even more distinct selection and for better separation of resonance ions from other ions, e.g. from other isotopes, one focuses the ions into a mass spectrometer, in which only ions of the wanted mass can be transmitted and counted on a detector.


Realization

The interested sample, in most cases in nitric acid solution, is brought into an graphite oven where it is electro-thermally heated up to 2000 K. The collimated beam is then perpendicular overlapped with three laser beams.

diode laser hardware
- click for big version -

For selective excitation we use three partially self constructed and partially commercial diode lasers (Toptica DL 100). Together with a reference Helium-Neon-laser partial beams of the diode laser are coupled into an Fabry Perot interferometer where we apply the technique of fringe-offset-locking for the laser frequency control. The desired stability of the laser system is about 5 MHz /h. Depending on the investigated element, the wanted isotopes are excited into an autoionizing state or a high lying rydberg-state from which they can be non-resonantly ionized into the continuum with a CO2 laser operating at a wavelength of 10.6 μm.

The laser ions are focused with an optimized ion optics array into a commercial quadrupole mass filter (ABB EXTREL). At an oscillator frequency of 2.9 MHz a neighbouring mass suppression of 1E+8 has been experimentally demonstrated in Ca-41 in very well agreement with our results from simulations. After transmission through the mass filter the ions are detected on an off-axis channeltron.

technical drawing of the QMS - click for bigger version

The detuning and control of the laser wavelengths, the mass filter control as well as the data acquisition and display is handled by a self-made software on a standard PC.