SHRIMP principles

Quick summary

SHRIMP uses a focused high-energy ion beam to sputter samples. Some of the material eroded from the surface becomes ionized and the secondary ions are extracted electrostatically into a mass spectrometer. The SHRIMP mass spectrometer has high mass resolving power to separate atomic and molecular ionic species. The large mass spectrometer maintains high sensitivity for the analysis of trace element ions in a few nanograms of material.  SHRIMP instruments can be changed through different configurations, such as changing polarity and sources to maximize sensitivity for ion emission.

Mass spectrometry

The principle purpose of a mass spectrometer is to disperse ions according to their mass to charge ratio. To first order, this is accomplished by an ion moving through a magnetic field. Specifically, an ion moving at velocity v through a magnetic field B will experience a force at right angles to both velocity and magnetic field (direction defined by right-hand rule). A constant force acting at right angles to velocity therefore results in the ions moving in a circular motion, with the radius a function of both mass and velocity (for a constant magnetic field).

In isotope-ratio mass spectrometry, the velocity characteristics of the ion beam are immaterial to the measurements. In secondary ion mass spectrometry, a large velocity spread is a characteristic and if not corrected, would cause degradation of peak shape, especially at the high mass resolving power required for SHRIMP analysis. The velocity spread is removed with double-focusing mass spectrometers. These mass spectrometers include an electrostatic sector (Electrostatic Analyzer or ESA) which compensates for the velocity spread produced by the magnet. The ESA consists of parallel plates held at specific potentials so that ions with different energy will travel different pathways, but refocus at the collector slit.

A feature of the SHRIMP mass spectrometers is the Matsuda lens, a quadrupole lens located between the magnet and ESA, that matches the ion optical emittance to the acceptance allowing high transmission and minimal aberrations.

A sector magnet disperses ions according to their velocity, charge, and the magnetic field through which the ions travel. The directions of the parameters are bound by a right-hand rule such that the resultant force is orthogonal to the velocity and magnetic field. The force results in ions dispersing according to their masses along circular trajectories.

Secondary ion mass spectrometry

Secondary ion refers to the emission of an ion from a solid target that is under bombardment from primary ions. The primary ion beam requires an ion source, from which ions are accelerated to energies typically in the 10-20 keV range. This primary beam is used to sputter the target and a fraction of the sputtered material is ionized. These ions are accelerated away from the sample through a potential drop of 10 kV and form the secondary ion beam that is passed into the mass spectrometer.

On SHRIMP, the primary ion beam is angled at 45° to the sample surface with secondary ions extracted at 90° to the sample surface.

Sputtering involves the bombardment of a sample with an energetic primary ion beam (red). These atoms physically disrupt solid targets and cause atoms and molecules to be ejected. Some of these atoms and molecular fragments become ionized (secondary ions) and are extracted for analysis in the mass spectrometer.

Peak shapes

In mass spectrometry, the optimal peak shape for isotopic ratio measurements is trapezoidal (see figure). This peak shape is most appropriate because any small fluctuations in the magnetic field do not cause variations in the peak intensity. The trapezoidal parameters can be adjusted by changing the width of the source and collector slits. The FWHM (Full Width at Half Maximum) of the peak reflects the collector slit width, while the slope on the side of the peak reflects the image of the source slit through the mass spectrometer. On SHRIMP II the beam is demagnified with a magnification factor of 0.44 while on SHRIMP RG it is 0.33. Any beam spread beyond that attributable to the demagnified source slit image can be attributed to beam aberrations. The peaks can be transposed between field, mass and geometrical distance through the mass spectrometer dispersion equation. The mass resolution of the peaks is expressed at a specified peak height (e.g. at 10 % peak height) or at a specified valley height (e.g. 1%) between two equal height peaks. In addition abundance sensitivity can be expressed as a function of signal level to peak height at a specified mass difference from a peak (e.g. unity mass separation from U-238 or Ca-40 are commonly used).

Trapezoidal peak shape is defined by the collector slit (c) and demagnified source slit (s) widths. Mass resolution is defined as M/ΔM

Primary beams

The primary beams most commonly used in SHRIMP are composed of oxygen and cesium ions. These primary species are used because they produce a chemical enhancement of ion yields. The electropositive Cs enhances yields of electronegative elements like O and S, while electronegative oxygen enhances yields of metals. The most efficient setup is to use a negative-ion primary beam [O] to sputter positive secondaries, and a positive ion beam [Cs] to sputter negative secondaries.

Oxygen ions are produced in a duoplasmatron ion source. This source is based around an electric glow discharge between a hollow cathode and an anode plate. The electrons in the discharge attach themselves to oxygen atoms and molecules to form the negative ion beams.

Cesium ions are produced in a Cs gun. A Cs-bearing charge is heated to evaporate the cesium which is then passed through a superheated frit, which causes electrons to be lost from the atom and hence ionizes the cesium.

Kimball Physics Inc. Cs gun ion source

Duo Plasmatron

Secondary ion mass spectrometry

Secondary ion refers to the emission of an ion from a solid target that is under bombardment from primary ions. The primary ion beam requires an ion source, from which ions are accelerated to energies typically in the 10-20 keV range. This primary beam is used to sputter the target and a fraction of the sputtered material is ionized. These ions are accelerated away from the sample through a potential drop of 10 kV and form the secondary ion beam that is passed into the mass spectrometer.

On SHRIMP, the primary ion beam is angled at 45° to the sample surface with secondary ions extracted at 90° to the sample surface.

Mass spectrum of mass 206 from zircon showing the Pb-206 peak and other peaks associated with molecular fragments.