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The Australian National University

Operation - 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).

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

 

Updated:  24 October 2011/Responsible Officer:  SHRIMP Group Leader /Page Contact:  WebAdmin