Mass Resolution and Abundance Sensitivity

Because slits in the range 0.1 to 1mm width are used in mass spectrometers, the separated ion beams have a defined breath b close to the slits. The capability of the mass spectrometer for separating ion beams with different masses m and m+Ara is characterized by the mass resolution R (or resolving power) using the following definition  

A second important property of mass spectrometric separation systems is the abundance sensitivity. Even under sufficient vacuum conditions there is a scattering of ions in the beam by the residual gas. Furthermore, by scattering of particles by the wall of the analyzer tube or by electrostatic repulsion in the ion beam itself or charging effects, so-called peak tails in mass spectra are observed, which result in an increasing energy spread of ions and consequently in a deterioration in the abundance sensitivity. The abundance sensitivity of a mass spectrometer is defined as  

Dietze, H.J., Massenspektroskopische Spurenanalyse, Akademischer Verlagsgesellschaft Geest Portig 

De Laeter, J. R., Application of Inorganic Mass Spectrometry, Wiley-Interscience Series on Mass Spectrometry, John Wiley Sons, Inc. New York, (2001). 

and Hintenberger, H., Methoden und Anwendungen der Massenspektroskopie, Verlag Chemie, Weinheim (1953). 

The abundance sensitivity can also be considered for ions beams with a mass difference of two mass units. For example, the ultrasensitive measurement of is limited by peak tailing from the major uranium isotope and °Sr detection at the ultratrace level is difficult due to peak 

In double-focusing sector field ICP-MS spectrometers with single ion detection, the best abundance sensitivity with a mass difference of two is observed at medium mass resolution m/Am = 4400 (9 x 10 ). At low mass resolution mlAm = 400) an abnndance sensitivity for °Sr determination of 2 x 10 was measured in the author s laboratory. In quadrupole ICP-MS with and without a hexapole coUision cell, an abundance sensitivity of 6 x 10 and 6 x 10 , respectively, for determination in the presence of abundant with high ion intensity was determined by Boulyga et al. ° A significant improvement in abundance sensitivity for the determination of the isotope ratio has been obtained by multi-collector ICP-MS (Nu Plasma 

Dietze, H.J., Massenspektroskopische Spurenanalyse, Akademischer Verlagsgesellschaft Geest Portig K.-G., Leipzig (1975). 

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A larger radius leads to higher dispersion of the masses (e.g., more separation between adjacent masses). High dispersion leads to better mass resolution and abundance sensitivity and enables the use of larger detectors and ion sources,... 

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