Mass abundance sensitivity

Figure 8. Schematic outline of a second-generation MC-ICPMS instrument (Nu Instalments Nu Plasma), equipped with a multiple-Faraday collector block for the simultaneous measurement of up to 12 ion beams, and three electron multipliers (one operating at high-abundance sensitivity) for simultaneous low-intensity isotope measurement. This instmment uses zoom optics to obtain the required mass dispersion and peak coincidences in place of motorized detector carriers. [Used with permission of Nu Instruments Ltd.]...

Neder H, Heusser G, Laubenstein M (2000) Low-level y-ray germanium-spectrometer to measure veiy low primordial radionuclide concentrations. ApplRadiat Isot 53 191-195 Palacz ZA, Freedman PA, Walder AJ (1992) Thorium isotope ratio measurements at high abundance sensitivity using a VG 54-30, an energy-filtered thermal ionization mass spectrometer. Chem Geol 101 157-165... 

For a nuclide of mass M, abundance sensitivity is the ratio between the signal at mass M+ arising from the same species to the signal at mass M. Off-peak ions are present because of collisions behind the magnetic filter, of reflections on the tube wall, or of space-charge effects. As a result of the collisions, the energy of these ions is different from the energy of the main beam. They alter the apparent peak baseline in a continuous way. Abundance sensitivity decreases with the mean free path of ions, i.e., when pressure near the collector assembly... 

Normal gas-source mass spectrometers do not allow meaningful abundance measurements of these very rare species. However, if some demands on high abundance sensitivity, high precision, and high mass resolving power are met, John EUer and his group (e.g., Eiler and Schauble 2004 Affek and Eiler 2006 EUer 2007) have reported precise (<0. l%c) measurements of CO2 with mass 47 (A47-values) with an especially modified, but normal gas-source mass spectrometer. A47-values are defined as %o difference between the measured abundance of all molecules with mass 47 relative to the abundance of 47, expected for the stochastic distribution. 

By using non-normal beam incidence and curved pole pieces, the effective radius of curvature of the magnetic sector field is extended, which results in an increase in dispersion and an improvement in the abundance sensitivity of the mass spectrometric system.4... 

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 ... 

Figure 3.16 Abundance sensitivity Influence of peak tailing (fi) from the abundant ion peak of mass m on neighbouring ion peak at mass m +1 with low intensity lB.

Instrumental layouts and developments in AMS are reviewed by Kutschera.195 Today AMS is the most powerful, sensitive and selective mass spectrometric technique for measuring long-lived radionuclides at the level of natural isotopic abundances (10-16 to 10-12). Accelerator mass spectrometry (AMS) allows uranium isotope ratio measurements with an abundance sensitivity for 236U in the range of l(rlo-10 l2.l98J"... 

In order to reduce the instrumental background in mass spectrometers, new materials are required or an improvement to the vacuum system. New ion detectors with very low noise should be developed for the measurement of very low isotope ratios. Multi-stage MS-MS instruments could be applied to improve abundance sensitivity. 

Isotope Dilution By Thermal Emission Mass Spectrometry. A three-stage thermal emission mass spectrometer (TEMS) was used for quantitatively measuring lead and uranium in coal and fly ash and lead in gasoline (Figure 3). The basic design of the instrument is modeled on that developed by White and Collins, 1954 ( 6) and modified at ORNL. The addition of an electrostatic third stage increased the abundance sensitivity to 108 as described by Smith et al. (7). 

High Abundance Sensitivity >Mass Range 1 - 240 >High Mass Fescluti on... 

Figure 10 Three-stage sector mass spectrometer with high-abundance sensitivity.

Figure 1 Outline of the multiple-collector inductively coupled plasma mass spectrometer (VG Elemental Plasma 54) at the University of Michigan (U-M). The U-M instrument is equipped with an extra 30-cm energy filter for high-abundance sensitivity measurements. (From Halliday et al., 1998b.)...

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