Abundance sensitivity

Before discussing the basic operating principles of those mass spectrometer types used in ICP-MS instrumentation, some important characteristics need to be introduced mass resolution, abundance sensitivity, mass range, and scanning speed [8]. 

Although very important in organic MS, this aspect of a mass spectrometer is of lower relevance in ICP-MS, as the heaviest nuclide occurring in Nature is As a result, a mass range of 0-250 u allows all signals of interest to be monitored. 

The speed with which the mass spectrometer can scan (a part of) the mass spectrum or can switch from monitoring one nuclide to monitoring another is also important, especially in the context of isotopic analysis. 

Continuous scanning of (a part of) the mass spectrum or peak hopping or peak jumping is accomplished by changing these voltages in such a way that the U/V ratio remains constant. The spectral peak width is constant over the mass range (mass resolution, on the other hand, varies as a function of the mass considered). 

Double-Focusing Sector Field Mass Spectrometer 

It can be seen in Fignre 7.7 that the tail of the spectral peaks drops off more rapidly at the high-mass end of the peak compared to the low-mass end. The overall peak shape, particularly its low-mass and high-mass tail, is determined by the abundance sensitivity of the quadrupole, which is impacted by a combination of factors, including 

FIGURE 7.8 Ions entering the quadrupole are slowed down by the filtering process and produce peaks with a pronounced tail or shoulder at the low-mass end m - 1) compared to the high-mass end (m -h 1). 

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Figure 6. Schematic outline of the first commercially available multiple collector ICPMS, the Plasma 54, after Halhday et al. (1995). This instrument uses Nier-Johnson double-focusing and is equipped with eight independently adjustable Faraday collectors. The axial collector can be wound down to provide access to a Daly detector equipped with ion counting capabilities and a second-stage energy filter for high abundance sensitivity measurements. The sample may be introduced to the plasma source by either solution aspiration or laser ablation.

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... 

Correction for tails from nearby large peaks (for instruments with abundance sensitivities worse than 0.1 ppm)... 

Mg = 0.5 1.1%0). Note that the slope of the isochron is 8 percent greater here than in our previous publications (excepting [12]) due to a recently discovered error in the ion probe calibration of Mg abundance sensitivity. All of the isochrons discussed in this paper reflect this change. Similar data showing the same linear correlation for other Allende inclusions have been reported in [6,23,24,25]. 

Figure 7. Effect of abundance sensitivity on isotope ratio determination...

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... 

Nuclei Spin Natural abundance Sensitivity Relative Absolute Range Gyromagnetic ratio Frequency ... 

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 ... 

A schematic illustrating the abundance sensitivity is shown in Figure 3.16. Measurements of the... 

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"... 

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