Mass Spectrometry (MS)

Mass Spectrometry (MS) is a powerful analytical technique used for the measurement of isotope compositions to very high precision. The working principle of MS is based on the fact that charged atoms can be separated according to their different mass-to-charge ratio using a magnetic field separator. Though, there are several types of mass spectrometers; generally, they consist of three major components: a source (or ionization source), a mass analyzer (i.e., a magnet), and an ion collector. A simplified scheme of a mass spectrometer is shown in Figure 3.7.

3.3.4.1 Magnetic Sector Mass Spectrometer

At first, the sample is charged and these ions are accelerated by high voltage field through a potential gradient and collimated by slits. The charged ions are then deflected according to their mass-to-charge ratio in a strong magnetic field and are separated. This technique is sensitive only for positively charged ions. Negatively charged and uncharged ions and isobars (i.e., atoms with the same mass number but with different atomic numbers) are not treated using this technique. Instead, they collide with the wall of the tube and are pumped away. In the magnetic field, the trajectory of the ion is ideally circular. The radius of the trajectory is inversely related to the magnetic field strength of the spectrometer. The radius of the trajectory can be computed based on the equation resulting from the balance of the magnetic centripetal force and the centrifugal force:

m/z = r²H² / 2V

where m = mass of the charged ion, z = charge of the ion, r = radius, H = magnetic field strength, V = potential gradient of the trajectory. The mass spectrum can be scanned by varying either the magnetic field strength (more common) or the potential gradient of the spectrum. The resolution and efficiency of a mass spectrometer is determined by the ability how well it separates ions close in mass. Abundance sensitivity tells to which extent the tails of the peak at mass m contribute to neighboring peaks at masses (m-1) and (m+1).

Figure 3.7 Simplified diagram of the main components of a mass spectrometer (after Gill, 1997).

3.3.4.2 Sample Preparation and Analysis

Whole-rock sample digestion for Sm–Nd analysis was executed in Savillex^TM beakers using an ultrapure 1:5 mixture of HNO3 and HF for about two weeks at 100–150°C on a hot plate.

The acids are evaporated and the residue is further dissolved in a 5.8 NHCl. During cooling, one component of the sample solution constituting ~7–20% was separated from and spiked for Sm and Nd concentration identification by isotope dilution (ID) using a mixed REE tracer (¹⁴⁷Sm–¹⁵⁰Nd spike). Using AG^TM 50W-X8 (200–400 mesh; Bio-Rad) resin and 4.0 NHCl, the REE fraction was extracted and then Nd and Sm separation from the fraction was followed using Teflon-coated HdEHP, and 0.24 and 0.8 NHCl, respectively. Both Sm and Nd had a maximum total procedural blanks of <50 pg. Using HF/HNO3 (4:1) sample dissolution for Rb–Sr analysis carried out, and element separation followed conventional procedures. Both Rb and Sr had a total procedural blanks of <1 ng. Sm, Nd and Sr were treated as metals from a Re double filament, using a ThermoFinnigan^TM Triton TI TIMS (for IC) and a Finnigan^TM MAT262 (for ID), whereas Rb was evaporized using a Ta filament. An ⁸⁷Sr ⁄ ⁸⁶Sr ratio of 0.710241 ± 0.000002 (n = 18) and an ¹⁴³Nd⁄ ¹⁴⁴Nd ratio of 0.51185 ± 0.000001 (n = 38) were determined for the NBS987 (Sr) and the La Jolla (Nd) international standards, respectively, during the period of investigation. Within-run mass fractionation for Nd and Sr isotope compositions (IC) was corrected for relative to ¹⁴⁶Nd⁄ ¹⁴⁴Nd = 0.7219 and ⁸⁶Sr ⁄ ⁸⁸Sr = 0.1194 respectively. Detailed information on this analytical method can be found in: Thöni et al.

(2008).

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CHAPTER 4: KINEMATIC ANALYSIS OF THE NORTHERN AND