## 5 Particle Detectors
### Mass Spectrometry
#### 5.2.2 Tims  
Thermal ionization mass spectrometry (TIMS) is a highly sensitive mass spectrometric technique able to perform very precise isotope ratio measurements despite a time-consuming sample preparation and analysis. Its main application is the measurement of isotope ratios of stable isotopes and radioisotopes, especially for heavy elements such as U, Sr, Th or Pb, which vary within small ranges and thus require high-sensitivity, precise and accurate measurements. Precision values of 0.005% or better are routinely measured. Thermal ionization is the process whereby ionization occurs on the surface of heated metal filaments (typical temperature in the range 1000-2000 K). They are usually made of refractory metals with relatively high work functions and high melting points; due to the necessary purity and mechanical properties Pt, Rh, Ta or W filaments are the most used. All elements with first ionization potentials below 7 eV may be efficiently evaporated and ionized, otherwise special ion-emitting reagents such as silica gel and H\({}_{3}\)PO\({}_{4}\) and/or H\({}_{3}\)BO\({}_{3}\) are used to make the formation of ions possible, in order that an enhancement  
Figure 15: Scheme of Element 2 high-resolution double-focusing ICP-MS (Thermo Fischer Scientific). of ionization by at least one order of magnitude is achieved. Both positive and negative thermal ionization can be applied. The ion source operates in high-vacuum conditions and it consists of the filaments, the acceleration voltage supply and the focusing plates. The ion source is separated by the analyzer flight tube and the detector units by means of the isolation valve to ensure a continuous high vacuum in the analyzer during the installation of the sample holder. The thermal surface ions formed in the ion source, characterized by low initial energies of approximately 0.1-0.2 eV, are accelerated towards the single magnetic sector field mass spectrometer using the extraction electrode together with the beam-focusing electrostatic lenses. Subsequently, accelerated ions enter the magnetic field through the flight tube and they are separated according to their mass to charge ratios. Along the focal plane, whereon ions of each m/z ratio are focused, multiple Faraday cups are arrayed in order to measure all isotopes of interest simultaneously. The flight tube and the detector units are kept under vacuum by means of two ion getter pumps. Each Faraday cup, connected to an amplifier, collects its own isotope and the resulting current measured is directly proportional to the number of ions hitting the cup. As TIMS makes use of multiple detectors, it can simultaneously measure different isotopes, which is not possible for example with HR-ICP-MS. In addition, TIMS uses Faraday detectors, in contrast to single-collector ICP-MS, typically fitted with a single secondary electron multiplier (SEM) detector. SEM detectors have worse long-term stability, compared to Faraday detectors, and also suffer from other issues, such as dead time, which can heavily affect measurements (e.g., when used to measure ratios for elements with very different isotopic abundances [265]). The strengths of the TIMS compared to ICP-MS include its high sensitivities for elements that ionize easily (in this case TIMS is able to analyze smaller sample size without affecting precision, for instance an isotope ratio precisions better than 0.1% can be obtained for Pb with few pg sample), the limited energy spread of the ions produced, the relatively low production of molecular interferences and doubly charged ions. In addition, the cross contamination from sample to sample is negligible, thus the limiting factor comes from potential contaminants introduced during the chemical separation of the element of interest from the matrix. On the other hand, the main disadvantages of TIMS include the need for extensive sample preparation prior to analysis, very low sensitivities for elements that do not thermally ionize efficiently, and particularly, the isotopic mass fractionation.