## 5 Particle Detectors
### Mass Spectrometry
#### 5.2.1 HR-ICP-MS  
High-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) is considered the most versatile technique for fast, sensitive, and multi-elemental analysis at trace and ultra-trace levels [254]. The inductively coupled plasma (ICP) is an effective ion source for elemental mass spectrometry operating at atmospheric pressure [255]. Normally, all commercially available instruments operate with Ar plasma obtained applying a radiofrequency of 27 MHz and a plasma temperature of approximately 8000 K. This setup enables the decomposition of the chemical compounds contained in the sample solution into their atomic constituents that are immediately and efficiently ionized in the plasma. In general, all elements with a first ionization potential below 8 eV are ionized more than 90%, while those with a first ionization potential between 8 and 12 eV are ionized by more than 10%. The ICP involves a 'hard' ionization because most molecules in the sample are completely atomized [256]. During the ionization process singly and doubly charged atomic ions as well as polyatomic ions are formed in the ion source. So, in addition to the singly charged atomic ions of the analyte of interest, other atomic and molecular ions are formed, thus resulting in potentially interfering species. Although quadrupole-based ICP-MS equipped with a collision/reaction cell is the most used worldwide, the development of high-resolution mass spectrometers, such as double-focusing sector field ICP-MS with single or multiple ion collectors, has been necessary to overcome limitations caused by spectral interferences (isobaric overlap, polyatomic ion interferences and double charged ions). By using an HR-ICP-MS operating in the high resolution mode (m/\(\Delta\)m = 10000), mosttarget elements may be resolved from polyatomic interferences. However, the enhanced mass resolution results in a reduced ion transmission and, therefore, the improvement in selectivity comes along with a loss of sensitivity [257]. A typical HR-ICP-MS system is as follows: the sample, previously digested, thus having an acidic aqueous solution, is aspirated by a peristaltic pump, converted to an aerosol by a nebulizer in a cyclonic spray chamber and transported to the plasma torch through an Ar gas stream. Here, desolvation, vaporization, atomization and subsequent ionization occur. The positively charged ions are extracted from the plasma at atmospheric pressure and focused into the high vacuum of the double-focusing analyzer via the ion transfer optics, which accelerates sampled ions to 8 keV kinetic energy, while shaping and focusing the ion beam on the entrance slit. The system is designed for maximum ion transmission efficiency, stability, low mass bias and low background. Among all available geometries coming from the different exploited combination of the magnetic with the electrostatic sector, Mattauch-Herzog and Nier-Johnson geometries are the most important to mention here [258]. The combination of the magnetic and electrostatic fields results in the high-resolution properties of the HR-ICP-MS. These instruments can operate at a mass resolution of approximately m/\(\Delta\)m = 10000, sufficient to resolve many interferences. Within INFN-CHNet, a HR-ICP-MS setup is installed at INFN-LNGS, based on a Thermo Element 2 double-focusing sector field, with a magnet designed for maximum speed measurements. Less than 150 ms is required for a \({}^{7}\)Li-\({}^{238}\)U-\({}^{7}\)Li switch. The magnet coils are water cooled, for maximum mass stability. Element 2 combines a small magnetic sector with a radius of 16 cm and a 10.5 cm radius toroidal electrostatic sector in a reverse Nier-Johnson geometry. The magnetic sector field is employed for a directional focusing and the electrostatic one for energy focusing of ion beams respectively [259; 260].