## 5 Particle Detectors
### Mass Spectrometry
#### 5.2.1 HR-ICP-MS
A schematic of Element 2 high-resolution double-focusing ICP-MS is reported in Figure 15.  
The instrument is equipped with a guard electrode inserted between the torch and load coil in order to eliminate secondary discharge in the plasma and to decrease the ion energy spread, thus increasing ion transmission. This and the high acceleration voltage both contribute to the enhancement of the overall sensitivity. The guard electrode is particularly useful to perform cold plasma measurements. The computer controlled slit system of Element 2 consists of a single slit holder with three fixed slits. Therefore, according to the mass resolution settings and thus using different slit systems, measurements can be carried out at low mass resolution (m/\(\Delta\)m = 300), medium mass resolution (m/\(\Delta\)m = 4000) and/or high mass resolution (m/\(\Delta\)m = 10000) guaranteeing unambiguous elemental spectra. An electron multiplier is commonly used as a collector and detector of the ions. It consists of a series of electrodes in cascade. When an ion arrives on the first electrode, the emitted electrons hit the second electrode, which in turn emits a greater quantity of electrons and so on, resulting in a strong amplification of the signal. The signal is then digitized and organized in the mass spectrum. Element 2 is equipped with a discrete dynode detection system, enabling both trace- and major element quantification in a single analysis. The ion beam does not directly strike the detector to initiate the electron cascade. In Element 2, the secondary electron multiplier uses a conversion dynode at \(-8\) kV, which produces a uniform response across the mass range, operating in both the counting and analog modes according to the signal intensity. The detector switches from the counting to the analog mode. The high acceleration to \(-8\) kV results in a mass-independent detector response, enabling fully automatic cross calibration between the counting and analog modes. The automatic cross calibration ensures a constantly updated detector response, allowing the linear dynamic range to be extended to 10 orders of magnitude. The dark noise measured with this instrument is less than 0.2 cps and the sensitivity achieved in the low mass resolution mode is greater than \(10^{6}\) cps/ppb (injecting a solution containing indium at 1 ngml\({}^{-1}\) with a standard inlet system). This enables quantification from \(10^{-6}\) to less than \(10^{-15}\) (from ppm to sub-ppq concentrations). HR-ICP-MS has the advantage to provide a rapid multi-elemental analysis, in addition to a high sample throughput, excellent sensitivity, precision and accuracy. Several studies in the CH field have been performed on metals and alloys, such as coins and lead ingots, to investigate particular aspects concerning the coin production, to reconstruct mining activities or to discover the origin of the lead used in a specific unit as well as to identify the geographical origin of raw materials and ancient trade routes [261,262]. [PERSON] et al. analyzed a set of lead artifacts [263] using quadrupole-based ICP-MS to determine minor and trace element composition and lead isotope ratios to establish the geological origin of the raw materials and the metallurgical technique used on the lead production. The elemental compositions and lead isotope ratios have been used to assess the degree of standardization and the question of source of brass ingots by [PERSON] [264]. At INFN-LNGS, HR-ICP-MS has been applied to CH findings to determine major, minor, trace and ultra-trace elements, thus providing their elemental fingerprint. These data, together with Pb and/or Sr isotope ratios, provide a useful method to discriminate between samples of different origin.