Multi-collector inductively coupled plasma - mass spectrometry (MC-ICP-MS) has gained substantial importance in isotopic analysis over the last two decades. In the beginning, MC-ICP-MS was almost solely deployed in geo- and cosmochemistry and in nuclear sciences and industry. Nowadays, many other scientific fields make use of the technique, which is mostly based on the high versatility of the ICP ion source and the high sample throughput in comparison to methods with equal or even slightly better precision, such as thermal ionization mass spectrometry (TIMS).
The major benefit of MC-ICP-MS is clearly the high ionization power of the ICP, compared to, e.g. that of thermal ionization. A major limitation of MC-ICP-MS is the omnipresent instrumental mass discrimination. It is the effect that light isotopes are discriminated against heavier isotopes during the measurement. The goal of this PhD research project was to identify and possibly quantify the major contributors to instrumental mass discrimination in MC-ICP-MS.
Commonly, instrumental mass discrimination is attributed to space-charge effects. Even though this is an easy to comprehend effect at first glance, it becomes more complicated once studied more closely. Firstly, space-charge effects are present in charged particle beams only. Secondly, space-charge effects are most severe for low energetic particle beams with high current. Certainly, the space-charge effects are not the only contributors to mass discrimination. Several other contributors have been identified in the past, namely: collisions, sample introduction and ion formation and energy-selective ion transmission.
The effect of the above mentioned processes in terms of mass discrimination were investigated by several strategies. The processes occurring during the ion beam formation were addressed by the Direct Simulation Monte Carlo method. Due to the nature of the ion source, the plasma is extracted from ambient pressure into the vacuum of the mass spectrometer; leading to drastically reduced fluid density. Yet, sufficient collisions between particles take place to possibly contribute to mass discrimination. The modeling results show a significant alteration of the fluid composition after the skimmer cone. Also a radial fractionation of the fluid was found.
The ion beam is formed shortly after the plasma is extracted through the skimmer cone, the electrons are lost; a process known as charge-separation. During this phase, the space-charge effects are strongest. Thus a radial dependence of the isotopic composition of the ion beam might occur. This particular effect was investigated by two experiments, one comprising of ion implantation for the subsequent determination of the radial composition of the ion beam, the second experiment with a variable aperture addressing the shortcomings of the ion implantation and provide reliable \emph{in situ} information about the beam composition and diameter. These beam diameters are in contradiction to those expected from typically reported ion beam current. In order to measure the gross beam current, a Faraday cup was placed after the first ion lens of the mass spectrometer. The results reveal a much lower ion current than reported in literature, but are in reasonable agreement with estimations by the Child-Langmuir law for space-charge limited beams.
Finally, it has to be pointed out that no dominant contributor to the mass discrimination could be identified. However, the energy-selective transmission can be excluded from the list of contributors, given the low ion beam current with the associated quasi complete beam transport. Since sample introduction a priory can be ruled out, only two contributors remain: collisions and space-charge effects. Both contributors can hardly be separated from one another experimentally, i.e., a higher throughput through the interface will lead to more collisions in the interface and consequently, to a higher ion beam current after charge-separation. Yet, the isolated treatment of both effects in computer simulations might provide a tool to solve this problem. Of course, the same simulator would need to have the capability to model both effects simultaneously, as well as separately, which is not yet possible
