Application of a portable quadrupole mass spectrometer to carbon isotope analysis

Abstract

Isotope analysis was present at the birth of mass spectrometry and has since found application in a diverse array of fields including biology, medicine, archaeology, geology, chronology, forensics and environmental science. Quadrupole mass spectrometry (QMS) revolutionised chemical analysis by obviating bulky magnets, thus enabling truly portable chemical sensors. The central focus of this thesis is to investigate the marriage of the two: application of quadrupoles to isotope analysis, This work takes a specific interest in the isotopes “carbon-13” (13C) and “carbon-14” (14C) because of their applicability to archaeology, a motivating factor behind the thesis. In the case of the trace abundance 14C, notable for its application to radiocarbon dating, a novel approach is explored in which the aim is to quantify the contribution of this species by analysing isotopologues of carbon dioxide. Such an approach requires a step change in portable QMS resolution without cost to transmission rates. Factors in this objective are investigated by simulation of both two-dimensional (2D) and three-dimensional (3D) QMF fields. A well-known formula for minimum achievable peak width (Δm) is revised through re-analysis of the empirical data, and evidence for the practical mass-dependence of minimum Δm is presented. The fundamental relevance of the ratio of the inscribed radius (r_0) to the source aperture radius (r_ie) to high-resolution aspirations is examined using a new simulation tool allowing rapid visual 2D analysis of multiple ion trajectories. Through combination with other known resolution-enhancing parameters, the theoretical possibility of extreme resolutions of the order of 10^4 and 10^5 is demonstrated; some aspects are tested experimentally. For 13C analysis, a key concern is measurement stability, such that instrumental error does not exceed analytical requirements. This is a fundamental requirement for all isotopic analysis, and so occupies most of the experimental work presented in this thesis. Experiments are presented investigating significant factors leading to improved measurement stability in portable QMS including single, dual and triple filter systems, having hyperbolic and circular profile electrodes. The work is conducted with specific attention to the δ^13 C figure, a relative measure of the 13C/12C ratio; a threshold of ≤±1‰ precision (1σ standard deviation) is set for useful application to archaeology. This threshold is exceeded for two of the four experimental setups. Optimisation of the transmission efficiency of the quadrupole mass filter (QMF) can not only extend resolution limits, but also improve measurement stability by maximising ion current at the detector. The QMF transmission efficiency is not constant in a given instrument, varying according to m/z. The nature of this variation is, in turn, strongly influenced by the gap from ion source to QMF electrodes (source gap). A series of simulations and experiments are presented which (a) confirm the high fidelity of the 3D simulation model, (b) characterise the transmission of a QMF according to m/z and (c) demonstrate how the source gap may be optimised to maximise transmission efficiency. Portable QMS shows strong potential for applicability to carbon isotope analysis. Trace radiocarbon analysis through CO2 isotopologues is a theoretical prospect which awaits advancements in areas including precision voltage control and ion guiding at the micro-level. Stable carbon isotope analysis is found to be a real and present possibility which only awaits further experimental optimisation and the development of a robust experimental protocol for in-situ application.