The measurement of isotopic ratios by inductively coupled plasma mass
spectrometry (ICP-MS) has the benefits of ionising all metallic
elements, simplifying sample preparation and reducing analysis time,
when compared with thermal ionisation mass spectrometry (TIMS).
However, the use of ICP-MS in isotopic ratio studies has been
somewhat restricted by Its failure to offer the precision and accuracy
required by a variety of applications. The precision achievable by ICPMS,
typically 0.2 to 0.3 % RSD, for isotopic ratios, has generally been
regarded as being primarily limited by instrumental instability.
An investigation of the sources of instrumental noise in ICP-MS has
been undertaken, utilising noise spectral analysis as a diagnostic md
Study of parametric variation upon noise production has identified the
methods by which modulation of the ion signal occurs Noise spectral
analysis has allowed an understanding of the limitations imposed upon
measurement precision by the various contributing noise sources to be
The key to improved measurement precision has been found to lie in
the development of data acquisition methods which allow the
predominant sources of instrumental noise to be effectively filtered
from the ion signal The methodology developed for sequential
measurement of isotopes, using a quadrupole mass analyser, to reduce
the deleterious influences of instrumental noise is discussed. Results
are given for isotopic ratio measurement which demonstrate that a
precision of approximately 0 05 % RSD can be attained
The factors which affect the accuracy of isotopic ratio measurement are
shown to be many and varied and depend to a large extent on the
particular Isotopes bemg studied Definition of an appropriate
measurement strategy for high accuracy isotope ratio measurement
involves consideration of all possible causes of bias and adoption of
methods for their elimination or correction. To facilitate this process a
protocol has been developed and subsequently applied to various
elements and instrument systems.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.