A study of isotope ratio measurement by inductively coupled plasma mass spectrometry

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 established 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.