Isobaric ions 744 Subject

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

External standards, so named because they are not added to the sample, are also occasionally used but are generally only applicable to samples requiring limited preparation and for which a consistent high degree of reproducibility and good recovery can be attained. Experiments should also be completed as quickly as possible to minimize instrumental variations (e.g., ion source contamination). In brief, instrument response is plotted against the concentration (or amount) of standard analyzed and this response curve is then used to calculate analyte concentration (or amount). However, unless the matrix is well characterized, this method can be subject to matrix effects (ion suppression) and to interference from isobaric matrix components. 

Figure 3.5 Use of three-isotope plots to check for spectral interferences in MC-ICP-MS. Each point represents the mean of an isotope ratio measurement of a standard (filled circles) or a sample (empty circle) of natural isotopic composition. Isotope ratios are plotted on the delta scale (5) as relative deviations in parts per thousand from the known isotope ratio of an isotopic reference material of natural isotopic composition. The diagonal line represents the theoretical fractionation curve as defined by the isotopic masses and an exponential fractionation law. (a) Absence of isobaric interferences. Data points from standard and sample plot on the theoretical curve, (b) At least one isotopic signal in the mass spectrum of the standard and the sample is subject to spectral interference from an isobaric nuclide, polyatomic ion, or doubly charged ion. (c) Matrix differences between sample and standard result in an offset of the sample data points from the theoretical fractionation curve.

Neither spike nor reference isotope should be subject to spectral overlap due to the occurrence of isobaric nuclides, doubly charged ions, or polyatomic ions. 

Recently, selective ion molecule chemistry has been applied as an altemative approach to reducing or eliminating isobaric interferences in ICP-MS. This approach, chemical resolution MS, has been developed into a viable analytical method, and its current status is described in detail in Chapter 8 of this book. This approach uses ion-molecule reactions inside the mass spectrometer to remove interferences from selected mass-to-charge ratio detection channels of interest. More than 200 peer-reviewed papers have been published on this subject, more than 500 conference presentations given and several review articles have now appeared. Our contributions in this area began with PSIT studies that we will describe below. 

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