Isotope isobaric interferences

Doubly-charged ions exist because the potential of second ionization of many metals is relatively low with respect to the plasma thermal energy. For instance, 11.9 eV are needed to remove two electrons, in contrast with 6.1 eV for one electron, from a calcium atom. At 8000K, a little less than 0.1% of Ca would be in the Ca + form and overlap with Mg+ isotopes. Likewise, Ba is rather easily formed and overlaps with Zn+ isotopes. Isobaric interferences with doubly-charged ions are easily identified as odd-mass atoms will produce peaks at half masses, such as Ca at mass 21.5. 

Isobaric interferences (especially those arising from the plasma itself, e.g., ArO+ on Fe) can be eliminated using cool-plasma conditions, sometimes in combination with a shield torch. This option is not suitable for seawater samples because a cool plasma, in the presence of a heavy matrix, cannot fully ionize elements with high first ionization potentials, notably Zn, Cd, and Hg. Protocols have thus been established for analysis of 10-fold diluted seawater on instalments with sufficiently high resolution to separate most of the affected isotopes from their isobaric interferences [1], To circumvent the issue entirely, others have used online chemical extraction to separate analytes of interest... 

Matrix effects are typically divided into spectral (isobaric) and non-spectral types. The spectral or isobaric effects include 1) elemental isobaric interferences such as Cr at " Fe, 2) molecular interferences such as Ca O at Fe and Ar N at Fe, 3) double charge interferences such as Ca at Mg. Non-spectral matrix effects are largely associated with changes in the sensitivity of an analyte due to the presence of other elements (Olivares and Houk 1986). Changes in sensitivity correspond to a change in instrumental mass bias, and therefore non-spectral matrix effects can have a significant impact on the accuracy of isotope measurements. 

The choice of the reference reservoir for calculating 5 or e values is quite variable among different laboratories. Table 1 lists all the laboratories that have published Fe isotope data in an abstract or peer-reviewed journal as of September 2003, and include the form in which the data are reported (e.g., 5 Fe or 5 Fe), as well as the reservoir or standard used to define 5 or s values. A common procedure of all labs currently conducting Fe isotope studies is to measure at least three Fe isotopes, which is done to provide a check on data quality, primarily evaluation of potential isobaric interferences. The choice of reference reservoir used in this chapter follows the approach of other stable isotope systems such as oxygen, which defines... 

The first modem observations of Mo isotope fractionations in geological materials were made using this method (Barling et al. 2001), employing Zr and Ru spikes with online correction for isobaric interferences from these spikes (Anbar et al. 2001). Precision of 0.25%o was reported in measurement of 8 Mo, with comparable results for other ratios. 

Resonance-ionization mass-spectrometry is still in the development stage in terms of its application to cosmochemistry. The Charisma instrument, which is operated by Argonne National Laboratories, uses multiple lasers to resonantly ionize only the elements of interest, which are then analyzed with a time-of-flight mass spectrometer. The Charisma instrument has made isotopic measurements of molybdenum, zirconium, strontium, ruthenium, barium and other elements in presolar grains. These measurements are made possible by the high ionization efficiency of the RIMS technique and its ability to completely eliminate isobaric interferences. Work is now underway to build a RIMS instrument that can be operated by an individual investigator in a university laboratory. If this succeeds, RIMS will play an increasing role in analysis of extraterrestrial materials. 

Instrumental developments (e.g., of sector field instruments with multiple ion collection, introduced in 1992, or the insertion of collision and reaction cells in order to reduce disturbing isobaric interferences), the progress in applications for ultratrace analysis, also in combination with on line hyphenated separation techniques (HPLC, CE), especially routine capability as well as decreasing price and user friendly maintenance mean that sales are increasing by 10 % every year. To improve the analytical performance of ICP mass spectrometers for precise isotope ratio measurements (e.g., for geochronology or for the study of fine isotope variation in nature) powerful instrumentation with high mass dispersion and multiple ion collector systems instead of single ion collection are commercially available on the analytical market. 

The figures of merit of quadrupole-based ICP-MS, such as the precision of isotope ratio measurements and the detection limits, can be improved significantly, especially for elements which are difficult to determine due to the appearance of isobaric interferences (e.g., by the trace, ultratrace and/or isotope ratio measurements of Ca, Fe, S, As, I or Se).16-22 The occurrence of interference problem can be minimized by the insertion of a collision/reaction cell in ICP-MS as the result of defined collision induced reactions using selected collision/reaction gases or gas mixtures (such as H2, He, NH3, 02, CH4 and others). For each analytical problem, which is different, e.g., for U or... 

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