Interferences isobaric

3 Isobaric Interferences A common issue encountered in SIMS is the presence of secondary ions from different elements and/or molecules that exhibit the same nominal m/q ratio. Such overlapping signals arise from what are otherwise referred to as isobaric interferences. More commonly, these are referred to as 

As noted in Section 5.1.1.1, isobaric interferences become more prevalent  

At higher m/q ratios (the number of possibilities increases quadratically as the secondary ion m/q ratio increases owing to the greater likelihood of molecular combinations of the lighter elements). 

When the spread in the m/q ratio of secondary ions increases (this arises from the fact that an increase in the spread increases the chance of overlap). 

When Static SIMS conditions are employed (this is due to the greater prevalence of molecnlar ions in Static SIMS mass spectra with respect to Dynamic SIMS mass spectra). 

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The optimal analytical GDMS instrument for bulk trace element analysis is the one providing the largest analytical signal with the lowest background signal, the fewest problems with isobaric interferences in the mass spectrum (e.g., the interference of with Fe ), and the least contamination from instrument com-... 

Figure 3.5 shows the positive SSIMS spectrum from a silicon wafer, illustrating both the allocation of peaks and potential isobaric problems. SSIMS reveals many impurities on the surface, particularly hydrocarbons, for which it is especially sensitive. The spectrum also demonstrates reduction of isobaric interference by high-mass resolution. For reasons discussed in Sect. 3.1.3, the peak heights cannot be taken to be directly proportional to the concentrations on the surface, and standards must be used to quantify trace elements. 

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. 

In addition to the potential problem of isobaric interference from CO+ (m/z 28) ions there are a number of additional factors that should be considered when attempting compound specific 815N analysis. First, the actual abundance of N in an organic compound is typically <10% (cf. C >60%), in a gas sample 1 molecule of N2 contains two N atoms (only 1 C in C02), the natural abundance of 15N is 0.732% (cf. 1.08% 13C) and N2 has an... 

LA offers several advantages over digestion/solution analyses such as 1) reduction in isobaric interferences, 2)... 

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... 

Rare-earth elements in minerals can be measured in situ by ion probe. It is observed that Gd oxide peaks overlap with Yb masses (isobaric interference). The... 

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. 

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. 

In the particular case we just discussed, the requirement of a <0.05%o bias necessitates that the Zn/ Zn ratio of the background be known for this particular run with a precision of 25%, which is a difficult goal to achieve on such a small signal. When isobaric interferences add up on the same masses, e.g., if minute traces of are present, accuracy and precision on the Zn/ Zn ratio may become disastrous. 

Higher mass resolution is becoming more common in MC-ICPMS technology. The result will be a reduction in the hindrances of isobaric interferences. With judicious use of narrow entrance slits and improvements in ion optics, even smaller radius instruments can resolve 50Ti from Mg+, for example. However, at this writing most studies have made use of low-mass-resolution instruments, and even with high mass resolution, care must still be taken to avoid changes in instrumental fractionation due the presence of elements other than the analyte in the plasma. 

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. 

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