Isobaric interferences, in ICP

Fig. 3. Example ofthe constitution of isobaric interferences in ICP-MS. Shown is the situation with the low-resolution ICP-MS instrument for the detection of bromine.

D. R. Bandura, V. I. Baranov, S. D. Tanner, Reaction chemistry and collisional processes in multiple devices for resolving isobaric interferences in ICP-MS, Fresenius J. Anal. Chem., 370 (2001), 454D470. 

Table 2 Typical isobaric interferences in ICP-MS elemental analysis...

TABLE 17.5. Isobaric Interference in ICP/MS and mass resolution required to separate them... 

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. 

Reducing oxide based isobaric interferences in the ICP mass spectrum via gas flow modulation was proposed by Wetzel and Hieftje. After a careful manipulation of the central chaimel gas flow to impact distinguishable frequency specific behaviour of analyte and oxide ion species and application of a Fourier transform (FT) correction method, contributions from an analyte and oxide species superimposed at a given mass can be mathematically umavelled with a degree of success. Through application of this correction method, a greater than ten-fold error at m/z 156 caused by tlie interference of Ce 0 on Gd has been effectively eliminated. ... 

The major limitations to atomic MS at this point are the inefficient introduction system to the ICP (a common problem with the ICP), matrix effects, and isobaric interference. In... 

What procedures are followed to eliminate isobaric spectral interferences in ICP-MS ... 

The major limitations to atomic M8 at this point are the inefficient introduction system to the ICP (a common problem with the ICP), matrix effects, and isobaric interference. In addition, quadrupoles are limited in sensitivity for traee amounts of light elements such as 8, Cl, P, and 8i due to significant interferences from C-, N-, and 0-based ions. ICP-MS cannot measure H, N, O, C, or Ar because it is open to the atmosphere and the plasma is Ar-based. Fluorine cannot be measured because of its ionization potential. 

As a counterpart to the previous fundamental study, it should be stressed that, as indicated above for the lanthanides, the exploration of the differences in reactivity of metal cations has found direct applications in elemental MS with the generalized use of colUsion/reaction cells in ICP-MS for the resolution of isobaric interferences. In the case of the actinides, and as described in previous sections, differences in reaction efficiencies can provide large mass separations for such important isobaric nuclides as U/ Pu and Pu/ Am. The measured oxidation efficiency of with CH2O is at least 500 times greater than that of Pu so that can be oxidized to produce... 

One of the maj or difficulties in making high-precision measurements involves accurate correction for spectral and isobaric interferences. In MC-ICP-MS, making interfering element corrections (lECs) for these interferences is not completely straightforward and introduces an additional component of error that must be propagated into the final uncertainty. In the Hf isotope system Lu and Yb isobarically interfere with Hf and this provides a good example of the mechanism of lECs. 

The difference in ionic radii between isobaric isotopes of different elements is too small for KED to be practical for suppression of atomic isobaric interferences. In our view, the most universal method of interference suppression in ICP-MS is chemical resolution via ion-molecule reactions, since it can provide a way of resolving both atomic and polyatomic interferences, and can be more specific compared to the use of energy and momentum transfer processes. 

In a relatively short time since their commercial introduction, r.f.-driven reaction cells and collision cells became a major every-day tool for reducing spectral interferences in ICP-MS. The major process that enables interference suppression in such cells is ion-molecule reactions that can be selected on the basis of different reactivities of an interference and an analyte. In many practical cases, many orders of magnitude of interference suppression are achieved through multiple reactive collisions of the interfering ions with a reaction gas. Efficient primary ion-molecule chemistry that distinguishes analyte ions from plasma-based interference ions is accompanied by efficient secondary reactions that create new isobaric interferences within the... 

Interferences in ICP-MS are similar to chemical interferences observed for ICP-OES. In addition, a number of isobaric interferences caused by molecules with similar mass-to-charge ratios of some analyte ions also influence the analytical results. These interferences can be minimized by judicious use of acids in sample matrices, optimized plasma power settings, use of different sample introduction schemes, and use of altermate mass ana-... 

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