Spectral Isobaric Interferences

Polyatomic ionic species also form from air, solvents, and matrix components, such as oxides of metals present in large amounts in the sample as well as from the acids and other reagents used to digest samples. For example, titanium has five isotopes, Ti, Ti, Ti, Ti, and Ti, which can form Ti 0 oxide ions that interfere with Ni, Cu, Zn, Cu, and Zn. Examples in addition to the argon ions mentioned earlier include C02 CO SH+, SO +, NOH+, C10+, and ArS+, which are derived from the plasma gas and from reactions of Ar with water, carbon, and other elements present in the solvent or the sample. Based on studies of these interfering ions. 

Isotope Abundance(%) Interfering Species Typical Reaction Gas 

The analysis of organic solvents presents some unique polyatomic interferences. Tables 10.24, 10.26, and 10.27 list potential polyatomic interfering species and the affected element. Some analysts add oxygen to the plasma when running organic solvents to minimize carbon (soot) formation on the cones. The additional oxygen can not only create the polyatomic species listed in Table 10.24 but can also react with some elements to form refractory oxides, as happens in aqueous solution. Examples include Ti 0, which interferes with Cu, and REE oxides, such as Nd 0, which interferes with Tb. 

Many of these interferences are well-documented, and in a well-controlled system, many can be corrected for mathematically. Most instrument companies have interference-correction software as part of their data handling software. A problem with this approach has been that sometimes the necessary data were not collected during analysis, requiring reanalysis if possible. Multiple scans or longer analysis times may have been needed. One instmment that has an advantage with respect to software corrections is the simultaneous ICP-MS, from SPECTRO, described in Chapter 9. Becanse the SPECTRO MS collects the complete mass spectrum simultaneously without scanning, aU data are available. Having been collected at the same time, measmanent-based corrections can be made more reliably. 

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One application involves the separation and measurement of rare earth species with IC-ICP-MS. A sihca-gel-based sulfonated cation-exchange column is used with a mixture of citric-hydroxyisobutyric acid as the mobile phase. Ion currents for each of the rare earth analytes are measured as a function of elution time to record a family of ion-specific chromatograms. This technique provides a means to eliminate several spectral isobaric interferences from rare earth oxides, hydroxides, and molecular hydrides on the analyte rare earth elements. For example, the isobaric interferences fiom the isotopes of Gd oxide and hydroxide on the primary isotopes ofYb and Lu were completely ehminated. In addition, the interference fiom the molecule, which directly interferes with 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. 

In inductively coupled plasma-mass spectrometry, isobaric interference occurs between species with the same mass and charge. Interference can be eliminated if the mass spectral resolution is sufficiently great or by dissociating an interfering polyatomic species with a collision cell. When laser ablation is used to sample a solid, matrix-matched standards are often necessary for quantitative analysis. 

Explain what is meant by spectral, chemical, ionization, and isobaric interference. 

Molecular ions present a more complex problem in ICP-MS. With a combination of molecular ion interferences and isobaric interferences, all of the isotopes of the analyte ion of interest may suffer from a spectral overlap. The molecular ion signals can also be strongly dependent on the sample composition and experimental parameters. It is often more difficult to identify and correct for molecular ion spectral overlaps than for isobaric overlaps. Because the resolution of the commercial quadrupole mass spectrometers is 0.5 dalton or less, isotopic patterns, rather than exact mass, must be used in an attempt to identify the interfering molecular ion. 

However, the most common interferences are the spectral interferences, also called isobaric interferences. They are due to overlapping peaks which can mask the analyte of interest and can give erroneous results. Such interferences may occur from ions of other elements within the sample matrix, elemental combination, oxide formation, doubly charged ions, and so on. 

Table 13.1 Spectral (isobaric and polyatomic) interferences in the detection of Rh, Pd, and Pt by ICP-MS...

The nature of an isobaric interference is illustrated in Fig. 17.13, where element B is the analyte of interest for which the isotope ratio indicated by the two spectral lines is to be measured. A lighter element, A, has isotopes whose diatomic oxides AO+ have three masses indicated as molecular ions in the figure, two of which occur at the element B masses of interest. The spectrum observed at the element B masses of interest is thus a composite of signals from the element B atomic... 

FIGURE 17.13. ICP/MS spectral analysis with isobaric interference. 

Interferences caused by spectral overlap were studied by spiking blank filters with 100 ppm single element solutions of potential interferents. Interefered elements were subsequently analysed to assess the extent of the interference. Non-oxide polyatomic and double charged species were considered (Table 3). Oxides are limited when using laser ablation and were therefore not considered. Isobaric interferences were corrected based on their natural abundances. Interfering elements caused an elevated analyte signal relative to the signal for the filter blanks. 

In mass spectrometry signals are obtained for each isotope present. With the low mass resolution of quadrupole mass spectrometers ( 1 dalton), this leads to a number of isobaric interferences, which can be corrected for with appropriate software. This type of interference depends only slightly on the working conditions, which is not the case for spectral interferences resulting from doubly charged ions, background species or cluster ions. The background species at low masses [512] cause considerable spectral interferences e.g. for Si+ (with (with... 

ICP-MS is a good option for determining various radioisotopes with limits of detection comparable to radiometric methods in a shorter time, especially sensitive for the determination of isotopes with very long lives. However, low-resolution quadrupole ICP-MS is susceptible to isobaric interferences, e.g. " Pu/ " Am, Tc/ Ru, U/ Pu, molecular interferences, e.g. Pu/ UH, and spectral interferences, e.g. also high concentration of dissolved solids should be avoided in order... 

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.

More complex examples of mass spectra common to the area of Materials Sciences, the Earth Sciences, and the Biosciences are shown in Figure 5.2(a-c). These were collected on Time-of-Flight (Figure 5.2a and 5.2c), Magnetic Sector (Figure 5.2b), and Fourier Transform-Ion Cyclotron Resonance (Figure 5.2c)-based SIMS instruments. In all cases, improved mass resolution was required to separate the increased prevalence of spectral overlaps or isobaric interferences, also referred to as mass interferences. 

Various chemical and physical interference effects can seriously impact the accuracy of ICP-MS analyses. In general, these interference effects are independent of the spectral overlap or isobaric interferences discussed in the previous section. These interferences can manifest themselves in either suppression or enhancement of the ion currents that are measured for quantitation. Some of these interferences can also have a deleterious effect on stability of signals and analysis precision. Because these effects can originate from multiple sources and often are very complex, they can be difficult to ascertain and mediate. In addition, the magnitude of these interferences can be hardware or apparatus dependent. 

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. 

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... 

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