Subject spectral interferences

Excitation of the Eu3+ or Tb3+ ions has traditionally been indirect, by broad-band UV excitation of a conjugated organic ligand which is followed by intramolecular energy transfer to the lanthanide ion / system, followed in turn by /- / emission.614 However, more recently, following the advent of tunable dye lasers, direct excitation of an excited / level is in many cases preferable. By scanning this frequency, an excitation spectrum can be obtained whose energy values are independent of the resolution of a monochromator and not subject to spectral interferences. 

D. The measurement of Li in brine (salt water) is used by geochemists to help determine the origin of this fluid in oil fields. Flame atomic emission and absorption of Li are subject to interference by scattering, ionization, and overlapping spectral emission from other elements. Atomic absorption analysis of replicate samples of a marine sediment gave the results in the table below. 

If the samples have not been subjected to strenuous chemical cleanup, or there are plasma-based spectral interferences to be considered, then high-resolution MS detection and/or interference correction equations may be required, rather than using quadrupole MS instrumentation. On the other hand, these two approaches are likely to introduce their own influence on the precision/accuracy of the measured isotopic ratio. 

Flame atomic absorption is subject to many of the same chemical and physical interferences as flame atomic emission (see Section 28C-2). Spectral interferences by elements that absorb at the analyte wavelength are rare in AA. Molecular constituents and radiation scattering can cause interferences, however. These are often corrected by the background correction. schemes discussed in Seetion 28D-2. In some cases, if the source of interference is known, an excess of the interferent can be added to both the sample and the standards. The added substance is sometimes called a radiation buffer. 

With electrothermal evaporation from a tungsten filament and quartz fiber optics, detection limits are at the 50-100 pg level for many elements, except for Fe which is subjected to spectral interferences from tungsten lines. This was established from the use of different working gases and especially from experiments with the addition of H2 to the argon. In the case of coupling with graphite furnace atomization Cu, Mg and Fe can be determined in serum samples without dilution for Fe and Cu and with a 1 100 fold dilution for Mg [434]. 

Electrothermal vaporization (ETV) in addition to its features for the analysis of microsamples, in ICP-MS has the additional advantage of introducing a dry analyte vapor into the plasma. Hence, it has been found to be useful for elements for which the detection limits are high as a result of spectral interferences with cluster ions. In the case of 56 Fe, which is subject to interference by 40ArO+, Park et al. [529] showed that the detection limit could be improved considerably by ETV. For similar reasons the direct insertion of samples in ICP-MS leads to the highest absolute power of detection (detection limits in the pg range and lower [530, 531]). 

Tables VIII, IX and X demonstrate that the accuracy of the SPD is generally similar to the accuracy of the PMT. In the case of some spectral lines the SPD results were less accurate than the PMT results. This is caused by the lower resolving power of the SPD causing it to be more subject to specific spectral interference. The adverse effects of spectral line interferences resulted in relatively large concentration errors for the low analyte concentration sample. The effect of these spectral interferences is twofold. First, there is the analyte signal intensity error caused by coincident spectral line interferences from unresolved matrix lines. Second, there is the background interpolation...

Aside from the extensively studied volatility interferences, it has been demonstrated that the conventional method of background correction which is based on the use of a continuum source (D2), is subject to spectral interferences from iron and for phosphate decomposition products (presumably PO and P2) (Saeed and Thomassen, 1981). Even though these spectral interferences are highly reduced by matrix modification with either nickel, a nickel/platinum or a nickel/palladium matrix modifier, the use of a Zeeman based instrument is highly recommended (Bauslaugh et al., 1984 Radziuk and Thomassen, 1992 Hoenig, 1991). 

From the sketch of Figure 1, it is possible to deduce some of the advantages of the atomic absorption technique. First, properly designed atomic absorption equipment, correctly used, is subject to very few spectral interferences, and from an analytical viewpoint these rarely present problems. 

The detection of Cd at 214.438 nm is subject to interferences from Fe and Al. In Figure 2, the spectral line interference of the 214.445 nm line of Fe is apparent, as well as the large sloping shift caused by the presence of the high concentration of Al. This latter effect is even more pronounced for the detection of Cr at 205.552 nm. The scan plotted in Figure 3 shows the large continuum shift caused... 

While detection of cations via porphyrin-ba.sed materials has been explored less than anion sensing, the ability of a porphyrin to coordinate different metals and the unique spectral signatures that result form the basis for metal ion detection. Use of free-base porphyrins in polymer matrices has allowed for the detection of heavy metal ions by Ache et Immobilization of 5,10,15,20-tetrakis(4-N-methylpyridyOporphyrin on Nafion membranes pennitted detection of cadmium and mercury in solution with detection limits of 5 X 10 M and 2 x 10 M, respectively over a 20-minute measuring period. The method is subject to interferences from other metal ions, but the researchers were able to detect several ions simultaneously using pattern-recognition techniques such as principal component analysis. Sol-gel films doped with 5,l0,l5,20-tetra(p-sulfonatophenyDporphyrin have also been used by Ache and coworkers for the fluorimetric determination of mercury in solution, with a detection limit of approximately 7 X 10... 

If a radionuclide D is formed out of another element C than the analyte element A, the determination of A by reaction A(a,b)B can be subject to spectral interference from C by the reaction C(c,d)D. Three methods can be applied to avoid spectral interference ... 

An objective evaluation of the methods on the basis of literature data is hardly possible, because of differences in experimental conditions, sample, amount of sample available, and measurement time. Therefore, the detection limit values stated in the literature given in Tables 1-4 should be considered, at best, approximate. But it is clear that there is no single instrumental technique that meets all the analytical requirements. For example, some methods may be applicable over only a limited concentration range, may be subject to matrix effects or spectral interferences, or have a limited availability. Also, sometimes nondestructive fast methods are needed. The choice of an instrumental method depends on the material to be analyzed and the type of analysis required. 

The detection method should be as species specific as possible, and ideally one would like to measure both reactant disappearance and product formation. The method must not be subject to interference from other reactants and should be applicable under a wide range of concentration conditions so that the rate law can be fiilly explored. Often there is a practical trade-off between specificity, sensitivity and reaction time. For example, NMR is quite specific but rather slow and has relatively low sensitivity, unless the system allows time for signal accumulation. Spectrophotometry in the UV and visible range often has good sensitivity and speed, but the specificity may be poor because absorbance bands are broad and intermediates may have chromophoric properties similar to those of the reactant and/or product. Vibrational spectrophotometry can be better if the IR bands are sharp, as in the case of metal carbonyls, but the solvent must be chosen to provide an appropriate spectral window. Conductivity change can be very fast but is r er unspecific, except for reactions that involve the production or consumptirm of the or OH ions, because of their unusually large specific conductivities. 

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.

For analysis lines subject to spectral interference as the detection limit is approached, the value given represents the lowest concentrations determinable when intensity ratios are measured. 

Choice of Atomization and Excitation Source Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these reasons a plasma emission source is usually the better choice. 

Other interferences which may occur in flame AAS are ionization of the analyte, formation of a thermally stable compound e.g., a refractory oxide or spectral overlap (very rare). Non-flame atomizers are subject to formation of refractory oxides or stable carbides, and to physical phenomena such as occlusion of the analyte in the matrix crystals. Depending on the atomizer size and shape, other phenomena such as gas phase reactions and dimerization have been reported. 

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