Interelement

High power pulsed lasers are used to produce plasmas and thus to sample and excite the surfaces of soHds. Improvements in minimum detectable limits and decreases in background radiation and in interelement interference effects result from the use of two lasers (99) (see Surface and interface analysis). 

INTERELEMENT INFLUENCE CORRECTION IN THE XRFA USING INTENSITIES RATIO... 

The standardless approach attempts to apply first principles descriptions of X-ray production to the calculation of interelement relative sensitivities. Several of the key parameters necessary for first principles calculations are poorly known, and the accuracy of the standardless method often suffers when different X-ray families must be used in measuring several elemental constituents in a specimen. 

The deviations of Class I, here called absorption and enhancement effects, are known in the literature also as matrix effects, as self-absorption, and as interelement effects. The authors consider the most important objection to each of the last three names to be as. follows. To matrix effect the element sought (ncrt included in the matrix) contributes to the absorption effect for the sample in the same kind of way as any element (free or combined) in the matrix. To self-absorption the name makes no provision for enhancement effects. To interelement effects it fails to recognize that an absorption effect occurs even when only one element is present (Equation 7-4). The term matrix is useful but requires precise definition. What is the matrix when an internal standard is added, or when a powdered sample is dissolved ... 

Interelement effects, see Absorption effect Enhancement effect Interference, by target lines, 102, 103... 

Mass absorption coefficient, measurement for interelement corrections, 174... 

For complicated samples where matrix or interelement effects are present, a linear calibration curve may not be valid, and one should consider using an empirical model for concentration correction. This usually requires a large set of standards of similar composition to the unknown, which generally makes analysis rather impractical. Inter-element effects can be calculated from a basic knowledge of physical parameters in combination with the appropriate use of samples of known composition, pure elemental standards or composite standards. 

Houben [256] has compared the determination of flame-retardant elements Br, P, S, K, Cl and F in polycarbonate using commercial (X40 and UniQuant ) software. For the X40 method, a calibration line for each element in PC or PC/ABS blends was mapped for the conversion of intensities to concentrations. With the universal UniQuant method, sensitivity factors (ks) were calibrated with pure standards. The X40 method turned out to be more reliable than UniQuant for the determination of FRs in PC and PC/ABS blends, even in the case of calibration of k values with PC standards. Standard errors of 5 % were achieved for Br, P, S and K, and 20% for Cl and F the latter element could not be determined by means of UniQuant (Table 8.44). GFR PC cannot be quantified with these two methods, because of the heterogeneous nature of the composites. Other difficult matrices for XRF analysis are PBT, PS and PP compounds containing both BFRs and Sb203 (10-30wt %) due to self-absorption of Sb and interelement effects. 

Thallium has been determined in 10 ml of ashed serum or in urine by extracting with sodium diethyldithiocarbamate into MIBK n°). More recently, Savory and co-workers 1131 described a wet digestion procedure for 50 ml of urine or 5 ml of serum in which the thallium is separated by extracting the bromide into ether, evaporating the ether and then taking up in dilute acid for aspiration. As little as 0.1 ppm is determined in urine. Curry et al.114) determined less than 1 ng of thallium in 200 /d of urine by using the tantalum sample boat technique. The sample in the boat is dried by holding the boat 1 cm from the flame and then it is inserted into the flame where it is vaporized. A similar procedure is used for >3 ng of thallium in 50-100/al of blood, except that the blood is preashed with 3 drops of nitric acid. Since the tantalum boat method is susceptible to interelement interferences, the method of standard additions is used for calibration. 

As a partial summary, in normal C3 V inclusions material from the neutron-rich nuclear statistical equilibrium nucleosynthetic process is in excess relative to the average solar system composition, as well as an O-rich component. Nevertheless, the exact composition of this material is somewhat blurred by secondary processes (nebular or interstellar) as the observations show no strict interelement correlation (Jungck et al. 1984 Birck and Lugmair 1988). 

Gillson GR, Douglas DJ, Fulford JE, Halligan KW, Tanner SD (1988) Nonspectroscopic interelement interferences in inductively coupled plasma mass spectrometry. Anal Chem 60 1472-1474 Gonfiantini R, Valkiers S, Taylor PDP, De Bievre P (1997) Adsorption in gas mass spectrometry. II Effects on the measurement of isotope amount ratios. Int J Mass Spectrom Ion Proc 171 231-242 Habfast K (1997) Advanced isotope ratio mass spectrometry. I magnetic isotope ratio mass spectrometers. In Modem Isotope Ratio Mass Spectrometry. Chemical Analysis Vol. 145. Platzner IT (ed). John Wiley and Sons, Chichester UK, p 11-82... 

An X-ray fluorescence method has been developed for the determination of technetium in solution . At concentrations of less than 1.0 mg Tc per ml there are not interelement effects. Therefore, it is possible to ascertain technetiiun in its compounds without previous decomposition, provided that the compounds are soluble in water or dioxane. The detection limit is about 4 Tc. 

Griffiths et al. [92] quantified Pt, Pd and Rh in autocatalyst digests by ICP with a CCD detector array. They compared univariate techniques (pure standards, pure standards with interelement correction factors and matrix-matched standards) and PLS, the latter being superior in general, although less effective... 

Whenever quantitative analysis is desired, care must be taken to use proper standards and account for interelement matrix effects since the inherent sensitivity of the method varies greatly between elements. Methods to account for matrix effects include standard addition, internal standard and matrix dilution techniques as well as numerous mathematical correction models. Computer software is also available to provide semi-quantitative analysis of materials for which well-matched standards are not available. 

To reduce the detrimental effects of spectral interferences on element quantitation, laboratories select the spectral lines that are least affected by the background, and use the background compensation and interelement correction routines as part of the analytical procedure. The instrument software uses equations to compensate for overlapping spectral lines the effectiveness of these equations in eliminating spectral interferences must be confirmed at the time of sample analysis. That is why laboratories analyze a daily interelement correction standard (a mixture of all elements at a concentration of 100mg/l) to verify that the overlapping lines do not cause the detection of elements at concentrations above the MDLs. 

The formed free atoms absorb the light at a characteristic wavelength from a hollow cathode lamp that is positioned on one side of the flame. A spectrophotometer with a grating monochromator located on the other side of the flame measures the intensity of the light beam. Because absorption is proportional to the number of free atoms that are produced in the flame, the light energy absorbed by the flame is a measure of the element s concentration. The FLAA technique is relatively free of interelement spectral interferences, but it has the sensitivity that is inferior to ICP-AES or GFAA. 

An alternative to MSA in ICP-MS analysis is the internal standard technique. One or more elements not present in the samples and verified not to cause an interelement spectral interference are added to the digested samples, standards, and blanks. Yttrium, scandium, and other rarely occurring elements or isotopes are used for this purpose. Their response serves as an internal standard for correcting the target analyte response in the calibration standards and for target analyte quantitation in the samples. This technique is very useful in overcoming matrix interferences, especially in high solids matrices. 

When results of ICP-AES analysis indicate possible interelement interference, request confirmation with A A methods. 

Environmental laboratories routinely determine IDLs in the course of ICP-AES analysis as a measure of background and interelement interferences at the lowest measurable concentration level above the background noise. The IDL is a trace element analyte concentration that produces a signal greater than three standard deviations of the mean noise level or that can be determined by injecting a standard to produce a signal that is five times the signal to noise ratio (APHA, 1998). 

In ICP-AES analysis, the IDLs must be determined every time an instrument has been adjusted in a way that may affect its sensitivity and interelement correction. The CLP SOW requires that the IDLs be determined at a minimum on a quarterly basis. Laboratories that are not part of the CLP determine IDLs annually. Table 4.4 shows... 

Fig. 10.40 A pair of bilobal elements that operate either in co- or counterrotation and impart mechanical energy in deforming solids or forcing flow in melts filling the available interelement volume. The shear intensity is depicted by the color code going from white to red with increasing intensity. [Courtesy of C. Martin, American Leistritz Extrusion Corp.]...

Bums, R. G. Fuerstenau, D. W. (1966) Electron-probe determination of interelement relationships in manganese nodules. Amer. Mineral., 51,895-902. 

Fair Lawn, NJ 07410) prepared from the metal (Mg, Ni, Mn, Mo, Co), the oxide (Cu, Cr), the chloride (Fe), or the carbonate (Sr) and contain dilute HC1, HNO3, or aqua regia as the solvent. For the emission studies, all solutions were prepared to contain 1000 mg/ , rubidium to reduce interelement effects observed for alkali and... 

The observation region in the plasma was selected to provide the best compromise in the signal-to-background ratio observed for the largest possible number of wavelengths. The effect of this compromise on sensitivity has been discussed as have the interelement effects among alkali and alkaline earth elements,... 

G. R. Gillson, D. J. Douglas, J. E. Fulford, R. W. Halligan, S. D. Tanner, Non-spectroscopic interelement interferences in inductively coupled plasma mass spectrometry, Anal. Chem., 60 (1988), 1472-1474. 

Solution aspiration rates, fuel and oxidant mixtures, gas flow rates, burner choice, matrix effects and interelement interferences must all be taken into account when using flame AAS. While optimal choices for the above parameters vary from instrument to instrument, recommendations which afford reasonable starting points for operation have been published by both the Intersociety Committee for Methods of Air Sampling and Analysis (ISC) [7] and the National Institute of Occupational Safety and Health (NIOSH) [8]. These recommendations were the result of ISC efforts supported by both the United States Environmental Protection Agency and NIOSH. 

Through calibration and re-calibration techniques possible interelement effects or influences from the matrix can be largely eliminated. Because of its worldwide importance the introduction of standard methods by atomic absorption started first with iron ores [67] and refractories. The ISO/TC 102 is presently occupied with draft proposals for the atomic absorption determinations of Na/K Ca/Mg, Zn, V, Pb and Al. 

The preparation of suitable standards depends on the composition of the sample, the method of decomposition, and the type of flame used in the determination of a given element. It is felt that many workers pay inadequate attention to the problems of standards preparation and interelement interferences. A concise and thorough, although rather dated, compilation of interferences arranged by element is presented by Angino and Billings (2). 

Many of the interelement interferences result from the formation of refractory compounds such as the interference of phosphorous, sulfate, and aluminum with the determination of calcium and the interference of silicon with the determination of aluminum, calcium, and many other elements. Usually these interferences can be overcome by using an acetylene-nitrous oxide flame rather than an acetylene-air flame, although silicon still interferes with the determination of aluminum. Since the use of the nitrous oxide flame usually results in lower sensitivity, releasing agents such as lanthanum and strontium and complexing agents such as EDTA are used frequently to overcome many of the interferences of this type. Details may be found in the manuals and standard reference works on AAS. Since silicon is one of the worst offenders, the use of an HF procedure is preferable when at all possible. 

The other major source of interelement interference is related to ionization. Since AAS depends on the absorbance of light by atoms, any change in the degree of ionization of an element will be reflected by a change in apparent concentration. Since the ionization of atoms in a flame represents an equilibrium with electrons within the flame, the ionization of one element affects the degree of ionization of another. The extent of ionization increases with flame temperature, so that these effects become exacerbated when the higher temperature acetylene-nitrous oxide flame is used to overcome the interference mentioned above. This ionization also leads to a lower sensitivity since a smaller proportion of the element is present in the atomic form. 

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