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

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. 

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... 

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). 

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. 

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. 

An ICP-MS instrument will not tolerate dissolved solids at concentrations that can be run with an ICP-atomic emission spectrometer. In addition to increasing the probability of interelement (isobaric) interferences and signal suppression, high levels of dissolved solids condense on the sample-cone orifice. This deposition degrades the sensitivity and stability of the analytical signal. Typically, a maximum of 0.1% dissolved solids is recommended for continuous nebulization with a pneumatic... 

Another procedure uses an alkaline flux method with lithium metaborate (41), and involves diluting the fused portion in nitric acid. Interferences and interelement effects must be eliminated by calibration of samples and standards of similar composition. A procedure used successfully in the analysis of archaeological ceramics is described by Gritton (19). The sample is decomposed with hydrofluoric acid and perchloric acid, fumed to a moist residue, treated twice more with perchloric acid, dissolved in more perchloric acid, and diluted with water. 

Indirect FAA methods, in which the hydrocarbon matrix is eliminated and the analyte is concentrated, could have been applied to all elements studied by the Project if a large enough sample were used. However, since indirect techniques still may encounter chemical (interelement) interferences, a minimum of 5 ml of solution must be available so that standard additions techniques can be applied. Some trace metals have been determined in petroleum by indirect FAA after a 100-g sample had been ashed (17), However, ashing such large samples, particularly crudes or residual fractions, is difficult and time consuming. Sulfated ash procedures were used in the Project to prepare various petroleum matrices for determining Cd, Co, Mo, Ni, and V by FAA. However, procedures were developed only for the first three elements, and cross-check data were collected only for cadmium. Since alternate techniques had greater sensitivity and allowed smaller samples to be ashed, flame atomic absorption was not widely used. 

External sample decomposition procedures, such as ashing or acid digestion, not only eliminate the matrix but also convert the analyte to some specific salt suitable for HVAA analyses. For example, the ashing used to obtain data in Table 3.II converted the nickel to its sulfate. In the Project, indirect HVAA procedures were successfully applied to six elements. The application of these procedures, however, may be restricted by contamination or by reagent blanks. For example, exposure to nichrome heating elements affects chromium (see p. 19). Although destruction techniques eliminate the hydrocarbon matrix, they do not compensate for interelement interferences. 

As with all analytical determinative techniques, interelement and matrix interferences exist and accurate quantitative determinations of concentration involve corrections for matrix effects. X-ray spectrometers comprise an excitation source, a means for the separation and isolation of emission lines, a device for intensity measurement, and typically a dedicated computer for calculation and applying corrections. The two basic types are wavelength-dispersive (in which the X-rays are characterized by wavelength) and energy-dispersive (in which the X-rays are measured by their energy levels) spectrometers. A disadvantage, as with many (all) other methods of analysis is the complication caused by the chemical composition of the matrix and as well by granulation, i.e., particle size. It remains the view... 

Especially for the determination of Pb and Cd, it is necessary to take into account possible interelement interferences (e.g. Al Pb Fe- Cd) which may lead to systematic errors. A critical evaluation of the spectra is necessary for finding the adequate background correction position and other parameters as well. The expenditure which must be done is again dependent on the spectral resolution power of the instrument used for the analysis. 

FIGURE 10-16 Internal standard calibration curves with an tCP source. Here, an yttrium line at 242.2 nm served as an internal standard. NoUce the lack of interelement interference. (From V. A. Fassel. Science. 1978. 202,187. With permission Copyright 1978 by the American Association for the Advancement of Science.)... 

Figure 7.41 Direct spectral overlap of Pt and Cr emission lines. No background correction technique can solve this problem. A line with no interference must be found, an interelement correction factor must be applied or the elements must be separated chemically. [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

Sources of error in the sample preparation should be recognized and interferences controlled. However, each analysis involves random (statistical) errors, and the whole error is the sum of cumulative errors at each stage of an analytical procedure. A number of effects contribute to the uncertainty of the final signal displayed on the readout system. In the measurement stage various sources of interference are fluctuations in radiation source signal, photomultiplier shot noise , electronic noise , flame fluctuations, nebuliza-tion and atomization noise , inaccuracies in the read-out system, and interelement interferences. 

Interferences of types 1 and 2 can occur not only from another element in the matrix but also from an argon line or molecular species such as OH or N2. An example of a type 1 interference is the Zn(I) line at 213.856 nm overlapping with the Ni(I) line at 213.858 nm. As type 2 can be eliminated only by improved spectral resolution, mathematical algorithms are used on a routine basis to solve this type. If the interference can be identified and quantified, then interelement correction factors can be inferred in a simple manner First, a calibration for the analyte is constructed in the usual way and then different concentrations of the interfering element are studied and the apparent analyte concentration calculated at the emission line(s) of interest. 

Because many elements have several strong emission Hnes, AES can be regarded as a multivariate technique per se. Traditionally, for quantitative analysis in atomic emission spectroscopy, a single strong spectral line is chosen, based upon the criteria of Hne sensitivity and freedom of spectral interferences. Many univariate attempts have been made to compensate spectral interferences by standard addition, matrix matching, or interelement correction factors. However, all univariate methods suffer from serious limitations in a complex and Hne-rich matrix. 

Fassel and Kniseley reported experimentally determined detection limits for 61 elements that compare favorably with detection limits obtained by atomic absorption, atomic fluorescence, and flame emission methods. Interelement interferences are lower than by other methods and interferences due to PO4" and apparently are negligible. The technique seems well adapted to simultaneous multielement analyses through use of direct reading spectrometers. 

One of the very first quantitative applications of x-ray fluorescence spectrometry involved the analysis of copper-based alloys for trace metals. IWenty years later the rapid development in the use of speciality alloys for, among others, the aircraft industry, required the availability of fast, accurate multielement instrumental methods. In the early 1950s two methods seemed to hold promise— x-ray fluorescence and ultraviolet emission (UVE). At that time, x-ray fluorescence was a technique limited to a wavelength range of about 0.5 to 8.0 A, in other words, all elements down to atomic number 14(Si). Even though it was unable to measure the lower atomic numbers, especially the important element carbon, it was able to provide data for S and R This, along with (at that time) a perceived minimum problem of interelement interferences, made x-ray fluorescence an ideal choice for the nonferrous industry. However, the UVE technique was the method of choice for most ferrous industry-based problems. This situation was to persist into the 1960s until the classic work of Shiraiwa and Fujino [14] provided the means for accurate... 

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