Multielement sensitivity

Comparison of single element and multielement sensitivities. (With permission, Anal. Chem., 50, 602 (1978).)... 

Element Single Element9 Sensitivity (nA/mq/JL) Multielement Sensitivity (nA/mq/H) Sensitivity1 Ratio (SE/ME) ... 

Neutron activation analysis has proven to be a convenient way of performing the chemical analysis of archaeologically-excavated artifacts and materials. It is fast and does not require tedious laboratory operations. It is multielement, sensitive, and if need be, can be made entirely non-destructive. Neutron activation analysis in its instrumental form, i.e. that involving no chemical separation, is ideally suited to automation and conveniently takes the first step in data flow patterns that are appropriate for many taxonomic and statistical operations. 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

Ix is the background-corrected net intensity of the principal peak of analyte X, Kx a proportionality factor for the absolute sensitivity of the standard reference, e. g. an Ni plate, and c the concentration of X. Multielement analyses are based on known relative sensitivities S ... 

AED Simultaneous multielement Superior to FPD for quantitative analysis (S compounds) High sensitivity Versatile Expensive Requires skilled analysts [33]... 

Activation analysis is based on a principle different from that of other analytical techniques, and is subject to other types of systematic error. Although other analytical techniques can compete with NAA in terms of sensitivity, selectivity, and multi-element capability, its potential for blank-free, matrix-independent multielement determination makes it an excellent reference technique. NAA has been used for validation of XRF and TXRF. 

The preponderance of work on multielement analysis in seawaters has been carried out using the graphite furnace technique, as this has the additional sensitivity over the direct technique that is required in seawater analysis. 

The palladium and magnesium nitrates modifier has a substantial equalising effect on the atomisation temperature of the nine elements investigated. The optimum atomisation temperature for all but one element (thallium) is between 1900 and 2100 °C. This means that all elements can be determined at a compromise atomisation temperature of 2100 °C with a minimum sacrifice in sensitivity. Such uniform conditions for as many elements as possible are of vital importance if simultaneous multielement furnace techniques are envisaged. Moreover, in conventional graphite furnace AAS, uniform conditions for a number of elements can greatly facilitate and simplify daily routine analysis. 

There can be no doubt that instrumental methods of analysis have revolutionized analytical chemistry, in terms of increased sensitivity, more rapid throughput, multielement capability, computerized calibration, and data handling, etc. There is a cost, too, of course - increased capital expenditure, increased instrumental complexity, and, above all, the current tendency to believe implicitly the output of a computer. Just because a machine gives an analysis to 12 places of decimals doesn t mean that it is true (see Chapter 13) ... 

One important factor which limits the performance of flame AAS is interference, both spectral and chemical. Spectral interference occurs where emission lines from two elements in the sample overlap. Despite the huge number of possible emission lines in typical multielement samples, it is rarely a problem in AA, unless molecular species (with broad emission bands) are present in the flame (in which case, a higher temperature might decompose the interfering molecule). If spectral interference does occur (e.g., A1 at 308.215 nm, V at 308.211 nm) it is easily avoided by selecting a second (but perhaps less sensitive) line for each element. 

The advantage of ICP is that the emissions are of such intensity that it is usually more sensitive than flame AA (but less sensitive than graphite furnace AA). In addition, the concentration range over which the emission intensity is linear is broader. These two advantages, coupled with the possibility of simultaneous multielement analysis offered by the direct reader polychromator design, make ICP a very powerful technique. The only real disadvantage is that the instruments are more expensive. See Workplace Scene 9.3. 

Advantages 1) more sensitive, 2) broader concentration range measurable, and 3) multielement analysis possible. Disadvantages cost. 

Quantitative trace element analysis of diamond by LA-ICP-MS using different synthetic multielement carbon based standards (e.g., cellulose pellets) is discussed by Rege et al 2, whereby 13C was used for internal standardization. Concentrations of 41 elements were determined in two fibrous diamonds from Jwaneng Botswana (JWA 110 and 115) by relative sensitivity coefficients measured using the synthetic cellulose standard. The analytical data were verified by means of instrumental neutron activation analysis (INAA) and proton induced X-ray emission (PIXE).72... 

Conventional external calibration uses pure standard solutions (single- or multielement) and is therefore unable to compensate for matrix effects, fluctuations or drifts in sensitivity. Matrix effects can be compensated for by using matrix-matched calibration solutions. In this case, the degree of compensation depends on the proper matrix adjustment. 

An alternative to quantitative analysis by ICP-MS is semiquantitative analysis, which is generally considered as a rapid multielement survey tool with accuracies in the range 30-50%. Semiquantitative analysis is based on the use of a predefined response table for all the elements and a computer program that can interpret the mass spectrum and correct spectral Interferences. This approach has been successfully applied to different types of samples. The software developed to perform semiquantitative analysis has evolved in parallel with the instrumentation and, today, accuracy values better than 10% have been reported by several authors, even competing with typical ones obtained by quantitative analysis. The development of a semiquantitative procedure for multielemental analysis with ICP-MS requires the evaluation of the molar response curve in the ICP-MS system (variation of sensitivity as a function of the mass of the measured isotope) [17]. Additionally, in the development of a reliable semiquantitative method, some mathematical approaches should be employed in order to estimate the ionisation conditions in the plasma, its use to correct for ionisation degrees and the correction of mass-dependent matrix interferences. 

Battelle has developed instrumental neutron activation analysis (INAA) techniques which permit very sensitive and accurate multielement analysis of approximately 40 elements in coal and fly ash. These techniques, which will be described in this work, form the basis for extensive environmental studies of the effluent from coal-powered generating facilities and other pollution sources. 

Spark source mass spectrometry (SSMS) is also a multielement technique. Conventionally the data obtained are semiquantitative, and the results have an uncertainty of 50% or less. If the stable isotope dilution technique is performed, the SSMS can be 3%. This latter technique was used for lead, cadmium, and zinc as noted in the results tabulations. NAA and SSMS complement each other quite well, and those elements for which one technique has poor sensitivity can usually be measured by the other. 

Multielement determination (sequential or simultaneous) faster analysis time minimal chemical interaction detection limits and sensitivity fall in between that of flame and graphite furnace measurements. 

Multielement determination sensitivity and detection limits exceptionally good (over 100 times greater than furnace techniques for some metals) isotopes also may be measured also has the capability to determine nonmetals (at a much lower sensitivity) broad linear-working range high cost. 

The fluorescence technique combines the advantages of the large dynamic range of emission techniques with the simplicity and high selectivity of absorption techniques. Flame sources have been extensively used, however, for elements with refractory oxides, the ICP source has been found to be more satisfactory for AFS. A system for hollow cathode lamp excited ICP-AFS, as proposed by Demers and Allemand (1981), is commercially available as a modular simultaneous multielement ICP system. Although fluorescence techniques often offer two orders of magnitude sensitivity improvement over absorption, the multielement approach for AFS has not yet been commercially successful. Also promising for the future is the laser-excited furnace AFS where the detection limits for most elements are comparable to those of ICP-AES and for some elements, for eg, As, Cd, Pb, Tl, Lu, even lower (Omenetto and Human, 1984). The future for AFS techniques has been discussed by Stockwell and Corns (1992). 

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