Multi-element capability

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 potential for the employment of plasma emission spectrometry is enormous and it is finding use in almost every field where trace element analysis is carried out. Some seventy elements, including most metals and some non-metals, such as phosphorus and carbon, may be determined individually or in parallel. As many as thirty or more elements may be determined on the same sample. Table 8.4 is illustrative of elements which may be analysed and compares detection limits for plasma emission with those for ICP-MS and atomic absorption. Rocks, soils, waters and biological tissue are typical of samples to which the method may be applied. In geochemistry, and in quality control of potable waters and pollution studies in general, the multi-element capability and wide (105) dynamic range of the method are of great value. Plasma emission spectrometry is well established as a routine method of analysis in these areas. 

The main advantages that ICP-MS has over other techniques are its low detection limits, in the 1-10 pg ml i range for quadrupole instruments (Fig. 5.9), large linear dynamic range and rapid multi-element capability. However, ICP-MS also suffers from a number of interferences. 

Unstable radionuclei result on subjecting the nuclei of some elements to neutron bombardment. During the decay process, in which the radionuclei return to more stable forms, characteristic radiation is emitted. The energy of the radiation is characteristic of the element, and its intensity forms the basis for quantitative elemental analysis. The advantages of NAA for trace analysis include low detection limits, good sensitivity, multi-element capability and relative freedom from matrix effects. However, for successful application of this technique skilled personel are required and because of the low sample throughput the amount of work involved in the analysis of column fractions, for example, is prohibitively high. In addition, it may take up to several weeks before the results are available. Further, only few laboratories have easy access to a neutron source. 

ID-TIMS 0.000 005-0.1 by isotope dilution + + +/ + + excellent quantification possibility, highest precision no multi-element capability, time consuming sample preparation... 

Water samples (drinking water, rain, sea, river or waste water and others) have been characterized by ICP-MS with multi-element capability in respect to metal impurities (such as Ag, Al, As, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe, Hg, K, Na, Sb, Se, Mg, Mn, Mo, Ni, Pb, Tl, Th, U, V and Zn) in many laboratories in routine mode with detection limits at the low ng I 1 range using ICP-QMS, and below by means of ICP-SFMS. Drinking water samples are controlled in respect of the European legislation (Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption). For quality control of analytical data, certified standard reference materials e.g. drinking water standard (40CFR 141.51), river water reference material SLRS-4 or CASS-2 certified reference sea-water material and others are employed. 

There are several factors which make neutron activation analysis (NAA) an appropriate technique for investigating potential pollutants in coal and the combustion process. First, the multi-element nature of NAA is useful because of the large number of potential elemental pollutants, such as Se, Hg, As, Zn, Ni, Sb, and Cd. Also, the use of elemental ratios made possible by the multi-element capability facilitates the understanding of chemical behavior during the combustion process. Elemental ratios have been used previously in urban (15) and upper atmospheric (26) studies. Secondly, the sensitivity and selectivity of NAA allows determination of many elements present at very low concentrations (ppm or lower), and the results are unaffected by matrix interferences. This sensitivity also allows analysis of very small samples. Finally, the cost of NAA when conducted as a multi-element analytical tool is competitive with more conventional and less sensitive techniques on the cost-per-element-per-sample basis. 

One application of great interest is depth profiling by means of GD-TOF-MS. Particularly in conjunction with pulsed GD sources, the simultaneous multi-elemental capabilities of TOF-MS permit the analysis of solid samples as a function of depth (or time) as the discharge operates. In such a case, one might envision complete trace elemental analysis of solid materials with resolution on the nanometer scale. 

Some refractory elements cannot be determined by ET-AAS at the levels usually present in waters. That is the case with M. El Himri et al. [28] developed a fast and accurate procedure, without any prior treatment, to analyze tap and mineral waters from Spain and Morocco for this highly toxic element. ICP-MS was employed. The analytical isotope selected was 238U, with Rh as internal standard. An LoD of 2ngl 1 was obtained. The estimated repeatability was 3 percent at the concentration level of 73 ng l-1. The method was validated by comparison with a radiochemical procedure devised for natural samples and by analysis of a Certified Reference Material (CRM). Multi-element capabilities of ICP-AES have also been employed for surveys of trace elements. Al-Saleh and Al-Doush [29] reported the concentrations of dissolved Be, Cd, Cr, Cu, Fe, Mg, Mn, Hg, Ni, Se, Sr, V, and Zn in 21 samples of retail bottled waters from Riyadh, Saudi Arabia. It was found that Cd, Fe, Hg, Ni, and Zn were present at concentrations higher than the limits recommended by the EU and World Health Organization (WHO) guidelines. 

The ideal analytical technique to be used in the challenging task of reconstructing past changes and recent variations in the concentration of trace substances in polar snow and ice should present several important features. Of these, extremely low detection limits, multi-element capability, low sample consumption and the possibility to avoid, as far as possible, any preconcentration step which could be a source of contamination are the most appreciated. Nevertheless, there is currently no technique with all the special features listed above several instrumental methods have been used in the past for trace element determination in polar snow and ice (see Table 3.4). 

Collision and reaction cell techniques have been used for many years in the study of organic and biological mass spectrometry, but only in the last few years in ICP-MS. The development of collision and reaction cells extended the capability of the technique by allowing the selective attenuation or removal of problematic spectral interferences. Today a variety of collision/reaction cells using various gasses (H2, He, CH4, NH3...) are available, virtually able to eliminate the problems associated with polyatomic interferences for most elements in food and beverage matrices. However, the simultaneous multi-element capability and maximum productivity of ICP-MS is partially reduced by the different CRC tuning conditions required to eliminate a specific interference in a specific matrix. 

Electrolytic solutions used for extra-renal infusion and dialysis contain metal chlorides of Na, K, Ca and Mg salts at concentrations that are critical for effective treatment. These solutions also contain dextrose, citrate and lactate additives as part of this special formulation. The analysis for these metals must be precise and accurate and this can be achieved with ICP-OES using yttrium or scandium as internal standard to correct for matrix affects. The method of standard addition may also be used with similar success but is a more tedious method. The ability to dilute the sample several fold due to the high concentrations of metals reduces/eliminates the effect of EIE (easily ionised elements) caused by other elements in the same solution. The dilution and the ease of detection and corrections with an internal standard using the multi-element capability make this an excellent method. 

Considering these limitations, a procedure for lead determination has been developed as a result of the Project and is described fully in Chapter 10. Preliminary work related to cadmium determination suggests that a similar technique can be used to determine other elements and can be expanded to a multi-element capability for trace metals in petroleum. 

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