Neutron activation limitations

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

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. 

The properties of such materials are not measurably altered until subjected to doses in excess of a million rads. At these higher doses, the principal changes are due to chem decompn which, with very few exceptions, resnlt in a decrease in sensitivity to mechanical stimulus and also in a dimunition of expl output. The radiation doses normally encountered in neutron activation procedures range from a few rads for 14 MeV fast neutron activation to several thousand rads for thermal neutron activations in a nuclear reactor. Thus, such doses are well under the limit at which measurable changes can occur... 

In addition to the aforementioned methods, TLC in combination with other instrumental techniques have also been used for quantification of inorganic species. For example, two-dimensional TLC coupled with HPLC has been utilized for the separation and quantification of REEs in nuclear fuel fission products using silaiuzed silica gel as layer material [60]. In another interesting method, REEs in geological samples have been determined by ICP-AAS after their preconcentration by TLC on Fixion plates [32]. TLC in combination with neutron activation has been used to determine REE in rock samples on Eixion 50 x 8 layers with the sensitivity limit of 0.5 to 10 pg/g for 10- to 30-mg samples [41]. A combination of TLC and A AS has been utilized for the isolation and determination of zinc in forensic samples [27]. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Holzbecker and Ryan [825] determined these elements in seawater by neutron activation analysis after coprecipitation with lead phosphate. Lead phosphate gives no intense activities on irradiation, so it is a suitable matrix for trace metal determinations by neutron activation analysis. Precipitation of lead phosphate also brings down quantitatively the insoluble phosphates of silver (I), cadmium (II), chromium (III), copper (II), manganese (II), thorium (IV), uranium (VI), and zirconium (IV). Detection limits for each of these are given, and thorium and uranium determinations are described in detail. Gamma activity from 204Pb makes a useful internal standard to correct for geometry differences between samples, which for the lowest detection limits are counted close to the detector. 

Lieser et al. [628] studied the application of neutron activation analysis to the determination of trace elements in seawater, with particular reference to the limits of detection and reproducibility obtained for different elements when comparing various preliminary concentration techniques such as adsorption on charcoal, cellulose, and quartz, and complexing agents such as dithizone and sodium diethyldithiocarbamate. 

May et al. [73] used neutron activation analysis to determine 237neptunium in waste waters. The determination used the 237Np(rc,y)238Np reaction. The detection limit was 5 x 10 6 xg of 237neptunium, which corresponds to 2.5 x 10 6 xg/kg for 200 ml seawater samples. 

Neutron activation analysis is an attractive method in many trace element problems, or where the total amount of sample is limited. Many geochemical studies of trace constituents and semi-conductor developments have used the technique, whilst in recent years pollution investigations have provided a new focus. In forensic science small flakes of paint, single hairs and a variety of other small samples have been analysed and identified by activation analysis. In recent years activation analysis has lost further ground to ICP-MS which provides more comprehensive information and is more readily operated. Sensitivity is also comparable in many cases. 

Figure 2.13 Schematic diagram of the nuclear processes involved in neutron activation analysis. Prompt gamma neutron activation analysis (PGNAA) occurs within the reactor delayed gamma NAA (DGNAA) occurs at some remote site. (After Glascock, 1994 Fig. 1. John Wiley Sons Limited. Reproduced with permission.)...

Trace amounts of Tc are also determined in filter paper and vegetable samples by neutron activation analysis The procedure consists of the following major steps separation of technetium from the sample, thermal neutron irradiation of the Tc fraction to produce °°Tc, post-irradiation separation and purification of °°Tc from other activated nuclides, and counting of the 16 s Tc in a low-background P counter. The estimated detection limits for Tc in this procedure are 5 x 10 g in filter paper and 9 x 10 g in vegetable samples. 

Elements chosen from the limited NURE multi-element geochemical packages that may be pathfinders for porphyry-style deposits (Lefebure Ray 1995) include Ba, Co, Cu, Mn, Pb, Ti, V, and Zn. Under the NURE program, two analytical techniques were used energy dispersive x-ray fluorescence (Cu and Pb) and neutron activation (Ba, Co, Mn, Ti, V, and Zn). Single element plots and element association plots were generated. Geochemical data for pond sediments collected over the Pebble deposit in 2008... 

Manganese in aqueous solution may be analyzed by several instrumental techniques including flame and furnace AA, ICP, ICP-MS, x-ray fluorescence and neutron activation. For atomic absorption and emission spectrometric determination the measurement may be done at the wavelengths 279.5, 257.61 or 294.92 nm respectively. The metal or its insoluble compounds must be digested with nitric acid alone or in combination with another acid. Soluble salts may be dissolved in water and the aqueous solution analyzed. X-ray methods may be applied for non-destructive determination of the metal. The detection limits in these methods are higher than those obtained by the AA or ICP methods. ICP-MS is the most sensitive technique. Several colorimetric methods also are known, but such measurements require that the manganese salts be aqueous. These methods are susceptible to interference. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

This analytical method measures two elements, K and Th, which were also determined via NAA. However, the neutron activation procedure is limited to samples weighing a few hundred milligrams. Thus, another set of internal standards was included in the analysis to insure not only consistent and accurate results, but also homogeneous samples for these elements over a sample size range of 1000-fold. 

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