Magnesium neutron activation analysis

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

The y particle is emitted virtually instantaneously on the capture of the neutron, and is known as a prompt y - it can be used analytically, in a technique known as prompt gamma neutron activation analysis (PGNAA), but only if such y s can be measured in the reactor during irradiation. Under the conditions normally used it would be lost within the nuclear reactor. In this reaction, no other prompt particle is emitted. The isotope of sodium formed (24Na) is radioactively unstable, decaying by beta emission to the element magnesium (the product nucleus in Figure 2.13), as follows ... 

Olehy DA, Schmitt RA, Bethard WF. 1966. Neutron activation analysis of magnesium, calcium, strontium, barium, manganese, cobalt, copper, zinc, sodium, and potassium in human erythrocytes and plasma. J Nucl Med 6 917-927. 

NOTE All values are given as weight percent ( 95% confidence limit). Sulfate and chloride were analyzed by ion chromatography carbonate was analyzed by classical titration and calcium, magnesium, chlorine, aluminum, and sodium were analyzed by neutron activation analysis. 

Two methods that have been used in tracer studies with are neutron activation analysis CNAA) and EIMS, mass spectrometry of a volatile chelate with electron impact ionization C15>19,20). Neither technique can match TIMS in accuracy or precision, but both may offer greater sample throughput. Until more precise techniques become more generally available, research utilizing stable metal isotopes will remain limited to the conditions imposed by the analytical facilities at hand. As will be shown below, research with stable magnesium isotopes to date, has had to be tailored to the limitations of available analytical techniques ... 

Neutron Activation Analysis. Magnesium-26 has a small cross section of 0.03 b. The product of irradiation with thermal neutrons is Mg (1 9.5m). As shown in Table 1, several elements commonly present in biological materials give rise to radioactive nuclides with radiations at energy levels close to those characteristic of Mg. Neutron activation was used in the first trials of Mg as an in vivo tracer when measurements were made with a well-type Nal-Tl crystal detector (14,21). Under these conditions the presence of sodium, altuninum and manganese in the samples interfered in the accurate detection of Mg, but could be reduced or eliminated by sample purification. 

Neutron activation analysis has low sensitivity for the detection of Mg. The best results were obtained in our hands by irradiating samples containing 200-300 yg natural magnesium (20-35 yg Mg) for one minute at a neutron flux of 10 cm sec followed by detection with a HP-6e detector. Since it is usually necessary to run activations in triplicate and to carry out additional analyses for total magnesium by an Independent method such as atomic absorption NAA is not suitable for specimens containing magnesium at low concentrations. It was found suitable for the analyses of fecal samples, marginal for urine and unsuitable for plasma (15,21). 

A reasonably complete analysis of the inorganic chemical composition of the aerosol requires much effort and involves, in addition to wet chemical methods, instrumental techniques such as neutron activation analysis, atomic absorption spectroscopy, or proton-induced X-ray emission (PIXE). These latter techniques yield the elemental composition. They furnish no direct information on the chemical compounds involved, although auxiliary data from mineralogy, chemical equilibria, etc. usually leave little doubt about the chemical form in which the elements occur. Thus, sulfur is present predominantly as sulfate, and chlorine and bromine as Cl- and Br-, respectively, whereas sodium potassium, magnesium, and calcium show up as... 

The determination of trace elements in urinary calculi is gaining importance in view of studies on the mechanisms of their formation. These human biological specimens usually have a low organic content but are rich in phosphate, calcium and magnesium and also have varying amounts of silicates. The specimens have been studied directly by non destructive techniques, e.g. instrumental neutron activation analysis (Jacimovic et al.. 

There are numerous methods for the analytical detection of magnesium. Only those methods which are applicable to biological material will be discussed. The variability of instruments required is immense, ranging from the colorimetric method, which requires only a spectrophotometer, to neutron activation analysis. 

Table 2.3 clearly shows that flame photometry after dope ashing at 500 "C gives a quantitative recovery of sodium relative to results obtained by a non-destmctive method of analysis, i.e., neutron activation analysis (NAA). Direct ashing without the magnesium ashing aid at 500 °C causes losses of >10% of the sodium, whereas direct ashing at 800 °C causes even greater losses. 

One source of error in activation analysis is interference reactions. These are reactions that produce the same isotope as the one being counted, through bombardment of a different isotope in the sample. As an example, assume that a sample is analyzed for magnesium by using fast-neutron activation. The reaction of interest is Mg(n,p) Na. Therefore, the activity of Na will be recorded, and from that the amount of Mg can be determined. If the sample con-tains Na and Al, two other reactions may take place which also lead to Na. They are... 

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