Monoelement standards

In general, activation analysis relies on the use of standards that are irradiated under the same conditions and in the same position, and are also measured under the same conditions. Monoelement standards contain a known amount of one element. If they are applied to the evaluation of other elements the ratio of the cross sections o x/o s under the special conditions of irradiation and the ratio H /Hs of the relative abundances of the decay processes that are measured must be known (subscript x is for the sample and subscript s for the standard). Knowledge of the ratio o x/o s may cause problems, because the cross sections may vary drastically with the energy of the projectiles, for instance in the energy range of epithermal neutrons. These problems are not encountered with multielement standards that contain all the elements to be determined. However, the preparation of such multielement standards may be time-consuming. 

The method is based on the simultaneous irradiation of the sample with standards of known quantities of the elements in question in identical positions, followed by measuring the induced intensities of both the standard and the sample in a well-known geometrical position. A relative standardization can be performed by means of individual monoelement standards, or by using synthetic or natural multielement standards. In the comparison method, one needs to correct the difference in decay between the miknown sample and the comparator standard. One usually decay corrects the measured counts (or activity) for both samples back to the end of irradiation using the half-life of the measured isotope. The equation used to calculate the mass of an element in the unknown sample relative to the comparator standard is... 

When a relative method is used in CPAA, the ratio of the beam intensity for standard and sample has to be determined experimentally. As knowledge of the absolute beam intensity is not required, it is common practice to cover sample and standard with a thin (much less than the range) Tmonitor foil. The induced activity in the I-monitor foil is then a measure of the beam intensity. Pure metal foils are the obvious choice they are good thermal conductors, monoel-emental, and available in different thicknesses. 

Atomic absoiption spectrometer with background correction Graphite furnace with autosampler and pyrolytically coated graphite tubes Monoelement aluminum hollow cathode lamp Aluminum standard, 1 g/Iiter (Al metal in 0.3 M HCl)... 

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See also in sourсe #XX -- [ ]

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