It is generally accepted that an analytical procedure can be regarded as an information production system yielding information on the composition of the analyzed sample. Thus, information theory can be useful and the quantities characterizing the information properties of an analytical method may be applied not only as evaluation criteria but also as objective functions in the optimization. The usability of information theory is demonstrated on the example of neutron activation analysis. Both precision and bias of NAA results are taken into account together with the possible use of reference materials for quality assessment. The influence of the above-mentioned parameters on information properties such as information gain and profitability of NAA results is discussed in detail. It has been proved that information theory is especially useful in choosing suitable reference materials for the quality assessment of routine analytical procedures not only with respect to matrix and analyte concentration in the sample but also to concentrations and uncertainties of certified values in the CRM used. In the extreme trace analysis, CRMs with relatively large uncertainties and very low certified concentrations can still yield rather high information gain of results.
Information properties of analytical results together with other important parameters especially economic ones can be used for the optimization of analytical procedures. Therefore, we have proposed a computational technique for the optimization of multielement neutron activation analysis (NAA) based on the information content and profitability. The optimization starts with the prediction of the -ray spectra to be expected during analysis under given experimental conditions (sample size, irradiation, decay and counting times etc.) and with the calculation of detection and determination limits. In the next step, the information contents for the determination of particular elements and for the simultaneous determination of element groups are computed. The information content depends or is closely connected with such properties of the method as selectivity, sensitivity, precision, accuracy and, as in the other cases of trace analysis, also with the detection limit. Then, the information profitability (IP) taking into account the information content and relevance (appreciation of specific information according to its contribution to the solution of a given problem) together with economic aspects can be calculated. This function can be used for the optimization of a particular NAA procedure, for the mutual comparison of different variants of NAA and also for the comparison with other analytical methods. The use of information profitability for the optimization of NAA is shown on a practical example of the INAA analysis of urban particulate matter SRM 1648 produced by NBS (USA).
Hair samples collected from free living mammals common hare (Lepus europaeus Pall.), common vole (Microtus arvallis Pall.) and wood mouse (Apodemus sylvaticus L.) have been subjected to instrumental neutron activation analysis (INAA). up to 18 elements As, Au, Br, Ce, Co, Cr, Cu, Fe, Hg, K, La, Na, Sb, Sc, Se, Sm, Th and Zn have been determined in each hair sample. Animal hair samples from areas polluted by thermal power plants burning coal were taken and compared with hair samples from the animals living in relatively non-polluted control areas. The results show that animal hair samples from areas with higher level of pollution contain usually higher concentrations of toxic and essential elements as As, Co, Cr, Fe and Se. Muride rodents can be used for more detailed monitoring of environmental exposure than hare. Moreover, hair of common vole shows usually higher levels of contamination as compared with wood mouse, which could be explained by different compositions of feed. Animal hair has been shown to be a rather sensitive indicator of environmental exposure and INAA has proved to be a suitable analytical tool for this purpose.
An epithermal neutron activation analysis (ENAA) procedure has been evaluated in which samples packed in polyethylene capsules are irradiated during 15 minutes only, and induced activities are counted using well-type Ge-spectrometry. The evaluation was carried out in order to improve on ease of operation in ENAA. Biological and sediment reference materials have been analyzed. Compared to routine INAA, an improvement in detection limits was observed for As, Au, Cd, Mo, Ni, Sb, Sm, Sr, Ta, U, W and Zn. By Au–Zr neutron flux monitors, epithermal flux gradients have been determined. Concentrations found in the reference materials were generally in agreement with certified and consensus values.
An ion selective electrode was employed to check a possible calcium release from human hair when washing samples prior to their neutron activation. Preliminary measurements in TRIS buffer /pH 7.4/ demonstrated the portion Ca release to be of 3.3% out of the total Ca content.
Instrumental neutron activation analysis (INAA) has been utilized for the analysis of 19 different fly ashes, 1 sample of
coal and 3 samples of industrial emissions. Both short and long irradiations in a nuclear reactor have been used. The irradiated
samples have been measured by means of a computer-based Ge(Li) gamma-ray spectrometer. The concentrations of 27 elements have
been determined in the samples.
Profiles of thermal flux, epithermal flux, f and have been determined for a multisample irradiation container in use with k0-based NAA. It could be concluded that for accurate NAA with the IRI-reactor, samples should be surrounded by at least two thermal flux monitors. Since f and values were found to vary considerably between irradiations as compared to the variations within one container, the possibility of determining f and just once for the irradiation facility used, and monitoring only the thermal flux in future irradiations, was ruled out. The use of average values for f and within a container was shown to yield unacceptable inaccuracies mainly for Zr and U determinations.
Performance characteristics (especially accuracy) of a routine INAA with k0 standardization were verified and compared with those of INAA with a classical single comparator. For this purpose, samples of three certified reference materials of environmental origin (Fly Ash, Orchard Leaves and Buffalo River Sediment—all supplied by NIST) were irradiated with both kinds of comparators (Au–Zr for k0 and Zn for classical k method) in one irradiation rabbit. Also the following steps of INAA procedure were practically the same for both standardization methods used (counting, spectral processing, etc.). The results have shown that the k0 method gives sufficiently accurate results comparable with those of the well established and routinely used single comparator (Zn) method, provided proper neutron flux monitoring, efficiency calibration and also coincidence summing corrections are applied. This work shows that modern k0 standardization method in INAA can be sucessfully used in routine practice and applied with an advantage in INAA laboratories subject to changes of neutron spectra or counting conditions.
Authors:I. Obrusník, O. Skřivánek, M. Umlaufová, and V. Hovorka
Instrumental neutron activation analysis has been performed on human head hair of newborns and mothers sampled in two areas with a different level of environmental exposure. The group of neonates from the exposed area (polluted by thermal power plants burning brown coal and by chemical industry) has exhibited higher levels of several trace elements in hair, e.g. Se, Zn, Hg and Sb in comparison with the control group. Moreover, the mean concentrations of Se, Hg, Zn and Br in neonate hair have been found to be higher than in mothers hair. Although the study revealed statistically significant differences in the composition of neonate hair sampled in areas with different levels of environmental exposure, the differences are relatively small. Only a thorough long-term study both with environmental and medical observations can prove a direct connection of the elevated levels of some trace elements in neonate hair with the higher frequency of mental diseases of children living in the exposed area.
Authors:N. Pilecká, M. Raković, I. Obrusník, and P. Broulík
The calcium concentration was determined in human hair of several normal individuals and of patients suffering from various bone diseases. A comparison of results achieved with an ion-selective electrode with those obtained with the neutron activation analysis demonstrated a good accuracy of the two methods. The precision of the determination with the ionselective electrode may be characterized by a relative standard error of 8.8%.