Errors activation analysis

Although sophisticated methods may constitute the core methods for certification it is useful to include good, well executed routine methods. In order to further minimize systematic error, a conscious purposeful attempt should be made to get methods and procedures with wide-ranging and different sample preparation steps, including no decomposition as in instrumental neutron activation analysis and particle induced X-ray emission spectrometry. 

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

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. 

Methods were described for diminishing the systematic errors in Pb activation analysis, stemming from the variable isotopic composition of Pb. Results with ICP-AES and ICP-MS were taken as standards for comparison106. 

Selected ion monitoring can be used for the determination of the relative amount of each component of a mixture, introduced into the mass spectrometer by the direct inlet probe However, such a determination requires reference mixtures of known composition for calibration. In the present experiment, since the monochloro pentaziridino derivative had not yet been isolated in the pure form, it was necessary to determine its concentration, by an auxiliary method, in a sample which could then be utilized as a reference mixture for further experiments. In order to do this we titrated chlorine in the toxic sample of MYKO 63 (B) by the classical method. The results indicated that the amount of N3P3AZJCI was between 0.5-1.5 %. The large statistical error is due to the low chlorine content in the sample examined. Thus, we used the remarkable possibilities provided by neutron activation analysis when the impurity to be quantified is a chlorinated moiety. It is well-known indeed that the C1 -f 2n peak is amongst the most easily detectable by neutron... 

By using measured values in structure-activity-analysis more reliable structure-activity-relationships may be obtained, but higher experimental expense is normally required. When is this expense justified This is only the case if the deviation between the calculated and the measured values of the physico-chemical property exceeds all the other errors which enter the respective structure-activity-relationships or if measuring requires less effort than calculation. 

Discussions of errors associated with the technique of activation analysis in general may be found in many of the books and monographs referenced in the introduction to this paper. Interferences unique to 14 MeV neutron activation techniques have been reviewed by Mathur and Oldham 42> and a discussion of precision has been published by Mott and Orange 43>. 

The time of irradiation necessary for the accumulation of long-lived isotopes is 30 days, but in the case of short-lived isotopes the time of irradiation was selected in accordance with the saturation conditions. One ml of the solution was taken for the activation analysis. For the other analytical methods, the sensitivity data was recalculated to correspond to a sample volume of 1 ml, and the relative error to 10%. The threshold of the determination corresponds to 40 decays per second. 

From all that has been said about activity and activity coefficients, it is apparent that whenever precise results are to be expected, activities should be used when expressing equilibrium constants or other thermodynamic functions. In the present text however we shall be using simply concentrations. For the dilute solutions of strong and weak electrolytes that are mainly used in qualitative analysis, errors introduced into calculations are not considerable. 

Activation analysis is a blank-free technique. In general, blanks not oiJy determine the limits of detection, but at low concentrations they cause the main problems with respect to accuracy, because the small amounts to be determined have to be conveyed through all the steps of the chemical procedures, from sampHng to detection, without introducing systematic errors. These problems are not encountered in activation analysis, because contamination by other radionucHdes can, in general, be excluded and losses of the radionuclides to be determined can easily be detected by activity measurements. 

While high sensitivity has been obtained in the examination of pure materials, a far more rigorous test of the activation method is found in its application to materials of a more complex matrix. Emission and X-ray spectrometry and direct spark source mass spectrometry are all restricted by the lack of suitable standards when applied to materials of complex composition. Provided that precautions are taken to avoid self-shielding errors radioactivation is largely independent of the nature of the matrix material. It is this advantage which has enabled activation analysis to prove such an invaluable tool in geochemistry. 

Activation analysis has become, because of its extremely high sensitivity, an indispensable tool in a wide variety of fields ranging from science and engineering to industry, minerals exploration, " medicine, environmental monitoring, and forensic science. " The purpose of this chapter is not to present all the aspects, details, and applications of this field, but to discuss the major steps that comprise the method, the interpretation of the results, the errors and sensitivity of the method, and certain representative applications. The reader will find many more details and an extensive list of applications in the bibliography and the references given at the end of the chapter. 

Activation analysis may be qualitative or quantitative. In a qualitative measurement, only identification of the element is involved. This is accomplished, as shown in Sec. 12.7.3, from the energies and intensities of the peaks of the spectrum. In a quantitative measurement, on the other hand, in addition to identification, the amount of element in the sample is also determined. To illustrate how the mass is determined and what the errors and sensitivity of the method are, consider the energy spectrum of Fig. 15.1 as an example. 

Sensitivity of the activation analysis method for a particular element refers to the minimum mass of that element that can be reliably detected. The minimum detectable mass is determined from Eq. 15.4 by assuming the most favorable conditions for the measurement and by setting an upper limit for the acceptable error of the result. The process is similar to the determination of the minimum detectable activity discussed in Sec. 2.20. 

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... 

Many papers have been published directed to the preparation of biological samples for analysis and to methods for avoidance of contamination. Versieck (1983) discussed errors arising from needles, vacutainers, etc, for Cr, Ni, Cd, Pb, Mn, Cu and other metals. He also discussed errors arising in neutron activation analysis. His emphasis is on problems relating to contamination. 

In fact, errors at the sampling stage can be more easily avoided with neutron activation analysis than with any other analytical technique firstly, because preirradiation sample preparation can generally be kept to a minimum and, secondly, because mineralization of the sample and chemical separations of the elements of interest can be postponed till after the end of the irradiation when extraneous additions from reagents and laboratory equipment no longer affect the final result. These points are discussed more in detail later in this chapter. 

Neutron activation analysis is an invaluable technique for trace element determinations in biological matrices. Probabiy its most important advantage is its relative freedom from errors due to extraneous additions of exogenous materiai from reagents, equipment, or laboratory environment. Characteristics which contribute further to the popularity of the technique are its outstanding sensitivity, excellent specificity, and multielement capability. In principle, the technique is able to produce relatively unbiased and precise measurements — at least in competent hands. That it is, however, necessary to warn against uncritical expectations is illustrated by the grossly inconsistent results obtained in several laboratories. 

The Pb and Br diurnal patterns were measured for a particular day using x-ray fluorescence techniques (4). Those patterns which demonstrated a possible non-automotive Pb component were then analyzed for many elements including Br but not Pb, by neutron activation analysis (NAA) (5). Measurements of Br concentrations by the two techniques were within experimental error. Both techniques are non-destructive. 

Each experimenter involved in the use of activation analysis has created particular techniques that he considers practical to his use of the method. The publications cited herein give specifics about each of these efforts. Other writers, however, emphasize special situations that are inherent to any activation analysis application. For example. Gibbons (317) has reported on the possible errors to be encountered in activation analysis applications, while Mesler (611) has provided potential users of... 

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