Activation analysis interference reactions

Devise an activation analysis scheme for determining the concentration of nitrogen in a sample of plant material. Assume the analysis must be nondestructive and rapid. Suggest an appropriate reaction, irradiation, and counting conditions, and indicate possible interferences in your analysis. 

Selenium forms a volatile derivative, piazselenol, which can be subjected to GC analysis (Scheme 5.39). Young and Christian [612] treated selenium with 2,3-diaminonaph-thalene at pH 2.0 and extracted the resulting piazselenol into -hexane. With the use of an ECD, down to 5 10-I° g of selenium could be detected. The procedure, applied to the analysis of selenium in human blood, urine and river water, led to results equivalent to those obtained by neutron activation analysis. Similarly, Nakashima and Toei [613] performed the reaction of selenium (as selenious acid) with 4-chloro-o-phenylenediamine at pH 1 and extracted the derivative into toluene. They reported a detection limit of 0.04 jug. Shimoishi [614] analysed the content of selenium in metallic tellurium by this method. The sample was dissolved in aqua regia, followed by reaction with 4-nitro-o-phenylenediamine and extraction into toluene. Down to 10 ng of selenium could be determined using only a few milligrams of sample. Common ions did not interfere even when present in a large excess. Selenium in marine water was determined after the same derivatization step [615],... 

Arsenic levels below 10 ng/g can be readily detected in petroleum by instrumental neutron activation analysis. The most convenient technique involves direct gamma counting based on the 75As (n, y) 76As reaction with a principle radiation of 559 keV. After a 1-hr irradiation at a neutron flux of 1012 n cm"2 sec-1, the arsenic may be counted in a relatively short time. The method requires a high resolution Ge(Li) detector to avoid interference from bromine (550 keV) or antimony (564 keV). 

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... 

Interference reactions are discussed in detail in many activation analysis books (see Rakovic, and Nargolwalla Przybylowicz). A few representative examples are given below ... 

Several investigators have used neutron activation analysis (NAA) to determine the aluminium content of biological specimens both with and without some chemical processing. Instrumental neutron activation analysis involves the bombardment of a sample with neutrons and the measurement of the radioactivity induced by nuclear reactions. No chemical processing is required. Upon activation Al (100% isotopic abundance) forms the radioactive AI nuclide by a (n,y) reaction. There are a number of attractive features in this technique which include excellent sensitivity with relative independence from matrix effects and interferences. Also, there is relative freedom from contamination since the sample is analyzed directly with minimal handling. One major problem is the need to... 

A scheme of activation analysis usually includes a chemical separation process or the addition of a carrier . It is sometimes desirable to remove chemically, before activation, any elements which would cause interference. For example, the presence of sulphur or chlorine in the sample could cause inaccuracies in the estimation of phosphorus content due to reactions (13.216) and (13.217) taking place and providing extra radioactive phosphorus. In a similar manner silicon can also cause interference with the phosphorus estimation owing to the side reaction. 

There are potential interferences in activation analysis that must be considered and corrected for these include interfering activation reactions and gamma-ray spectral interferences. Several well-known cases for NAA will be discussed other forms of AA will need an assessment according to each experiment. 

Charged Particle Activation.—Charged particle activation analysis is most often applied to the measurement of the light elements Be to F using p, d, He, or a-particles. The various reactions used for the determination of these elements are subject to many mutual interferences. Table 3 shows the various reactions possible for the determination of O using charged particles. 

Table II-ll Examples of competing nuclear reactions liable to cause interferences in the determination of carbon, nitrogen and oxygen by (r,n) activation analysis...

Proton activation analysis via the reaction B(p,a) Be is one of the most interesting methods as it is precise and accurate at + 5 %, provided Be is separated radiochemically before counting and the lithium concentration in the sample is very low, which is generally the case. The use of protons of lower energy (e.g. 2.75 MeV) theoretically allows the analysis to be carried out in an instrumental way, but experience proved that it is difficult to obtain a good precision. Deuteron activation analysis via the B(d,n) C reaction is an alternative. This method is free from nuclear interferences but requires a radiochemical separation of... 

Despite the variety of methods that had been developed, by 1960 kinetic methods were no longer in common use. The principal limitation to a broader acceptance of chemical kinetic methods was their greater susceptibility to errors from uncontrolled or poorly controlled variables, such as temperature and pH, and the presence of interferents that activate or inhibit catalytic reactions. Many of these limitations, however, were overcome during the 1960s, 1970s, and 1980s with the development of improved instrumentation and data analysis methods compensating for these errors. ... 

This NAA technique is based on the nuclear reactions 23Na(n,7)24Na and 41K(n/y)42K. Half-lives of the activated products are 15.0 hrs and 12.4 hrs, respectively. For Na analysis, the samples were irradiated in a specially designed thermal column to suppress the fast neutron reaction of 27Al(n,a)24Na which interferes with the reaction for Na. For K analysis, the proplnt samples were irradiated at a standard irradiation position of the reactor. For the Na irradiations, the neutron flux in the thermal column was in the order of 1010, whereas for the K assays it was approx 1012 neutrons/cm2-sec... 

Stoichiometric reaction of 5 with phenylsilane produced the iron(O) bis(silane) c-complex 18, which was confirmed by the single-crystal X-ray analysis as well as SQUID (Superconducting QUantum Interference Device) magnetometry (Scheme 19). Complex 18 as a precatalyst showed high activity for the hydrosilylation of 1-hexene. 

FIA has also found wide application in pharmaceutical analysis.214,215 Direct UV detection of active ingredients is the most popular pharmaceutical analysis application of FIA. For single component analysis of samples with little matrix interference such as dissolution and content uniformity of conventional dosage forms, many pharmaceutical chemists simply replace a column with suitable tubing between the injector and the detector to run FIA on standard HPLC instrumentation. When direct UV detection offers inadequate selectivity, simple online reaction schemes with more specific reagents including chemical, photochemical, and enzymatic reactions of derivatization are applied for flow injection determination of pharmaceuticals.216... 

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