Interference Reactions

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 

If this is the case and the investigator does not consider these last two reactions, the mass of Mg will be determined to be higher than it is. 

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

In the last two reactions, the proton is produced by the incident fast neutrons interacting with the target nuclei. 

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If such fillers are to be used, they should have a neutral or slightly alkaline pH, otherwise additives such as ethylene glycol and triethanolamine, which are preferentially adsorbed on the surface of the filler, should be used, preventing any undesirable interference reactions between the filler and the crosslinking peroxide. These additives must, however, always be added to the mix before the peroxide. With some mineral fillers, such as some types of clay, the polymer may be bound to the filler by means of silane treatment, and the surface of the filler becomes completely non-polar. Consequently, the interaction with the polymer matrix increases, while the adsorption of the crosslinking peroxide by the filler is severely suppressed. 

Primary interference reactions. These are nuclear reactions on elements other than the element to be determined which yield the same indicator radionuclide. For example, silicon is determined by the 28Si(, >)28Al reaction. However, the same indicator radionuclide is produced from phosphorus by the 31P(w,a)28Al reaction. Hence, a high phosphorus abundance in a sample will lead to erroneously high values for silicon. Corrections may be applied to the data if concentrations of the interfering elements can be determined independently. 

Secondary interference reactions. These are nuclear reactions induced by secondary particles produced in the sample or its immediate environment which will produce the indicator radionuclide by interaction with elements other than the one to be determined. For example, nitrogen is usually determined by the 14N(n, 2n) 13N reaction. The 14 MeV neutrons may generate recoil protons by collision with hydrogen atoms in the vial, transfer tubes, or sample support assembly. These recoil protons may induce the 13C(/, )13N reaction with the carbon of the vial, leading to the formation of the same indicator radionuclide. This type of interference is ordinarily not serious in cases other than the nitrogen determination. 

Beckmann rearrangements [43] of oximes of steroid ketones are quite normal in forming amides or lactams [44,43,46] unless special structural features interfere. Reactions are effected by the usual reagents, particularly thionyl chloride and sulphonyl chlorides. In one case the use of benzene- or toluene-j -sulphonyl chloride in a basic solution permitted the... 

Unsaturated polyesters with terminal hydroxyl or carboxyl groups can also react at the same time with isocyanate groups and a vinyl monomer, such as styrene, to produce hybrid polymers. Two kinds of reactions, the NCO-OH addition reaction and radical polymerization by the vinyl groups, can occur. In parallel to the two reactions, interference between the two reactions also can occur. Detailed studies on the interference reactions were studied by Hsu et al. (190). 

The foil material should be available in high purity, to avoid interference reactions caused by impurities. 

Composite based electrodes have shown improved selectivity by inhibiting the interference reaction at the electrode. The large surface area, electrochemical properties, catalytic abilities and inherent biocompatibility make composites suitable for use in amperometric biosensors. Composites of gold nanopaiticles carbon based materials, Prussian blue nanopaiticles have been utilized for the fabrication of electrochemical... 

If the interference is not too important it can be corrected for. If uranium or other fissionable material is present in the sample, the fission products with high fission yields (Sr, Mo, Zr, Ce, Ba) can induce important positive errors. The interference can easily be estimated when the uranium content is known. In FNAA, secondary interference reactions may occur when fast neutrons interact with other elements and produce particles that induce a nuclear reaction that forms the same indicator nuclide. These particles are usually protons ejected by fast neutrons from a matrix with a high hydrogen content. Examples are ... 

Problems encountered in acquiring stable gas-phase conditions in the laboratory also contribute to the relative lack of atmospheric pesticide reaction rate and product data. Semi-volatile organics, which conoprise the majority of pesticides, can sorb onto the surfaces of the laboratory reaction vessel. The wall interference reaction rates and products may or may not be similar to those occurring under actual atmospheric vapor-phase conditions (P,/0). Experimental designs that can provide environmentally relevant reaction rates, clwacterization of gas-phase and oxidative transformation products, and maintain material balance at environmental tenq>eratures has yet to be established. 

If it is suspected that the composition of the product is not correct, for example that the antioxidant or correct cure system has not been incorporated into the compound, then the appropriate species specific tests that have been discussed earlier can be carried out. If possible, it is advisable to conduct the analysis alongside that of a sample known to be of the correct composition. This ensures that no product specific matrix effects/interference reactions affect the data obtained and hence the conclusions reached. 

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

Ferric chloride solution sometimes contains a large excess of HCl which would interfere with the following reactions. If it is very markedly acidic add dil. NaOH solution, drop by drop, to the ferric chloride solution until a small but permanent precipitate of ferric hydroxide is obtained. Filter this off through a small fluted filter paper, and use the clear filtrate. The latter is still not quite neutral owing to hydrolysis, but this feeble acidity does not interfere with the tests given below. 

The hydrochloride of the nitroanilin may separate out at this stage, but this does not interfere with the reaction as the hydrochloride separates in fine, feathery crystals which readily redissolve and hence are very reactive. 

The principle of this test is as follows The liquid suspected of containing urea is treated with dilute acid or alkali until its pH is about 7. A solution of the enzyme is also made and its pH adjusted to 7. The two solutions are mixed and the resulting conversion of urea to ammonium carbonate causes the pH of the solution to rise to over 8 this change is noted by the use of a suitable indicator, phenol-red being the one usually employed. Proteins do not interfere with the test, but the reaction is inhibited by traces of heavy metals. 

Surely one would hope not. What if one just used the mercuric nitrate monohydrate that is at hand. One s only real concern would be if the monohydrate water would interfere with the acetonitrile in a competing oxymercuration reaction. But could it really considering the massive excess of acetonitrile present All Strike can say is that someone, somewhere is gonna try it. And Strike would really, really like to hear about it. 

Stereoselectivities of 99% are also obtained by Mukaiyama type aldol reactions (cf. p. 58) of the titanium enolate of Masamune s chired a-silyloxy ketone with aldehydes. An excess of titanium reagent (s 2 mol) must be used to prevent interference by the lithium salt formed, when the titanium enolate is generated via the lithium enolate (C. Siegel, 1989). The mechanism and the stereochemistry are the same as with the boron enolate. 

Analytical chemistry is more than a collection of techniques it is the application of chemistry to the analysis of samples. As you will see in later chapters, almost all analytical methods use chemical reactivity to accomplish one or more of the following—dissolve the sample, separate analytes and interferents, transform the analyte to a more useful form, or provide a signal. Equilibrium chemistry and thermodynamics provide us with a means for predicting which reactions are likely to be favorable. 

Since an analyte and interferent are usually in the same phase, a separation often can be effected by inducing a change in one of their physical or chemical states. Changes in physical state that have been exploited for the purpose of a separation include liquid-to-gas and solid-to-gas phase transitions. Changes in chemical state involve one or more chemical reactions. 

Other types of reactions can be used to chemically separate an analyte and interferent, including precipitation, electrodeposition, and ion exchange. Two important examples of the application of precipitation are the... 

In a liquid-liquid extraction, the analyte (or interferent) is extracted from one liquid phase into a second, immiscible liquid phase. When the analyte is involved in secondary equilibrium reactions, it is often possible to improve selectivity by carefully adjusting the composition of one or both phases. 

The presence of a time limitation suggests that there must be a kinetically controlled interference, possibly arising from a competing chemical reaction. In this case the interference is the possible precipitation of CaCOs. 

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