Radiochemical degradation

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

The highly radioactive end-labeled oligonucleotides should not be used for more than 1 week after preparation because they are subject to radiochemical degradation. Labeled probes not used immediately are stored at — 20°. We routinely prepare several groups of slot blots and perform the hybridization to oligonucleotide probes on the day they are labeled. 

Radiochemical degradation occurs by three simultaneous competitively distinct processes [48, 49] oxidative scission in amorphous phase, crosslinking and crystal destruction and/or chemicrystallization. The increase of polymer density was explained by crystallization in amorphous regions. In fact, chemicrystallization induces only an apparent increase of crystallinity and of density, observed in PE oxidation too, due to the formation of polar groups (—OH, >C = 0, —COOH) which would increase intermole-cular forces of attraction, resulting in a high density [50]. 

A related mechanism of degradation involves the direct interaction of the radioactive emission with other tracer molecules in the preparation. This phenomenon is likely to occur in high specific activity compounds stored at high radiochemical concentrations in the absence of free-radical scavengers. 

As pointed out previously, controlled degradation reactions are very difficult with aliphatic or alicyclic hydrocarbons, and most of the relabeling work has been concentrated on aromatic reaction products. Procedures have been extensively described by Pines and co-workers (e.g., 97, 96, also 87, 89-98, 95, 98). For the present purpose, it suffices to note that the 14C contents of the methyl side-chains and the rings in aromatic reaction products are readily estimated by oxidation of the methyl to carboxyl, followed by decarboxylation, while ethyl side-chains may be oxidatively degraded one carbon atom at a time. Radiochemical assays may be made on CO2 either directly in a gas counter, or after conversion to barium carbonate, while other solid degradation intermediates (e.g., benzoic acid or the phthalic acids) may be either assayed directly as solids or burned to CO2. Liquids are best assayed after burning to CO2. 

Variously astatinated nucleic acids, At-DNA and At-RNA, have been obtained via their chloromercuri derivatives with radiochemical yields of >90%. These compounds have been isolated and proved stable to purification by gel filtration. There was no evidence of any deastatination at pH 2-11 on incubation for 20 hours, nor at neutral pH in the presence of small amounts of reductants or oxidants at room temperature. However, heating to 50° C caused slow deastatination with 15-20% astatine loss in 20 hours. On heating of the At-nucleic acids there was some degree of degradation hut this did not appear to involve breakage of the C—At bond (i 70). 

Radiochemical purity should be demonstrated by one or preferably two separate dilution analyses as described in Section 6b, or by degradation experiments together with dilution analysis. Neglect of this important... 

In many situations, the experimenter will prefer to buy labeled compounds from commercial suppliers rather than attempt to synthesize them. The radiochemical purity of such purchased compounds cannot be assumed. Radiation-induced selfdecomposition (radiolysis) can result in the formation of a variety of labeled degradation products, which must be removed before experimental use of the compounds. The extent of radiolysis depends on the nature of the labeled compound, how long it has been stored, and the manner of storage. Radiolysis is most significant with low-energy (3 emitters (especially tritium) since the decay energy is dissipated almost entirely with the compound itself. Furthermore, impurities involving other radionuclides may be present. 

This method characterized by a very high quantum yield seems to be very attractive because it would prevent any important degradation reaction, which is usually concomitant with the radiochemical activation discussed next. 

Nowak, Z., Nowak, M. 1973. Radiolytic and thermal degradation of dodecane - 30% TBP-HNO3 systems. Radiochem. Radioanal. Lett. 14(3) 161-168. 

D.P. Bishop and D.A. Smith, Combined pyrolysis and radiochemical gas chromatography for studying the thermal degradation of epoxy resins and polyimides. I. The degradation of epoxy resins in nitrogen between 400° and 700°C. J. Appl. Polym. Sci., 14, 205 (1970). 

The nature of our concern is best illustrated by a specific example. Blank and Kidwell use a cocaine solution of 100,000 ng/mL for their contamination experiments, to which they add approximately 1 pCi of tritium-labeled cocaine, i.e., approximately one million counts per minute. Therefore, they have approximately a sensitivity of 10 cpm/ng of sample. Decontamination of hair means that residual drug concentration must drop below the endogenous cutoff level of 5 ng/10 mg of hair, i.e., to 50 cpm/10 mg hair. Now if the labeled cocaine has a radiochemical impurity of as little as 0.1%, this corresponds to 1000 cpm or to 100 ng of residual cocaine equivalents. Since self-irradiation of tritium-labeled material tends to form polymeric impurities, and since these are likely to preferentially bind to hair, one incurs a major risk of concluding erroneously that the residual radioactivity represents residual cocaine contamination rather than contamination by polymeric degradation products. 

The degradation of thyroxine in liver occurs primarily by conjugation with glucuronic acid. In this assay, [12SI]thyroxine glucuronide was quantitated by on-line radiochemical detection. 

The main limitation of the technique is the problem of achieving radiochemical, as distinct from chemical, purification of the final labeled product. Purification of the irradiated sample is necessary to remove radiation-induced degradation products and labile tritium. Many of the degradation products are not only chemically similar to the parent compound, but also possess much higher specific activities. For complete purification it is necessary to use multistage processes, such as gas and column chromatography, countercurrent distribution, and fractional distillation. Distribution of isotope within a molecule is generally random and nonuniform however, in some circumstances useful specificity can be achieved.17... 

The basic methodologies of measuring JH titer and rates of biosynthesis will be mentioned here only briefly. JH titer is determined by either radioimmunoassay (3) or preferably by gas chromatography coupled with mass-spectrometry (4-6). Alternatively, JH biosynthesis can be determined with excised CA in vitro by the radiochemical (RCA) method (2). JH values obtained by titering reflect the equilibrium of rate of JH synthesis, its degradation and clearance from the hemolymph, its tissue uptake and excretion. The rate of de novo synthesis, as measured by the RCA method, is argued to correlate well with JH hemolymph titer (8.9). As we shall later show, the correlation of in vitro synthesis and ija vivo titers is not always maintained in L. migratoria under all experimental situations. 

Iavarone, L., Scandola, M., Pugnaghi, F. and Grossi, P. Qualitative analysis of potential metabolites and degradation products of a new antiinfective drug in rat urine, using HPLC with radiochemical detection and FIPLC-mass spectrometry. ]. Pharm. Biomed. Anal. 13 607-614, 1995. 

Such phenomena dictated two major philosophies for our work with pyrethroids (a) a mass balance approach was taken to account for all losses in the experiments (b) as long as the pyrethroids maintained their chemical integrity (did not hydrolyse or otherwise degrade) then use of radiolabelled analytes was desirable. This allowed the low concentrations of the pyrethroids in the aqueous phase to be accurately and reproducibly determined and the mass balance to be computed. The chemical integrity of the radiochemicals was determined by radio-chromatographic techniques (radio-TLC and radio-HPLC). 

The in vitro stabilities of the radioiodinated compound were evaluated by incubating the radiolabelled protein in PBS and in human serum. The radiochemical purities were evaluated at 12, 24, 48 and 72 h post-labelling. The stability of the labelled compound with low specific activity was compared with that of the labelled compound with high specific activity. The results showed complete retention of the radiochemical purity of the low specific activity preparation for up to 48 h in both media. However, in the case of the high specific activity preparation, rapid degradation of the radiolabelled protein was observed (Fig. 4.1), with a loss of 54% of the radioiodine in PBS and even higher losses in human serum. These results show a dependence of the stability on the specific activity and/or the radioactive concentration resulting from... 

Field studies with single applications of phenoxyalkanoic acid herbicides have indicated that breakdown is rapid under temperature and moisture conditions that favour microbiological activity (5). Enhanced degradation of these herbicides, under field conditions, was first noted in the late 1940s. The use of plant bioassay procedures, led to the discovery that the persistence of 2,4-D, but not 2,4,5-T, was decreased,by pretreatment of the soil with 2,4-D (26, 27). This enhanced breakdown was later confirmed using (14C)2,4-D and radiochemical analytical techniques (29). The breakdown of the (14C)2,4-D being more rapid in soil from the treated plots, tested 8 months after the last field application, than in soil from plots treated for the first time. 

Nitration and oxidation. Nitric acid does not react appreciably with TBP at temperatures up to 70°C. At sufficiently high temperatures, however, nitration and oxidation take place. In two instances reaction of TBP-hydrocarbon mixtures with hot, concentrated solutions of nitric acid and uranyl nitrate led to destructive explosions. At Savannah River in 1953 [Cll], an evaporator was destroyed while concentrating a solution of nitric acid and uranyl nitrate that contained TBP and a kerosene diluent. At Oak Ridge in 1959 [A8], an explosion occurred in a radiochemical plant evaporator that was concentrating a nitric acid solution of plutonium nitrate possibly contaminated by TBP, diluent, and their radiation degradation products. 

A number of reviews of the effects of radiation chemistry on polymer systems have been published. The classic texts are those of Charlesby, Chapiro and Dole. " Since then a number of extensive reviews of the effects of radiation have been published. For example, the Polymer Handbook has previously tabulated lists of the radiochemical yields reported elsewhere.-" The ACS has published a number of collections of papers presented at radiation chemistry meetings,- -- and the two journals Radiation Physics and Chemistry and Polymer Degradation and Stability are of much interest. As mentioned above the proceedings of major meetings on radiation processing,-"and the text by Singb and Silverman provide an excellent overview of the field. 

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