Artefacts stable

In any form of analysis it is important to determine the integrity of the system and confirm that artefacts are not produced as a by-product of the analytical procedure. This is particularly important in enantiomeric analysis, where problems such as the degradation of lactone and furanon species in transfer lines has been reported (40). As chromatography unions, injectors, splitters, etc. become more stable and greater degrees of deactivation are possible, problems of this kind will hopefully be reduced. Some species, however, such as methyl butenol generated from natural emissions, still remain a problem, undergoing dehydration to yield isoprene on some GC columns. 

Sayre, E.V., Joel, E.C., Blackman, M.J., Yener, K.A. and Ozbal, H. (2001). Stable lead isotope studies of Black Sea Anatolian ore sources and related Bronze Age and Phrygian artefacts from nearby archaeological sites. Appendix new central Taurus ore data. Archaeometry 43 77-115. 

Hoffmann GF, Sweetman L, et al. (1991) Facts and artefacts in mevalonic aciduria development of a stable isotope dilution GCMS assay for mevalonic acid and its application to physiological fluids, tissue samples, prenatal diagnosis and carrier detection. Clin Chim Acta 198 209-227... 

Unfortunately, this synthetic route results in the isolation not of the natural compound CP-263,114 (1), but of the C-7 epimer 15, which proved to be the more stable stereoisomer. The formation of the unwanted C-7 epimer is a consequence of the OSO4-catalyzed dihy-droxylation of intermediate 12, which occurs from the face of the C=C double bond opposite to that desired. Epimerization at C-7 takes place under basic conditions, and on closer inspection it was possible to identify both isomers in the original sample extracted from the fungus [16]. However, the isolation of both isomers could also be an artefact of the extraction process, during which the configuration at C-7 could be partially flipped during treatment with acid and base [2]. 

A single 3/8-hydroxy-steroid dehydrogenase A5-3-oxo-steroid isomerase complex is present in human adrenal301 but a similar complex isolated from rat mitochondria was thought to be a redistribution artefact.302 The dimeric form of the A5-3-oxo-steroid isomerase from Pseudomonas species has a molecular weight of 26 800 dalton and is unusually stable to dissociation on dilution 303 it may be the catalytically active species. The enzyme from a bacterial source is irreversibly inhibited by an acetylenic analogue of the natural substrate.304... 

The detection of nucleosomes in archaebacteria should be taken with caution since particles have been also visualized on the E. coli chromosome in vitro by some investigators [46]. Such structures could be artefacts of specimen preparation (compactosomes) produced by DNA supercoiling [47]. In fact, it is likely that the archaebacterial chromatin resemble more the dynamic chromatin of eubacteria, in which all genes can be activated at any time, than the stable chromatin of eukaryotes. Reeve and coworkers [24] have shown for example that the binding of HMf to the DNA is dynamic and readily reversible. If archaebacterial nucleosomes exist, they should be therefore less stable than eukaryotic ones. This could explain contradictory results about their existence in archaebacteria. 

In the absence of active biological attack, the two dominant processes leading to natural deterioration of cellulose are acid-promoted hydrolysis and oxidation. The latter is quite slow, and we would expect cellulosic artefacts that are above neutrality and kept in a stable environment to remain in relatively good condition for hundreds of years. However, those that are somewhat acidic can show significant signs of deterioration within a few decades. 

The minerals from which the metals are extracted, existed for millions of years in the earth s crust and are the most stable form of the metal. A considerable amount of energy is required to convert this mineral into the metal. Once this pure metal comes into contact with the natural environment such as sea-water or soils, the metal slowly converts back to its original starting material. Iron, for example, is obtained from the mineral, haematite, an oxide of iron. Once the pure iron comes into contact with water and air (oxygen), it slowly converts back to the oxide. This is called corrosion and the product is familiar to everyone as red rust. Nearly all metals will corrode in natural environments although the rates of corrosion will vary from metal to metal and alloy to alloy. In addition, the rates of corrosion will vary from one natural environment to another. Iron will corrode at approximately 50 pun per year in freshwater but at 120 pm per year in seawater. The reason for this is due to the difference in chemical composition between freshwater and seawater. The latter contains salt (sodium chloride) and this is very deleterious to the corrosion behaviour of the metal. Silver artefacts may be excavated after several hundred years buried in soils with only minimal amounts of corrosion. Those recovered from marine sites after a similar period of burial, have completely corroded and have reverted back to 100% mineral. This is entirely due to the presence of chlorides in seawater. 

Dissolution of the chlorides from the corrosion products is an essential part of the conservation process. It is essential that the artefact is immersed in an electrolyte that will not corrode the metal any further, while this dissolution is taking place. Corrosion scientists have developed redox potential - pH diagrams from thermodynamics in order to predict the most stable form of the metal. These diagrams are divided into three zones. Where metal ions are the most stable phase, this is classed as a zone of corrosion. If the metal itself is the most stable species, this is said to be the zone of immunity. The third zone is where solid metal compounds such as oxides, hydroxides, etc, are the most stable and may form a protective layer over the metal surface. This zone is termed passivity and the metal will not corrode as long as this film forms a protective barrier. The thickness of this passive layer may only be approximately 10 nm thick but as long as it covers the entire metal surface, it will prevent further corrosion. 

As an alternative to PEG treatment of waterlogged wood, attempts have been made to use a radiation curing method involving a range of monomers and resins. The aim of such treatments is to reduce treatment times and achieve a more stable artefact. Styrene, vinyl acetate, acrylonitrile, acrylates and methacrylates are the most studied monomers in the treatment of waterlogged archaeological wood. The most widely used of these monomers is the water-soluble vinylpyrrolidone and methacrylamide and the non-water soluble chemicals such as n-butyl methacrylate monomer or unsaturated polyester resin. 

Also here, an areawise test can be performed to sort out false positive patches being artefacts from time/frequency resolved analysis. The procedure is exactly the same as for the wavelet spectrum, only the critical patchsize Pcrit b, a) has to be reestimated. Areawise significant patches denote significant common oscillations of two processes. Here, common means that two processes exhibit a rather stable phase relation on a certain scale for a certain time intervall. 

In contrast to liquid chromatography which is the technique of choice when considering high molecular mass constituents, gas chromatography (GC) is well-suited for the fractionation of low molecular mass species which are volatile, thermally stable and preferably neutral. Some species that do not already possess the above properties can be converted into forms that are amenable to GC. Various derivatization procedures that can be used to perform such conversions have been described by Poole and Schuette (1984). However, it Is worth mentioning that hydride formation and alkylation are the two most commonly used derivatization methods that have found application in the study of the speciation of various elements like As, Bi, Ge, Hg, Pb, Sb, Se, Sn, Te and Tl. Two points have to be considered when derivatization is performed first, the specificity of the chemical conversion second, the percentage yield of the reaction. A specific reaction is desirable in order to avoid the introduction of artefacts. Moreover, if a quantitative estimate of the amount of the original species is required, then it is essential that the extent of the conversion is known. 

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