Extraction artifacts

For thin-film samples, abrupt changes in refractive indices at interfrees give rise to several complicated multiple reflection effects. Baselines become distorted into complex, sinusoidal, fringing patterns, and the intensities of absorption bands can be distorted by multiple reflections of the probe beam. These artifacts are difficult to model realistically and at present are probably the greatest limiters for quantitative work in thin films. Note, however, that these interferences are functions of the complex refractive index, thickness, and morphology of the layers. Thus, properly analyzed, useful information beyond that of chemical bonding potentially may be extracted from the FTIR speara. 

The tightly bound chromophore could be extracted from the protein with methanol [186], and the major component of the extract was determined to have the enediyne structure 116 (Figure 11.21), related to chromophores of other chromoprotein antitumor agents such as neocarzinostatin. Additional minor components were extracted, variously containing an OH group instead of OMe attached to the enediyne core, with Cl instead of OMe when chloride was present in the buffer salt, or with OEt instead of OMe when ethanol was used for the extraction. Another byproduct was isolated in the form of structure 117, consistent with a facile cy-doaromatization reaction as observed for all other enediyne antibiotics. Surprisingly, 117 also displayed antibiotic and antitumor activity, perhaps due to alkylation of DNA or protein by the aziridine. The interpretation of these results was that 116 and the other enediyne byproducts were merely artifacts of the extraction procedure and that the true structure of the maduropeptin chromophore is the aziridine 118. 

Electrochemical cells play important roles in both the purification and the preservation of metallic materials. Redox reactions are used throughout the chemical industry to extract metals from their ores. However, redox reactions also corrode the artifacts that industry produces. What redox reactions achieve, redox reactions can destroy. 

The concentrations of nitrosamines were reduced to undetectable levels by ultraviolet treatment of the amine solutions and were not increased by addition of 2 ppm NaN02> indicating that the nitrosamines were present originally in the amines and were not formed in the GC injection port. Similar concentrations were found when the amine samples were analyzed using the column extraction method. Direct injection is appropriate for analysis of relatively simple mixtures, if adequate precautions are taken ( ), but can result in significant artifact formation in more complex systems (42). 

The typical isocyclic ring E present in chlorophylls is susceptible to a number of different modifications such as epimerization, which produces stereoisomers by inversion of the configuration at C-13 of their parent pigments. These 13 -epichlorophylls, known as chlorophylls a and b, are minor pigments. They are considered artifacts produced in the course of handling plant extracts and sometimes are also found in small amounts in heated and deep-frozen vegetables, hi the old Fischer systan of nomenclature that can still be found in some literature, these epimers were named 10-epichlorophylls. 

Figure 2. Fhase adjusted Fourier transforms of Ft metal, 1% Ft/ Cabosil catalyst in H2 and 0.5% Ft/Cabosil catalyst in H2, all at 90 K. All are plotted to the same scale to emphasize the diminished magnitude because of the smaller average coordination numbers in the catalysts. The INSET shows the Ft-0 peak area retransformed with the appropriate Ft-0 phase shift. The artifact at low R is due to the EXAFS extraction procedure.

Bioom ns, Colman JA, and Barber L 1997) Artifact formation of methyl mercury during aqueous distillation and alternative techniques for the extraction of methylmercury from environmental samples. Fresenius J Anal Chem 358 371-377. 

However, when considering the use of acid or base in organic solvents for sample extraction, care must be taken to avoid potential artifacts that may arise from side reactions. For example, methylation of active hydroxyl groups or acidic functions on the analyte may sometimes occur when acidic methanol is used as the extractant. Another example is acetylation of an active alcohol on the analyte following partition of the analyte into ethyl acetate from aqueous solution acidified with glacial acetic acid. 

Improved Methods for Collection, Bioassay, Isolation, and Characterization of Compounds. Techniques used to characterize natural products are evolving rapidly as more sophisticated instrumentation is developed. Plant physiologists and chemists should work closely together on this aspect, since rapid and reproducable bioassays are essential at each step. There is no standard technique that will work effectively for every compound. Briefly, isolation of a compound involves extraction or collection in a appropriate solvent or adsorbant. Commonly used extraction solvents for plants are water or aqueous methanol in which either dried or live plant parts are soaked. After extracting the material for varying lengths of time, the exuded material is filtered or centrifuged before bioassay. Soil extraction is more difficult, since certain solvents (e.g. bases) may produce artifacts. 

When discussing seco alkaloids the question of their genesis should not be disregarded. Are they true alkaloids or artifacts of isolation It is difficult to answer this question with certainty. Some of them, e.g., secophthalide-isoquinoline ene lactams, are postulated to be formed during the extraction process however, most of them are believed to be metabolites produced naturally. This may be evidenced by the fact that some of these alkaloids retain optical activity, and in addition many of them can be synthesized in biomimetic syntheses in the laboratory. Thus, one can generalize the opinion of Shamma (10), whose significant contribution to the field of secoisoquinoline alkaloids should be acknowledged, that [a process]. .. could presumably occur in vivo at least as readily as it could in vitro. ... 

It has been postulated that secophthalideisoquinoline ene lactams and hydroxy lactams are most probably artifacts of isolation resulting from the reaction of enol lactones or keto acids with ammonia during the extraction process. The hydroxy lactams are probably formed initially and then undergo dehydration to give ene lactams (5,8). For this reason, this section covers the hydroxy lactams in addition to the ene lactams. The hydroxy lactams are... 

Rapin F., Tessier A., Campbell P.G.C., Carignan R. Potential artifacts in the determination of metal partitioning in sediments by a sequential extraction procedure. Environ Sci Technol 1986 20 836-840. 

Some investigators described artifactual DNA sequence alterations after formalin fixation, when testing DNA samples extracted from FFPE tissues. Williams et al.46 reported that up to one mutation artifact per 500 bases was found in FFPE tissue. They also found that the chance of artificial mutations in FFPE tissue sample was inversely correlated with the number of cells used for DNA extraction that is, the fewer cells, the more the artifacts. However, they mentioned that these artifacts can be distinguished from true mutations by confirmational sequencing of independent amplification products, in essence comparing the product of different batches. Quach et al.47 documented that damaged bases can be found in DNA extracted from FFPE tissues, but are still readable after in vitro translesion synthesis by Taq DNA polymerase. They pointed out that appropriate caution should be exercised when analyzing small numbers of templates or cloned PCR products derived from FFPE tissue samples. 

The presence in molluscs of molecules structurally related to typical dietary metabolites could be ascribed either to selective accumulation of minor compounds acquired through the diet, or to an in vivo chemical transformation of major metabolites acquired from the prey. However, all reports on this topic have to be carefully evaluated before drawing hurried conclusions. In particular, interaction among molecules from different organs could favor formation of artifacts when the secondary metabolites are extracted from the whole mollusc and not from individual dissected tissues. Only some cases, where the ability of the molluscs to modify dietary metabolites seems to be well supported, are reported in this chapter. 

Hammerschmidt and Fitzgerald [ 127] have studied the formation of artifact methylmercury during extraction from a sediment reference material. 

The ethyl ester of (-)-stachydrine (22) has been isolated from the root of Courbonia virgata (60) , it is not considered to be an extraction artifact from stachydrine. (-)-3-Hydroxy-l,l-dimethylpyrrolidinium (23) is not a betaine even though it may exist in a neutral form it has been isolated for the first time from a natural source, Courbonia glauca, in 1973 (45). 

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