Artefact formation

Alternatively (or initially) the mixture is treated as a whole and tested in its crude state. The advantage of this strategy includes the relevancy of the tested sample to its environmental counterpart, decreased potential for artefact formation, and inclusion of combined effects of chemicals in the mixture. Moreover if the mixture is representative of others in its class (e.g., diesel emissions from different sources would share certain characteristics), it may be possible to extrapolate results across samples. This method also circumvents the labor-intensive process of individual testing of multiple chemicals. But sometimes a complex mixture is too cytotoxic to be tested directly in a bioassay. Furthermore, it may be incompatible with the test system because of the physical matrix. Other disadvantages include the inability to specify the constituent of the mixture responsible for the toxicity, as well as potential masking effects (e.g., the masking of mutagenicity by cytotoxicity). 

There are a number of difficulties involved with the comparative analysis of Khat samples. At the time of writing, there are no detailed reports in the literature of optimized methods for comparison of such materials. While thin layer chromatographic methods have been used, this technique suffers from lack of resolution. Coupled to the problems associated with artefact formation during the process of analysis, it still remains difficult to achieve a meaningful comparison with any degree of certainty. 

With the slow-stirring method a precise and accurate determination of Kow of compounds with log Kow up till 8.2 is allowed (De Bruijn et al, 1989). For highly lipophilic compounds the shake-flask method is prone to produce artefacts (formation of microdroplets), and with the HPLC method Kow needs to be extrapolated beyond the calibration range to obtain estimates of Kow-... 

Lestremau, F., Andersson, F.A.T. and Desauziers, V. (2004) Investigation of artefact formation during analysis of volatile sulphur compounds using solid phase microextraction (SPME), Chromatographia, 59(9-10), 607-613. 

There are other cases where steam distillation is responsible for more specific problems of artefact formation which can be avoided if C02-extraction is applied for essential oil separation. One example is the blue colour of distilled German camomile oil. This is caused by the transformation of matricin, which is the genuine plant ingredient, into chamazulene [4]. Both substances have anti-inflammatory properties but the matricin, which is preserved during COj-extraction, is believed to have the better efficacy. Also, the sensitive fragrance of camomile flowers is better preserved in the C02-extract than in the steam distillate. 

Since this approach is based on the scientific proof of natural occurrence, the International Organisation of the Flavour Industry (lOFI), Brussels (B), established the Working Group on Analytical Methods (WGMA), which examines published data with regard to the validity of chromatographic (e.g. GC retention time) and spectroscopic (e.g. mass spectra, infrared and NMR spectra) data. In this context reference data, artefact formation and the nature of the source material (i.e. food use) are also considered. 

However, these workers have noted the ability of monoterpene dlols and triols, which are also present In musts and wines, to act as precursors of volatiles (6) Accordingly, variations In monoterpene abundance between samples could be reflective of the method used in preparing the sample or of the analysis Itself. A similar questioning of the emphasis given to the analytical characterization of grapes and wines based on quantification of free monoterpenes had been expressed previously when the lability of various precursor forms of these compounds was first recognized (18,19). Nevertheless these caveats questioned only the detailed interpretation of the statistical data and the need for appropriate techniques to avoid artefact formation. The fundamental soundness of the correlations remains intact. 

Preparation of the fruit. Since the early research of Webb et al. (33) several workers have recognized the problem of artefact formation during work-up of grapes. Accordingly, these investigators have employed additives such as NaF and ascorbic acid or SO. and sorbic acid to minimize microbial and oxidative degradation of the juice (2,10,34). 

Problems of contamination and artefact formation in nitrosamine sampling and analysis in Environmental car- 15A24. 

Andersen KB, Hansen MJ, Feilberg A (2012) Minimisation of artefact formation of dimethyl disulphide during sampling and analysis of methanethiol in air using solid sorbent materials. J Chromatogr A 1245 24... 

No solvent other than methanol is necessary if free fatty acids alone are to be methylated (also only 20 minutes at reflux, or 2 hours at 50°C, is required), or if polar lipids such as phospholipids are to be transesterified. The reaction can be scaled up considerably for example, 50 g of lipid in 100 mL of toluene can be transesterified with 200 mL of methanol containing 4 mL of concentrated sulfuric acid. A/-a( l lipids are transesterified very slowly with these reagents (see below). If acidic reagents are permitted to super-heat in air, some artefact formation is possible. 

Note that aldehyde derivatives prepared from trihydroxy bases will be one carbon shorter than those from equivalent dihydroxy bases, and the number of hydroxyl groups must be determined from the TLC behaviour of the base or its DNP derivative. The proportions of the various isomers within each class of base can be determined with reasonable accuracy by GC, but artefact formation during the hydrolysis stage may distort the apparent relative proportions of the various classes of base to each other. 

Many foods contain components that are thermo-sensitive or labile and are changed by the application of heat resulting in artefact formation. One of the most commonly used extraction methods is simultaneous distillation-extraction (SDE). This method was first proposed by Likens and Nickerson in 1964 (16). Due to the high temperature used in atmospheric SDE, thermal artefacts may be generated during the extraction process and thus vacuum SDE systems have been developed to overcome the generation of thermal artefacts. One of the more recent vacuum SDE systems was developed by Maignial et al (10). This Static Vacuum SDE apparatus is an "easy-to-use device to isolate volatile... 

These studies on model enzyme complexes imply that the organic compounds which stabilize exocellular enzymes in soil are brown-coloured humic materials, probably aromatic in character. However, knowledge of the nature of the organic ligands and of the manner in which they may complex active enzymes, rests in part on the extraction of enzymes from soil, preferably in high yield and without artefact formation, and their characterization after fractionation and purification. 

DS Mottram, DJ Puckey. Artefact formation during the extraction of bacon volatiles in a Likens-Nickerson apparatus. Chem Ind 1978 385-386, 1978. 

A more detailed discussion on this topic can be found in Section 7.5.5(e) on the use of isotopically labelled species for the determination of artefact formation or species degradation during extraction and derivatisation reactions. 

IsotopicaUy labelled MeHg was used to investigate artefact formation in water samples during derivatisation with NaBEt4. Halide ions were found to induce different transformation processes. While chloride and bromide converted MeHg into Hg , iodide hindered the ethylation of methylmercury. The effect was shown to occur only during the derivatisation step, and in contrast to ethylation, propylation with NaBPr4 did not cause any transformation. 

In sediment samples, methylmercury levels are typically only a few percent or less of the inorganic mercury level and even minor artificial methylmercury formation during sample preparation will result in significant overestimation of the actual methylmercury content. A correction can be carried out with a single measurement, when stable Hg isotopes are used, by measuring different isotopes to determine the artefact formation factor. 

H. Verhoeven, T. Beuerle, and W. Schwab, Sohd-phase microextraction artefact formation and its avoidance, Chromatographia 46 63 (1997). 

The simultaneous distillation-extraction (SDE) under atmospheric pressure (53) is not always the most appropriate technique, and its use should be carefully considered. This technique is an elegant and rapid extraction method resulting in an aroma extract that is ready to be injected into a GC system after concentration. However, heat-induced artefact formation, decomposition of labile compounds, and loss of very volatile compounds are serious drawbacks. Furthermore, only steam-distillable volatiles are extracted. Polar compounds, such as hydroxyfura-nones and phenols, are particularly poorly recovered. 

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