Line Sampling

Onions, dehydrated Onium Onium salts On-line databases On-line sampling ONO802, [64318-79-2] Onsager model Onychomycosis Onyx... 

The development of analytical strategies for the regulatory control of dmg residues in food-producing animals has also been reviewed (128). Because of the complexity of biological matrices such as eggs (qv), milk, meat, and dmg feeds, weU-designed off-line or on-line sample treatment procedures are essential. 

H. G. J. Mol, H.-G. Janssen, C. A. Cramers and U. A. Th Brinkman, On-line sample enrichment-capillary gas clir omatography of aqueous samples using geometr ically deformed open-tubular extraction columns , 7. Microcolumn Sep. 7 247-257 (1995). 

Figure 11.16 Chromatograms of plasma samples obtained by using SPE-SFC with super-aitical desorption of the SPE cartridge (a) blank plasma (20 p.1), UV detection at 215 nm (b) blank plasma (20 p.1), UV detection at 360 nm (c) plasma (1 ml) containing 20 ng mitomycin C (MMC), UV detection at 360 nm. Reprinted from Journal of Chromatography, 454, W. M. A. Niessen et al., Phase-system switching as an on-line sample pretreatment in the bioanalysis of mitomycin C using supercritical fluid cliromatography, pp. 243-251, copyright 1988, with permission from Elsevier Science.

A. Farjam, G. J. de Jong, R. W. Frei, U. A. Th Brinkman, W., Haasnoot, A. R. M. Hamers, R. Schilt and F. A. Huf, Immunoaffinity pre-column for selective on-line sample pre-tr eatment in liigh-perfor mance liquid cliromatogr aphy determination of 19-nortestosterone , J. Chromatogr. 452 419-433 (1988). 

A. Farjam, R. de Vries, H. Lingeman and U. A. Th Brinkman, Immuno precolumns for selective on-line sample pretr eatment of aflatoxins in milk prior to column liquid cliro-matography , Int. J. Environ. Anal. Chem. 44 175-184 (1991). 

W. M. A. Niessen, R J. M. Bergers, U. R. Tjaden and J. van der Greef, Phase-system switclring as an on-line sample preti eatment in the bioanalysis of mitomycin C using supercritical fluid clrromatogi aphy , 7. Chromatogr. 454 243-251 (1988). 

M. E. Swaitz and M. Merion, On-line sample preconcentration on a packed-inlet capillary for improving the sensitivity of capillary electi ophoretic analysis of pharmaceuticals , 7. Chromatogr. 632 209-213 (1993). 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

Although on-line sample preparation cannot be regarded as being traditional multidimensional chromatography, the principles of the latter have been employed in the development of many on-line sample preparation techniques, including supercritical fluid extraction (SFE)-GC, SPME, thermal desorption and other on-line extraction methods. As with multidimensional chromatography, the principle is to obtain a portion of the required selectivity by using an additional separation device prior to the main analytical column. 

The sample dissolved in a weak (nonpolar for siliea gel), volatile solvent is applied as a narrow band across the plate. Manual applieation can be aehieved with a pipet or syringe gnided by a ruler, or roimd spots ean be placed close together, side by side, in a line. Sample applieation instruments are available commercially, e.g., a meehan-ical streaker from Analtech and an antomated spray-on apparatus from CAMAG. 

On-line sample-stacking techniques " and, more recently, the use of isotacho-phoresis have added to the potential benefits of CE by permitting the concentration of analyte in a large volume by exploiting the difference in the electric field between the dilute sample and system buffer. The electric field is much stronger in the dilute buffer-sample and hence analyte ions move faster until they reach the border with the separation buffer. At this point they slow down, causing the analyte to concentrate as a sharp sample band at the interface. 

Garn, M. B., Gisin, M., Gross, H., King, P., Schmidt, W. and Thommen, C., Extensive flow-injection dilution for in-line sample pre-treatment, Anal. Chim. Acta, 207, 225, 1988. 

Miller and Hawthorne [416] have developed a chromatographic method that allows subcritical (hot/liquid) water to be used as a mobile phase for packed-column RPLC with solute detection by FID, UV or F also PHWE-LC-GC-FTD couplings are used. Before LC elution the extract is dried in a solid-phase trap to remove the water. In analogy to SFE-SFC, on-line coupled superheated water extraction-superheated water chromatography (SWE-SWC) has been proposed [417]. On-line sample extraction, clean-up and fractionation increases sensitivity, avoids contamination and minimises sources of error. 

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