Off-line separations

HPLC-NMR analysis in a closed-circuit reveals the stereochemical information for elucidating the structures of unknown compounds (Albert 2002). In contrast to the technique of off-line separation, sample collection, and peak identification closed-circuit analysis guarantees the absence of isomerization and degradation. Very often only small amounts of sample are available after extraction. 

To avoid interferences of isobaric atomic ions of different elements and polyatomic ions at the same nominal mass, off line separation of matrix elements or analyte separation can be applied and/or enrichment and hyphenated techniques such as HPLC- or CE-ICP-MS. Analytical procedures have been developed in the author s laboratory for the determination of spallation nuclides in an irradiated tantalum target using HPLC and CE coupled on line to ICP-MS after dissolution and separation of the tantalum matrix.12,17 The mass spectra of rare earth elements (REEs)... 

ISq and Groundwaters fi-om boreholes GoHy 611, 572, 573 and 2227 were also sampled for U series radionuclides as well as for colloid in-situ and off-line separation and characterisation. 

Multicomponent samples can be analyzed directly without prior laborious off-line separation steps, resulting in a minimal sample loss and saving of time. 

The perfusate after microdialysis sampling is often compatible with both subsequent liquid chromatography and capillary electrophoresis analysis. In an off-line separating system, the microdialysate fractions are first collected and then analyzed. Analysis of the microdialysate can also be performed online. For this, the microdialysis and the separating unit need to be coupled. 

Two basic operating modes exist in preparative OPLC, linear and circular for on-line and off-line separations, respectively. For linear separations, impregnation of the plate is always necessary. The separation must, in addition, be started with a suitable mobile phase inlet pressure, otherwise the mobile phase front cannot migrate regularly. 

Figure 18 Scale-up steps and on-line U-RPC separation of furocoumarin isomers, (a) Off-line separation of 50 pg extract on HPTLC plate, (b) Off-line separation of 8 mg extract on TLC plate, (c) On-line separation of 160 mg extract on preparative layer. (1, Sphondin 2, Iso-pimpinellin 3, Bergapten 4, Pimpinellin 5, Iso-bergapten) Conditions Stationary phase 11-C and HPTLC precoated silica 60 P2S4. TLC silica 60 GF2S4 for preparative separation Layer thickness for preparative separation 4 mm Separation distance 8 cm for analytical and 10 cm for preparative separations Row rate 0.17 ml/min for analytical and 3.S ml/min for preparative separations Detection UV 313 (off-line) and UV 279 nm (on-line) Mobile phase n-hexane-dichloromethane-chloroform-tetrahydrofuran 72.8 10.8 8.3 8.1.

Advantage of On-Line versus Off-Line Separation of Elemental Species 497... 

In the previous paragraph, a general example of isotope fractionation between different species (Sp) of an element was described. It is clear, however, that these types of processes and fractionation will depend on the specific physicochemical properties of the element. To determine the isotopic composition of the target element in the species, development of analytical techniques is required. These can rely on either on-line or off-line separation of the species using chromatography or another technique, followed by the measurement of the isotopic composition of the target element that it contains by mass spectrometry. 

Kilar, A., Dornyei, A. and Kocsis, B. (2013) Structural characterization of bacterial hpopolysaccharides with mass spectrometry and on- and off-line separation techniques. Mass Spectrom. Rev. 32, 90-117. 

Matrix-assisted laser desorption ionization-time of flight is based on the desorption of molecules from a solid surface and, therefore, the compatibility with online SEC is difficult to achieve. A number of research groups carried out off-line separations and investigated the resulting fractions with MALDI/ToF to make use of MALDI/ToF capabilities in... 

Feng,Y.-L., Chen, H.-Y.,Tian, L.-C., and Narasaki, H. (1998). Off-line separation and determination of inorganic arsenic species in natural water by high resolution inductively coupled plasma mass spectrometry with hydride generation combined with reaction of arsenic(V) and L-cysteine. Anal. Chim.Acta 375(1), 167. 

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