Elution-based techniques

Whereas, most elution-based techniques (exceptions are the microcapillary arrow and the surface... 

Due to the open planar surface of UTLC layers, especially ambient desorption- or elution-based techniques were used to transfer analytes from the layer to the ionization region (Table 9.2). In addition, MALDI applications under vacuum were reported and the introduction of the whole ultrathin layer plate was possible without any special mounting devices, due to the small plate dimension. As UTLC-MS is a very new hyphenation, only a few applications of MS detection were reported after separation of different substances on UTLC layers. In addition, desorption- or elution-based approaches for detecting analytes without a separation directly from the UTLC layer are mentioned to show the capabilities of this hyphenation technique. However, as there is no chromatography (separation), it may not be termed UTLC-MS. 

SEC-ESI-FTMS combines the size separation based technique of SEC with one of the most powerful mass spectrometric techniques of FTMS offering high mass accuracy (ppm), ultrahigh resolving power (>10(i) 6) and the capability to perform tandem mass spectrometry. The technique enables generation of oligomer elution profiles, which can be used for accurate calibration of standard SEC data. Coupling of SEC to ESI-MS is further described in ref. [710],... 

The most important techniques of the chromatography principle are column elution based on a solvent — non solvent gradient, and column elution with a superimposed temperature gradient. 

GC was introduced very early as the technique of choice for the detection and identification of accelerants in debris from arson cases because of its high selectivity and sensitivity. But to use the full potential of the technique the methods of recovery of traces of common accelerants from fire debris had to be developed and adjusted. The used methods include solvent extraction, direct headspace analysis, and enrichment by adsorbent-based techniques. In the past, the most common concentration steps prior to the analysis have been (heated) headspace direct injection using a gastight syringe for analyte collection and GC injection or headspace adsorption techniques, mostly using charcoal followed by carbon disulfide (CSi) elution. Some of these procedures have been quite effective and are standardized by... 

The increasing technological significance of lanthanides will continue to pave the way for development of rapid and efficient separation methods with sensitive detection. Several reviews on lanthanide analysis indicate the importance of these materials and the need for the development of new and better techniques. Many HPLC based techniques have been developed for the separation of individual lanthanides using cation exchange, dynamic ion-exchange methods, etc. The selection of gradient elution techniques has been... 

An entirely different concept in analytical separations is provided by capillary electrophoresis (CE) in which the flow of liquid is generated by electro-osmotic flow (EOF) driven by an external electric field. The major advantage of this approach is the essentiaUy flat plug flow profile that leads to intrinsically more narrow elution peaks than the parabolic flow profiles characteristic of pressure-driven viscous flows. In capillary zone electrophoresis (CZE) separation is achieved by superimposing the different electrophoretic mobihties of the solutes on to the EOF. In electrochromatography the separation is achieved as in packed column HPLC but using an EOF to generate flow of the mobile phase past the stationary phase particles. The importance of these EOF-based techniques is their application to miniaturized devices, lah-on-a-chip or micro total analysis systems. Such devices that can be directly interfaced to a mass spectrometer via an ESI source are currently under intense development. 

In general, as newer detection techniques and sensitivities are developed in the analytical laboratory, the demands are more stringent regarding the purity of the new products whether they are solvents or thin-layer plates. This is particularly true for the silica gels, bonded phases, or binders used in HPTLC plates to be used in TLC-MS work. If doing elution-based TLC-MS or desorption-based TLC-MS, the requirements are the same. 

In the end, each approach—either offline or online elution-based TLC-MS—has its advantages and disadvantages. The technique that will be most suitable to our... 

Compared to densitometry and desorption-based approaches (atom and ion bombardment, laser light beam, spray beam, excited gas beam) where only a few micrometers of the plate surface is being sampled, elution-based TLC-MS is a destructive technique as it extracts all or most of the sample, respectively. This sets aside one of the main advantages of TLC, which is the possibility to reevaluate chromatograms at a later time (i.e., after weeks or months). 

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

Physical Methods. Vitamins D2 and D exhibit uv absorption curves that have a maximum at 264 nm and an (absorbance) of 450—490 at 1% concentration (Table 8). The various isomers of vitamin D exhibit characteristically different uv absorption curves. Mixtures of the isomers are difficult to distinguish. However, when chromatographicaHy separated by hplc, the peaks can be identified by stop-flow techniques based on uv absorption scanning or by photodiodearray spectroscopy. The combination of elution time and characteristic uv absorption curves can be used to identify the isomers present in a sample of vitamin D. 

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