Retention factor sampling techniques

There is considerable discussion in the literature regarding the adsorption mechanism of ions from aqueous solutions onto RPLC stationary phases [87-90]. It has been shown that, under certain conditions, organic ions are adsorbed as ion pairs [87,89,91], and that, under other conditions, they may be adsorbed as separate ions. In this case, the model derived by StMUberg [92] may be useful. In his theory of the retention mechanism in ion-pair chromatography, StMilberg focused on the derivation of the isotherm of the amphiphilic compoimd, that is, the counter-ion used in this technique to adjust the retention factors of the sample components and their separation factors e.g., the cation tetrabutulammonium). The counter-ion (Br, Cl , H2PO4 ) may not be strongly associated with the cation in a mobile phase that is a mere aqueous buffer. Other cations, rmder other experimental conditions may adsorb as true ion pairs, in which case the isotherm behavior is quite different. 

Thin-layer chromatographic (TLC) techniques use very small quantities of material. A sample is deposited in the form of a spot and submitted to the action of a solvent (eluant), which causes component substances to migrate on the TLC plate. Detection is performed using a redox reaction with tetramethylbenzidine or ort/>o-tolidine. Positive and negative controls are tested on the same plate. The retention factor k) is used to identify haemoglobin in the sample. 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

After a particular chromatographic technique has been selected a chromatogram should be recorded. This is not always straightforward, since recording a chromatogram requires that the components in the sample show reasonable retention times (capacity factors). Often, therefore, we have to adjust the chromatographic system in order to get all sample components to appear as a peak (but not necessarily a well-resolved one) in the... 

It is apparent, from Eq. (1), that the primary sample property measured by flow FFF is the diffusion coefficient. Secondary information includes the hydrodynamic diameter which can be obtained via the Stokes-Einstein equation and the molecular weight if the molecule shape factor is constant. Unlike other FFF techniques, the retention time in flow FFF is determined solely by the diffusion coefficient rather than a combination of sample properties. As a consequence, flow FFF is well suited for analyses of complex sample mixtures and the transformation of the fractogram to a diffusion or size distribution is straightforward. In addition, flow FFF is applicable to a wide range of samples regardless of their charge, size, density, and so forth. 

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