Gradient capabilities

Appropriate gradient-capable solvent delivery system Sample injection valve... 

Not every NMR spectrometer or probehead is equipped with pulsed field gradient capabilities, yet it may still be possible to execute the basic heteronu-clear shift correlation experiments described above. In such cases it is desirable to employ an alternative scheme to remove the interfering parent resonances and produce spectra devoid of objectionable artefacts. In the BIRD (bilinear rotation decoupling) variant (Fig. 6.12), unwanted resonances are... 

Hydrogen transfer is one of the most pervasive and fundamental processes that occur in biological systems. Examples include the prevalent role of acid-base catalysis in enzyme and ribozyme function, the activation of C-H bonds leading to structural transformations among a myriad of carbon-based metabolites, and the transfer of protons across membrane bilayers to generate gradients capable of driving substrate transport and ATP biosynthesis. 

Figure 5.4-6. Online MDLC with gradient elution. This type of MDLC system is similar to the setup with salt-plug elution, but in this case, the elution is performed with a step- or linear-gradient that requires a second gradient capable HPLC-pump.

The modern NMR instrument will have z-axis pulsed field gradient capabilities. This capability allows the collection of the absolute... 

It is always recommended to use the same column with the same type of surfactant. A column should be dedicated to the anionic surfactants, a second one to the cationic surfactants, etc. The reproducibility of the results in MLC depends on the column equilibration. The adsorbed layer of surfactant should be done correctly. It was shown that the time to reach the equilibrium between the stationary phase and the mobile phase could be very long in ion-pair chromatography with sub-micellar mobile phases. Two days at 1 mL/min were necessary to equilibrate a 15 cm x 4.6 mm i.d. column of Hypersil ODS with a mobile phase containing 0.0003 M CTAB [19]. These low surfactant concentration solutions do not contain micelles. So, they are not used in MLC. With a micellar phase, the equilibration time is reduced. It is possible to use the rapid gradient capability just mentioned above. Typically, a mobile phase containing a high surfactant concentration (10 to 100 cmc) can be used to quickly saturate the column with surfactant. Then 5 to 10 column volumes are used to rinse the column with the mobile phase containing the desired amount of surfactant. 

Transport properties determine a polymer s ability to move through some medium or to have some penetrant medium move between its constituent segments. This definition encompasses processes with diverse driving forces such as concentration and pressure gradients, and even electrical or temperature gradients capable of motivating one component relative to another. This article emphasizes polymer transport properties under conditions of low to intermediate penetrant concentration, where extraordinary differences can exist between the diffiisivities of penetrants having relatively small differences in molecular size or shape. 

Landy J S, Dorsey J G 1984 Rapid gradient capabilities of micellar liquid chromatography. J Chromatogr 22 68-70... 

Figure 2 Sketch of a probe possessing 6, gradient capabilities. The single-turn coil serves tor generating a RF gradient in the X spatial direction. The saddle coil (in principle electrically orthogonal to the latter) is used essentially for detection purposes. Possibly it can produce a RF gradient in the Z direction.

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