Phase-sensitive gradient spectroscopy

In the future, gradient spectroscopy may prove to be a better approach to suppression of undesired components. At the time this review was written preliminary results obtained in our laboratory indicate this to be the case provided gradients are used in conjunction with the appropriate phase cycle schemes, which are then usually reduced to their minimum possible size. Spectra usually reveal less artefacts, and, in favourable cases, higher sensitivities. Systematic investigations are underway. 

Fig. 2. Pulse scheme for the gradient-selected, sensitivity-enhanced X/Y se-HSQC experiment as employed for 31P/15N correlation spectroscopy in Ref. 25. 90° and 180° hard pulses are denoted by solid and open bars, respectively. 2 are delays of length 1/(4 /x,y)> and is a short delay of the same length as the gradient pulse (typically rj 1 ms). Pulse phases are x, unless specified. The ratio of gradient pulse strengths is set to G2/Gi = Yy/Yx, and quadrature detection in Fi is achieved by recording every transient twice and changing the sign of G2 in the second scan.

The solubility of hydrocarbons in water is greatly enhanced by the presence of surfactants. Similarly, the addition of hydrocarbons to aqueous surfactant solution induces change in the physical properties of the surfactant (24, 25). Thus, the quantitative analysis of the partitioning of hydrocarbons between micelle interior and aqueous phase has been a subject of numerous studies. Among the many techniques used to determine the partitioning, only fluorescence (26), FT-pulsed-gradient spin echo NMR (27), and recently, Raman spectroscopy (28) have demonstrated sufficient sensitivity. 

Modern interfaces joining LC and Fourier transform infrared spectroscopy (FTIR) function in two ways The first technique is based on a flow cell ( on-line ), in which sensitivity is limited by the dilution of the analyte in the mobile phase. In the second technique the mobile phase is evaporated, leaving the analyte as a small spot that is scanned ( off-line ) by FTIR. The advantage of this approach is a significant enhancement in sensitivity. Compared to the refractive index detector the IR detector has a lower sensitivity but theoretically offers the possibility of gradient runs. 

Measurement of diffusion using pulsed field gradient NMR (PFG-NMR) is a powerful analytical tool because it combines the high specificity and information content of NMR spectroscopy with the size selectivity of diffusion coefficients. PFG-NMR employs timescales of tens of ms and has a displacement sensitivity of the order of 100 nm. PFG-NMR can determine molecular self-diffusion coefficients in liquid phases down to a lower limit of 10 " m s Due to the combination of experimental convenience and straightforward interpretation, PFG-NMR has become the method of choice for studying translational diffusion. PFG-NMR experiments have been reported using H, H, Li, C, F and other nuclei. The time A over which PFG-NMR measurements are possible is limited. 

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