NMR observe volume

As Beer s law in absorption spectroscopy has a path length dependence, the observe volume, Vobs, or active volume of an NMR probe is an important determinant of the sensitivity of NMR measurements. The observe volume is the fraction of the total sample volume, Vtot. that returns a signal when a sample is inserted in an NMR tube or is injected into a flow system. The relationship between chromatographic peak shape, peak volume and flow rate, and sensitivity in hyphenated NMR measurements is complex and is discussed in greater detail in Section 7.2. For the purpose of this discussion, the sample is assumed to be present at a uniform concentration in a sample volume, Vtot. The probe observe factor, /o, is calculated as shown in the following equation ... 

The signal intensity and line shape can be significantly affected in continuous-flow detection and, in the case of CE, additional broadening is produced by the electrophoretic current. The amount of time that a nucleus spends in the NMR flow cell or the residence time, t, is related to the observe volume and the applied flow rate, F,... 

Novel instrumentation integrates capillary HPLC (cHPLC)-NMR, CE-NMR, and CEC-NMR and provides extreme versatility [29,49]. A number of CE-NMR and CEC-NMR applications performed with this integrated instrumentation have been described. In this system, saddle-type coils with 240-400-nL observe volumes increase the sensitivity and avoid current-induced magnetic field gradients. A mixture of lysine and histidine has been analyzed in a continuous-flow CE experiment with a detection limit of 336 ng (2.3 nmol) [29],... 

An example of the low-volume flow cells is the CapNMR probe (Prota-sis/MRM Corp., Savoy, Illinois). The CapNMR probe is operated at ambient temperature and has a flow-cell volume of 5 /iL, where the solenoidal RF coil encloses an observed volume of about 1.5 /iL. Typically, the mass-limited sample is dissolved in a few microliters of an appropriate NMR solvent and is transported to the achve probe volume via a microsyringe pump or a capillary HPLC system. The mass sensihvity [48] of the CapNMR probe is reported to be similar to that of a cold-probe [49]. 

Our on-line GPC/NMR system consists of a JASCO TRI ROTAR-V chromatograph and a JEOL JNM-GX500 spectrometer (a 16-bit A/D converter was installed). A 2- or 3-mm (i.d.) glass tubing with tapered structure at both ends is employed as an NMR observation flow cell (Fig. 30) the detection volumes are 0.060 and 0.140 ml, respectively. A similar type of flow cell was used in the on-line HPLC/NMR experiments.340,341 The tapered structures at both ends are needed to prevent turbulent flow in the inlet path of the cell, which would cause broadening of the elution band. When a cell without tapering was used, some broadening of the band was observed.334... 

Figure 3.47. The active (observe) volume of an NMR coil is the region of the sample that sits within the coil and so contributes to the detected response.

Strength and the sample concentration, po is the permeability of free space, Q is the quality factor of the coil, coo is the Larmor angular frequency, K is the volume of the coil, F is the noise figure of the preamplifier, k is Boltzmann s constant, is the probe (as opposed to sample) temperature, and A/is the bandwidth (in Hz) of the receiver. It can be seen that the concentration sensitivity 5c (SIN per pM concentration of analyte) is poor for microcoils. This is due to the fact that microcoil probes have very small observation volumes and therefore contain a very small amount of analyte. However, if the sample can be concentrated into a small volume, then the microcoil can more easily detect the signal. This high mass sensitivity 5m (SIN per pmol of analyte) is characteristic of microcoil NMR probes. In essence, the use of microcoil probes enhances the mass sensitivity 5m at the expense of the concentration sensitivity 5c. To better understand the relationship between sensitivity and coil diameter, a detailed analysis was reported by Peck et Their results showed that mass sensitivity increases monotonically with decreasing coil diameter within the 1mm to 50 pm range they studied. However, the concentration sensitivity decreases, and therefore there is a trade-off between Sc and 5m that depends on coil diameter. 

Since the time dependence of variations in magnetization, dipolar or quad-rupolar coupling, and chemical-shift anisotropy are inherent parts of the NMR phenomenon, it is a natural tool for studying molecular motions. These relationships are usually well understood, and NMR data can therefore be used to test dynamic models. Conformations, free volume, diffusion, and mechanical properties have all been correlated with NMR observations. This section will first review mobile systems (solution, melts, and elastomers), and then discuss solids. 

With semicrystalline polymers having moderate to high crystallinity, Tg may be poorly resolved. Other static methods used to measure Tg are the following refractive index gas difiusion/solubility thermal conductivity chain mobility [nuclear magnetic resonance (NMR)] specific volume (dilatometry). Experimentally observed Tg is a function of several variables including molecular weight, plasticizer content, test rate/fi equency, sample size, copolymers/blends, cross-linking, crystallinity, and tacticity. 

Two physically reasonable but quite different models have been used to describe the internal motions of lipid molecules observed by neutron scattering. In the first the protons are assumed to undergo diffusion in a sphere [63]. The radius of the sphere is allowed to be different for different protons. Although the results do not seem to be sensitive to the details of the variation in the sphere radii, it is necessary to have a range of sphere volumes, with the largest volume for methylene groups near the ends of the hydrocarbon chains in the middle of the bilayer and the smallest for the methylenes at the tops of the chains, closest to the bilayer surface. This is consistent with the behavior of the carbon-deuterium order parameters,. S cd, measured by deuterium NMR ... 

Spatially localized NMR spectroscopy was performed using a sample composed of a monoolein-D20 dispersion forming a cubic liquid-crystalline phase [41]. The spectra were selected from a slice of 300-pm thickness. The direction of the gradient was along the spinning axis. A spectral resolution was obtained that had never been observed before when selecting the whole volume of the sample. 

The alternative method is continuous-flow , in which the reactants flow through the detection coil during data acquisition. Continuous-flow NMR techniques have been used for the direct observation of short-lived species in chemical reactions [4—6]. The main difference between stopped- and continuous-flow NMR is that in the latter the sample remains inside the detection coil only for a short time period, termed the residence time, x [7], which is determined by the volume of the detection cell and the flow rate. The residence time alters the effective relaxation times according to the relationship in Eq. (2.5.1) ... 

The development of microcoil techniques has been reviewed by Minard and Wind [24, 25] and by Webb [26]. In a more recent publication Seeber et al. reported the design and testing of solenoidal microcoils with dimensions of tens to hundreds of microns [27]. For the smallest receiver coils these workers achieved a sensitivity that was sufficient to observe proton NMR with an SNR of unity in a single scan of 10 pm3 (10 fL) of water, containing 7 x 1011 proton spins. Reducing the diameter of the coil from millimeters to hundreds of microns thus increases its sensitivity greatly, allowing analysis of pL to pL sample volumes. 

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