Microcoil Probes

In the size regime larger than 100 pan in diameter, the S/N per unit sample volume of solenoids can be expressed as follows  

To date, most NMR microcoils have been wound directly on to a capillary tube which functions as both sample container and coil form [26,27], According to electromagnetic field theory, a sample enclosed by a perfectly uniform and infinitely long hollow cylinder (e.g. a fused silica capillary) experiences a uniform static magnetic field [28]. In reality, the susceptibility variation of the 

In order to carry out complete structural elucidation of unknown compounds (especially for complex molecules), the RF probe should enable a variety of heteronuclear NMR techniques to be performed. In particular, inverse detection H-15N and 1H-13C experiments such as heteronuclear multiple quantum coherence (HMQC) [29,30] and heteronuclear single quantum coherence (HSQC) [31] find almost ubiquitous application in myriad research environments. Although the microliter-scale probes described above feature both heteronuclear and homonuclear capabilities, no commerical product is 

These types of experiments call for efficient doubly tuned coils, ideally with a separate deuterium lock channel. For more complex molecules, such as proteins, considerably more intricate NMR pulse sequences, such as (HNCO) [32,33], require the probe to operate at three or four distinct frequencies. High efficiency is demanded from the proton observe channel. Ideally, the additional circuitry allowing multiple tuning should not interfere with the proton efficiency when compared to a singly tuned proton coil. In practice, some reduction is tolerated. The two most important design criteria for such 

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FIGURE7.14 Instrumental setup for CE-NMR with online NMR detection using a solenoidal microcoil probe. 

Figure 6.45 Microbore LC-NMR layout. A Microbore HPLC system with a 0.5 mm X 150 mm C18 column is interfaced to a solenoidal microcoil probe. The transfer capfllary is connected to the NMR flow cell with a polyamide resin. Reproduced from [85] with permission. Copyright 1999 American Chemical Society.

Figure 7.3.1.7 A 13C-decoupled HMQC spectrum of 54 mM chloroquine diphosphate in D2O acquired with an inverse detection microcoil probe. The 740-nl F0bs contained 40 nmol (13 jig) of chloroquine. The data, 32 transients per slice, 1024 x 128 (x2, hypercomplex) points, were acquired in 3.6h. The data were zero-filled to 256 points in the 13C dimension. A 40° shifted sinebell function was applied, followed by Gaussian multiplication prior to Fourier transformation...

Figure 11 Description of dual microcoil probe. (A) Two coils wrapped around a polyimide sleeve (B) Dual-coil probe mounted on top of the probe head. (C) Schematic of balanced tank circuit used for each microcoil. Coil (L), series capacitors (Cs) 3.3 pF, tuning capacitors (CT) 0.6-4.5 pF, matching capacitor (CM) 0.6-4.5 pF, bridge capacitors (CB) 24 pF. (Reproduced with permission from Ref. 41. Copyright 2002 American Chemical Society.)...

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. 

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