NMR sample tubes

Because most common solvents, including water, contain protons, and most NMR analyses involve the measurement of protons, a solvent without protons is generally used in NMR spectroscopy. Commonly, solvents in which the hydrogen atoms are replaced with deuterium (i.e., solvents that have been deuterated) are used, the most common being deuterochloroform. In addition, an internal standard, most commonly tetramethylsilane (TMS), is added to the sample in the NMR sample tube (see Figure 14.3, D) and all absorption features are recorded relative to the absorption due to TMS. 

In our final realization (Fig. 18), the probes use the Helmholtz coil geometry, favoring ease of use and efficient sample temperature control over a wide range of temperature values. The tunable, broad-band probe is inserted into the magnet from below and fixed to the bottom part of the magnet assembly in a simple way reminiscent of most high-resolution NMR systems. Thanks to this design, it is possible to use standard 10 mm NMR sample tubes which are inserted comfortably from above without any need to manipulate the probe. 

Table F1.4.2 NMR Sample Tube Requirements for NMR Instruments with Different Field Strengths"...

Data provided by the NMR sample tube producers Wilmad and Norell. 

Dry the glass equipment (i.e., NMR sample tube without plug and glass pipettes) for 2 hr in an 100°C oven. 

Dry the sample, pipet tips, and the NMR sample tube plug 2 hr in a desiccator under vacuum at room temperature. 

Transfer the sample to an NMR sample tube using a dry pipet. 

Plug the NMR sample tube and seal with Parafilm. 

A. Typical Systems. A simple system for the transfer of samples to an infrared gas cell or to a NMR sample tube consists of a fore pump, diffusion pump, trap, and manifold (Fig. 5.1). At the other extreme is a general-purpose chemical vacuum line, which permits the separation of volatile compounds, transfer of noncondensable gases, and storage of reactive gases and solvents (Fig. 5.2). When attack of stopcock grease is a serious problem, grease-free de-... 

In a typical experiment 0.8 mL portions of solutions made from 0.029 g (p-pdt)[Fe(CO)2(PMe3)]2 in 1 mL CH2C12 were placed in medium-pressure NMR sample tubes (Wilmad, 528 -PV-7) together with 2 pL H20. The tubes were degassed, pressurized with 10 bar D2 and exposed to sunlight as shown in Fig. 2. 2H NMR spectra were taken at time intervals to follow the formation of HOD. 

Fig. 2. Medium pressure NMR sample tubes containing solutions of the diiron complexes, pressurized with 10 bar D2 and were exposed to sunlight on the windowsill.

The external reference method is common in 31P NMR. The reference substance, 85 % H3P04 (phosphoric acid), is transferred from a cylindrical or spherical capillary tube to an NMR sample tube containing the same solvent as the test sample. Owing to its chemical reactivity, H3P04 cannot be added as an internal reference substance. Although not an exact method (21), the resonance of H3P04 is set at 0.00 ppm when the coaxial capillary tube system is used. 

The quality of the results in NMR largely depends on the care with which the sample has been prepared. Not only should the NMR sample tube be of good (routine) quality but also the sample solution in the tube should be free of nondissolved particles and dust. 

These coherences will have a helical phase twist in the NMR sample tube and will add to give a net signal of zero in the probe coil during the FID. 

Figure 11. For the purpose of this discussion, the most important features in the spectrum are the two methanol peaks labeled b and c, where b is the methyl proton resonance for methanol trapped within the hexameric sphere, and c is the methyl proton resonance for methanol in the bulk solvent. The identity of these peaks was proved by a spiking experiment in which additional methanol added to the NMR tube had the effect of increasing the intensity of peak c. Similar results were obtained in acetone-, DIVISOR and toluene- - It should also be noted that NMR spectra for hexamer 8 synthesized in ethanol or 2-propanol also showed two sets of resonances for each inequivalent proton in the ethanol or 2-propanol molecules. Indeed, in a pressurized NMR sample tube we observed no change in the intensities of peaks b and c at 150 °C in acetone- -...

Figure 3.6. Bottom section of an NMR sample tube and the sample cavity where the NMR signal is generated.

Many substances that are vapors at room temperature and atmospheric pressure may be used as NMR solvents in sealed tubes or at reduced temperature. For example, S02 has a vapor pressure of about 3 atm at room temperature and can be easily contained in sealed thin-walled, 5 mm diameter NMR sample tubes. Supercritical fluids are also used as NMR solvents in specialized sample tubes. For NMR studies of nuclei other than hydrogen and carbon, suitable solvents that do not contain the nucleus being studied are usually readily available. Frequently, the use of two or more solvents can provide valuable information on molecular structure, as indicated in Chapter 4. 

One other application where Nano-probe technology excels relative to conventional tube or flow NMR probe formats is in the area of heterogeneous samples. One of the early applications demonstrated for the Nano-probe was the acquisition of NMR spectral data for chemically modified polymer beads used in solid-phase-assisted peptide synthesis and related chemical transformations.21 23 When chemically modified beads are interrogated in a conventional NMR sample tube, the resin bead behaves as an insoluble material and at best very broad and poorly resolved spectra may be recorded. In contrast, when the same beads are placed in a Nano-probe and spun at several kHz at the magic angle, there is sufficient solvation of the pendant chemical moiety and the linker to resin bead nucleus to allow the modified portion to behave as if it is in pseudo solution, which allows reasonable NMR spectra to be recorded. Various factors affect the quality of the NMR data that can be obtained for the pendant molecule, which include the tether length and the solvent used for the measurement.23 There have been a diverse assortment of applications of Nano-probe applications reported in the literature that are discussed in further detail in Section 6.3. 

Fig. 18. Comparison spectra for a sealed 3 mm NMR sample tube containing 40 pg (120 nmol) of strychnine (5) dissolved in 165 pL CDC13.234 All of the spectra were acquired and processed identically. The non-gHSQC spectrum shown in Panel A was acquired in 90 m using a 3 mm inverse-detection cryogenic NMR probe operating with an rf coil temperature of 12 K. The sealed 3 mm sample was used to acquire the 90 m spectrum shown in Panel B in a conventional 3 mm gradient inverse-detection probe. All parameters were identical. Panel C shows the results obtained for the sealed 3 mm sample in a conventional 3 mm NMR probe with an overnight (17.5 h) acquisition. (Reprinted with permission from J. Nat. Prod., 63, 1049 (2000). Copyright 2000, American Chemical Society and American Society of Pharmacognosy.)...

Each solution should be prepared in a small test tube or vial that can be sealed airtight for refrigerated storage prior to transfer to an NMR sample tube. In order to calculate the concentration in each solution, the molarity of the concentrated HCl is needed. If this is not given by the instructor, it can be determined either by titration or by a density measurement (using tables in the CRC Handbook of Chemistry and Physics). 

Dependence on Hydrogen-Ion Concentration. Solutions 1 through 6 will be studied at a constant temperature of 25°C in order to determine the acid-catalyzed rate constants and. If the NMR sample tubes have been filled with cold solutions, bring these... 

Operation of the NMR instrument will be desoibed by the instructor. Particular care should be taken in inserting and removing the NMR sample tubes to prevent damage to the probe. All tubes should be wiped clean prior to insertion to avoid contamination of the probe. 

No change in the splitting is observed when tire NMR sample tube is rotated by 180° after the steady-state separation is reached, indicating that the magnetic-field orientation is caused by the induced (magnetic) dipoles of the polymer molecules in contrast to the electric-field orientation in high dielectric solvent... 

Basically, the flow control and sampling unit allows three alternative methods of operation. Firstly the eluent from the column can flow directly from the UV detector to the NMR sample tube and the spectra can be continuously monitored during the development of the separation. The success of this procedure will depend on the volume of the cell, the sample size, the column flow rate, the resolution of the NMR spectrometer and the rate of data acquisition by the computer. In general, unless the new micro-cell facilities mentioned above are exploited, this procedure will rarely be successful, particularly if microbore columns are used and multi-component mixtures are being examined. 

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