NMR and EPR Spectroscopy FAQs
(Frequently Asked Questions)

General

1. What is Camber?

   A measurement of the degree of curvature or bow of a tube.

2. What is the importance of Camber?

   Failure to meet Camber requirements leads to the intensity of modulation sidebands, particularly harmful at higher fields, where optimum sensitivity and resolution are required. When Camber becomes excessive, sample tubes can make contact with the inner surfaces of the probe causing time consuming and expensive repairs to the probe.

3. What is Concentricity?

   A measurement which determines how centered the I.D. of an NMR or EPR tube is to the O.D.; this affects the uniformity of the tubing wall thickness.

4. What is the importance of Concentricity?

   Failure to conform to concentricity requirements causes portions of the sample to fall outside the perfect cylindrical volume, exposing that portion to a magnetic field that may not be homogeneous. This leads to reduced resolution along with modulation sidebands.

5. What are Outside Diameter (O.D.) and Inside Diameter (I.D.)?

   A measure of the distance across the center of the tube from the outer surface for the O.D. and the inner surface for the I.D.

6. What is the importance of O.D. and I.D.?

   Failure to adhere to diameter tolerances can produce a diversity of undesirable performance characteristics in NMR spectroscopy. If the I.D. is too large, coaxial inserts can move during spectral accumulation. If the I.D. is too small, any insert can burst the tube making sample loss a catastrophic possibility. If the O.D. is too small, the tube might slip in the spinner turbine. This can cause the tube to become unsteady and cause modulation sidebands. If the O.D. is too large, the tube may not fit in the spinner turbine. Also, the tube can come in contact with the inner surfaces of the probe causing damage and costly repairs.

7. What is the best method to join (seal to avoid leaking of boiling water) pieces of 1/2 inch O.D. 1/16 inch wall thick pyrex tubing? Or to use metal tube nipples to effect watertight joints?

   There are several methods. You could use part no. LG-1455-112 which connects 11-14mm to 11-14mm tubing, or you could use Cajon Ultra-torr union SS-8-UT-6, 1/2" TO 1/2" available through Swagelok. You could use simple laboratory tubing too. Alternatively, you could have a glass shop join the tubing by heat to assure no leakage when boiling water.

NMR

1. What is NMR?

   NMR(Nuclear Magnetic Resonance) is the study of molecular structure through measurements of the absorption of radio frequency by the nuclei that are exposed to a strong magnetic field.

2. How does NMR Work?

   NMR samples and a known reference material are subjected together to an applied external magnetic field and measured by a spectrophotometer to create a spectra graph. The NMR spectra is characterized by the frequency at which the proton absorbs in the field of the magnet.

3. What is the shelf life for an NMR Reference Standard?

   If the standard is kept in a cool, dry place away from heat and sunlight it should be good for 5 years.

4. I am looking for a long thin-walled tube, 8 inches long. I think this means 0.38mm. Would you recommend a thick or thin walled tube? We are using thin walled tubes from another supplier at the moment but they break in the probe.

   Wilmad 5mm Precision NMR tubes have a 0.38mm wall thickness. Our 5mm Economy tubes have a .43mm wall thickness. Our Precision NMR tubes are made of Borosilicate Type 1, Class A glass that has a Thermal Expansion Coefficient of 33, a part number example is 528-PP-8. Our Economy NMR tubes are made of Borosilicate Type 1, Class B that has a Thermal Expansion Coefficient of 49 to 55; a part number example is WG-1228-8. The working temperatures are different between the two types of glass. Borosilicate Type B will soften at a lower temperature than Borosilicate Type A. However, rapid changes in temperature for both types of glass can create undue thermal shock possibly causing breakage. A 528-PP or WG-1228-8 could be suggested here depending on the application.

   Sometimes choosing a tube is not totally based on MHz. For example, there is a MHz difference between 528-PP-8 and WG-1228-8 because the 528-PP is a Precision tube, recommended for 500 MHz, and the WG-1228 is an Economy tube recommended for 400 MHz. The camber and concentricity are the same for the Precision tube 5XX-PP series and the Economy tube WG-XXXX series. The 5XX-PP has a Precision O.D. at .1955, while the Economy version WG-XXXX has a selected O.D., .193 to .1955. The wall thickness is greater on the Economy WG-XXXX tube, which makes it more robust, especially for student use. As long as NMR tubes are handled carefully and washed carefully they could last indefinitely.

5. Is Part Number 335-PP-8 good for the 700 and 800 MHz spectrophotometer? Your catalog states only up to 600 MHz? Can you suggest a good tube for this field?

   When the tube will be reused, our best 3mm is Part Number 335-PP. Our manufacturing process allows us to create a Precision O.D./I.D. on this tube. The MHz rating is > 600 so this tube should perform well for 700 and 800 MHz.

6. I am using Chloroform (CHCL3) or Dimethylsulfoxide (DMSO) to dissolve my sample. The caps that are supplied with the NMR tubes are leaching impurities into my sample. How do I prevent this?

   Only use the PTFE (Teflon) tube caps to seal for these solvents. The part Number series is WG-1264, then add a dash then the O.D. size of the cap, for example WG-1264-5 for 5mm.

7. I am doing Boron NMR. What kind of sample tube should I use?

   We suggest the Clear Fused Quartz or the Suprasil Synthetic Quartz tubes. Quartz does not contain enough Boron to produce interference.

8. I am doing Variable Temperature studies. Which Spinner Turbine can you suggest?

   We suggest the PEEK material Spinner Turbine for NMR tubes. PEEK Spinner Turbines are available for both Bruker and Varian Systems.

9. What is the temperature range that I can use the PEEK Spinner Turbine?

   Between -150 degrees C to +200 degrees C.

10. My NMR sample is light sensitive. Which NMR tube should I use?

    Order amber NMR tubes. Amber tubes give added protection against light exposure between the wavelengths of 300 to 800nm.

11. Which cap should I use to hermetically seal my biological or air sensitive sample in the 5mm NMR tube?

    Use our 5mm NMR Serum Cap, #521-S series, made of #990 natural gum rubber.

12. Do you have equations that allow for the volume of the NMR tube to be determined by the height of the solution level for your 535 5mm NMR tubes?

    Our 5-mm NMR 535-PP Tube is 5.0-mm OD and has a 0.38mm wall thickness. The volume of a cylinder 4.24-mm diameter (the tube ID) x H height (mm) is

    Volume = Pi x R^2 x H = Pi x 2.12^2 x H = 14.12 x H mm^3

    Allowing for the fact that the bottom end of the tube approximates a half-sphere will give a truer volume calculation and yields the formula:

    V = ((14.12 x H) - 9.978) mm^3 where H is greater than 2.12-mm
    (click here for the derivation)

    If you need this much precision, be sure to allow for miniscus effects in determining H. You may want to consider weighing the NMR tube dry and then with solution, then determine the weight of the solution and use its density to determine the actual volume occupied.

EPR

1. What is EPR?

   EPR (Electron Paramagnetic Resonance) is a spectroscopic technique which detects species that have unpaired electrons. It is also often called ESR (Electron Spin Resonance). A large number of materials have unpaired electrons. These include free radicals, many transition metal ions, and defects in materials. Free electrons are often short-lived, but still play crucial roles in many processes such as photosynthesis, catalysis, and polymerization. As a result EPR crosses several disciplines including: chemistry, physics, biology, materials science, medical science and many more.

2. How Does EPR work?

   EPR is a magnetic resonance technique very similar to NMR (Nuclear Magnetic Resonance). However, instead of measuring the nuclear transitions in our sample, we are detecting the transitions of unpaired electrons in an applied magnetic field. Like a proton, the electron has spin, which gives it a magnetic property known as a magnetic moment. The magnetic moment makes the electron behave like a tiny bar magnet similar to one you might put on your refrigerator. When an external magnetic field is supplied, the paramagnetic electrons can either orient in a direction parallel or anti parallel to the direction of the magnetic field. This creates two distinct energy levels for the unpaired electrons and allows measurement of them as they are driven between the two levels. The microwave frequency radiation is applied to the sample from outside the microwave source. The microwave energy is absorbed when the magnetic field is equivalent to the energy differences between the two energy levels.

3. What is the difference between the EPR tissue cells Part Numbers WG-806-Q and WG-806-A-Q? It seems the difference is the base plate size and the coverplate size. Why is the cover plate shorter than the base plate on these tissue cells?

   When the tissue cell is made the rod is fused to the base plate. During this process the base plate takes on a different shape, therefore, the cover plate is shorter so that it can accommodate the base plate.

4. I want to be able to seal my EPR sample tube with a butane torch (for many reasons it is difficult to have a stronger torch in the lab that can seal quartz). I am wondering if this graded seal means that above this grading is where I would seal the tube; is the glass Pyrex/Borosilicate?

   Many EPR samples are degassed; the Graded Seal tube provides a good closure for long time storage. The vacuum racks that are used for degassing usually have Pyrex/Borosilicate components; therefore a Pyrex/Borosilicate seal is needed. Our Quartz Graded Seal tubes (Part Number example 710-GRD-275M) have an approximate 1 inch Pyrex/Borosilicate Seal where there is room to seal.