CAESR

ESR Facility - Department of Chemistry

Concentration / quantity 

As a rule of thumb, a desirable concentration of paramagnetic centres is around 5 mM for frozen solutions, powdered samples, or for solutions of paramagnetic molecules with fast relaxation rates (e.g. transition metal complexes). The quantity required is approximately 50 μL or less, see EPR tubes below for filling heights and tube diameters. For the measurement of organic radicals in solution the concentration needed for the best resolution will be usually much less, c.a. 0.1 mM. 

EPR tubes 

The centre provides an extensive range of EPR tubes suitable for the two EPR spectrometers. You MUST record (in the EPR record book) and pay for the tubes out of your grant. You are strongly encouraged to re-use in particular the high precision EPR tubes, and cleaning facilities are provided in the Sample Preparation Laboratory. There is equipment for sealing all EPR tubes on the vacuum line so that air sensitive samples can be prepared at X- and W-band microwave frequencies. The EPR sample tube and sample volume depends upon the resonator to be used. 

Table: Range of EPR tube diameters (OD, ID) and lengths (L).  5mm NMR tubes don't fit any of the cryostats

Tube Dimensions
  OD          ID           L
(mm)      (mm)     (mm)
 

Microwave

Band

Comment

0.4

0.3

100


 

W

For lossy samples

0.84

0.6

100

 

W

General, for powders and frozen solutions

0.87

0.7

100

 

W

For weak signals. The thin wall makes them unsuitable for solutions that expand when frozen or thawed.

3

-

150

 
  

X

For split ring resonator

3.8±0.013

2.79±0.013

150

 
  

X

For ENDOR pulsed resonator, high precision wall thickness

4

3

250

 

X

For all CW resonators, and the dielectric resonator with 5mm sample access

 

Air-Sensitive, frozen samples from a glovebox

For the samples in the glovebox, first use a Pentel or Sharpie marker to label the 10 cm tubes with a marking above halfway.  You can also take a piece of white or light coloured electrical tape and cut ca. 8 mm x 20 mm, label it and wrap it around the tube, leaving 1 cm clear at the top.  Bring the labeled tubes and two Subaseals into the glovebox with your materials.  In the glovebox, load one tube with a 12 cm syringe needle and put a Subaseal on it (should fit closely around the outside top of the tube).  Pass the tube out the small transfer chamber and have a bowl dewar with liquid N2 ready.  Take your sample from the transfer chamber and touch the tip to the top of the liquid N2, until you hear "fizzle" and then lower at ca. 2 mm / sec. until the sample material is frozen solid (ca. 30 seconds), but DON'T freeze the whole tube and Subaseal --just the bottom portion with the sample material.  Remove the Subaseal from the top of the tube and submerge the bottom portion of the Falcon tube into the bowl dewar.  The Falcon tube should have two 5 mm holes at ca. 4 cm from bottom two 5 mm holes at 2-3 cm from top and 4-5 1.5 mm holes in top cap.  Place your frozen, uncovered tube into the Falcon tube.  Repeat this process, but also add the second frozen sample with a different label into the same Falcon tube.  Freeze the samples prepared outside of the glovebox in the same way, again with different labels, and place those in the same Falcon tube that I gave to you, such that there are then four tubes in the 15 mL Falcon tube.  This Falcon tube you can store until tomorrow