JOURNAL OF MAGNETIC RESONANCE 33, 465-467 (1979) COMMUNICATIONS An Inexpensive Technique for Obtaining Low Temperature EPR Spectra Usually, EPR spectra can be taken at low temperatures if the sample is maintained at a constant appropriate temperature inside a Dewar flask. For liquid nitrogen temperatures, a new method has been developed, involving continuous-flow cry- ostats (1, 2) which have several advantages over the more common reservoir-type cryostats (3). A recent note by D. Chasteen (4) provides a convenient solution to the cryogenic bubbling problem encountered with the classical narrow tailed quartz Dewar. We now wish to report a simple method, based on the same idea as that operating in continuous-flow cryostats, which we use for obtaining quick-freeze liquid nitrogen temperature EPR spectra. Several problems are common to all low temperature techniques and will be pointed out for the ordinary Dewar-type installation. For K and Q band experiments the cavity is small and the entire system is cooled. For ordinary X band spectra the cavity is larger but only the sample container must be cooled. This can be achieved rapidly by inserting the quartz tubing containing the sample into a cooled Dewar flask, and positioning the assembly within the cavity. Some precautions must be taken to maintain proper operating conditions. First, dry room-temperature nitrogen must flow continuously through the waveguide and cavity to eliminate temperature variations outside the thin Dewar flask and prevent moisture conden- sation on the cavity walls. Second, that portion of the Dewar flask which is inside the cavity n~ust be unsilvered and free of dust and other particles to preclude interference with the experiment. If the sample tube is relatively large (2 or 3 mm outer diameter) there will be too little nitrogen left between the walls of the Dewar and sample container, resulting in excessive bubbling of the coolant. With smaller sample tubes, bubbling is also a problem which can cause jumping of the sample and may damage the detector crystal during operation. Our technique is a simple modification in the installation of available variable temperature equipment. We place the sample tube in an open-end Dewar flask (Varian insert Dewar 961180-02) positioned in the cavity (Varian E-231, TE 102 mode) as indicated in the drawing (see Fig. 1). The sample tube can be as large as 3 ram; convenient for dilute solutions. The liquid nitrogen flows, as indicated in Fig. 1, from the source to the Dewar flask thrugh a rubber tube attached to the arm at A. The nitrogen gas exits from the Dewar at the openings B and C. When the system approaches liquid nitrogen temperature, the flow of coolant is kept at a minimum, such that only a few drops of liquid nitrogen appear at C. This ensures a nominal temperature of 77 K at the sample site. In our experiments, the Dewar flask is attached to the cavity with a collet ring and teflon-lined screw at the top port. The dry 465 0022-2364/79/020465-03~$02.00/0 Copyright © 1979 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain