Simple Setup for Quantitative Electron Paramagnetic Resonance Measurements Raymond Chang Department of Chemistry, Williams College, Williamstown, Mass. 0 1267
A dual cavity is commonly used in the quantitative measurement of unpaired spin concentrations. The procedure is to compare the area under the first derivative of the electron paramagnetic resonance (EPR) spectrum of an unknown sample with that of a standard. Averaged values are obtained by interchanging the samples to eliminate possible different magnetic and/or microwave field a t the samples. We report here a very simple and accurate method for spin measurements that is applicable to EPR spectrometers with a single, rectangular cavity such as the Varian E models. Figure 1 shows the schematic diagram for this setup. The sample tube, which can be either a 3-mm 0.d. quartz or Pyrex tubing, is held in both lateral and vertical positions by two Teflon plugs, C and E. Plug E is itself held rigidly in position with a collet fitted inside a ring. The solution to be studied is introduced into the tubing by suction with a syringe a t the other end. A small piece of Tygon tubing, which is closed a t one end with a piece of glass rod, is then attached to one end of the sample tubing. In this manner, the downward flow of the solution is prevented when the sample tube is inserted vertically through holes in C and E as shown in Figure 1. By using the identical spectrometer settings (microwave power, modulation amplitude, etc.) and with careful tuning, we have been able to obtain routinely reproducibility to about 5%. Of course, the volume of the unknown and standard solutions should be as close to one another as possible. This is achieved by bringing up the solutions between the same upper and lower mark on the sample tube. The error limits are determined by the slight variation in volumes as well as in tunings. We have found this technique useful in the titration of Mn2+ ions with enzymes for binding site and binding constant determinations. I t should also be applicable to pollution studies; for example, the determination of transition metal ions and free radicals such as C102 in water and paramagnetic species in air. The only restriction is that the dielectric constant of the standard and unknown must not
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E Flgure 1. Schematic diagram of a simple setup for quantitative EPR
measurements A, Tygon tubing which is closed in one end with a small piece of glass rod: 8 , 3-mm 0.d. sample tube; C and E, Teflon plugs; D, coilet ring. The shaded area represents the solution
differ greatly to ensure similar tunings. The spin concentration of the unknown can then be obtained by comparing either the intensity of the lines (as in the case of Mn2+ ions) or, for poorly resolved spectra, the area under the curve with that of the standard. The area is readily measured from the first moment of the EPR spectrum ( I ) . Received for review January 7, 1974. Accepted April 26, 1974. Financial support of the Research Corporation is gratefully acknowledged. (1) R. Chang and C.
S.Johnson, Jr., J. Chem. Phys., 46, 2314 (1967)
Preparation of High Purity Concentrated Hydrofluoric Acid Michael S. Lancet and James M. Hueyl Department of Chemistry, Carnegie-Mellon University, 4400 Fifth Ave. Pittsburgh, Pa. 15213
Recently Mattinson ( I ) presented a new method for the preparation of high purity hydrofluoric acid. Mattinson’s method, subboiling distillation in Teflon bottles, yielded 48% HF with a lead concentration of from 0.002 to 0.005 Present address, Department of Surgery, L.S.U. School of Medicine, New Orleans, La. 70112. (1) J M Mattinson Anal Chem 4 4 , 1715-16 (1972)
1360
ng/g. Earlier, Tatsumoto (21, in preparation for his lead studies of lunar samples, described a distillation-sublimation procedure for 100% HF which produced a 50R hydrofluoric acid solution with a lead content of 0.08 ng/g. Others (3-5) had previously described methods of HF preparation but these yielded lead concentrations of 0.2 to (2) M Tatsumoto, Anal Chem 41, 2088-9 (1969) (3) W. Kwestroo and J Visser, Analyst (London). 90, 297-8 (1965)
A N A L Y T I C A L C H E M I S T R Y . VOL. 4 6 , NO. 9, AUGUST 1974
