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
W
n
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)
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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)
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1.0 ng/g. This paper describes a simple method of preparing ultra-pure hydrofluoric acid containing less than 0.001 ng/g of lead.
I CM Teflon Tube
EXPERIMENTAL Apparatus and Procedure. Our apparatus consists of two 1liter narrow-mouth Nalgene polyethylene bottles with a 10-mm hole drilled in each cap, and a 1-cm diameter by 50-cm long flexible Teflon tube fitted with polyethylene connector a t each end (Figure 1). Mattinson’s ( I ) apparatus would work equally well for our purposes, b u t its machined Teflon parts appear t o us t o be a n unnecessary expense. After thorough cleaning in 12M nitric acid, one bottle is cooled in a dry-ice b a t h a n d filled with 100% H F liquid via a Teflon tube from a tank of H F gas. When t h e bottle is approximately 314 full of liquid H F , it is removed from t h e HF gas t a n k a n d connected t o the apparatus as shown in Figure I. The dry-ice b a t h is then switched from t h e feed bottle (bottle 1 in Figure 1) t o t h e collection bottle (bottle 2 ) ; and as t h e 100% H F warms to room temperature, it automatically distills into t h e collection bottle. After about 90% of t h e H F in the feed bottle has distilled into t h e collection bottle, t h e distillation is halted and t h e feed bottle is rinsed in triply-distilled water. T h e positions of t h e two bottles are then reversed and t h e distillation is repeated. Five such distillations produce 100% hydrofluoric acid of the desired purity (Table I ) . The total time required for five distillations is about 10 hours and thus the whole preparation can easily be managed in one day. If desired, the distillation time can be considerably shortened by using a heat l a m p on the feed bottle to increase t h e vaporization rate; however, this lowers t h e purity of t h e distillate somewhat and t h u s requires more distillations t o achieve the same purity (Table I). After t h e distillation is complete, a hydrofluoric acid solution of any desired strength can be prepared by carefully mixing t h e distilled 100% H F with high-purity water. Thus, unless water which has been distilled five or more times in a sub-boiling quartz still is used, the final purity of t h e acid will depend on t h e purity of the water used for mixing. Finally, t h e freshly prepared high-purity solution is stored in a 32-ounce FEP Teflon bottle (Nalgene).
RESULTS AND DISCUSSION The results of mass-spectrometric analysis of lead contamination from the distillate of each distillation, by isotope dilution analysis using a 208Pb spike, is shown in Table I. About 50 ml of HF plus 100 ng of *08Pb spike was evaporated in a Teflon beaker in a laminar-flow hood located inside a class 100 clean room, and no corrections were made for lead introduced during the evaporation. Loading blanks for lead in our laboratory were always less than 0.0001 ng and no corrections for loading blanks were made here. The lead concentration in our fifth distillate (0.0009 W F. Hillebrand, G . E F Lundell. H. A. Bright. and J. I . Hoffman, “Applied Inorganic Analysis with Special Reference to the Analysis of Metals. Minerals and Rocks,” 2nd ed., John Wiley and Sons, New York. N . Y . . 1953, p p 38-9. G R. Tilton. C. Patterson, H . Brown. M . Inghram, R. Hayden. D. Hess, and E. Larsen. Jr., Geoi. SOC. Arner. Buii., 66, 1131-48 (1955).
R-”’ M 1 0 0%
Figure 1. Distillation apparatus for the preparation of ultra-pure 100% hydrofluoric acid
Table I. Lead C o n c e n t r a t i o n s in Multi-Distilled 100% Hydrofluoric Acid Lead concentration, ng,‘g No. of distillations
Room temperature
With heat lamp
3.68 0.546
3.68 0.925
0.105 0.0210 0 ,0048
0.314 ... ...
0.0009
...
ng/g) is about one-half to one-fifth that of Mattinson’s 48% HF ( 1 ) and close to one-hundredth that of Tatsumoto’s 50% solution (2). The ability to prepare H F solutions of concentrations greater than 48% is a distinct advantage in the dissolution of silicate materials. We have found that rocks that require u p to one week for dissolution in 4870 HF can be dissolved in less than one day using 55-6070 HF. Thus our procedure [and that of Tatsumoto (211 has a distinct advantage over that of Mattinson ( I ) . Since Tatsumoto’s system could conceivably be used to redistill the 100% H F and achieve purity similar to that obtained by us, the main advantage of our method is lower cost and shorter distillation times as Tatsumoto’s procedure requires about (lh) day per distillation for approximately the same volume of acid. Thus our system seems to be the simplest; yet it produces hydrofluoric acid of the highest purity.
ACKNOWLEDGMENT We are grateful to T. P. Kohman for his valuable assistance with this project. Received for review August 20, 1973. Accepted March 13, 1974. This work was supported by the U.S. Atomic Energy Commission through contract AT( 11-1) -3236.
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