finned extension tube described which enabled the septum to be maintained between 60-80 'C irrespective of the temperature of the injection block. An efficiently designed water cooled device would undoubtedly serve the same purpose but would be more difficult to install and would necessitate the use of an additional pipeline to the instrument. Figure 4 shows the effect of fitting the modification to an F & M 810 gas chromatograph, all spurious peaks being completely removed despite an injection port temperature of 250 "C. Figure 5 shows the temperature/time variation of the septum and injection port: at no time is the critical temperature reached despite an injection block temperature of 250 "C. Since the completion of the above experiments, injection-port temperatures of up to 350 "C have been used at comparable sensitivities without any detectable septa bleed occurring. The device undoubtedly increases the dead volume of the injection port. However, on injection the greater part of this increase in volume is occupied by the syringe needle, and
provided the heated portion of the injection block into which the injection is made is packed (glass wool), no peak broadening was experienced. CONCLUSION
This method described for eliminating ghost peaks has the advantage that it does not alter the properties or shorten the life of the septa. It is B simple modification and once installed it is permanent unlike pretreatment methods which must be carried out on each batch of septa. Although this modification is described for an F & M instrument, it has been applied with equal success to a Phillips PV.4000 despite the fact that this instrument has a vertical injection line above the oven. Therefore, it is likely that similar modifications can be made to any instrument irrespective of injection port position. RECEIVED for review March 18,1969. Accepted July 8,1969.
New Method for Preparing Ultrapure Hydrofluoric Acid Mitsunobu Tatsumoto U.S . Geological Suroey, Denver, Cola 80225 THE PURITY of hydrofluoric acid is extremely critical in the chemical analysis of trace amounts of metals in silicates and in the determination of the isotopic composition of elements in silicates. The commercial reagent-grade acid is customarily purified by distillation in platinum apparatus ( I ) . Tilton and others (2) prepared pure hydrofluoric acid by passing hydrogen fluoride gas through a filter of fine Teflon (Du Pont) shavings and bubbling the gas into water in an ice-cooled platinum vessel. The hydrofluoric acid that has been prepared in our laboratory by these methods usually contains 0.0002 to 0.001 pg/ml of lead. Recently Kwestroo and Visser (3) reported an isothermal distillation method for the purification that can be carried out at room temperature. Their hydrofluoric acid prepared by this method contains 0.0002 pg Pb/ml. Because of the need to obtain hydrofluoric acid in as pure a state as possible for lead isotopic studies in the analysis of the returned lunar sample, a new method for the preparation of pure hydrofluoric acid has been developed. The new method does not require an expensive platinum still, and it yields hydrofluoric acid of higher purity than did previous techniques. Principle of the Method. The principle of the preparation method is (1) to pass hydrogen fluoride gas through a Teflon filter and to freeze it in a Kel-F tank by cooling it with liquid nitrogen; (2) to then distill and freeze the gas in another Kel-F tank and subsequently to introduce the gas into pure distilled water in a Teflon (Du Pont) bottle.
(1) W. F. Hillebrand, 6.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, New York, 1953, pp 38-9. (2) G. R.Tilton, C. Patterson, H. Brown, M. Inghram, R.Hayden, D. Hess, and E. Larsen, Jr., Bull. Geol. SOC.Amer., 66, 1131
(1955). (3) W. Kwestroo and J. Visser, ,4nalyst, 90, 297 (1965). 2088
*
TEFLON
1 -
CHECK BACK-FLOW VALVE
I11 - I1
/I
u
I
HF CRYS H20
IN TEFLON
BOTTLE
Figure 1. Schematic diagram of HF purification system A : commercial monel valve B, D , F, G, and I: Teflon (Du Pont) valves C and E: Kel-F made tank, capacity about 2 liters
EXPERIMENTAL
Method and Apparatus. The purification system, shown in Figure 1, is made of Teflon (Du Pont) and Kel-F. Steps of the method are as follows : (1) Close all valves and start pumping down. Open Teflon valves G , F, D, and B in succession to determine whether any component is leaking. After obtaining a good vacuum (usually lo-* mm Hg), close all the open valves. ( 2 ) Cool tank C with liquid nitrogen and open valves A and B. Hydrogen fluoride gas now diffuses into tank C through the Teflon filter (commercial product, 9p porosity) and condenses on the wall as a solid, since the melting temperature of hydrogen fluoride gas is -83.1 "C. The transfer of 700 grams of hydrogen fluoride gas into tank C usually takes about a day. (3) Close valves A and B. Remove the liquid nitrogen from tank C and place it around tank E. Hydrogen fluoride crystals melt in tank C. The use of an infrared heat lamp speeds up the melting and also makes it convenient to observe the height of the melted hydrogen fluoride. However, caution should be exercised not to heat the tank too much (boiling point of hydrogen fluoride is 19.54 "C).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
(4) Open valve D and gently warm tank C by a heat lamp so that hydrogen fluoride is transferred from tank C to tank E where it is frozen. Leave a small portion of the liquid hydrogen fluoride in tank C and close valve D. This transfer usually takes about half a day. (5) Remove the liquid nitrogen from tank E and warm up the tank with a heat lamp. After the hydrogen fluoride has melted, place the liquid nitrogen on copper-trap H and then open valves F and G (observe that hydrogen fluoride in tank E is boiling). After 5 minutes, close valve G and open valve 1 so that the hydrogen fluoride gas is bubbled into triply distilled water contained in a Teflon bottle. Care must be exercised. Sucking back of the distilled water into tank E is dangerous. The distilled water in the Teflon bottle is cooled by ice; however, the use of a mechanical cooling device would be more convenient considering the prolonged time of the bubbling. Discard the hydrogen fluoride in the Cu-trap. Stop bubbling by closing valve I and leave about 20 ml of hydrogen fluoride liquid in tank E. DISCUSSION
Purity. The purity of hydrofluoric acid made by this method has been determined by isotope dilution for lead and potassium contents. Fifty per cent hydrofluoric acid contained only 0.002 pg K and 0.00008 pg Pb per ml, which is lower than the values mentioned above. These results were obtained by analyzing different volumes of the acid (4). The acid in the amount of 100 ml and 200 ml was evaporated in
Fluoride Microelectrode-Fabrication
Teflon dishes under nitrogen atmosphere and the residues from the evaporation were analyzed for lead and potassium. The blank values of the analyses which were obtained simultaneously with concentration determinations by the different volume analyses were 0.004 pg for lead and 0.02 pg for potassium whereas the determined values for 100 ml and 200 ml of the acid were 0.010 pg and 0.018 pg for lead and 0.20 pg and 0.37 pg for potassium, respectively. ACKNOWLEDGMENT
I am grateful to J. D. Obradovich for assisting in the potassium determination and to N. V. Carpenter for constructing the purification system. RECEIVED for review June 25, 1969. Accepted August 13, 1969. This work was supported in part by the National Aeronautics and Space Administration under contract T-75445 and was presented as a part of “Plans for Chemical Analysis of Returned Lunar Samples” at the Division of Nuclear Chemistry and Technology, 157th National ACS meeting Minneapolis, Minn., April 1969. Publication authorized by the Director, U. S. Geological Survey. (4) M. Tatsumoto and C. C. Patterson, “Earth Science and Meteoritics,’’ North-Holland Pub. Co., Amsterdam, 1963, pp 74-89.
and Characteristics
Richard A. Durst Institute for Materials Research, National Bureau of Standards, Washington, D. C. 20234 THE FEASIBILITY of constructing a fluoride microelectrode suitable for studies in microliter volumes of sample solutions has been demonstrated. This basically simple and straightforward technique illustrates the capabilities of such ionselective electrodes in studies similar to those in which glass microelectrodes are currently used. Although still about two orders of magnitude larger than the smallest glass microelectrode ( I ) , the characteristics of the first fluoride microelectrode are encouraging enough to prompt further work in the miniaturization of this and other ion-selective electrodes. Previously, small volumes (5 to 50 p1) could be studied with a conventional ion-selective electrode using either the inverted electrode technique, as was demonstrated with the fluoride electrode ( 2 , 3), or the drilled-well electrode, as used with the silver sulfide electrode (4). In this latter sensor, 5- and 10-p1 holes were drilled into the polycrystalline silver sulfide membrane of a commercial electrode (Orion Model 94-16). These depressions then served as sample containers when the electrode was used in the inverted position. Although these systems allowed measurements on small volumes of solution, (1) R. N. Khuri in “Ion-Selective Electrodes,” R. A. Durst, Ed., National Bureau of Standards Special Publication 314, U. S. Government Printing Office, Washington, D. C., 1969, Chapter 8. (2) R. A. Durst and J. K. Taylor, ANAL.CHEM., 39, 1483 (1967). (3) Ibid.,40,931 (1968). (4) R. A. Durst in “Microchemical Analysis Section: Summary Qf Activities, July 1968 to June 1969,” J. K. Taylor, Ed., National Bureau of Standards Technical Note 505, U. S. Government Printing Office, Washington, D. C., in press.
these electrodes could not be used for measurements in situ but, rather, the sample had to be placed in the microelectrode cell. In contrast, while this new fluoride microelectrode is still too large for most in vivo measurements, it should be applicable to certain in situ studies where the solution volume is a limitation or in conjunction with other microelectrodese.g., H+, Na+, Kf--where simultaneous measurements must be made on a limited volume of sample or on a continuous basis. In such cases, the inverted microelectrodes would not suffice. CONSTRUCTION
The fluoride microelectrode is constructed from a section of polyethylene tubing drawn out to a 2-mm (0.d.) neck at the lower end. A small, cone-shaped piece of europiumdoped lanthanum fluoride (5) is inserted into the narrow end of this tube and heat sealed into place. In the first design, the microelectrode was used in this form, but was later refined as shown in Figure 1. In this latter design, the tip was painted with polystyrene coil dope which, after drying, was scraped away to expose a small, well-defined area of the crystal tip. The first form of the electrode utilized a mercury contact between the inner surface of the lanthanum fluoride membrane and the lead to the pH-millivolt meter. This type of contact proved unsuccessful, and the internal reference electrode system was used. This consisted of an internal 0.1M NaF( 5 ) M. S. Frant, U. S. Patent 3,431,182, issued March 6, 1969.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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