Curie temperature can be obtained at zero-field magnetization by this method, and Tc values calculated for zero-field magnetization are in excellent agreement with these results. For example, the literature value for Ni obtained in a low-field magnetic measurement is 358 "C, but calculations (5) for zerofield indicate that Tc should be 354 "C. This is the same value that we report from the DSC experiment. Similarly, the zero-field magnetization value calculated (6) for gadolinium is 17 1 "C, while the DSC measurement shows a Tc of 18 "C. (5) A. Arrott, Phys. Rel;., 108, 1394 (1957). (6) J. S. Kouvel, General Electric Development Center, Schenectady, N.Y. 12301, General Electric Laboratory Report 57-RL1799, September 1957.
In addition to the determination of transition temperature, the DSC experiment can also yield additional thermodynamic data such as latent heat of the transition. ACKNOWLEDGMENT
The authors acknowledge the help of R. A. Baxter, who suggested this approach; W. H. Cloud, who advised us on the magnetic interactions; and D. D. Bly for technical suggestions.
RECEIVED for review June 9, 1969. Accepted September 25, 1969.
AIDS FOR ANALYTICAL CHEMISTS Injection Port Modification to Prevent Septa Bleeding in Programmed Temperature Gas-Liquid Chromatography 0.G . Tucknott and A. A. Williams Long Ashton Research Station, University of Bristol, Bristol, England
WHENPERFORMING programmed gas-liquid chromatography at high sensitivity, trouble is often experienced with ghost peaks, caused either by the bleeding of volatile material from the flow controllers or injection ports. During the cooling period, these volatile compounds concentrate on the front of the column and are hence analyzed in the next programmed run giving rise to spurious peaks. When operating isothermally, bleeding gives rise to continual background noise and hence is not noticed. The phenomenon of ghost peaks is not so important when working at low sensitivities, but can be very confusing when operating at high sensitivities. Since the completion of this work, the production of volatiles from flow controllers has been reported by Mizuno et al. (1)who came to similar conclusions, with regard to their source and cure, to those reached in our laboratory. These contaminants were easily removed by the incorporation of a 12411. x 1I2-in. molecular sieve (B.D.H. Type 5A, 1/16-in. pellets) trap between the flow controller and injection port. The second source of contamination is the injection port septa. It is this source of spurious peaks which is the concern of the present publication. Both Kolloff (2) and Tamsma et al. (3)have reported overcoming the problem of septa bleed by heating the septa before use. Kolloff used a temperature of 300 "C in a stream of nitrogen for 90 hr and Tamsma et al. heated to 250 "C under vacuum in a rotary evaporator overnight. Relatively little success was obtained with similar methods in our laboratory, the major disadvantage being that the properties of the septa were considerably modified and their usable life shortened. The injection port modification described below overcomes the problem of septa bleed without any need for pretreatment. (1) G. Mizuno, D. Jarvis, and J. R. Chipault, J. Gas Chromatogr., 6, 179 (1968). ( 2 ) R. H. Kolloff, ANAL.CHEM., 34, 1840 (1962). (3) A. Tamsma, F. E. Kurtz, N. Rainey, and M. J. Pallansch, J . Gas Chromatogr., 5 , 271 (1967). 2086
D
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Figure 1. Injection port modification EXPERIMENTAL
Apparatus. The instrument used for this investigation was an F & M 810, temperature-programmed, dual-channel gas chromatograph with flame ionisation detectors, fitted with two ]/%-in.12-ft columns (60:80 mesh 10% Carbowax 20M on Chromosorb W). Nitrogen flow was maintained at 40 ml per min. Size X 12-in. molecular sieve traps (B.D.H. Type 5A l/&n. pellets) were incorporated between both flow controllers and injection ports. The injection port on the reference column was sealed off to permit spurious peak production on one column only, thus simplifying the resulting chromatograms. All chromatograms were obtained by programming from 65 "C to 210 "C at 8 "C per min. with a detector temperature of 250 "C. The injection-port temperature, except where stated, was held at 250 "C. The maximum sensitivity of the amplifier was such that it was
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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Figure 3. Chromatogram from septum at injectionport temperature of 165 "C left overnight before running
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terial only or without fins do not possess the ability to keep the septum sufficiently cool. With the injection port modification in place, it is essential to use a syringe with a 21l2to 3-in. long needle which protrudes beyond the modification into the heated zone.
24 Mln
Figure 2. Chromatograms from septum at various injection-port temperatures
RESULTS AND DISCUSSION
able to give a full-scale deflection on a I-mV recorder with an input of 4 x 10-12 amp. Septa. Septa were obtained from Hewlett-Packard, Varian Aerograph, Pye Unicam, and Perkin-Elmer as standard supplies. They were also cut from silicone rubber sheets supplied by Esco (Rubber) Ltd., Seal Street, London, E.8. Details of Construction of Injection-Port Modification. The injection-port modification as fitted to the F & M 810 is shown in Figure 1. It consists of a stainless-steel locating nut, A , silver soldered to a stainless-steel tube, B. Brass disks, Cyare attached at right angles to B and the septum is held in place by means of a second removable nut, D, possessing a small central hole. The materials and mode of construction are such that heat transfer is maintained at a minimum in the line of injection, whereas at right angles to this it is at a maximum. Thus, heat travelling from the hot injection zone is quickly dispersed before reaching the septum. Injection-port modifications constructed of one ma-
Typical chromatograms resulting from septum bleed are shown in Figures 2, 3, and 4. The intensity of these peaks depends not only on the temperature of the injection port but also on the time interval between cooling and restarting the programmed run (Figures 2c and 3); the longer this time interval, the more material accumulating on the front of the column. Several makes of septa were used during the investigation and all were found to suffer from similar ghostpeak production. Those purchased from Hewlett-Packard in July 1968 were improvements over earlier septa; nevertheless the problem still existed. Solvent extraction of septa using pentane/ether (1 : 1) or ethylene chloride only reduced the problem and was nowhere near as efficient as the injection-port modification. While being partly successful when using a flame ionisation detector, the use of chlorinated hydrocarbons as extracting solvents would increase noise when using an electron-capture detector despite the fact that such detectors are normally run isothermally. By keeping the injection port cool, it was found that ghost peaks were not produced, thus confirming Kolloff's observation. It therefore seemed reasonable to assume that the removal of volatile material from the septum was a function of temperature. Several runs were performed at various injection-block temperatures (Figure 2) showing that ghost peaks were only produced above 80 "G. The solution was therefore to keep the septum cool independently of injectionport temperature. This was achieved by fixing the short-
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Figure 4. Chromatograms before and after fitting injection-port modification ( A ) Before (B) After Injection-port temperature, 250 "C
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Figure 5. Temperature variation with time of septum and injection block ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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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
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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
