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Anal. Chem. 1985, 57, 2419-2421
Overall, the system for sampling and desorption proved effective and simple t o use. The system described here is a new combination and application of proven analytical technologies. Thus, the basic advantages and limitations of the system are those inherent in the individual trapping materials and desorption techniques. For the materials used in this system (Tenax and charcoal), the range of applicability has been adequately studied ( 4 , 5 , 1 0 , I I ) . With these materials, the combined technique should be applicable to a wide range of organic compounds in the atmosphere. Improvements in the system may come with development of improved sampling resins and sorbent materials. Registry No. C, 7440-44-0;Tenax-GC, 24938-68-9;naphthalene, 91-20-3.
LITERATURE CITED ( 1 ) McClenny, W. A.; Plell, J. D.; HoMren, M. W.; Smith, R. N. Anal. Chem. 1984, 56, 2947. (2) Fox, D. L.; Jeffrles, H. E. Anal. Chem. 1983, 55, 233R. (3) Melcher, R. G. Anal. Chem. 1983. 55, 40R.
National Institute for Occupational Safety and Health “NIOSH Manual Analytical Methods”,2nd ed.; U.S.Department of Health, Education and Welfare: Cinclnnati. OH, 1977; Vol. 1-5. (5) Krost, K. J.; Pelllzzari, E. D.; Walburn, S. G.; Hubbard, S. A. Anal. (4)
of
Chem. 1982, 5 4 , 810. (6) Giam, C. S.;Chan, H. S.;Neff, G. S. Anal. Chem. 1975, 47, 2319. (7) Billings, W. N.; Bldleman, T. F. Atmos. Envlron. 1983, 77, 383. (8) Doskey, P. V.; Andren, A. W. Anal. Chim. Acta 1979, 110, 129. (9) Chang, L. W.; Atlas, E.; Giam, C. S. J . I n t . Envlron. Anal. Chem. 1985, 19, 145. (IO) Grob, K. J . Chromatogr. 1973, 8 4 , 255. (11) Brown, R. H.; Purnell, C. J. J . Chromatogr. 1979, 778, 79.
RECEIVED for review March 29,1985.
Accepted June 5,1985.
Capillary Tube Sealing for Microcell Nuclear Magnetic Resonance Spectrometry Ben V. Burger* a n d Hendrik S. C. Spies Department of Chemistry, University of Stellenbosch, Stellenbosch 7600, South Africa Microcell NMR is used as a routine analytical technique in our research on insect and mammalian chemical communication ( 1 , 2 ) . Although the capillary tube sealer developed by Skrabalak and Henion (3)is considered to be a handy piece of equipment in the NMR laboratory, we have found the sealing of micro NMR samples under a slight vacuum to result in a higher success rate, especially if such samples are to be used for temperature studies, if very long data acquisition periods are required, or if highly volatile solvents are employed. The application of vacuum during the sealing process eliminates the formation of a fine capillary which is normally formed when a glass tube is drawn out. For this purpose we have been using the extremely simple sealing aid described herein for more than 7 years without a single failure.
EXPERIMENTAL SECTION Apparatus. The device which is used for the sealing of sample tubes is shown in Figure 1. It consists of a gastight 500-pL syringe (B) and a connecting capillary tube holder (C) made of Teflon (15 X 4 X 4 mm). This connection is provided with a hole in one end to fit the tip of the syringe needle as tightly as possible and a connecting hole in the opposite end, the size of which is selected so as to give a vacuum-tight connection with, for example, a 1.7 mm diameter precision micro NMR sample tube (A). Procedure. The open end of the sample tube is fire-polished to facilitate proper insertion into the holder and to avoid damaging the Teflon surface. The sample tube is then purged with dried (molecular sieve) and filtered ultra-high-puritynitrogen or treated with D20 to remove adsorbed moisture ( 4 ) , if required. The sample is introduced into the tube by rapidly ejecting 5-8 pL of a concentrated solution of the material from a 10-pL syringe into the capillary. This procedure is repeated with more of the sample or with solvent until the required sample size (for example, 15 pL,corresponding to ca. a 15 mm column in a 1.7 mm sample tube) has been reached. The upper half of the sample tube is then carefully purged for 10-15 s with a slow stream of ultra-high-purity nitrogen from a short piece of uncoated fused silica tubing or a drawn out melting point capillary, so as to remove residual solvent and solvent vapor from this part of the sample tube, care being taken not to disturb the surface of the sample itself. The sample tube is then immediately connected to the 500-rL syringe by means of the Teflon connector, the plunger drawn back to the 100 pL mark, and the sample tube, held at an angle of 45”, cooled yith solid COP During the cooling process the plunger is gradually drawn back to the 250 pL mark. Degassing of the sample takes place. The rising air bubbles are an indication of a proper vacuum-tight connection between syringe, Teflon connector, and
sample tube. As soon as the sample is frozen solid, the plunger is drawn back to the 400 p L mark, the syringe is rotated with the right hand, and the sample tube is sealed and drawn out in the side of the flame from a small Bunsen burner, care being taken to grasp the sample tube well above the surface of the frozen sample (Figure 1). If necessary, the sealed tip of the sample tube can be trimmed or straightened in the flame while the sample is still frozen.
RESULTS AND DISCUSSION Two problems are often encountered with the conventional sealing of NMR capillary sample tubes with a microburner or with the sealing apparatus of Skrabalak and Henion (3). The first is the formation on the inside capillary wall of a black carbon deposit by pyrolysis of the solvent, preventing the glass from forming a gastight seal. Furthermore, DC1 produced by the pyrolysis of a halogen-containing solvent, such as CDC13, could lead to the catalytic decomposition or rearrangement of the sample. Even with solvents, such as ethanol-c16which do not readily produce such a carbon deposit, pinholes are often formed, and at a probe temperature of, for example, 34 “C aJl the solvent can be lost during overnight data acquisition. We have found that these problems are eliminated by the procedure described herein. Care is taken to avoid getting the sample on the capillary wall at the point where it is to be sealed. If a sample is ejected slowly into an empty capillary, it will invariably form a plug and the vapor pressure of the solvent will push this plug upward. The plug could even be pushed out of the sample tube, and although this can be avoided by quickly shaking the plug down into the bottom of the tube, some material may be left on the upper parts of the tube wall. When a sample is ejected slowly, a film of liquid will also be trapped between the needle and the glass of the tube by capillary forces. When the needle is afterward withdrawn, some material will adhere to the needle and will be lost. The rest of the trapped liquid will be deposited at the upper end of the tube, forming a plug which, in most cases, will be pushed out of the tube before it can be shaken down. If, however, the solution is “shot” into the sample tube, the formation of a plug and the trapping of solvent between the needle and the glass surface are totally eliminated, and if care is taken to ensure that the needle of the syringe is clean and dry before it is introduced into the sample tube, no solvent or sample should come into contact with the glass at the point where it is to be sealed. Should
0003-2700/85/0357-2419$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
of the tube has been avoided, residual solvent vapor in the upper part of the tube, especially when highly volatile solvents such as CDzClz are used, can still result in unsatisfactory results. Purging of the upper half of the tube with pure nitrogen eliminates this possibility. Instead of a machined Teflon connection, Teflon shrink tubing can also be used, provided that several coaxial layers of tubing are used on the needle to compensate for the different diameters of needle and sample tube. Solid C 0 2 is normally used to freeze samples before sealing. Direct cooling in liquid nitrogen is not advisable as air may be liquified in the sample tube if sufficient vacuum is not applied or if the Teflon connector develops a leak. The sealed tube may thus be fractured when the cooling medium is removed. Cooling with a wad of cotton wool dipped in liquid nitrogen can, however, be used for this purpose if a lower temperature than that of solid COz is required to freeze the sample or if only a few tubes are to be sealed. Care has to be taken to ensure that the Teflon connection is not cooled, as Teflon loses its sealing ability at low temperatures. In the sealing of NMR sample tubes, the application of vacuum during the sealing process has the added advantage that the glass does not have to be heated to such a high temperature, as the seal will be formed as soon as the glass is soft enough to be drawn together. The possible transfer of heat to the solvent is thus minimized. The risk involved in the application of vacuum to a liquid sample in a narrow tube and the use of a cumbersome vacuum tube is eliminated by using suction from a syringe. Pheromones can normally be isolated only in very small quantities and the acquisition of 13C NMR data for 50 pg of a sesquiterpene may require more than 600 000 pulses over a period of 5 or more days. No solvent loss from sample tubes sealed by this procedure has been observed during such long acquisition periods. This is illustrated by the 80-MHz 'H and 20-MHz I3C NMR spectra, shown in Figures 2 and 3, re(80 pg in 12 p L spectively, of (Z,Z)-pentadeca-6,9-dien-l-ol of CDClJ isolated from the preorbital gland of the small antelope Raphicerus melanotis. These spectra were run at 34 "C on a Varian F T 80 NMR spectrometer using the standard insert-based switchable probe (receiver coil diameter 3 mm)(5) with internal heteronuclear lock, and CDC13 as the lock solvent. Although some decomposition, polymerization, or rearrangement of certain terpenoid compounds could be
Figure 1. Sealing an NMR capillary sample tube (A) under reduced pressure generated by drawing back the plunger of the gastight syringe (B) which is connected to the sample tube with a Teflon connector (C).
I l l II
IO
9
3
7
6
5
4
3
2
1
0
6 (PPd
Flgure 2. 80 MHz 'H NMR spectrum of (Z,Z~pentadeca-6,9dien-loI (80 pg in 12 pL of CDC13) obtained with 1800 pulses, a tip angle of 45" and a pulse repetition tlme of 2.048 s. CHCI3 with 6 7.25 ppm from Me,SI was used as internal reference.
contamination of the glass surface nevertheless occur, all traces of the sample have to be removed from the upper part of the tube, especially if an involatile compound is present in the solution. This can be effectively accomplished by producing a small plug of solvent high up in the tube and moving it slowly downward by cooling the lower part of the tube with ice or solid COz. Since the tube will already be saturated with solvent vapor, there is little risk of this solvent plug being pushed out of the tube. The rinsing should be repeated with several solvent plugs, whereafter solvent in excess of the required volume can be removed with a slow stream of ultrahigh-purity nitrogen. Even if contamination of the upper part Carbon atom 1
2 3 4
5 + I1 6 7 8 9
IO 12 13
14 15
I3C Chemical shift
Intensity
63.06 32.75 25.68 29.37 27.24 129.85 127.92 25.45 128.32 130.32 31.59 29.49 22.59 14.06
39 66 59 59 80 59 44 42 39 35 22 71 71 51
200 Figure 3. 20 MHz I3C NMR spectrum of
100
(Z,Z)-pentadeca-6,9-dlen-l-ol (80 pg in 12 pL of CDC13)obtained with 531 600 pulses, a tip angle of
45', and a pulse repetition time of 0.4092 s. CDC13 with 6, 77.04 from Me,Si was used as internal reference.
Anal. Chem. 1985, 57, 2421-2423
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(2) Burger, B. V.; Munro, Z.; Roth, M.; Turner, V.; Tribe, G. D.; Crewe, R. M. Z . Naturforsch., C : Blosci. 1983, 38C, 848-855. (3) Skrabalak, 0.S.; Henion, J. Anal. Chem. 1883, 55, 1184-1186. (4) Shoolery, J. N. "Use of Microsample Techniques in 'H and '% NMR Spectroscopy to Study Mlcrogram and Submilligram Amounts of Sarnple wlth the CFT-20", Varlan Associates: Palo Alto, CA, 1976; p 4. (5) Shoolery, J. N. in "Topics in Carbon-I3 NMR Spectroscopy"; Levy, G. (Z,Z)-Me(CH2)4(CH=CHCH2)2(CH2)40H, C., Ed.; Wiley: New York, 1979; Vol. 111, pp 28-38.
deteded when the NMR analyses of a number of samples had to be repeated about a year later, no solvent was lost from any of the sealed capillaries that are stored in a filing system for possible future reference.
Registry No. 77899-11-7.
LITERATURE CITED ( 1 ) Burger, 8. V.; Le Roux, M.; Spies, H. S. C.; Truter, V.; Bigalke, R. C. Tetrahedron Left. 1978, 5221-5224.
RECEIVED for review April 17,1985. Accepted June 11,1985. assistance was provided by the University Stellenbosch and the CSIR.
Of
Use of a Nebullzer in Pyrohydrolytic Decomposltion of Silicate Materials for Determination of Fluorine and Chlorine David Whitehead* and John E. Thomas Sedimentology Research Laboratory, Department of Geology, University of Reading, Whiteknights Park, P. 0. Box 227, Reading, Berkshire, England Pyrohydrolysis has been established for many years as an effective method for the release of fluorine from silicate rocks (1-6) and, more recently, for the release of chlorine (5). The pyrohydrolytic method described below uses a nebulizer (from an atomic absorption spectrophotometer) to transfer water to the combustion tube instead of using either a steam generator or a water-boat placed in the tube. The fine aerosol spray from the nebulizer is instantly converted to steam inside the combustion tube. The efficiency of the nebulizer allows a single flux (vanadium pentaoxide) to be used for release of fluorine and chlorine from a range of silicate materials. Furthermore, use of a nebulizer simplifies the apparatus, facilitates operation, and is safer than using a steam generator.
EXPERIMENTAL SECTION Apparatus. The assembled apparatus is shown in Figure 1. A tube furnace (Model C.F.M. 1470 "C, Carbolite, Ltd., U.K.) was used with the temperature set at 1230 "C. The stainless steel nebulizer (Model 0303 0358, Perkin-Elmer, Ltd., Beaconsfield, U.K.)was adjustable and enabled flow rates to be varied. PTFE tape was used to seal the Teflon connection to the outlet end of the alumina combustion tube and also to secure the nebulizer in the Teflon inlet bung. Figures 2 and 3 show details of the connections to the alumina combustion tube. Reagents. An acetate buffer solution was prepared by dissolving 150 g of sodium acetate in deionized water, adding 50 mL of glacial acetic acid, and diluting to 1-Lwith deionized water. All the reagents used were of analytical reagent grade. Sample Preparation. International reference samples were analyzed as received with no further grinding. For routine rock analysis, samples should normally be ground to
