604
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Anal. Chem. 1982, 5 4 . 604-605
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gas chromatographic method to the parts-per-billion range using an electron capture detector, but the detector became unresponsive after the foil and walls became coated with triheptylamine which derived from decomposition of tetraheptylammonium cation. If n-butyl tosylate was employed as the alkylating agent in a similar procedure, however, the nonvolatile tetraheptylammonium tosylate decomposition products should not deactivate the electron capture detector and low-parts-per-billion-level sensitivity for simultaneous halide determinations in water might be possible. When attempting to extend the use of this method to other systems such as to the determination of inorganic halides or to the determination of other nucleophiles, it should first be verified that the derivatization reaction (eq 2) proceeds rapidly and quantitatively in the injection port of the gas chromatograph.
CETONE
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B
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Figure 1. Comparison of gas chromatograms for acetone solutions containing: 0.0672 M benzyltriethylammonium chloride, 0.0652 M TBAB, 0.0615 M TBAI, and 0.25 M n-butyl tosylate (A) and 0.0671 M l-chlorobutane, 0.0652 M l-bromobutane, and 0.0615 M l-iodobutane (B). Experimental conditions were the same as those described at the bottom of Table 11.
100% a t this relatively low concentration. These experiments demonstrate that the simultaneous determination of quaternary ammonium chlorides, bromides, and iodides is feasible in nonaqueous solvents such as acetone, acetonitrile, and methanol in which the halide salts and nbutyl tosylate are mutually soluble. The determination of these halide ions in aqueous solution by this procedure, however, would probably require that the halides first be extracted into a nonaqueous solution containing n-butyl tosylate and was not attempted here. Matthews et al. (3) attempted to extend the sensitivity limit of their extraction-
ACKNOWLEDGMENT The author thanks M. Finkelstein and S. D. Ross for the preparations of benzyltriethylammonium chloride and n-butyl tosylate, respectively. LITERATURE CITED (1) Moore, W. M.; Finkelstein, M.; Ross, S. D. Tetrahedron 1980, 36, 727-730. (2) MacGee, J.; Allen, K. G. Anal. Chem. 1970, 4 2 , 1672-1673. (3) Matthews, D. R.; Shults, W. D.; Dean, J . A. Anal. Lett. 1973, 6 , 513-522. (4) Rudd, E. J.; Finkelsteln, M.; Ross, S. D. J . Org. Chem. 1972, 37, 1763-1767. (5) Ross, S. D.; Finkelstein, M. J . Am. Chem. SOC. 1957, 7 9 , 6547-6554. (6) Sekera, V. C.; Marvel, C. S.J . Am. Chem. SOC.1933, 55, 345-349. (7) Hammett, L. P. "Physical Organic Chemistry", 1st ed.; McGraw-Hill: New York, 1940; Chapter 4. (8) March, J. "Advanced Organic Chemistry", 2nd ed.; McGraw-Hill: New York, 1977; Chapter 10, p 326.
RECEIVED for review October 22, 1981. Accepted November 30, 1981.
Optically Transparent Thin-Layer Electrode for Organic Solvents Edward P. Muth and John E. Fuller Optical Instrument Division, National Bureau of Standards, Washington, D.C. 20234
Lawrence M. Doane and Elmo A. Blubaugh" Organic Analytical Research Division, National Bureau of Standards, Washington, D.C. 20234
Optically transparent thin-layer electrodes (OTTLE) have been used in voltammetry, coulometry, and spectroelectrochemistry (1). OTTLEs are generally constructed by bonding two microscope slides (optical flats) together with a transparent minigrid sandwiched between (see Figure 1, ref 1). Applications of the OTTLE in nonaqueous solvents have been limited because of solvent attack on the bonding adhesive. Since electrochemistry in nonaqueous solvents is common, an OTTLE possessing long-term resistance to commonly used organic solvents would have wide appeal. In this paper, we describe the construction of an OTTLE that is resistant to materials that do not attack quartz and gold. In our laboratory, electrochemical experiments with a conventional OTTLE in solvents such as propylene carbonate, N,N-dimethylformamide (DMF), and acetonitrile show that after a dozen or so experiments or lengthy exposure to these solvents, particularly DMF, the bonding material deteriorates and the optical flats separate. Attempts at mechanically
binding the OTTLE together with minature C-clamps which are subsequently covered with a silicone rubber adhesive (a bonding material relatively resistant to attack by many organic solvents but mechanically weak) have been unsuccessful because of the inability to form a uniform seal at the OTTLE edges. Because glass sealing compounds that form vacuum seals when sufficiently heated are available (Vitta Corp., Wilton, CT), an attractive alternative is to make all solvent surfaces of glass. Quartz was selected because of its excellent transmission qualities. The OTTLE, along with an enlargement illustrating the edge detail, is shown in Figure 1. One optical flat is made from a quartz Beckman cuvette cut in half lengthwise. (This process actually produces two optical flats from which two OTTLEs can be made.) The cut sides are ground parallel to the flat to a depth of 0.1 mm, measured from the ground surface to inner face of the flat (edge c in Figure 1). A slot, which serves as a port to draw in new solution when the
This article not subJect to US. Copyright. Published 1962 by the American Chemical Society
(305
Anal. Chem. 1982, 5 4 , 605-607
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Drawing of OTTLE: (a)quartz seallng tape, (b) gold minigvld, (c) Interior edge that approximately defines electrode width, (d) hemitube. Flgure 1.
OTTLE is finally assembled, is ground into the edge that was originally the bottom portion of the Beckman cuvette. The other flat is cut to size from a quartz microscope slide. Two applications of quartz sealer are fixed to the ground surfaces of the Beckman flat. (The quartz sealer comes as a ribbon, 25 km thick, and is held in place by its own adhesive which burns off during the sealing process.) With the flat face down, an appropriately sized gold minigrid is laid across the flat so that a portion of the grid material overlaps both raised lateral edges of the flat. The overlapping grid material provides electrical contact to the interior portion of the grid (vide infra). After the other quartz flat is positioned on top, a larger quartz flat and a 60-g weight are placed on the entire assembly, which is then placed in an oven at room temperature. The weight, in addition to securing the assembly from misalignment, provides adequate pressiure to force the tape through the grid material when the tape melts. Melting the tape to form a seal normally requires an oven temperature above 1000 "C. This temperature is impracticable because of the softening point of gold is below 1OOO OC. However, the pressure of the weight is sufficient to achieve a vacuum seal without deformation of the gold minigrid, when the temperature of the oven is allowed to rise to only 850 "C and then cool down to room temperature. At this point the OTTLE is a fused assembly and therefore impervious to separation by normal use. Electrical connection is made to the grid material extending beyond the lateral edges of the OTTLE. This portion of the grid is isolated from the solvent with quartz hemitubes (d in Figure 1) sealed to the OTTLE edges. The hemitubing is made from 2 mm diameter quartz tubing closed at one end and then cut in half lengwise. The cut edges of the hemitube are ground flat and quartz sealing tape is applied (vide supra).
05
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Figure 2. Thin-layer cyclic voltammogram of 1.04 mmol/L mei hyl viologen in propylene carbonate 0.5 mol/L TEAP supporting electrolyte vs. AgIAgCIO,. Scan rate = 2 mV/s.
The grid material is trimmed leavirAgapproximately 1-2 mm exposed. With the quartz hemitubing positioned appropaiately, a seal is formed with gentle heating from a torch. Sirice the edge of the OTTLE is not ground smooth, additional applications of the quartz sealer are made to the exterior seam. The procedure is repeated on the opposite edge. Electrical connection is made by filling the quartz tube with conducting cement (ACME Chemicals-Insulation Co., New Haven, CT) and inserting a length of wire. An OTTLE constructed in this manner has been used c'xclusively in DMF solutions for over 50 h in our laboratory. No discernible change in the OTTLE has been detected. A typical thin-layer cyclic voltammogram with this OTTLE is shown in Figure 2. We have also used this procedure to seal quartz tubing to a quartz flat with a gold minigrid between and subjected it to several freeze-thaw cycles from 77 to 373 K. Since the seal maintained a vacuum (0.01 torr), it is evidence that this method of sealing gold mesh to quartz introduces no sevme stress.
LITERATURE CITED (1) DeAngeiis, T. 594-597.
P.; Heineman, W. R. J . Chem. Educ. 1976, !53,
RECEIVED for review July 31,1981. Accepted September 2!3, 1981. The identification of commercial products does not imply endorsement by the National Bureau of Standards.
Detachable Hydride Introduction Device for Inductively Coupled Plasma Torch Scott Stieg
and Allan Dennis2
Laboratory of the Government Chemist, Cornwall House, Stamford St., London SEI 9NQ, United Kingdom
The preconcentration method of hydride evolution is used extensively for extending detection limits and reducing matrix effeds in atomic absorption and plasma emission spectroscopy. 'Present address: Department of Chemistry, Harvey Mudd College, Claremont, CA 91711. 2Present address: Warrell Spring Laboratory, Gunneb Wood Rd., Stevenage, Herts, U.K.
The method, which converts an analyte to a volatile hydride, introduces a much larger fraction of analyte to the atom reservoir than the small fraction provided by droplets from a nebulizer spray. Two approaches are used to form arid introduce analyte hydrides (1). In the batch approach, the hydrides are formed above the reacting liquid and then suddenly swept into the detector along with other headspace
0003-2700/82/0354-0605$01.25/00 1982 American Chemical Society
