Metal Rod and Wire Electrode Holders Gaston A. East"' and Edmund Bishop Chemistry Department, University of Exeter, Stocker Road, Exeter, EX4 400, England
In constant current stripping coulometry with electrometric end-point location, two aspects have to be considered: first, the metal rod electrode must be readily weighed on a microbalance and therefore a demountable assembly is highly desirable, and second, the indicator wire electrode(s) has to be leak-free to avoid erratic end points. Gold and silver wire electrodes deserve particular attention in this respect since these metals do not seal well with glass, even if blue cobalt and soft soda glass are used ( I ) , and the electrode will crack a n d leak after prolonged used. Consequently, another sheathing material should be searched for. T h e present paper describes novel electrode holders made of Teflon and silicone rubber t h a t fulfil the requirements demanded above. P T F E was selected regarding its valuable properties: thermal and chemically inert, water repellent, readily machined, and it does not break. Silicone rubber washers of various diameters (used in Quickfit screw-cap adaptors) were employed and the chemical resistance of the material was tested in a variety of common inorganic electrolytes, namely, aqueous solution of perchloric acid, nitric acid, sulfuric acid, hydrochloric acid, potassium chloride, sodium hydroxide, ammonia, and it proved to be resistant to all of them and did not release any harmful surfactant t h a t fouled the electrode surface in the range of pH 1 t o 12.
ELECTRODE HOLDER FOR METAL RODS This is a fully demountable assembly that permits metal rods to by mounted and demounted a t will. Two types of these holders were designed, vertical and horizontal, Figure 1, and they were made by machining P T F E stock on a lathe; their size was scaled to suit the size of the metal rods used, but there is nothing critical about the dimensions. In Figure l A , the stem, a , and the shoulder, b, are machined out of a rod stock and a cavity is turned in the shoulder to accommodate the spring, f , which bears on the inner end of the concentric cavity in b, and on the brass disk, d , into which is welded the tinned copper connecting lead, e . The disk, d , is a sliding fit in the cavity so that the spring maintains good electrical contact with the electrode, h, when the assembly is screwed together. The nut, c, is also machined from P T F E t o screw tightly on the stem and drilled to take the machined end of the electrode, h, with a press fit. Flutes are machined on the outer surface for finger grip. T h e silicone rubber washer, g, when rammed t o the base of the nut under compression from the bottom bearing face of the stem provides a leak-proof seal. The washer is a squeeze fit on the electrode collar. T h e machined collar is of such a length t h a t the top of the electrode will penetrate the cavity of the stem containing t h e spring and still leave a few millimeters gap between the shoulder on h and the nut face. The contact end face is machined square and lapped flat. Figure 1B shows an horizontal holder developed to decrease t h e over-all cell resistance by orientating the electrode horizontally directly beneath the sintered glass separator of the auxiliary compartment in the coulometric cell (2). A pair of these electrodes is made from a single length of P T F E rod stock by using a nested-L cut. A PTFE cone, i, is machined t o a sliding fit on the stem, with an outside taper machined to fit a standard taper, e.g., B14 or B19, hole machined in the Present address, Departamento de Quimica, Universidade de Brasilia, Campus Universitario, 'iO.OOO-Brasilia-DF,Brazil. 1880
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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Figure 1. Metal rod electrode holders. (A) Vertical type: (B) Horizontal type. ( a )Stern. ( b ) Shoulder. ( c ) Nut. (d) Brass disk. ( e )Lead. ( f ) Spring. (9) Silicone rubber washer. ( h )Rod electrode. ( i )Fitting cone. ( j ) Slit
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Flgure 2. Wire electrode holders. (A) Single configuration. (B) Twinned configuration. ( a ) Knurled cone. ( b )Cell lid. ( c ) Slit. ( d ) Stern. ( e ) Lead. ( 0 Insulated leads. ( 9 ) Nut. (13Welded joint. ( k ) Silicone rubber cone. ( p ) Wire electrode
cell lid. A fine slit, j , cut across the diameter of the cone for a little over half its height, closes on the stem and clamps it rigidily in the cell lid when t h e join is made.
ELECTRODE HOLDER FOR WIRE ELECTRODES Based upon the same principles applied t o metal rod electrode holders, the following holders for wires were constructed from P T F E and silicone rubber. In the electrode holder depicted in Figure 2A, the wire, p , is directly welded to the connecting lead, e , which must retain its insulation in
the twinned configuration, Figure 2B. A drop of paraffin wax was poured onto the welding point to prevent oxidation of bare copper. The silicone cones, h, carry a slightly slower, but still sharp, taper than the seating in nuts, g, so that on screwing up the nut, the silicone rubber exerts strong compression on both nut face and wire electrode(s) affording a completely leak-free and water-repellent seal of great simplicity. Silicone rubber is not easily machined to precise limits, but it was found that if a disk or sheet of the material is penetrated by a cork borer of appropriate size under pressure, the resultant core is tapered as desired. Insertion of soft electrode wire, as gold, is facilitated by piercing the silicone rubber cone with a fine steel needle. T h e appropriate exposed length of wire is adjusted before screwing up the nut. The bottom bearing face of the stem, d , compresses the cone on screwing on the nut, the weld and sleeving, f, rise up the central hole of the stem and when a twinned electrode is made, both cables rotate freely in the cavity. Individual electrodes were fitted on the vessel lid by means of Quickfit screwcap adaptors. When a
twinned electrode was employed, a knurled cone, a , with a slit, c , illustrated affords a rigid mount in the machined reaction vessel lid. The main features of the assembly are: totally demountable, leak-proof, length of exposed wire varied a t will, electrode material readily interchanged and most important, saving of precious metal. The electrode holders described above are particularly suitable for zero current potentiometry, amperometry, and differential electrolytic potentiometry (DEP), where the exposed length of the wires, p , must be adjusted to exact equality. Metal rod and wire electrode holders described here have been continuously used for about two years and they proved to be 100% reliable.
LITERATURE CITED ( 1 ) E Bishop and P H Hitchcook. Ana/yst(London). 98 465 (1973) (2) G A East, PhD Thesis, University of E,eter, 1975
RECEIVED for review May 23, 1977. Accepted July 18, 1977.
Solvent-Free and Splitless Injection Method for Open Tubular Columns J. W. de Leeuw," W. L. Maters,' D. v.d. Meent,* and J. J. Boon Delft University of Technology, Department of Chemistry and Chemical Engineering, Organic Geochemistry Unit, de Vries van Heystplantsoen 2, Delft, The Netherlands
T h e introduction of samples into capillary gas chromatographic columns is a matter of great delicacy; extremely small sample quantities are required to avoid overloading the column. In practice this is often realized by using splitting devices. These splitters-of which various types are in use presently-often have major disadvantages: the greater part of the injected sample is lost, the composition of the sample actually entering the column may significantly differ from the original one due to splitter nonlinearity and quantitative analysis is hampered severely. Other problems with conventional splitting devices can arise because of relative high injector temperatures, the necessity of using injector septa causing bleeding, and the use of solvents. T o overcome these problems, a splitless injection technique is necessary. Several methods for splitless injection on capillary columns are known. By means of the so-called "Grob" injector, a splitless injection is possible ( I ) . A solid injection system using a "falling needle" technique is reported by van den Berg ( 2 ) . In this report, another splitless injection technique is described, based on a very rapid and reproducible evaporation of a very small amount of solvent-free sample. The Curie-point technique, used for this injection method, is based on high frequency induction heating of a ferromagnetic wire coated with a thin layer of the sample to be analyzed (3). The vaporized sample is directly introduced into the capillary column. T h e technique is simple and the results are reproducible, as is shown by the preliminary results. Exactly the same equipment can be used for pyrolysis gas chromatography.
EXPERIMENTAL Gas Chromatographic Equipment. The experiments were carried out with a Perkin-Elmer 990 instrument equipped with FID detectors. The capillary columns were obtained from Chrompack N.V.. The Netherlands. These were stainless steel columns coated with OV-101 (length = 30 m; i.d. = 0.25 mm). The Present address, Koninklijke Shell Exploratie & Produktie Laboratorium, Volmerlaan 6, Rijswijk, The Netherlands. 'Present address, Chemistry Department, University of California, Berkeley, Calif. 94720.
conventional splitting device was a Perkin-Elmer Sample Splitting Injection System 009-0598. The injection temperature using this device was 300 "C. The detector temperature was 300 "C. The oven temperature was programmed from 120 to 300 "C with 4 "C/min. N, was used as carrier gas, t h e flow rate being 0.65 mL/min at 120 "C. In several experiments, the instrument was connected to an Infotronics CRS 101 electronic integrator. Curie Point Evaporation Injector. The evaporation injector is the Curie point pyrolysis unit developed by Meuzelaar et al. ( 4 ) (see Figure 1). The Viton O-ring (Figure 1, (16))was replaced by a gold ring, and the PTFE seal (Figure 1, (17)) in some experiments was replaced by a graphite seal. The injector temperature was kept at 30 "C, the connection with the column (column coupling device) at 300 "C. The total evaporation time was set at 0.2 s using a ferromagnetic wire with a Curie temperature of 300 "C. The back flush ratio was 1:15 (Figure 1, (2)). The high frequency generator is a Fisher Labortechnik instrument, type 0310 (1.5 kW, 1.1 MHz). Sample Handling. The sample to be analyzed is brought on the ferromagnetic wire from a solution of known sample concentration by means of a micro syringe. The solvent is evaporated by slow rotation of the wire. The sample-coated wire is inserted into a quartz tube as shown in Figure 2. The coated part of the wire should be located finally in the central part of the high frequency coil. Samples. ( a )Methyl Esters. A standard mixture of methyl esters (CI2-Cl9),synthesized in our laboratory, was used. ( b ) Cholesterol. Cholesterol was purchased from E. Merck A. G., Darmstadt (art. 3670) and purified by recrystallization from ethanol. ( e )$-Hydroxy Methyl Esters. A mixture of $-hydroxy methyl esters was synthesized from the saturated acid fraction of Mackerel lipids as described elsewhere ( 5 ) . ( d ) Vicinal Dihydroxy Methyl Esters. A mixture of vicinal dihydroxy methyl esters was obtained from the monoenoic fatty acid fraction of Desulfovibrio desulfuricans bacteria after treatment with OsO,and H2S (6). ( e )Farnesyl Methyl Ethers. Commercial farnesol was treated with BF3/MeOH, yielding a mixture of farnesyl methyl ethers as described previously ( 7 ) . ( f ) Alkanes. A saturated hydrocarbon fraction (CI8-C3J of a crude oil was used as a standard mixture. (g) Insoluble Organic Sediment Sample. This sample was ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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