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ANALYTICAL CHEMISTRY, VOL. 52, NO. 3 , MARCH 1980
Table I.
til
P e a k Current V a l u e s
drop expt. type' 1 2 3 4 5 6 7 8 9 10
amalgamated platinum disk, m: mercury contact, c: wire connection, r: mercury reservoir, t: storage tube Figure 1. d:
benzophenone (2 mM) in 0.1 M tetraethylammonium fluoroborate/DMF using a 3-electrode system (platinum wire secondary electrode; Ag/O.l M AgNO, in DMF reference electrode) with a sweep rate of 240 mV s-l, and taking the cathodic peak current (iP,J to be proportional to the electrode area (I, p 338). See Table I for results. Between each experiment, the HMDE was removed from the cell, washed with distilled water and acetone, air dried, and returned to the storage tube. I t was then removed from the tube in the appropriate manner and returned to the cell. For each drop, the voltammogram was recorded 5 times, the solution being stirred and then allowed to come to rest between each recording. The experiments were run in the order shown within a period of 1.5 h. A statistical analysis (3) of the 25 results from experiments 1-5 (method B) gave a mean i,,c of 3.026 FA, with an overall standard deviation, s, of f0.032 pA, an s due to redipping, i.e., between experiments, of k0.027 PA, and an s due to recording of the voltammograms of 50.022 pA. [For experiments 6-10 (method A), the corresponding s values were zk0.360, f0.394, and f0.016 PA]. Method B therefore produces a H M D E of reproducible area over a period of 1-2 h (area determination for longer periods is limited by other experimental factors e.g., stability of solution concentrations). It
B B B B B A A A A A
b
i ~ , e / ~ A
2.96, 3.00, 3.00, 3.00, 3.00 3.02, 3.04, 3.00, 2.98, 3.00 3.06, 3.04, 3.02, 3.02, 3.00 3.08, 3.08, 3.06, 3.04, 3.04 3.08, 3.06, 3.04, 3.02, 3.02 4.32, 4.32, 4.30, 4.30, 4.30 4.04, 4 . 0 4 , 4 . 0 2 , 4 . 0 2 , 4.00 4.70, 4.68, 4.68, 4.68, 4.68 4.00, 3.96, 3.96, 3.96, 3.94 4.86, 4.84,4.88, 4.84, 4.84
2.992 3.008 3.028 3.060 3.044 4.308 4.024 4.684 3.964 4.852
a HMDE types A and B were formed by methods A and B, respectively, (see text). Conditions: benzophenone ( 2 mM) in 0.1 M Et,NBF,/DMF, sweep rate 240 mV s - ' , sweep range -1.8 to -2.5 V, Ep,c -2.28 V vs. Ag/O.l M AgNO, in DMF.
also gives good reproducibility from day to day, and even over extended periods; similar experiments performed with the same electrode 3 years earlier gave a mean ip,cof 3.116 f 0.065 pA (from 17 results). Usually any contamination of the mercury drop which occurs during individual experiments is removed by exchange when the electrode is returned to the storage tube. If the area of the electrode is required, it may be determined by the usual electrochemical methods using a standard test solution ( I , p 74), or may be estimated using an optical microscope. By the latter technique, the electrode prepared by method B was found to be spherical with width 0.630 f 0.005 mm, depth 0.090 f 0.005 mm, calculated radius of curvature 0.595 i 0.01 mm, and calculated surface area 0.335 f 0.02 mm2 (area 7-8% larger than that of a flat disk of the same diameter). LITERATURE CITED (1) D. T. Sawyer and J. L. Roberts, "Experimental Electrochemistry for Chemists", Wiley, New York, 1974. (2) L. Ramaley. R. L. Brubaker, and C . G. Enke, Anal. Chem., 35, 1088 (1963). (3) K. Eckschlager, "Errors, Measurements and Results in Chemical Analysis", Van Nostrand, New York, 1969, p 120.
RECEIVED for review July 16, 1979. Accepted December 6, 1979.
ADDENDUM Liquid Sample Introduction in Gas Chromatography
Sir: The main purpose of Figure 1 appearing in this paper ( 1 ) is to illustrate the flow path of carrier gas. I t is a general diagram which is not limited to the Hewlett-Packard model. In fact, the schematic of Figure 1 is that of the injection port of the Perkin-Elmer Model 3920 gas chromatograph (2). Most commercial instruments provide a similar flow path even though the detailed design may not be the same. This liquid sample introduction is a general method applicable to all gas chromatographic models.
LITERATURE CITED (1) John Chih-An Hu, Anal. Chem., 51, 2395 (1979). (2) The Perkin-Elmer Model 39208 Series Gas Chromatographs. Brochure No L-450. November 1975.
John Chih-An Hu Quality Assurance Laboratory Boeing Aerospace Company Seattle, Washington 98124
mean
