Anal Chem 1980, 52, 607-608
U!GITOXINE
113 NG
7 6 3 .E
607
is easy to operate, and is protected against mechanical damage during insertion through the vacuum lock.
ACKNOWLEDGMENT The author is grateful to J. Roede for building the negative ion source supply, to F. P. Buss and A. Oosterhoff for constructing the Pt wire probe, and to J. v.d. Greef, University of Amsterdam, for practical information on field desorption.
LITERATURE CITED
1 BO
2Oci
f
SECCINO5 1
Figure 5. Selected ion current profile of the [M - HI- ion of digitoxine by OH- negative ion CI. Current program r a t e 2 A/min. Source temperature 1 7 0
OC
performance of the instrument. It thus seems to be possible to record a full spectrum of 100 ng under favorable conditions, but 1 pg, collected for example by preparative chromatography, would be a more realistic sample size. When 10 ng of digitoxine was introduced, however, no signal was observable by single ion monitoring. Essentially the same results have been reported recently: 100 ng of a disaccharide gave both molecular weight information and fragment ions using the Ribermag desorption CI probe with NH, as the reactant gas, but 10 ng only produced fragment ions (15). Apparently, a certain amount of sample is consumed by decomposition on active sites on the metal wires. In conclusion, the Pt wire probe is constructed from inexpensive components, can handle complex sample molecules,
Baldwin. M. A,; McLafferty, F. W. Org. Mass Spectrom. 1973, 7, 1353. Hansen. G.: Munson. B. Anal. Chem. 1978. 50. 1130. Cotter, R. J. Anal. Chern. 1979, 51, 317. Cotter, R. J.; Fenselau, C. Biomed. Mass Spectrom. 1979, 6 , 287. Carroll, D. 1.; Dzidic, I.; Horning, M. G.; Montgomery, F. E.; Nowlin, J. G.; Stillwell, R . ; Thenot, J. P.; Horning, E. C. Anal. Chem. 1979, 51, 1858. Ohashi. M.; Yamada. S.; Kudo, H.; Nakayama, N. Biorned. Mass Spectrom. 1978, 51, 578. Hunt, D. F.; Shabanowitz, J.; Botz, F. K.; Brent, D. A. Anal. Chem. 1977, 49, 1160. Soltmann, B.; Sweeley, C. C . ; Holland, J. F. Anal. Chsm. 1977, 49, 1164. Anderson, W. R.; Frick, W.; Daves, G. D. J . Am. Chem. SOC.1978, 100, 1974. Raaymakers, J. G. A. M.; Engel, D. J. C. Anal. Chem. 1974, 46, 1357. Thenot, J. P.: Nowlin, J.; Carroll, D. 1.; Montgomery, F. E.; Horning, E. C. Anal. Chem. 1979, 51, 1101. Stafford, G.; Reeher, J.; Smith, R . ; Story, M. I n "Dynamic Mass SDectrometry", Vol. 5, Price, D., Todd, J. F. J., Eds.; Heyden and Son: London, 1977, p 55. Smit, A. L. C.; Field, F. H. J . Am. Chem SOC.1977, 99, 6471. Bruins, A. P., paper presented at the 8th International Mass Spectrometrv Conference. Auoust 12-18. 1979 Oslo. Norway; Adv. Mass siectrom. VOI. 8 , in bress. Prome, J. C.; Beaugrand, C., papers presented at the Ribermag GC/ MS/DS Symposium, October 17-18, 1979, Antwerpen, Belgium.
RECEIVED for review August 14, 1979. Accepted December 10,1979.
Simple Procedure for Producing a Hanging Mercury Drop Electrode of Constant Surface Area Anthony J. Bellamy Chemistry Department, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland
T h e use of the hanging mercury drop electrode (HMDE) for many quantitative electroanalytical measurements, e.g., voltammetry, anodic stripping analysis, requires that the electrode area is constant, and in some cases known. This is usually achieved in two ways: (i) one or more drops of mercury from a dropping mercury electrode, of known characteristics, are collected in a scoop and transferred to a small amalgamated platinum disk, (ii) a glass capillary tube connected to a closed mercury reservoir is fed by advancing a calibrated, threaded plunger into the reservoir ( I , p 85). We have found that the following simple procedure produces a HMDE with a reproducible surface area (standard deviation *1 YO),and is more convenient than either of the above methods. A small platinum disk electrode was prepared by sealing a short length of 0.635-mm diameter platinum wire into the end of a soft glass tube (7-mm 0.d.); the sealed end of the glass tube and wire were ground flat and then polished with 0.3-pm alpha alumina. The platinum disk was made the cathode (Pt wire anode) in the electrolysis of 1 M perchloric acid (3-V battery); hydrogen was discharged for 10 min; and then the disk was amalgamated by plunging it below the surface of a mercury pool while still connected to the battery (2). The 0003-2700/80/0352-0607$01 O O / O
electrical connection was removed, the electrode was withdrawn, washed with distilled water and acetone, and, after drying in air, was stored in a test tube (ca. 10-mm i.d.) containing mercury to a depth of ca. 20 mm, with the electrode tip ca. 5 mm below the surface. The surface area of the mercury drop adhering to the platinum disk when the electrode is removed from the storage tube was found to be critically dependent upon the method of withdrawal. If the storage tube is held vertically and the electrode is withdrawn vertically (Figure lA, method A), the area is irreproducible (see Table I, experiments 6-10). If, however, the storage tube is held almost horizontally, so that the mercury is about to run along the tube, and the end of the electrode is moved from below to above the mercury surface by downward movement of the outer end of the electrode (Figure 1B,method B), the area is very reproducible (see Table I, experiments 1-5). The mercury drops produced by method A are visibly larger and more rounded than those produced by method B. T h e constancy of the area of the mercury drops produced by method B was demonstrated by using the HMDE for the linear sweep voltammetry of the first electron transfer of C 1980 American Chemical Society
608
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 B 2 B 3 B 4 B 5 B 6 A 7 A 8 A 9 A 10
Figure 1. d: amalgamated platinum disk, m: mercury contact, c: wire
connection, r: mercury reservoir, t: storage tube 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
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
