Anal. Chem. 1986, 58, 2087-2088
stabilizer to regulate for constant power output from the MIP power unit; and varying the depth of the plasma cavity for optimal sample residence time in the plasma. This system seems most suitable as a detector for high-performance liquid chromatography and this possibility is currently under investigation in this laboratory.
ACKNOWLEDGMENT The authors thank Sherritt Gordon Mines for the gift of the MAK nebulizer and David Zellmer for drawing Figure 1. Registry No. HzO, 7732-18-5; Cr, 7440-47-3; Mn, 7439-96-5; In, 7440-74-6; V, 7440-62-2; Pb, 7439-92-1; Sr, 7440-24-6; Zr, 7440-67-7.
LITERATURE CITED (1) (2) (3) (4)
Hieftje, G. M. Spectrochim. Acta, P a r t 8 1983, 388, 1465. Zander, A. T.; Hieftje, G. M. Appl. Spectrosc. 1981, 35. 357. (bode. S. R.; Baughman, K. W. Appl. Spectrosc. 1984, 38, 755. Beenakker, C. I. M.; -man, 6.; Boumans, P. w. J. M. spectr&/m. Acta, Part B 1978, 338, 373.
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Beenakksr, C. I. M.; Boumans, P. W. J. M. Spectrochh. Acta, Part8 1978, 338, 53. Beenakker, C. I. M. Spectrochim. Acta, Part 8 1977. 328, 173. Beenakker, C. I. M.; Spectrvchim. Acta, Part 8 1978, 318, 483. Beenakker, C. I. M.; Boumans, P. W. J. M.; Rommers, P. philips Tech. Rev. 1980, 39, 65. Haas, D. L.; Caruso, J. A. Anal. Chem. 1984. 56, 2014. Michkwicz, K. G.; Carnahan. J. W. Anal. Chem. 1985, 5 7 , 1092. Urh, J. J.; Carnahan, J. W. Anal. Chem. 1985. 5 7 , 1253. Deutsch, R. D.; Hieftje. 0.M. Appl. Spectrosc. 1985, 39, 214. Deutsch, R. D.; Keilsohn, J. P.; Hieftje, G. M. Appi. Spectrosc. 1985, 39, 531. Rezaalyaan. R.; Hieftje, G. M. Anal. Chem. 1985. 57, 412. Isaaq, H. J.; Morgenthaler, L. P. Anal. Chem. 1975, 4 7 , 1748. Kaiser, H.; Meddings, B. Eastern Analytical Symposium, New York, 17-19 November 1982; Paper No. 120. Ng, R. C.; Kaiser, H.; Meddings, B. Winter Conference on Plasma Spectrochemistry, San Diego, CA, 2-6 January, 1984; Paper No. 16. Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33. 206. (19) Haas, D. L. University of Cincinnati, personal communication, 1983.
RECEIVED for review November 22,1985. Accepted April 21,
1986.
CORRESPONDENCE Homogeneous Precipitation of Nickel as the Dimethylglyoxime Complex: Revisted Sir: The gravimetric determination of nickel by precipitation with dimethylglyoxime (DMG) to form the nickel dimethylglyoxime complex (Ni-DMG) is one of the classical methods in analytical chemistry (1-3). This method can use either the direct precipitation of the complex from a basic solution or the homogeneous precipitation using the hydrolysis of urea to shift the p H of a slightly acidic solution to the necessary basic conditions for precipitation. Recently, while using the latter procedure for determining the nickel content of samples of nickel ore, we obtained unsatisfactory results. In some cases Ni-DMG precipitates were obtained contaminated with an unknown material. In others, large variations were observed in the amount of complex formed in replicate determinations. We had not encountered this before and to our knowledge it has not been reported in the literature. This note identifies the source of these problems. EXPERIMENTAL SECTION Procedure Used in Ore Analysis. Approximately 1.5 g of each ore sample was dissolved in 20 mL of aqua regia and then diluted to 200 mL with deionized water and the pH adjusted to about 2. About 20 g of urea and 50 mL of a 1% dimethylglyoxime solution dissolved in 1-propanolwere added to each solution. The resulting solution was then gently heated to hydrolyze the urea and precipitate Ni-DMG. Visual inspection of replicate determinations on the same sample after 0.5 h of heating suggested that in some instances uneven amounts of Ni-DMG were being precipitated. This situation was not rectified even on heating for several hours. Many samples had replicates with quite obviously different amounts of precipitate. The precipitate in all determinations was filtered off by use of sintered glass crucibles and weighed. It was noticed that some replicates, those with the smaller amounts of Ni-DMG, were contaminated with a pale yellow, needlelike, crystalline precipitate. We will refer to this material as compound A. The larger crystals of compound A were removed from the finely divided Ni-DMG. Samples of compound A were then analyzed 0003-2700/86/0358-2087$01.50/0
with a Du Pont 21-490B mass spectrometer. The resulting mass spectrum showed a molecular ion at mass 170 and did not have any peaks characteristic of the presence of the lfNi and 2Ni isotopes. This suggested that compound A was not one of the original constituents of the homogeneous precipitation analysis and also not the expected product. A series of simple experiments to determine those reactants required for the production of compound A showed that it was produced when urea and DMG were heated together in an acidic solution. Subsequently,the optimum conditions for the synthesis of compound A were investigated primarily in order to obtain sufficient material for a complete analysis. Solutions were prepared containing identical amounts of hydrochloric acid but varying mole ratios of urea to DMG, including a control containing only DMG. These solutions were heated for 4 h and were monitored at 0.5 h intervals for the presence of DMG. The maximum amount of compound A was obtained with solutions containing the largest urea to DMG mole ratios. From these experiments, approximately 1.5 g of compound A was obtained for investigation. While the optimum conditions for the production of compound A were being determined, it was also found that butane-2,3-dione was produced when DMG was heated under acidic conditions both with and without urea present. Butane-2,3-dionewas identified in aliquots of the reaction mixture with gas chromatography/mass spectrometry and by isolation and preparation of the semicarbazone derivative. Analysis of Compound A. Compound A was found to be remarkably insoluble in most common solvents. However, it was slightly soluble in boiling water and the material was recrystallized from hot water. It was subsequently found to be soluble in trifluoroacetic acid (4). The recrystallized material melted at about 345 "C with decomposition. C, H, and N analysis (Galbraith Laboratories, Inc.) yielded C, 42.46; H, 6.26; and N 33.08. The mass spectrum of the compound showed major peaks at mass 170, 142,127,109,101,85,69,58 and 42. IR spectra (KBr disk in a Perkin-Elmer Model 621 IR) showed major peaks at 3235 cm-I (N-H stretch), 2970,2940,2860 cm-' (C-H stretch) 1720,1665 cm-' (C=O stretch), 1430, 1380 cm-' (C-H bend), 1160 cm-I (C-N stretch), and 725 cm-' (N-H out-of-plane bending). The 0 1986 American Chemical Society
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elemental analysis and the mas8 spectrum with an apparent molecular ion at nupi 170 suggested a compound with the formula C6H1&02 (C, 42.39; H, 5.93;N, 33.H). This combined with the IR data suggested structure I for the compound: 3a,6a-dimethyltetrahydroimidazo[4,5-d]imidazole-2,5-dione. Structure
imately 2-3, the hydrolysis of urea will rapidly increase the pH to 4.5 and Ni-DMG will be precipitated quantitatively without being contaminated with compound A (even if a small amount of A is produced, it is soluble enough to stay in solution). If the initial solution is more acidic than pH 2-3, then both compound A and Ni-DMG will be formed; if the solution is very acidic, only compound A will be formed. Each of these results has been observed. CONCLUSION We attribute this problem to an inadequate control of the initial pH of the Ni2+ solutions prior to addition of urea and DMG. We suggest that when the homogeneous precipitation method in the determination of Ni is used,the pH be carefully adjusted to lie between 2 and 3. Obviously the problem does not arise if urea is omitted from the analysis. Registry No. I, 28115-25-5;Ni, 7440-02-0; dimethylglyoxime, 95-45-4; urea, 57-13-6; butane-2,3-dione, 431-03-8.
I suggested ari alternative synthetical route using butane-2,3-dione and urea in an aqueous acidic solution. This reaction was extremely facile and produced a crystalline proddct that had ideritical mass spectra, IR spectra, C, H and N analysis and melting point as compound A. A mixture melting point was identical with that of the indivitld kterials. A literature search revealed that compound A was a known compound, with it and ita near derivatives having been breRafed from the corresponding diketones and substituted ureas (4-9). We conclude that our compound A has structure I. This was further confirmed by comparing the proton N M R of compourid A and the synthesized material run in deuterated trifluoroacetic acid using a JEOL FXSOQ NMR spectrometer. DISCUSSION The acidic solution chemistry of DMG in the presence of urea is obviously more complex than the hydrolysis of urea to ammoriia and formation of Ni-DMG. We suggest that the formation of compound A occurs as a result of the slow hydrolysis of DMG to butane-2,3-dione and its subsequent reaction with urea. If the initial pH of the solution is approx-
LITERATURE C I T E D (1) Tschugaeff, L. BBr Dtsch. Chem. as.1905, 3 8 , 2520. (2) Tschugaeff, L. 2.Anorg. Chem. 1905. 4 6 , 144. (3) Brunck, 0.Z . Angew. Chem. 1907, 20,824. (4) Butler, A. R.; Hassain, I J. Chem. Soc., Perkin Trans 2 1981, , 310-16. (5) Biltz, H. Ber. Dtsch. Cbem. Ges. 1908, 41, 167-173. (6) Seekles, L. R e d . Trav. Chim. Pays-Bas 1927, 48, 77-84. (7) Imidazole and Its Derlvatives ; Hofmann, K., Ed.; Interscience: New York, 1953;pp 227-232. (8) Kuhling, D. Justus Liebigs Ann. Chem. 1973, 263-277. (9) Suvorova, L. 1.; et al. Izv Akad. Nauk SSSR, Ser. Khim. 1979, 6 , 1306-13 13.
J e n s Hemmingsen David Larkin* Thomas M a r t i n Department of Chemistry Towson State University Towson, Maryland 21204
RECEIVED for review January 13, 1986. Accepted April 14, 1986.
Micro Carbon Electrode for Intracellular Voltammetry Sir: Since the earlier studies of Adams’ group (1,2)electrochemistry has proved to be a powerful method for the determination of amine neurotransmitters and ascorbic acid in vivo ( 3 , 4 ) . The most frequently used microelectrodes are constructed from carbon paste or a single carbon fiber enclosed in a glass micropipet. The carbon fiber either protrbdes forming a cylindrical electrode of 250-500 pm length and 8 pm diameter or is cut on the end of the glass coat so that the active surface is a single disk of 20-500 pm diameter (5-8). The size and form of these dlectrodes make them inadequate for intracellular measurements. Yet there is no doubt that much could be learned if measurements could be performed inside individual cells. In this paper we report a technical method to prepare a needle-tip micro carbon electrode with a tip diiuneter ranging f r o b 0.5 to 2 pm which works in oxidation as well as in reduction. This microelectrode could be implantd for several hours into identified neurons of Aplysia witbout damaging them or affecting their electrophysiological properties. We tested this new microelectrode by studying, by differential p& voltammetry (DPV), the penetration into a neuron of two electroactive drugs (antipyrine and metronidazole, respectively, in oxidation and reduction) and by measuring endogenous intracellular ascorbic acid. 0003-2700/86/0358-2088$01 S O / O
EXPERIMENTAL SECTION Working and Reference Microelectrodes. The working microelectrodes were prepared with carbon fibers (external diameter 5-12 pm, Le Carbone Lorraine) sealed to a copper wire with graphite powder in polyester resin (Figure 1A). The sharpening and insulation of the carbon fiber bear some similarities to the preparation of microplatinum electrodes used in oxygen determination (9, IO). The carbon fiber was electropolished (Figure 1A) by dipping the extreme tip onto the surface of an half-saturated solution of sodium nitrite containing detergent, TWEEN 40 (Sigma). A similar technique as been described earlier (11). An alternating current (50 Hz, 2-6 V) was passed through the solution between the carbon fiber and a platinum macroelectrode. Under these conditions, the carboli dissolved away slowly. At the end of this treatment, the carbon fibei. had a very sharp tip in the range of 0.4-1.5 p m diameter. A soda glass tube, 1 mm o.d., was heated locally and pulled down to a diameter slightly larger t h d that of the carbon fiber. A hook was made on the end of the pipet. The carbon fiber was pushed down inside the tube uhtil the end of the copper wire was blocked against the taper on the inside of the capillary tube (Figure 1B). The copper wire was fixed to the glass with adhesive and the glass tube was placed in a de Fonbrune microforge, point downward, and a small weight (about 10 mg) was hung on the 0 1986 American Chemical Society
