Anal. Chem. 1980, 52, 2400-2402
2400
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rate of 0.500 L min-I. The sensitivity remained constant t o within f O . l ppm SOz until the eleventh day. The rapid change in sensitivity a t longer time appears to have resulted from a failure in the mechanical seals holding t h e various parts of the sensor together. The drift in the cell zero over a 24-h period is on the order of 1 ppm SOz. This zero drift exhibits no large short-term fluctuations and resetting the zero before a measurement permits SOz measurements to be made with an uncertainty of f0.2 ppm. LITERATURE CITED
2.0
0
1 ..o
' 5
10
15
20
TIME !DRYS1
Figure 8. Time stability of sensor: (0)curve A , stability vs. time: (A)
curve B, sensitivity vs. time.
SOz in a stepwise fashion. The current change reaches 99% of t h e final value in 30 s. T h e time required for a sensor that has had its electronics turned off to reach a stable zero value on turning on the electronics is about 15 min. Time Stability. Figure 8 illustrates the time stability of a typical sensor using 10 ppm SOz, iref= 695 MA,and a flow
(1) Fed. Regist. 1971, 36, 6684. (2) Shaffer, P. A., Jr.; Briglio, A. B., Jr.; Brockman, J. A., Jr. Anal. Chern. 1948, 20, 1008-1014. (3) Strauss, W. "Air Pollution Control, Part 111"; Wiley: New York, 1978; p 325.
(4) Hollowell, C. D.; Gee, G. Y.; Mclaughlin, R. D.Anal. Chern. 1973, 45, 63A-72A. (5) Werner, P. 0."Analysis of Air Pollutants"; Wiley: New York, 1976; p 178. ( 6 ) Sherwood. W. G.; Martinchek, G. A,; Gal-Or, L.; Bruckenstein. S. Tech. Prog. Rep.-U.S. Bur. Mines, Grant No. 155007, March 1, 1975.
RECEIVED for Review July 7, 1980. Accepted September 3, 1980. This research supported by Contract No. AFOSR 783621A from the Air Force Office of Scientific Research.
Neutral Carrier Based Ion-Selective Electrode for Intracellular Magnesium Activity Studies F. Lanter, D. Erne, D. Ammann, and W. Simon' Department of Organic Chemistry, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland
A Mg2'-seiective microelectrode based on a ligand belonging to a new class of neutral synthetic Ion carriers Is described. The selectivities in respect to Na', K', and Ca2' are sufficlent for intracellular measurements of Mgz' ion activities 20.4 mM at an ionic background of 10 mM Na', 100 mM K', and 1 pM Ca2+. Electrodes with tip diameters of around 1 pm have an electrical resistance of about 3 X 10" s2 and a 90% response time of 1 5 s. An example of an intracellular appllcatlon is given.
Several methods have been proposed (1-5) for the estimation of the concentration of ionized magnesium in single cells. T h e most reliable technique for such an assay is based on t h e intracellular injection of Eriochrome Blue SE as indicator dye and spectrophotometric determination of the absorption of its Mg2+complex in the range of 550-592 nm (6-9). It was pointed out that due attention should be paid t o the interference of Caz+ and H+ (6). A direct determination of the intracellular Mgz+ activity has not been possible so far. Here we report on a liquidmembrane microelectrode which, based on a ligand belonging to a new class of neutral carriers (IO), has sufficient selectivity for a restricted range of intracellular Mg2+ assays. EXPERIMENTAL SECTION Electrode System. Cells of the type 0003-2700/80/0352-2400$01 .OO/O
Ag:AgCl,KCl(std)/l M CHSCOOLi/sample solution// external m a c r o reference electrode membrane//O.Ol M MgC12.AgCI:Ag
ion-selective microelectrode
were used. The external reference electrode was a double junction silver/silver chloride electrode Philips R 11. Ion-Selective Microelectrodes. Glass micropipets were drawn from single-barreled Pyrex glass tubing (W. Krannich KG, D-34 Gottingen, GFR; 80 mol % SiOz, 2% A1203,13% Bz03,1 % KzO, 3 % NazO, 1% Fe203)which had been stored over silica gel. The tip was dipped into pure hexamethyldisilazane and baked at 200 " C for at least 15 min. By use of a syringe and as fine as possible plastic tubing, a small volume of internal filling solution was injected into the top of the micropipet shank (height -5 mm). A solution of 20 wt % of the Mg2+-selectiveneutral carrier (ETH 1117, see Figure 1) and 1 wt % of sodium tetraphenylborate (NaTPB) in propylenecarbonate (PC) was sucked into the tip to a height of about 700 pm. The micropipet was then back-filled with the rest of the internal filling solution (see also ref 11). Emf Measurements. The emf measurements were performed at about 22 "C with a FET operational amplifier (AD 515L, Analog Devices, Norwood, MA; input impedance 1013Q/L6 pF differential, 1015Q/0.8 pF common mode; input bias current
