Electrolyte-Filled Glass Microelectrodes

Of the three commonly available types of glasses, borosilicate (Pyrex-type) glasses appear to be the most satisfactory. They combine the properties of good electrical resistance, resistance to thermal shock, and mechanical 

A suitable glass for micropipette applications is Corning 7740 (Frank and Becker, 1964). Tubing with the dimensions 1.0 mm o.d. and 0.5 mm i.d. is satisfactory in most applications. Stock tubing should be selected to the tolerances of 0.1 mm for the outside diameter and 0.05 mm for the wall thickness to insure uniformity and reproducibility in pulling electrodes. 

---

The series resistance of a glass microelectrode is generally quite high. If isotonic saline solution (0.1 5n) is used as the electrolyte, a typical glass electrode will exhibit a series resistance of the order of 1000 MQ. Such a resistance would cause too high a voltage drop with respect to the usual input impedance values of microelectrode preamplifiers (see Chapter 7 on preamplifiers). For this reason, 2 or 3n KCl solution is used to fill the electrode lumen. This results in an electrode series resistance of 50 to 100 MQ. 

A recent study by DeFelice and Firth (1971) has shown that the electrical noise present in glass microelectrodes is in excess of the Johnson noise predicted by the Nyquist formula given in Section 4.4.3. If the microelectrode lumen is filled with an electrolyte of concentration ri2 and the external electrolyte in which the electrode is immersed has the concentration when = ri2 2i noise voltage is observed as predicted by the Nyquist formula. When Ml offset voltage is noted (tip potential) because of the... 

The electrical connection from a glass microelectrode to its accompanying electronics is made by a metal wire inserted in the stem end of the lumen and in contact with the filling electrolyte. Early literature frequently refers to tungsten wire for this purpose, but there seems to be no valid basis for using this metal. Apparently it was available and had been used in place of antimony in certain pH electrodes. Because of its metallurgical properties, tungsten is difficult to form, and it has undesirable electrical characteristics. 

There appear to be two factors contributing to the tip potential associated with a glass microelectrode. One is independent of electrode tip size and is produced by the liquid junction formed by the electrode filling solution and the electrolyte in which the electrode is placed. The other potential source is related to tip size and the type of glass used. It appears to be related either to ionic gradient at the tip and/or selective permeability of the glass to certain ions in the electrode environment. 

---