Guarded electrode

ICP-SFMS (Thermo Finnigan, Flement) with cold vapour generation was developed with a guard electrode and a gold amalgamation device using an Au-sorbent for sample pre-concentration to improve the sensitivity. Instrumental parameters of ICP-SFMS such as take-up time, heating temperature of Au-sorbent, additional gas flow, and sample gas flow were optimized. Detection limit calculated as 3 times the standard deviation of 10 blanks was 0,05 ng/1, RSD = 7-9 %. 

The focused current system employs the guarded electrode design shown in Figure 4-274. 

A Thermo Finnigan Element 2 Inductively Coupled Plasma Sector-Field Mass Spectrometer (ICP-SF-MS) with guard electrode was employed for trace element analyses. RSD values derived from internal check standard never exceeded 10%. Accuracy was better than 15% for all elements as determined by analyzing the certified reference standard NWRI TM-RAIN 95 trace metal fortified rainwater, every 5 to 8 samples. 

Figure 2.11 Sensitivity for selected long-lived radionuclides measured by ICP-SFMS (Element, Thermo Fisher Scientific) with (yellow) and without (green) shielded torch (CE - guard electrode). (]. S. Becker and H. ). Dietze. Int. ). Mass Spectrom. Ion Proc. 197, 1-35 (2000). Reproduced by permission of Elsevier.)...

The detector of the PIMMS is a U-shaped Faraday cup surrounded by guard electrodes. 

Stray capacitances, e.g. between a lead and the opposite electrode, must be avoided by proper shielding. For this reason it is preferable to run the guarded electrode at zero potential. Then the guard electrode can be earthed and a completely shielded 3-terminal cell constructed (Fig. 5.5), where the only stray capacitances are between the electrodes and earth, giving an equivalent circuit as shown. 

Unfortunately, with most high-frequency measurement methods it is not possible to use a guarded electrode system and, in addition, lead inductances become appreciable at high frequencies. To meet the high-frequency requirement... 

In this condition, earthed screens can be used around all bridge components, and a guard electrode can be used in the sample cell. 

Fig.6. Configuration of a vertical parallel-plate cell [10]. a Side view b Cross section view (X-X ) (1) silicone tube (outer diameter, 4 mm), (2) glass plate (length x width x thickness, 50x50x3 mm), (3) copper wire (diameter, 1 mm), (4) electrode (gold-plated copper foil diameter, 20 mm thickness, 0.035 mm), (5) guard electrode (gold-plated copper foil diameter, 22 mm thickness, 0.035 mm), (6) electrode (gold-plated copper foil length x width x thickness, 50x50x0.035 mm), (7) epoxy adhesives, (8) epoxy resin (thickness, 1.5-2.5 mm)...

FIGURE 3-24 Left Four plane capacitance sensor with driven guard electrodes located near the distributor in fluidized bed (distances in mm) Adapted from Beck et. al. (1995) Right Bubbles (dark) attached to a wall of bed in time sequence (see also Wang et. al. 1996)... 

FIGURE 11-5 A micro-Faraday array ion transducer. The transducer elements are 5, 2.45, and 1,60 mm long and 145 jm wide with 10 gm spacing. There Is also a lO-pm guard electrode between each element, (Reprinted from A. K. Knight IV, R. P. Sperline, G, M, Flieftie, E, Young,... 

The dielectric material in the form of a flat circular disc of thickness d, is placed between the electrodes to form a parallel-plate capacitor of plate area A. The upper or high electrode (H) is directly connected to the high-voltage terminal of the measuring unit, whereas the lower electrode (L) is similarly connected to the low-voltage terminal. These two terminals are Isolated from the metal screen of the BNC-type connector and cables. The third or guard electrode (C) is held at the same potential as the lower electrode, but is isolated from it by a Teflon guard. The... 

Figure 5. Three-terminal parallel-plate capacitor showing uniform field lines (dashed lines) between the high-voltage (H) and low-voltage (L) electrodes. Fringing fields (distorted dashed lines) are present only between the electrode H and the guard electrode, C. The ensemble is enclosed by a metal screen connected to ground (detailed scheme in the Supplemental Material of reference [28]).

The guard electrode consists of a third electrode mounted as close as possible to the electrode from w-hich the current is to be measured. It is maintained at a potential as close to the potential of the latter as possible, but not in electrical contact with it. so that the... 

For most materials the volume resistivity will be greater than 10 Qm, and the measured resistance using the electrodes illustrated in Fig. 3 will be above about 10 Measurement in this range will require the use of an electrometer instrument drawing bias current less than about I0 A. Such instruments are almost invariably provided with a third terminal for connection to a guard electrode (see Fig. 2). The initial charging current for... 

A guard electrode may be placed on the opposite side of the specimen from the electrodes alternatively, the specimen may be supported on the flat surface of a material of very high resistivity, such as P.T.F.E. 

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