Capacitance measurements with PULSE

The EPC 10 and our PULSE software combine to provide you with everything you need if you are interested in capacitance measurements. The EPC 10 is the ideal patch clamp amplifier for capacitance measurements because all the relevant parameters are under its control. Digital control of filter settings, gains and compensation networks allows the admittance of the signal source at the amplifier's input to be determined, based on the amplifier's own calibration. No external filters are required for standard measurements.

In 1982 Neher and Marty introduced the lock-in amplifier into the patch clamp field for measurement of membrane capacitance using a single sine-wave frequency. For determining the appropriate phase setting, they used a simple and easy trick - dithering of the compensation network while changing the phase for obtaining a maximum signal. This method is appropriate under stationary conditions.

Commonly used extensions, however, particularly the automatic 'phase tracking', is prone to errors (see Gillis in B. Sakman & E. Neher Eds. Single Channel Recording 2nd Edition, Plenum). For this reason several novel schemes, including 2 frequency methods, have been suggested in the meantime. The minimum requirement for determining the three unknowns of the equivalent circuit of the cell under study involves three experimental parameters. The 1 1/2 frequency method (or Lindau-Neher technique) uses real and imaginary part of a sinewave signal plus the DC-conductance to determine membrane capacitance, membrane conductance, and access resistance. This technique was adopted long ago by leading patch clamp laboratories. It is fully implemented in the EPC 10 together with two further variants of capacitance measurement.

The PULSE software implements both the classical single frequency and the 1 1/2 frequency (or Lindau-Neher) technique in a convenient user-friendly way. It provides 'internal calibration' which automatically corrects for phase-shifts and frequency dependent attenuation, so that the admittance of the external circuit is obtained in calibrated SI units. On this basis the parameters of the equivalent circuit are calculated 'online' according to Lindau & Neher.

In addition, the EPC 10 offers automatic capacitance compensation. When activated repetitively (capacitance tracking!) this provides a continuous readout of capacitance. The experimentor can observe the residual capacitive current for a validation of correct compensation.

Since the amplifier 'knows' the settings of the compensation network, the 1 1/2 frequency method can operate while the bulk of the capacitance is compensated. Consequently the amplifier can be operated at high gain without saturating. High gain implies low noise. The high resolution admittance measurement of the residual capacitance together with the compensation circuit provides an effective dynamic range which has yet to be achived with any other instrument.