Series resistance compensation for fast activating ion channels can help remove voltage clamp errors and increase clamp accuracy. All Sophions patch clamp amplifiers utilize a patented technique enabling up to 100% Rs compensation. In addition, Sophion amplifiers are equipped with automatic clip detection to avoid loss of cells due to fatal oscillations, thereby increasing data throughput while maintaining high quality recordings.
- Increase clamp accuracy
- Up to 100% Rs compensation using patented algorithms
- Automatic clip detection increase data throughput, while maintaining high quality recordings
Read more about Sophion Rs compensation and clip detection in this Application Report or this poster….or continue reading below.
Sophion amplifiers are designed for Rs and clip detection
When performing whole cell voltage clamp measurements, the membrane potential is controlled or clamped to a user specified potential. The command potential does not translate directly to membrane potential due to the so-called access resistance. The access resistance is made up of the resistance from the electrode through the orifice in the substrate and the whole in the cell membrane. These resistances combined are electrically in series with the membrane resistance, hence the name series resistance (Rs), and they constitute an “obstruction” to obtaining precise membrane potential.
The series resistance has several effects when performing whole cell recordings. Firstly the time constant at which one can perform the compensation, can be limiting how good and how fast the compensation is. Will you record the true nature of your ion channel target, or are you being limiting by the amplifier setting or limited by your amplifiers capability? If a fast channel e.g. sodium channel, is being recorded the amplifier needs to be equally fast.
Increases data throughput, maintaining high quality recordings
The electronic of the amplifier have a speed of a few µs so this might not be the limiting factor, but the RS circuit loop of the amplifier might be significant slower > 50 µs. The Sophion amplifier utilizes a patented technique invented by Adam Sherman (US Pat. 6163719, US6700427B1) characterized by its unique algorithm. In addition, it is equipped with automatic clip detection allowing Rs to be temporarily turned off and thus avoid loss of cells due to fatal oscillations. The Rs compensation is re-enabled in the following sweeps and clipped sweeps can automatically be removed in the analysis process. Automatic clip detection thereby increases the data throughput, while maintaining high quality recordings.
Regular and Fast Series Resistance Compensation
Sophion QPatch amplifiers are equipped with two different series resistance compensations algorithms, regular and fast series resistance compensation. Sophion Qube amplifiers are equipped with fast series resistance compensation. Regular series resistance compensation is able to compensate for series resistance as well as simultaneously compensating for cell capacitance. This however has a down side, namely that the speed of the R S compensation time constant is limited to 400 µs and slower. Fast series resistance compensation is able to compensate for series resistance a lot faster (in theory down to a time constant of 2 µs), but with the cost of cell capacitance compensation. It should be noted that leak compensation will remove any cell capacitance which would otherwise interfere with the ion channel measurements.
The cell membrane constitute a capacitor and the time constant for charging a capacitor is given by multiplying the series resistance with the cell capacitance. In case of a 10 MΩ series resistance and 15 pF cell, the time constant is thus 150 µs. Using the rule of thumb that one needs 6-7 time constants to reach equilibrium this becomes far too slow to compete with the kinetics of the sodium channel. On the other hand, if charging with a time constant much faster there is risk of oscillations in the current response. To prevent these oscillations the fraction of compensation must be lowered to e.g. 80% compensation. Another dimension of series resistance is that the voltage error due to Rs increases with current. In case of a series resistance of 10 MΩ and a current of 3 nA, it will give rise to a voltage clamp error of 30 mV (Sakmann, Bert and Neher, Erwin, Single channel recording, 2 nd ed. New York, 1995). The Sophion RS compensation is designed to be able to handle voltage errors in the range up to 100 mV (10 nA current with a RS of 10 MΩ)