Figure 1. Measurement of anesthetic concentration using Ca^2+-sensitiveelectrodes. The voltage output of the Ca²⁺electrode increased linearly with anesthetic concentration when measured in the perfusion line (filled squares; the voltages at the end of a 20-min application of enflurane are plotted). When ACSF bubbled with 4.3% enflurane was tested in the recording chamber, the voltage output of the electrode corresponded to about 3.6% enflurane (open squares), according to the linear regression line, corresponding to about a 14% loss in this example. The inset shows the raw voltage output from the calibration measurement in the perfusion line and the measurement in the recording chamber, both in ACSF bubbled with 4.3% enflurane. The voltage response had not reached steady state at the end of a 20-min application of anesthetic, because of a slow response time of the electrode or a slow exchange time in the perfusion line. Exponential fits to the voltage traces indicated that the responses were typically within 85-90% of their steady state values, and that the steady state loss (18% in this case) was not substantially different from the loss measured at 20 min.