Reducing noise in a simple current measurement

I need to measure the impedance across a specialized electrochemical cell with as low noise as possible, for a research project. The impedance of the cell is about 30 Mohms, and the potential across the cell is a 50 mV 500 Hz sine wave. The circuit measures the current, from which the impedance can be determined.

I am using an AD797 op amp with a carbon 1 Mohm resistor in the feedback loop. Can the noise of the output signal be reduced by using a different op amp? What about replacing the feedback resistor with a metal film resistor? Is there a better overall circuit design that would provide lower noise? I am seeing a noise level of about 100 Kohms in the 30 Mohm measured impedance, and need to reduce the noise as much as possible.