This could be a reasonable application for analog switch IC's... like the DG211. The DG211 will provide an analog switch closure between 2 points based on a TTL level control input. The device is rated to work with up to +18/-18V power source (which you will likely find in your instrument to be controlled). With a common GND and a TTL level control signal from the Arduino, you can create switch closures that tolerate your voltages recorded.
4 switches to a package. http://datasheets.maximintegrated.com/en/ds/DG201A-DG211.pdf
Prices seem to vary between $1 to $5 with the DIP package being the most expensive.