MOSFET with *actual pin* for body
The parts on that schematic are very low capacitance (and, unfortunately, that generally means relatively high 'on' resistance) compared to the otherwise nice ones that Ignacio mentions in his comment. 

The switch is designed to have low charge injection since that directly affects the accuracy of the nulling. 

You might consider something like the ADG5236 in a modern design, which has 0.6pC of charge injection, which is only 60nV on a 10uF capacitor. 

Or use an amplifier that has the autozero circuitry internally, but I see you're actually working on a sample-and-hold circuit.