I'm building a control panel for an old altitude training machine and I need to measure the output of an oxygen sensor with an arduino. The oxygen sensor (medical oxygen sensor OOM202) is a micro fuel cell which outputs a voltage between 200μV and 20mV depending on the oxygen level. I'm using a LMC6482 CMOS dual rail-to-rail op amp running on a supply of 5 volts to amplify the signal before feeding it into the analogue input of an Arduino Uno. The resistor connecting pins 1 and 2 is 270kΩ and the resistor connecting pin 2 to ground is 1kΩ, though later I changed these to 20kΩ and 100Ω respectively (to no avail).

I've done about as much as I can to reduce noise and interference in the circuit, but the op-amp is behaving weird. I've measured the voltages of each of the pins and got the following:

Pin 1: 185mV
Pin 2: 29mV
Pin 3: 13.8mV
Pin 4: 0mV
Pin 8: 4.92V

What am I doing wrong? My calculations say the output at Pin 1 should be around 2V.