vmandin:
Thank u so much sir. How about if 0ppm is 1V and 2000ppm is 4V? how is that calculated?
ppm = (analog_reading * 3000 * 5)/(1024 * 4) - 1000
However, if your 5V supply (the default analog reference) is not very accurate then you will have a significant zero error. If the sensor has a digital output, it would be better to use that.