In clean air, I calculated the value of R0 by using MiCS quick evaluation board. Formula applied :(ADC/(RESOLUTION - ADC)) * 51820---for NO2 and (ADC/(ADC-RESOLUTION))*100820 -- for CO and in polluted air I applied same formula and gave it a name Rs Then took ratio of Rs/R0 which is less than one. So I had a slope and its constant meand y = m*x + c But i do not know how to derive equation to find out the value of ppm from Rs/0