you will need to find out the output voltage at 350ppm and at another point (such as 3500ppm or 1000ppm), so you can get the electromotive force variation. Then you can use both points to figure out the equation y = m*log(x) + b ; where y is the gas concentration, x the frequency and m and b the two constants of a linear relationship.
You can take a look at this post by AmroQuandour, where he did the job for the CO sensor (but be careful, since both frequency and concentration are in logarithmic scale for this sensor):viewtopic.php?f=15&t=2447
You may get a better accuracy by getting more concentration values and linearly approximating the different stretches, but you can get a good result just with one.
I have one question..from the graph of both TGS2442 & TGS4161 sensor what I got is x axis representing the gas concentration and in case of TGS2442 y axis is RS/R0 and for TGS4161 y axis is the difference in voltage between 350ppm of CO2 and x ppm of CO2
pls let me know my understanding is correct or not..