1. The problem statement, all variables and given/known data I'm supposed to find the first ionization energy of a gas (Argon), and to do that I used mass spectrometer. The data I plotted was amplifier voltage (acts as Ion current) vs. electron volts. The graph appears in the attachment as data2.jpeg. To find the ionization energy I'm supposed to take the first and second derivative of the data, and the max x-value of the data is the ionization energy. When I do that the plot of the derivatives is not smooth; the points fluctuate. So I decided to fit a function to the data, and then take the derivative. 2. Relevant equations I fitted using curve_fit on numpy, python 2.7. and tested the goodness of fit using chi-squared/dof. The fit was bad. 3. The attempt at a solution I used the logistic function defined here http://en.wikipedia.org/wiki/Logistic_function, with parameters, (0.026, 0.745, 19.0, 0.0005). The fitted function is shown here. Since the fit was not good, is there a better function to fit this kind of data, or is there a better way to find the derivatives of the data?