A method for deriving a background-corrected portion of a measured optical emission spectrum comprising the steps of identifying two or more background correction points from the portion of the measured emission spectrum; deriving a background correction function fitted to the identified background correction points, and applying the background correction function to the portion of the measured emission spectrum so as to produce a background-corrected portion of the emission spectrum, wherein the background correction points are identified from the measured data points by consideration of the gradients between the measured data points.