1. A method of obtaining mass spectrographic data describing a substance, comprising: ionizing the substance; obtaining a current mass spectrogram of a portion of the ionized substance, and retaining data representing the current mass spectrogram; obtaining subsequent mass spectrograms of at least two further portions of the ionized substance, and retaining data representing the at least two subsequent mass spectrograms; generating information useful for describing at least one extracted ion chromatogram, using data associated with the current and at least two subsequent mass spectrograms and a non-linear curve approximation algorithm; using the generated information useful for describing at least one extracted ion chromatogram to generate further information useful in further analysis of the ionized substance.