Described systems and methods allow a classification of electronic documents such as email messages and HTML documents, according to a document-specific text fingerprint. The method may provide the ability to restrict the range of pixels in the first color space that may ordinarily transform to \u201cillegal\u201d values with erroneous brightness in the second color space, to a sub-space containing the entire second color space, while preserving the brightness of the pixels. The invention discloses a method and apparatus for separation of ionic mixtures and characterization, identification, or quantification of ions in a gas based substantially on the terms of third or higher order in a series expansion of ion mobility as a function of electric field intensity.