Sample preparation is an essential part of most bioanalytical methods. As discussed in Chapter 1, it is required because biological samples cannot normally be introduced directly into the analytical instrument and some “sample cleanup” and/or concentration is usually essential for optimal performance. Sample preparation may vary from a simple dilution or protein precipitation technique to complex multistep procedures involving liquid-liquid and solid-phase extraction (SPE) or derivatisation (chemical modication) of the analyte.