Sample preparation for planar chromatography

High‐performance thin‐layer chromatography has favorable properties for high‐throughput separations with a high matrix tolerance. Sample preparation, however, is sometimes required to control specific matrix interferences and to enhance the detectability of target compounds. Trends in contemporary applications have shifted from absorbance and fluorescence detection to methods employing bioassays and mass spectrometry. Traditional methods (shake‐flask, heat at reflux, Soxhlet, and hydrodistillation) are being challenged by automated instrumental approaches (ultrasound‐assisted and microwave‐assisted solvent extraction, pressurized liquid extraction, and supercritical fluid extraction) and the quick, easy cheap, efficient, rugged, and safe extraction method for faster and streamlined sample processing. Liquid‐liquid extraction remains the most widely used approach for sample clean‐up with increasing competition from solid‐phase extraction. On‐layer sample, clean‐up by planar solid‐phase extraction is increasingly used for complex samples and in combination with heart‐cut multimodal systems. The automated spray‐on sample applicator, the elution head interface, biological detection of target and non‐target compounds, and straightforward mass spectrometric detection are highlighted as the main factors directing current interest toward faster and simpler sample workflows, analysis of more complex samples, and the determination of minor contaminants requiring high concentration factors.