Question: I-m trying to understand how to code chromatography lab procedures based on stationary phase/mobile phase. Please explain how to code these two scenarios: 1. The lab performs a column chromatography using a single stationary and mobile phase to identify two different unlisted analytes. 2. The lab performs a column chromatography using a single stationary phase and two mobile phases to identify two different unlisted analytes. Maine Subscriber Answer: Chromatography is a method that labs use to physically separate components of a complex mixture by distributing the components between two phases. The stationary phase is the substance that is fixed in place, such as silica gel packed into a tube for column chromatography. The mobile phase is the portion that moves, such as a solvent that carries the mixture components up the column, separating them by physical or chemical properties. Because running a single mobile phase/stationary phase requires the same amount of work whether the test identifies one or more analytes, chromatography codes often use the stationary phase/mobile phase as the unit of service rather than the analyte. Solution: For your example 1, report the service as 82492 (Chromatography, quantitative, column [e.g., gas liquid or HPLC]; multiple analytes, single stationary and mobile phase). For example 2, this constitutes two separate procedures, so you should list two units of 82491 (- single analyte not elsewhere specified, single stationary and mobile phase).