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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Liquid Chromatography

In LC/MS/MS, the sample can be diluted, undiluted, or extracted. It is injected into the liquid stream of the liquid chromatograph. The liquid stream originates from the mobile phases that are pumped through the system by the LC pumps. Once injected, the sample rides with the mobile phase to the column. The analytes dissolved in the mobile phase then begin to partition between the mobile phase and the stationary phase that is packed inside the column. Separation takes place based on the analytes' affinity for each of the phases. The rate at which compounds elute from the column can be optimized based on the polarity of each phase, the pH of the mobile phase, the particle size of stationary phase, the length of the column, and the diameter of the column.
After the analytes elute from the column they are still in the liquid phase. This creates a unique problem in contrast to GC/MS. In GC/MS, the solvent is vaporized in the injection liner. In LC/MS/MS, the eluate from the LC has to be converted to the vapor phase before it enters the detector. There needs to be an interface that will eliminate the liquid and generate gas phase ions. This is accomplished by placing the source outside of the high vacuum of the detector. Therefore, evaporization and ionization takes place in the same area and under atmospheric pressure.