In LC/MS/MS the sample can be diluted, undiluted, or extracted. The preparation of the sample depends on the specimen type, the drug being detected, the need to derivatize the sample, etc. Once prepared, samples are injected into the liquid stream of the liquid chromatograph. The liquid stream is a mobile phase liquid that is pumped through the system. Once injected, the sample rides with the mobile phase through the column. Just as with GC/MS, 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 the 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. This means there needs to be a way to eliminate the liquid and generate gas-phase ions. This is accomplished by placing the ion source outside of the high vacuum of the detector. Therefore, vaporization and ionization takes place in the same area and under atmospheric pressure.
