The analyte ions produced via ESI then enter the high vacuum area of the mass spectrometer and are drawn into a small quadrupole or hexapole (akin to the quadrupole mentioned with GC/MS). The hexapole or quadrupole is tuned to select the ions they wish to analyze. The selected ions are then drawn into a component known as the collision cell. The collision cell contains a 'collision gas,' either nitrogen or argon. As the selected ions enter the collision cell, they collide with atoms of collision gas and fragment. The ion fragments are directed into a second quadrupole where the operator has selected the fragments desired to exit the second quadrupole and impact the electron multiplier. The two 'MS' abbreviations in the term LC/MS/MS refer to the fact there are two quadrupoles.
Consider the degree of separation and identification achieved with this method. First, a specific compound has to elute from a particular chromatography column at a precise time (for example, the operator knows that a given drug will elute at precisely 43 seconds). This specific chromatography elution (43 seconds) is then ionized, and only particular ions corresponding to the drug of interest are allowed to pass. Then, these specific ions are reacted in the collision cell to produce particular daughter ions specific to a given drug. Using an LC/MS/MS method can thus lead to a precise compound identification. Confirmation using LC/MS/MS can positively identify the presence of a compound with a near-certainty not seen with other methods.
