MIDAS

MIDAS

 

SRM

Selected or Multiple Reaction Monitoring (SRM or MRM) is a mass spectrometry based technique involving a triple quadrupole mass spectrometer. As the diagram shows, the instrument is split into three discrete areas, Q1 , Q2 and Q3.

In an SRM experiment, ions enter the mass spectrometer and get passed onto Q1. At this point, Q1 acts as a filter, excluding all ions except those with one specific mass to charge ratio. Ions with this mass to charge ratio are then passed on to the second stage of the mass spectrometer, Q2 . At this point, these ions, known as precursor ions, are hit with an inert gas which is typically nitrogen or helium. As a result of this, the precursor ions break apart into smaller fragment ions, known as product ions. This process of fragmentation is known as Collisionally Induced Dissociation, or CID. Following this, these daughter ions are passed into Q3, which acts as in a similar way to Q1, excluding all product ions except those with one specific mass to charge ratio. It is only these selected daughter ions which reach the detector  and give a response. The process of selecting a specific precursor ion in Q1, fragmenting them with a specific collision energy in Q2, and selecting one specific product ion in Q3 is known as a “transition”.

Due to two separate stages of ion selection, as well as fragmenting precursor ions with a specific collision energy, SRM is a very selective process, ensuring only very specific ions reach the detector of the instrument. This means that other contaminating ions are removed, and the vast majority of the ions detected by the mass spectrometer are those of interest, with very little contaminating ions reaching the detector. In addition, SRM allows quantitative data to be generated over a large dynamic range (often in excess of 4 orders of magnitude), even in complex backgrounds such as whole cell lysate digests and plasma. These benefits make selected reaction monitoring a powerful tool in mass spectrometry.