MadSci Network: Biochemistry

Re: What is delayed extraction,linear mode,reflector mode in mass spectrometry?

Date: Sat Mar 3 10:03:49 2007
Posted By: Luis Sojo, Faculty, Chemistry, Simon Fraser University
Area of science: Biochemistry
ID: 1172867679.Bc


Delay extraction, linear mode and reflector are terms from a mode of mass spectrometry called MALDI- Time of Flight (aka MALDI-TOF). MALDI stands for "matrix assisted laser desorption ionization" and it is a mode of ion generation in mass spectrometry. The sample is mixed with a matrix substance and placed on a small metal plate called the target. The sample is allowed to dry and is placed under vacuum and shot at with a laser beam. This generates ion in the gas phase into the mass spectrometer.

The most common configuration for MALDI mass spectrometry is with a time of flight mass spectrometer attached to the MALDI source. This is usually a tube with a series of electromagnetic lenses that permits the guiding of the ions generated at the MALDI source to fly down the tube until they reach the detector. The ions generated at the source have very similar kinetic energy, once they are accelerated down the tube into the detector by an electromagnetic field and will separate by virtue of their different masses. This is because with the same kinetic energy they will have to have different velocities (Kinetic energy = ˝ mv), so ions with different masses will travel down the tube with different velocities arriving at the detector at different time, therefore mass separation is achieved.

This configuration with is called linear mode (See FIgure 1 at (below).

What you do not see in the Figure above is the MALDI source, but it is a schematic of both linear and reflecting instruments. Reflecting instruments permit the ions to travel a longer path, improving the resolution of ions with very similar masses since the longer the ions travel the larger their arrival time difference is.

Now when you generate the ions they do not have exactly the same kinetic energy as stated above. They will have a distribution of kinetic energies. This causes loss of resolution since all ions of the same mass will not arrive at the detector at the same time. This is compounded when ions have to travel longer distances. You see, we try to improve resolution by increasing the time of flight, but by doing so we loose some resolution because the longer the ions travel the more chances of ions of the same mass to have a distribution of velocities. This can be improved by doing a “delay extraction”. This is basically a time delay between ion generation and allowing the ions to go into the flight tube. This, in a way, cools the ions and narrows their initial kinetic energy distribution, so they start with more uniform kinetic energies improving resolution. See Figure 3 at (below).

Notice the better resolution on the lower panel (delay extraction) in comparison to the top panel (continuous extraction). The lower panel shows the many different components underneath the peak on the top panel.

Hope this helps!

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