[18uos]

What is the use of magnet in mass spectrometer,?
why only +ve ions are only studied in MS ,not  -ve ions , although these are also produced in the mass spectrometer.//// answer

[Yggdrasil]

The answer to your first question has a lot to do with the answer to the second (note that it is possible to observe negative ions in a mass spec, however, when you are observing negative ions, you cannot observe positive ions).

A mass spectrometer must separate ions based on their mass.  One way of doing this is by using a magnet.  The MS will ionize your compound and shoot it through a magnetic field.  If you recall from introductory physics, magnetic fields exert a force on moving charges.  In the case of most MS machines, the force will alter the trajectory of the ions, causing them to bend around a corner and hit the detector (see diagram here) .

As it turns out, the force on our ion is proportional to the charge of the ion.  Since most of the ions we see in mass spec are positively charged, all of the ions experience the same force.  So, how do we distinguish between ions of different masses?   If you again think back to introductory physics, you'll remember Newton's Second Law: F = ma.  That is to say, given the same force on each ion, the acceleration experienced by each ion depends on the mass of the given ion.  The acceleration determines the amount that the ion curves as it moves through the magnetic field.  Therefore, very light ions will curve very sharply and hit the detector at a much different place than heavier ions that curve less sharply.

What happens to negative ions?  Well, as one might expect, they curve in the opposite direction as the positive ions.  So, when they enter the magnetic field, the negative ions curve away from the detector and therefore, cannot be detected.

[18uos]

Beautiful explaination !!!!!!!!very good..


the last sentence is very technical and precise but comprehensive