Question

Why are positive ions analyzed in Mass Spectroscopy? Is it theoretically possible to analyze negative ions? Give an example, when it is desirable to monitor negative ions.

Answer 1

To answer the your questions, let me tell you the principle of how a mass spectometer work

If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a curve - deflected out of its original path by the sideways force.

Suppose you had a cannonball traveling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose-pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course.

But suppose instead, you tried to deflect a table tennis ball traveling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection.

The amount of deflection you will get for a given sideways force depends on the mass of the ball. If you knew the speed of the ball and the size of the force, you could calculate the mass of the ball if you knew what sort of curved path it was deflected through. The less the deflection, the heavier the ball.

You can apply exactly the same principle to atomic sized particles.

toms can be deflected by magnetic fields - provided the atom is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't.

The sequence is :

• Stage 1: Ionization: The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions.
• Stage 2: Acceleration: The ions are accelerated so that they all have the same kinetic energy.
• Stage 3: Deflection: The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.
• Stage 4: Detection: The beam of ions passing through the machine is detected electrically.

Now, for the second question, yes it is posible but not as ions, but instead as chemicals system containing electronegative compounds.

Example, consider a system with Formic acid (HCOOH), and formic d-acid (DCOOH). the spectrum for these compounds contain only 3 components. Since each compound has it's own characteristic negative ion, there is no need to measure the O- or OH- ion beam intensity, and you just need to quantify the system with a mixture of these ions.

Hope this helps