How a quadrupole mass filter works

 

This is a simple explanation of a complicated device. A quadrupole is a set of four rods with a space down the middle. The ions enter this space. The rods are electrically connected to eachother in opposite pairs. A constant (DC) voltage and an alternating (AC) voltage are applied to the two pairs of electrodes.
The alternating electric field makes the ions go off into spirals as they pass down the quadrupole. The constant voltage drags them in one constant direction, towards one pair of electrodes.
A small ion will be dragged a large distance by the alternating field, and will find itself in stronger and stronger regions of field. It will quickly collide with an electrode and disappear.
A very large ion will not be affected much by the alternating field, but will gradually drift in the constant part of the field (the DC part). The alternating field is not strong enough to drag it back as it wanders, so it also collides with an electrode, and is lost.
An ion that is the right size drifts slightly in the constant part of the field, but is always dragged back by the alternating part. The alternating part, however, is not quite strong enough to make it spiral out of control into an electrode. Thus an ion just the right size is stable in this quadrupole field and reaches the end, where it can be measured.

The stability of an ion in a quadrupole (its chance of making it through the quadrupole without wandering so far from the “safe” region in the middle that it hits an electrode and is lost) therefore depends on the sizes of the alternating and constant fields. It is possible to draw stability diagrams describing whether an ion is stable or not at any given pair of voltages, AC and DC.

 

A conventional quadrupole mass spectrometer works by scanning the voltages applied to the rods of the quadrupole mass filter. The scan is arranged to follow the red dotted line on the diagram, right. Therefore for most of the scan, the ion is unstable, and won’t make it to the other end of the filter. For a brief moment the scan-line passes through the stable region, and during this time, the ion will pass through the filter unmolested, to the detector at the other end.
Different ions have different stability regions. The diagram left shows two ions, one larger than the other. As the spectrometer scans its voltages, the scan-line passes through the two stable regions one after the other.
The line above the red, dotted scan-line shows how the signal intensity at the detector at the end of the quadrupole mass filter will vary. There will normally be little signal, but as the scan-line passes through each stable region, there will be a peak as that ion emerges.

 

At a simplistic level, this is more-or-less all there is to be said. Sorting out the scan-line is a part of the process of tuning the mass spectrometer. The scan-line must be arranged so it just slices off the very tip of each stable region, from the smallest to the biggest ions. It must also be calibrated in units of mass rather than voltage.

 

How an ion trap works

 

An ion trap has electrodes like a single quadrupole, but wrapped into a circle. There are thus two convex end-cap electrodes, and a ring electrode shaped like a dough-nut. The ions enter and leave through the end-caps.

 

A voltage is applied between the ring electrode and the two end-caps, so an ion inside the trap will find itself in a potential well. The wood and cardboard model to the left is an inaccurate model of this potential well. The red areas represent a cross-section of the end-caps at a positive voltage, the blue bits a cross-section of the ring electrode, which in this model is negative.

 

A positive ion will tend to run down the slope towards the blue, negative electrode.

However, the voltage applied to the trap is an alternating one. As the ion falls down towards the negative electrode, the field changes, and 90 degrees later this electrode is actually the positive one. If the ion were still moving in the same direction, it would now be “falling” up hill.

 

The only truly stable point in this field is right in the middle, where the potential never moves up or down as the field rotates.

The ions will find themselves moving round in little circles in the trap, the largest tending to end up right in the middle at the “dead point” because of their inertia. The smaller ions will always be dragged around a bit more in the field.

Although the wooden model above is inaccurate, it needn’t have been. In 1989 Wolfgang Paul was awarded the Nobel prize (shared) for his part in the discovery of the ion trap, and he was able to demonstrate the principle using a ball-bearing on an accurate model of a potential well, made up in perspex, and set to rotate on an overhead projector!

The ion trap has a stability diagram not unlike that of the single quadrupole, but it’s not symmetrical above and below the x-axis because of the slightly different geometry. Unlike single quadrupoles, ion trap instruments tend to operate AC-only, holding their ions, in effect, along the x-axis of this diagram. fig 1
fig 2 As for the single quadrupole, large ions have large stability regions, small ions small ones, but rather than draw it that way, it is easier to draw one stability region, and represent the big ions to the left of the small ions. This is the same as imagining that we have a particular AC voltage applied, and at this voltage, the big ions will find themselves to the left of their stability diagram, while the small ions will be to the right of their (smaller) stability region.

 

Therefore at a very simplistic level, all that is necessary to convert an ion-trap into a mass selective system is to ramp the AC voltage gradually upwards, and the ions will fly out of the trap into the detector in order of mass, smallest first.

There are a few (a lot!) of extras. Firstly, if the trap operated this way and with a good vacuum, the ions would tend to accelerate into wider and wider orbits, and leave the trap anyway. Therefore a dampening gas, Helium, is allowed to leak very slowly into the trap to maintain a slightly raised pressure. Secondly, resolution of the trap can be vastly improved by bringing the ions into more coordinated orbits by adding a small AC voltage between the two end-cap electrodes. If this is not done, then as the main voltage forces an ion to become unstable, the ion may have a long, or a short, distance to travel to get out of the trap. There would be a difficulty in telling the difference between ions that had a long way to go, and slightly heavier ions that started to leave the trap later, but happened to have a shorter path to the nearest exit.

The end-cap AC voltage is also ramped as the main voltage is ramped, to encourage the right size of ion to leave.

Collision induced fragmentation

 

Fragmentation can be carried out to some extent even in a simple single quadrupole instrument. This is done simply by accelerating the ions into the detector through the first stages of the instrument a little too fast. “Too fast” means fast enough that when they collide with molecules of air, they strike with enough energy to break a bond, and smaller daughter ions are produced.

 

Source ionization is a messy thing, and it can be hard to work out which parent gave rise to which daughter. Ion traps (and triple quadrupoles) provide a much neater solution.

In the ion trap, a particular parent ion can be trapped, and then fragmented after first expelling all the other, unwanted ions. There are therefore three stages:

  • Expulsion of unwanted ions
  • Fragmentation
  • Scanning out of fragments

 

The scanning of fragments can be achieved in exactly the same way as the scanning of a full trap in normal full MS mode (see how an ion trap works).

Expulsion of unwanted ions is achieved by resonance. All the ions are going round in little orbits. The rates (revolutions per minute!) that the ions go round vary with mass, and are all slower than the normal AC frequency applied to the trap. The frequencies of the ions’ orbits are called their secular frequencies, and large ions go round slowest.

By applying a mixed wave-form to the end-caps, containing all frequencies up to the secular frequency of the ion you want to trap, and all frequencies above it, but NOT the actual frequency of the ion you want, all the others can be pushed out.

Having trapped the ion that is interesting, and got rid of the rest, it must be fragmented. This can be done by applying just a little of its secular frequency – enough to make it orbit more violently at higher speed, but not enough that it actually leaves the trap. In this new orbit it will strike the Helium dampening gas at increased energy, enough to break bonds.

Note that as soon as the ion has broken, its daughter fragments have lower masses. If you are unlucky, they might be so low that they cease to be stable in the trap, and are lost. More often they remain in the trap, but are no longer excited by the frequency across the end-caps. Therefore ion-traps tend to make few fragments compared to other collision induced systems (e.g. triple quad). But this doesn’t matter: in an ion trap you can always collect the daughter, and carry out another round of fragmentation…

 

Source: copy paste from : https://www.jic.ac.uk/services/metabolomics/topics/lcms/cid1.htm

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