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There are several approaches to measuring the mass of an ion; this page is a very brief summary of the major systems available. More details are available on single quadrupole machines and ion traps because in small-molecule work they and time-of-flight machines are currently dominant. All the machines work by pushing the ions around in some sort of field, magnetic or electrical. Thus they don't actually measure mass, they measure mass per unit charge. Very occasionally, for this reason, you may meet the unit, the Thompson, which is one Dalton per electron's worth of charge. Daltons and atomic mass units (amu) are interchangeable.

This is the sort of mass spectrometer that the well-educated non-spectrometrist is likely to know about. The ions are deflected by a large magnet, and usually also by a large electrical field. The angle through which they are deflected depends on the size of the field, and the mass to charge ratio of the ion. Thus mass can be measured for a known field strength. The machines are utterly huge, and quite sensitive to people wandering past them. They are also expensive, and because of their size, need a very good vacuum system (see creating a vacuum for why this is so). Therefore they are not an efficient method for routine measurement and identification of small molecules.

The crèmè-de-la-crèmè of mass spectrometers, and priced accordingly. I recommend ionspec's tutorial site for details on how they work. Their main advantage is incredibly accurate mass measurements, better even than time-of-flight instruments. This allows reliable calculation of empirical formula.

A quadrupole is a simple mass filter, and its development has been revolutionary in GC- and LC-MS. It is available in various flavours. The basic single quadrupole machine, such as our HP/Agilent instrument, measures mass. The accuracy is about ±0.1amu. The triple quadrupole can do all a single quadrupole can, and also carry out more complicated experiments. The first quadrupole is used to select an ion you are interested in. The middle quadrupole doesn't function as a mass-filter. It is a collision cell, where the ions are collided with an inert gas, to make them fragment. The third quadrupole can then be used to look for the fragments. This is the basis of neutral loss experiments, for example.

These are closely related to single quadrupoles. While a quadrupole is a linear filter, an ion trap is a circular filter, where the ions can be kept going round in circles. Once an appropriate mass has been selected (by throwing out all the other masses), the filter can be adjusted to hold a wide range of masses in the circular pattern. Then the ions can be allowed to collide with an inert gas, and the fragment ions can be analysed. Of course it is quite possible to select one of these daughter ions, readjust the ion trap again to hold a wide range of grand-daughters, and repeat the fragmentation. Thus ion traps, in contrast to triple-quadrupoles, can carry out "MSⁿ, i.e. multiple levels of fragmentation. This is the ideal instrument for identifying an unknown chemical from a finger-print of fragments.

These include anything with "tof" in the title. They are the simplest to explain. All the ions are given a "push" into a tube. The heaviest ones move off most slowly, the lightest fastest. It's a bit like imagining a wide range of different sizes of people on roller skates. If you prod each equally, a small child will move off at quite a rate, an overweight business man will roll away much more sedately.
The instrument records how long it takes the ions to go down the tube, bounce and come back, and calculates their mass from that. This method is very accurate, and allows at least some calculation of empirical formula. The Q-tof is a popular instrument for high-quality small molecuel work. These are the equivalent of a triple-quadrupole instrument (see above), but the third quadrupole is replaced by a time-of-flight analyser to yield accurate mass.

Mass detectors are so useful and combine so well with hplc it seems inevitable that they will become more and more standard. One of the limiting factors is undoubtedly all the vacuum equipment. Miniaturisation would be doubly beneficial in LC-MS, because not only is a small detector cheaper and more convenient in the lab, but it would run adequately on a normal vacuum pump. A quadrupole only 30mm long will work quite well at pressures a up to a thousand times higher than are normally used (Taylor et al., 2000).

Taylor S., Srigengan B., Gibson J.R., Tindall D., Tate T., and Ahmad M. (2000) Proc. SPIE 4036 187-193

John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK
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