You are here - Home ::
How a mass detector sorts out masses
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.
Magnetic sector systems
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.
Ion cyclotrons
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.
Quadrupole Instruments
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.
Ion trap systems
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.
Time-of-flight instruments
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.
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.
The future - miniaturisation
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).
Reference
Taylor S., Srigengan B., Gibson J.R., Tindall D., Tate T., and Ahmad M. (2000) Proc. SPIE
4036 187-193