sample is mixed with a solvent containing a low relative molecular mass compound to act as a matrix
and which strongly absorbs radiation at the emission wavelength of a suitable laser. Nicotinic acid is
commonly used for this purpose. After evaporation of the solvent to leave a solid dispersion of the
sample molecules in the matrix, the laser beam is focused onto the surface. The matrix absorbs large
amounts of energy from the beam by electronic excitation which results in the desorption and ionization
of sample molecules accompanied by proton transfer from the matrix to yield (M+H)+ sample ions. The
technique is extremely sensitive, and the relative molecular masses of proteins and peptides up to at
least 300 000 can be determined on as little as a few picomoles (10–^12 mole) or even femtomoles (10–^15
mole).
Spark source mass spectrometry is used for the examination of non-volatile inorganic samples and
residues to determine elemental composition. An RF spark of about 30 kV is passed between two
electrodes, one of which may be the sample itself, causing vaporization and ionization. Powdered
samples or residues from ashed organic materials can be formed into an electrode after mixing with
pure graphite powder.
Analyser or Separator
After ionization and fragmentation of the sample, positive (or sometimes negative) fragment ions are
accelerated into the analyser chamber by means of a potential gradient and slit system. The fragment
ions are then separated in space by allowing them to move through a magnetic and/or an electrostatic
field or by measuring the time taken for them to drift through space to a detector (time-of-flight
spectrometer). Single magnetic analysers are the simplest and a schematic diagram of this type of
instrument is shown in Figure 9.51(a). Double-focusing instruments use an electrostatic followed by a
magnetic analyser to improve the resolution. Some mass spectrometers employ a quadrupole analyser
which consists of a set of four metal rods placed symmetrically around and parallel to the direction of
travel of the fragment ions (Figure 9.51(b)). Application of a DC potential and an oscillating RF field
across the rods causes all ions except those with a particular mass to charge ratio (m/z) to follow an
unstable path leading to collisions with the rods. Ions with the appropriate m/z value follow a stable
path to the detector. By progressive alteration of dc potential and RF field, ions with different m/z
values can be made to follow a stable path to the detector which enables a mass spectrum to be
'scanned'.
Fast scanning quadrupole analysers are commonly used in HPLC-MS systems. In time-of-flight (TOF)
analysers, groups of ions, produced by a series of rapidly delivered short duration voltage pulses, are
accelerated into a straight tube that provides a field-free zone through which they drift to a detector. The
rate of drift of an ion is determined by its momentum (kinetic energy) therefore the heaviest ions reach
the detector first and the lightest last, their time-of-flight being measured with great accuracy.