Sample Inlet System
Volatile or volatilizable compounds may be introduced into the spectrometer via a pinhole aperture or
molecular leak which allows a steady stream of sample molecules into the ionization area. Non-volatile
or thermally labile samples are introduced directly by means of an electrically heated probe inserted
through a vacuum lock. Numerous methods of sample ionization are available of which the most
important are electron impact (EI), chemical ionization (CI), field ionization (FI), field desorption (FD),
fast atom bombardment (FAB), and radio-frequency spark discharge.
Electron impact ionization (EI) involves the bombardment of sample molecules with a high-energy
electron beam (~70eV) which results in a considerable degree of fragmentation into positive ions of
varying mass. Chemical ionization (CI) is achieved by first introducing a reagent gas such as methane
or isobutane into the electron beam where it is ionized to form several species including.
On subsequent introduction of the sample at a level of about 10^3 times less than that of the reagent gas,
these species react with sample molecules to yield (M + 1)+ and (M – 1)+ ions by hydrogen addition or
abstraction, where M represents the mass of the sample molecule. There is much less fragmentation into
ions of smaller mass. Field ionization (FI) employs a pointed or knife-edged anode and a slit cathode
between which a very large potential difference is applied (5–20 kV). Sample molecules entering this
high potential gradient or electrical force field are ionized to form M+ and (M + 1)+ ions with very little
accompanying fragmentation. Both CI and FI are much gentler methods of ionization than EI and are
particularly useful where the relative molecular mass of a compound is sought. The more complex
fragmentation patterns produced by EI are useful for the identification of structural features.
Field desorption (FD) is similar in principle to FI. It enables ions to be produced directly from solid
samples which are deposited from solution onto an anode fitted to a probe that can be inserted into the
instrument via a vacuum lock. It is even more gentle than CI and FI, producing molecular ions and
virtually no fragmentation. However, the ionization process decays very rapidly so spectra must be
scanned quickly and cannot be re-recorded without introducing more sample.
Fast atom bombardment (FAB) involves the production of a high-speed stream of energetic neutral
argon atoms (~5 KeV) which are directed onto the sample that is introduced into the spectrometer on
the end of a probe. The samples are first dissolved in a relatively non-volatile matrix, such as glycerol,
monothioglycerol or carbowax, which aids the ionization process yielding (M + H)+ and (M – H)+
sample ions together with some fragmentation. FAB is proving particularly useful for non-volatile
compounds, including those of a biochemical nature such as peptides, nucleotides and their salts, which
hitherto have yielded mass spectra only with difficulty.
Matrix assisted laser desorption ionization (MALDI) enables molecular ions to be obtained from large
molecules such as peptides and proteins. The