Mass spectrometry is the technique of
separating and counting the constituent atoms of a
sample according to their mass. Accelerator Mass Spectrometry (AMS)
requires that a charged particle
accelerator be added to the equipment conventionally
used in mass spectrometry, increasing the sensitivity of the
system for detecting rare atoms or isotopes.
The most common type of sample analysed by AMS is radiocarbon or
14C. The following paragraphs use the analysis
of 14C as an example to
introduce the concepts involved in AMS.
A New Approach to Carbon Dating
In 1977, a major breakthrough took place in the technique of radiocarbon dating.
It was discovered that individual carbon-14 atoms from a natural sample
could be counted directly, a process which takes considerably less time
and sample material than the conventional method of counting the infrequent
carbon-14 beta-decays. The equipment used to make this discovery was
similar to that used in mass spectrometry but with the addition of
higher voltage acceleration and charged particle counting apparatus used by
nuclear structure physicists.
The Advantages of Higher Energy
The carbon-14 isotope is such a small fraction of modern carbon
(1.2 x 10-12) that it lies well below the
detection limit for conventional mass spectrometry.
This limit is set by the presence of interfering atomic and
molecular species of the same mass as 14C,
e.g. 14N,
12CH2
and 13CH.
14N, the only atomic interference,
can be eliminated by choosing negative ions to analyse,
because these are unstable and do not live long
enough to enter the accelerator.
The molecular interferences are eliminated by removing a
sufficient number of electrons to break apart the molecules,
a process which requires acceleration of the ions to higher
energies than in conventional mass spectrometry.
The increased energy acquired by the ions has the additional
advantage that the high velocity 14C
ions can be individually identified and distinguished from other
background ions.
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Accelerator mass
spectrometry (AMS)
