Page 28 - Radiochemistry and nuclear chemistry

Nuclei, Isotopes and Isotope Separation 17

the masses m I and m 2 alternately are registered by the detector, producing ion currents I 1
and I 2, respectively, the abundance of each isotope can be calculated from the ratios x I =
11 I(I 1 + 12) and x 2 = 1 - x 1 when only two isotopes are present. The resolution of modem
mass spectrometers can be extremely high, as indicated by the values in Table 2.1.

2.3.2. Applications

For several decades, mass spectrometers were used primarily to determine atomic masses
and isotopic ratios. Now they are applied to a large variety of chemical problems and low
resolution mass spectrometers are used for routine chemical analysis. For example, a
modern mass spectrometer can easily distinguish between species such as 12CH4+ and
160+, having a mass difference of 0.03686 u.
Some uses of mass spectrometry of interest to chemists involved in nuclear science are:
(a) Molecular weight determination can be made by mass spectrometry if gaseous ions can
be produced with MIq values not exceeding about 400. This method is of great importance
in radiation chemistry (Ch. 7) where often a large number of products are produced which
may be quite difficult to identify by other means and in particular for the analysis of
organic compounds.
(b) The study of chemical reactions directly in the gas phase by mass spectrometry is
possible. Using an ion source in which molecules are bombarded by a stream of low energy
(_< 100 eV) electrons, ionization and dissociation reactions can be studied, e.g.

+
C8H18 + e- ~C8H18 + 2e-
l
C4H9 + + C3H6 + + CH 3" + e-
This technique has practical application e.g. in the petroleum industry for determining the
composition of distillation and cracking products.
(c) Isotopic dilution is a technique for determination of the number of atoms of an element
(or isotope) in a composite sample (e.g. rock or biota), from which is difficult to recover
the element reproducibly and determine it quantitatively. The technique is simple to use for
radioactive nuclides, as described in w but more complicated and time consuming for
stable nuclides. However, the high precision of mass spectrometry makes the latter
alternative more accurate, and the technique is widely used for geologic dating.
1) Suppose we have a sample with unknown concentration of a certain element,
consisting of the two isotopes 1 and 2, and want to determine the number of
atoms, N 2, of isotope 2 in the sample (which, of course must be of known weight).
The number of atoms of each isotope is N 1 and N 2, thus N = N l + N 2, in unit
sample weight. By mass spectrometry we determine the isotope ratio ~'N = NI/N2"
2) We have a reference with the same isotopes, in which the isotopic ratio ~'p - P1
/P x is known; P1 is the number of isotope 1, and t'2 of isotope 2 per unit weight.
P-PI +P2.
3) The amount P of the reference (a "spike') is mixed with a known weight of the
sample containing the unknown isotope amount N 2. The mixing must be isotopic,
i.e. a complete exchange must take place between the isotopes; this is not a