Page 29 - Radiochemistry and nuclear chemistry

18 Radiochemistry and Nuclear Chemistry

difficult problem when well planned. A sample is then withdrawn from the
mixture, containing M 1 atoms of isotope 1 and M 2 atoms of isotope 2. The sample
is measured by the mass spectrometer, yielding ~M = MI/M2"
Since M 1 = N 1 + Pl and M 2 = N 2 + P2 simple arithmetic gives

NE = P(rP - ~'M)/[(~P + I)(~'M -rN)] (2.12)
Thus from 3 mass spectrometrically determined isotope ratios (~'N, ~'P and ~'M) and
known standard amount P = PI + P2, the unknown number of atoms N 2 in the sample is
determined. The precision of this technique is largest when sample and standard are added
in such proportions that N ~ P. Examples of the use of this technique are given in w on
geologic dating.
Because the variables in these equations often need multiple indexing (e.g. element,
isotope and source) the amount of indexing can be reduced by using italic element symbols
to refer to the specific radioactivity, or concentration, of that nuclide in the sample; e.g.
238U means 8238o (radioactivity), or N238u (atoms) per unit volume or weight of the
sample. Extensive use of this formalism will be found in w 1.
(d) Analysis of gas purity (e.g. in plants for separation of 235U and 238U) is done
conveniently by mass spectrometry. Not only are the ratios of the uranium isotopes
determined, but also the air and water vapor which leaks into the system can be measured.
This produces such ions as O2 +, N2 +, CO2 +, and HF +, which can be measured easily
because mass spectrometry detects the presence of impurities in parts per million (ppm)
concentration.
(e) Instrumental chemical analysis can be done by using mass spectrometers having ion
sources of the inductively coupled plasma type (ICP-MS systems), sometimes as on-line
detectors for various chromatographic techniques. Due to the high resolution and sensitivity
of this technique it is widely used in analysis of pollutants in the environment.
(f) Measurement of products from nuclear reactions can be made with special mass
spectrometers on line to the irradiated target. With suitable arrangements both A and Z of
the re, coiling reaction products can be determined.

2.4. Isotopic ratios in nature

Mass spectrometric investigations of geologic material has shown that isotopic ratios vary
slightly in nature with the largest variations observed for the heaviest and lightest elements.
For the heaviest dements the reason for the variation in the ratio of isotopes in an dement
can be related directly to the consequence of naturally occurring radioactive decay. For
example, in a thorium mineral the decay sequence terminates in the formation of an isotope
of lead with mass number 208. By contrast, in a uranium ore, in which 238U is the most
abundant isotope, the primary end product of the decay is an isotope of lead with mass
number 206 (see Fig. 1.2). This means that thorium minerals and uranium minerals contain
lead with different isotopic composition. In fact, one of the best conformations of the

1 The Student making notes may have difficulties in distinguishing between italics and normal letters and is
therefore recommended to adhere to the use of multiple indexing.