Page 37 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OTHER TECHNIQUES 1.8

1.8 OTHER TECHNIQUES
In addition to the main general methods of analysis outlined above there are
also certain specialised techniques which are applied in special circumstances.
Among these are X-ray methods, methods based upon the measurement of
radioactivity, mass spectrometry, the so-called kinetic methods, and thermal
methods.
X-ray methods. When high-speed electrons collide with a solid target (which
can be the material under investigation), X-rays are produced. These are often
referred to as primary X-rays, and arise because the electron beam may displace
an electron from the inner electron shells of an atom in the target, and the
electron lost is then replaced by one from an outer shell; in this process energy
is emitted as X-rays. In the resultant X-ray emission it is possible to identify
certain emission peaks which are characteristic of elements contained in the
target. The wavelengths of the peaks can be related to the atomic number of
the elements producing them, and thus provide a means of identifying elements
present in the target sample. Further, under controlled conditions, the intensity
of the peaks can be used to determine the amounts of the various elements
present. This is the basis of electron probe microanalysis, in which a small target
area of the sample is pinpointed for examination. This has important applications
in metallurgical research, in the examination of geological samples, and in
determining whether biological materials contain metallic elements.
When a beam of primary X-rays of short wavelength strikes a solid target,
by a similar mechanism to that described above, the target material will emit
X-rays at wavelengths characteristic of the atoms involved: the resultant
emission is termed secondary or fluorescence radiation. The sample area can
be large, and quantitative results obtained by examining the peak heights of
the fluorescence radiation can be taken as indicative of sample composition.
X-ray fluorescence analysis is a rapid process which finds application in
metallurgical laboratories, in the processing of metallic ores, and in the cement
industry.
Crystalline material will diffract a beam of X-rays, and X-ray powder
diffractometry can be used to identify components of mixtures. These X-ray
procedures are examples of nondestructive methods of analysis.
Radioactivity. Methods based on the measurement of radioactivity belong to
the realm of radiochemistry and may involve measurement of the intensity of
the radiation from a naturally radioactive material; measurement of induced
radioactivity arising from exposure of the sample under investigation to a
neutron source (activation analysis); or the application of what is known as the
isotope dilution technique.
Typical applications of such methods are the determination of trace elements
in (a) the investigation of pollution problems; (b) the examination of geological
specimens; (c) quality control in the manufacture of semiconductors.
Mass spectrometry. In this technique, the material under examination is
vaporised under a high vacuum and the vapour is bombarded by a high-energy
electron beam. Many of the vapour molecules undergo fragmentation and
produce ions of varying size. These ions can be distinguished by accelerating
them in an electric field, and then deflecting them in a magnetic field where they
follow paths dictated by their mass/charge ratio (mle) to detection and recording