Advanced instrumental analysis

Recent developments in electronics and materials science have brought many changes in instrumental analysis. A variety of advanced analytical instruments and procedures are in use today.

Mass spectrometry

Chemical compounds that are bombarded with streams of electrons or beams of laser light undergo certain changes. Some molecules lose electrons and form cations (positively charged ions); others are broken down into smaller fragments that also carry positive charges. The overall result is that each molecular species gives rise to a series of cations characteristic of the original compound and useful for its identification.

Mass spectrometry is the name given to the study of these ions. An instrument called a mass spectrometer separates them from each other and measures them. One type of mass spectrometer generates a powerful magnetic field into which the fragments of the bombarded molecules are directed. Within the field, the different ions follow individual paths related to their mass. By varying the strength of the field, the ions are brought to focus separately on a machine called an electron multiplier detector. The mass spectrum of a compound consists of a series of peaks that differ in height. These differences depend on the relative numbers of ions of different mass values. On each spectrum, the highest peak often corresponds to the mass of the positive ion of the compound itself. The rest of the spectrum has peaks obtained from progressively smaller and smaller fragments.

Compounds can be identified from their mass spectra by the shape and distribution of the various peaks. The fragmentation of similar molecules follows a pattern. Some chemical groupings are even more readily detached than others. The loss of such groupings in the molecules can also be an aid to identification.

A spectrophotometer top measures concentrations by comparing the optical density of a liquid sample with that of a solution of known concentration. More recent computerized machines can display information as “three-dimensional” colored graphics.

Surface analysis

A variety of instrumental procedures are used for investigating the surfaces of materials or thin films deposited on solid surfaces. They lend themselves to detailed study of catalysts, oxidation of surfaces, and rates of corrosion. All these methods depend on the impact of various beams of energy on the surfaces. The particles—or energy released by the beam-are also measured. The activating beam may consist of electromagnetic radiation, ions, or electrons, depending on the depth of surface to be examined. Low-energy electrons, for example, penetrate only very short distances, whereas X rays are used for greater depth studies. Such techniques have made it possible to examine surfaces to a depth of less than 1 micron (0.001 mm). These methods of analysis reveal the ways in which the surface layer of a material differs from the material underneath, as well as irregularities in distribution of metals in alloys and sprayed coatings.

In a focusing mass spectrograph, a beam of positively charged ions (the ion beam) is deflected by curved electrically charged plates. It is then further deflected and focused by a magnetic field. Ions of different masses come to a focus in slightly different places. They can be recorded on a photographic plate. Here, they are collected as an electric current that is then amplified and plotted by a chart recorder or computer screen. The mass spectrogram produced takes the form of a bar chart (as shown below). It relates the intensity—that is, abundance—of particular ions to their mass-to-charge ratio (m/e).

Radiochemical analysis

Radioisotopes are very useful in analyzing small concentrations of materials. They undergo the same chemical processes as the stable isotopes of their elements and can be followed by tracing their radioactive emissions. Often, only very small amounts of the radioisotopes are needed. They can be incorporated into chemical compounds to produce “labeled” (compounds containing radioisotopes) substances. These can be used and followed through various reactions. The radioactivity at any step in a process can be measured to establish how efficient that step has been by the proportion of radioactivity carried through to the next step.

Isotope dilution analysis is a special application of radioactivity. A known weight of a ra-dioactively labeled compound is added to a mixture containing an unknown amount of the same—but unlabeled—compound. After complete mixing, a small amount of the mixture is removed and treated selectively for the particular chemical compound. By measuring the degree of dilution of the radioactive isotope, the amount of nonradioactive compound can be calculated.

In activation analysis, a nonradioactive sample is bombarded with high-energy neutrons, thereby converting some of the inactive atoms into radioisotopes. By measuring the amount of resultant radiation, the amount of the element originally present can be calculated.

The mass spectrogram of bromomethane (CH3Br) reveals that the bromine in the compound exists as a nearly equal mixture of two isotopes of atomic masses 79 and 81.