Chemical analysis
has grown rapidly

Ancient philosophers thought everything was made of four basic elements: earth, water, air and fire. Medieval alchemists thought that sulfur, mercury and salt were the fundamental substances.

Chemical analysis is the study of the chemical composition and structure of substances, encompassing all techniques that obtain any exact chemical information using the properties of atoms.

Three decades ago, detecting levels of one atom per million was the cutting edge of chemical analysis. Technology has grown rapidly, and today fractional part per billion analysis is routine.

There is no single test that will detect all of the chemical elements in a substance, and no test that will determine both composition and structure simultaneously. Despite our sophisticated techniques, we have not achieved the one-button, total analysis of "Star Trek's" Tricorder.

Classical methods of chemical analysis involved chemical reactions that are characteristic of a known substance. Others used reactions with various chemical reagents in algorithmic sequences to isolate atoms belonging to certain chemical families. To further isolate elements within a family required more refined reactions.

Chemical methods are limited by the precision of weighing scales and the purity of reagents. Compositions were expressed as percentages, in fractions of parts per hundred.

The development of electric power and the advances in electronic instrumentation in the 20th century allowed the development of new methods, using precise high-tech equipment that relies on the rather esoteric properties of atoms.

Flame spectroscopy is a sensitive, accurate and relatively quick method of analysis. A few milligrams of a sample is sufficient to detect levels with parts per million precision.

It relies upon the quantum properties of atomic electrons, which give each substance a unique and characteristic color signature in its emission spectrum.

When energized, electrons jump to higher levels, then emit light when they fall back to the "ground" level. The wavelengths of the emitted light form a unique spectrum of colors and intensities.

A solution containing a sample is injected into a flame, which energizes the atoms. The light from the flame is dispersed into a spectrum and measured by a sensitive spectrometer. The intensity peaks of the spectrum reveal the composition of the sample.

This is a variation of the method astronomers use to determine the composition of the sun and distant stars.

Mass spectroscopy requires stripping electrons away from the atoms or molecules of the sample, then beaming the resulting positive ions through a vacuum tube similar to the cathode ray tube used in CRT monitors and television screens.

A magnetic field deflects the beam of ions according to their weight.

Lightweight ions deflect more than heavier ones, separating the beam into distinct groups by molecular or atomic weight.

A disadvantage of mass spectroscopy is that different substances can have the same molecular weight. For example, a mass spectroscope cannot distinguish between acetone and carbon dioxide.

X-ray spectroscopy bombards a sample with a stream of electrons, causing it to emit X-radiation. There is a specific relationship between the atomic number of the element and the wavelength of the X-rays it emits, so each chemical element emits radiation that is uniquely characteristic.

We can tune in on a certain wavelength of X-rays to find a particular element by placing a crystal at various angles to reflect a pattern that depends on the angle and the wavelength of the X-rays, just like tuning a radio to find a particular station.

X-ray diffraction is used to identify crystalline substances that consist of atoms or molecules arranged in a repeating three-dimensional pattern.

When X-rays reflect from the atoms within a crystal, they produce an interference pattern that is unique for each crystal structure.

X-rays have wave properties and so they superimpose, their intensities adding or subtracting depending on whether waves arrive in phase or out of phase at different locations on the detector.

The resulting diffraction reveals the structure of the crystal and, along with an elemental analysis, identifies the substance.

X-ray diffraction has been essential in determining the structure of many organic substances, the most famous of which is the double helix of DNA.

Neutron activation analysis (NAA) is a highly sensitive and accurate technique for analysis, to the order of parts per billion or better.

Because of its accuracy and reliability, NAA is generally recognized as the reference method of choice when new procedures are being developed or when other methods yield results that do not agree.

NAA bombards atoms of a substance with neutrons, transmuting the nuclei of some atoms, changing them into radioactive isotopes of neighboring elements in the periodic table. The radioactivity emitted then identifies the type and number of atoms that were changed by the activation.

Radioactive elements have distinct and unique radioactive energy and particle signatures, even radioactive isotopes of the same element.

Chromatographic methods pass a sample mixed with liquid or gas through a solid or liquid adsorbent that attracts substances to its surface.

Different substances are adsorbed at different rates, so the substances separate from one another as the mixture moves through the material.

Electroanalysis measures the response of different substances in a sample to electric fields, and is mostly useful for identifying components of organic chemicals.

Chemical analysis has added immensely to our knowledge about virtually every aspect of the natural world, here on earth and throughout the visible universe.

It provides important information for science, technology, medicine and industry, analyzing samples of air, soil and water to identify pollutants; analyzing biological fluids and tissue; analyzing raw materials and products in the food, drug and other industries for quality control; aiding in the synthesis of chemicals for medical, dietary, industrial, chemical, electronic and consumer use.

Yet it is only one branch of the amazing science that we call chemistry.