The days are long past when
one person could hope to have a detailed knowledge of all areas of chemistry.
Those pursuing their interests into specific areas of chemistry communicate
with others who share the same interests. Over time a group of chemists with
specialized research interests become the founding members of an area of
specialization. The areas of specialization that emerged early in the history
of chemistry, such as organic, inorganic, physical, analytical, and industrial
chemistry, along with biochemistry, remain of greatest general interest. There
has been, however, much growth in the areas of polymer, environmental, and
medicinal chemistry during the 20th century. Moreover, new specialities
continue to appear, as, for example, pesticide, forensic, and computer
chemistry.
Analytical chemistry
Most of the materials that
occur on Earth, such as wood, coal, minerals, or air, are mixtures of many
different and distinct chemical substances. Each pure chemical substance (e.g.,
oxygen, iron, or water) has a characteristic set of properties that gives it
its chemical identity. Iron, for example, is a common silver-white metal that
melts at 1,535° C, is very malleable, and readily combines with oxygen to form
the common substances hematite and magnetite. The detection of iron in a
mixture of metals, or in a compound such as magnetite, is a branch of
analytical chemistry called qualitative analysis. Measurement of the actual
amount of a certain substance in a compound or mixture is termed quantitative
analysis. Quantitative analytic measurement has determined, for instance, that
iron makes up 72.3 percent, by mass, of magnetite, the mineral commonly seen as
black sand along beaches and stream banks. Over the years, chemists have
discovered chemical reactions that indicate the presence of such elemental
substances by the production of easily visible and identifiable products. Iron
can be detected by chemical means if it is present in a sample to an amount of
1 part per million or greater. Some very simple qualitative tests reveal the
presence of specific chemical elements in even smaller amounts. The yellow
colour imparted to a flame by sodium is visible if the sample being ignited has
as little as one-billionth of a gram of sodium. Such analytic tests have
allowed chemists to identify the types and amounts of impurities in various
substances and to determine the properties of very pure materials. Substances
used in common laboratory experiments generally have impurity levels of less
than 0.1 percent. For special applications, one can purchase chemicals that
have impurities totaling less than 0.001 percent. The identification of pure
substances and the analysis of chemical mixtures enable all other chemical
disciplines to flourish.
The importance of analytical
chemistry has never been greater than it is today. The demand in modern
societies for a variety of safe foods, affordable consumer goods, abundant
energy, and labour-saving technologies places a great burden on the environment.
All chemical manufacturing produces waste products in addition to the desired
substances, and waste disposal has not always been carried out carefully.
Disruption of the environment has occurred since the dawn of civilization, and
pollution problems have increased with the growth of global population. The
techniques of analytical chemistry are relied on heavily to maintain a benign
environment. The undesirable substances in water, air, soil, and food must be
identified, their point of origin fixed, and safe, economical methods for their
removal or neutralization developed. Once the amount of a pollutant deemed to
be hazardous has been assessed, it becomes important to detect harmful
substances at concentrations well below the danger level. Analytical chemists
seek to develop increasingly accurate and sensitive techniques and instruments.
Sophisticated analytic
instruments, often coupled with computers, have improved the accuracy with
which chemists can identify substances and have lowered detection limits. An
analytic technique in general use is gas chromatography, which separates the
different components of a gaseous mixture by passing the mixture through a
long, narrow column of absorbent but porous material. The different gases
interact differently with this absorbent material and pass through the column
at different rates. As the separate gases flow out of the column, they can be
passed into another analytic instrument called a mass spectrometer, which
separates substances according to the mass of their constituent ions. A
combined gas chromatograph–mass spectrometer can rapidly identify the
individual components of a chemical mixture whose concentrations may be no
greater than a few parts per billion. Similar or even greater sensitivities can
be obtained under favourable conditions using techniques such as atomic
absorption, polarography, and neutron activation. The rate of instrumental
innovation is such that analytic instruments often become obsolete within 10
years of their introduction. Newer instruments are more accurate and faster and
are employed widely in the areas of environmental and medicinal chemistry.
