主题：原子发射光谱简史

Flames and electrical discharges have been an important part of chemical analysis for a long time. In 1752, 26-year old Thomas Melville of Glasgow wrote of his observations of a bright yellow light emitted from a flame produced by burning a mixture of alcohol and sea salt. When the alcohol contained no salt, the yellow color disappeared. It has been said that if Melville had not died a year later, spectrochemical analysis might have gotten a much earlier start.One of the first uses of sparks for chemical analysis was reported in 1776 by Alessandro Volta. Volta had discovered a way to produce a static electric charge strong enough to create sparks. He was fascinated by the different colors of sparks that he could obtain by sparking different materials. Eventually he was able to identify certain gases by the colors emitted when he applied a spark to them.During the late 18th and early 19th centuries Fraunhofer and others looked at spectra emitted by flames and sparks, often comparing them to spectra emitted from the sun and planets. In 1826, W. H. Talbot reported a series of experiments in which he observed the coloring of flames by a variety of salts. Unfortunately, the utility of
his work was not recognized for several decades.
It was not until 1859, when Kirchhoff and Bunsen surmised that that the sharp line
spectra from flames were produced by atoms and not molecules, that the nature of
emission spectra was beginning to be understood. Much of their work was made
possible by Bunsen’s development of a burner which produced a nearly transparent,
nonluminescent flame. This is the same burner that practically every chemist since
Bunsen has used at one time or another. Credited with the discovery of spectrochemical
analysis, Kirchhoff and Bunsen developed methods based on emission
spectroscopy that led to the discovery of four elements, Cs, Rb, Tl, and In, between
1860 and 1864.
In the beginning of the 20th century, the sharp lines that appeared in the light emitted
from electrical arcs and sparks were a driving force for science. These atomic lines
were used to define the discrete energy levels that exist in atoms and were thus one
major test of theories developed with quantum mechanics. The lines were also used
analytically for qualitative analysis. Indeed, the appearance of sharp spectral lines
that had not been previously observed was the proof that most scientists required
for the verification of the discovery of a new element.
During the middle of the 20th century, quantitative arc and spark spectroscopy was
the best tool that analysts had to probe trace concentrations for a wide range of
elements. The sample preparation techniques used were, for many samples, difficult
and/or time-consuming. Conductive solids were relatively easy to handle; they were
simply machined into electrodes that could be used to support the electrical
discharge. Liquid samples, however, had to be either dried or plated onto electrodes
by various means. Nonconductive solid samples were mixed with a conductive
matrix, usually graphite, and pressed into the end of a graphite electrode. In addition
to difficulty in handling all but the conductive solid samples, the quality of the data
obtained was not very good. Precision of analysis of 5 to 10% RSD was typical for
a very good and careful analyst. Standards and samples had to be made very similar
since effects arising from the composition of the sample matrix were often large and
difficult to predict. This type of analysis is still used today in foundries where the
samples can be made easily into electrodes, the range of sample concentrations is
limited, and a library of matched standard materials already exists.