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原子发射光谱简史

renzhihai

2005/03/30

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.

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第1楼2005/03/30

While arc/spark emission techniques enjoyed widespread popularity for the determinationof metals, flame emission spectrometry, also known as flame photometry,was used extensively for determination of the alkalis and other easily excitedelements. A Swedishagronomist named Lundegårdh is credited with beginningthe modern era of
flame photometry in the late 1920’s. His apparatus for
elemental analyses of plants, shown in Figure 1-4,used pneumatic nebulizationand a premixed airacetylene flame and is remarkably
similar to equipment used today.While the atomic spectra emitted from flames had the advantage of being simpler than those emitted from arcs
and sparks, the main limitation of the technique was that the flames were not hot enough to cause emission for many elements. Despite
that limitation, several successful commercial instruments
were based on the technique, many of which are still in use today. The
most widespread use of the technique is in clinical labs for determining sodium and potassium levels in blood and other biological materials.In the 1960’s and 1970’s both flame and arc/spark optical emission spectrometry declined in popularity. Many of the analyses that had been performed using optical emission were increasingly performed using atomic absorption spectrophotometry (AAS). While advances in flame emission spectrometry allowed the determination
of about half of the elements in the periodic table, the technique could no longer
compete well with AAS. Since absorption of light by ground state atoms was used as the mode of detection, the need for very high temperatures to populate excited states of atoms was no longer a limitation. The instabilities and spectral interferences
which plagued arc/spark emission techniques were also greatly reduced by atomic absorption techniques.Spray chamber, nebulizer and burner
such as those used by Lundegårdh for flame emissionspectrography. (Used with permission from the
Division of Analytical Chemistry of the American
Chemical Society.)

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第2楼2005/03/30

At the time of its greatest popularity, flame atomic absorption was used primarily in
the analysis of solutions for trace metals. For solid samples, the technique requires
that samples be dissolved. With the exception of a few well-documented interferences,
samples and standards need not be made very similar. Flame atomic
absorption offers the analyst high precision (0.2 to 0.5% RSD) determinations and
moderate detection limits. Electrothermal atomization (graphite furnace) atomic
absorption spectrometry, on the other hand, offers high sensitivity and low detection
limits. Graphite furnace AAS (GFAAS) does provide poorer precision and a higher
level of matrix interferences than are experienced with the flame-based technique.
However, advances such as the use of stabilized temperature platform furnace
(STPF) technology and Zeeman background correction have reduced or eliminated
most of the interferences previously associated with GFAAS.
Both the flame and graphite furnace AAS techniques are used widely today and
both provide excellent means of trace elemental analysis. Most atomic absorption
instruments are limited, however, in that they typically measure only one element
at a time. The instrumental setup or operating conditions may require changing
hollow cathode lamps or using different furnace parameters for each element to be
determined. Because of the different operating conditions and furnace parameters
required for each element, conventional atomic absorption techniques do not lend
themselves readily to multi-element simultaneous analysis.
Also, despite advances in nonlinear calibration, the need for sample dilution is
greater than for present-day OES techniques, due to the limited working (calibration)
range for the AAS techniques. Consequently, devices for automatic sample dilution
when a sample concentration exceeds the calibration range are available. For those
samples that require element preconcentration for lower detection limits, flow-injection
techniques coupled with cold vapor mercury or hydride generation equipment
and GFAAS can not only provide significant improvements in detection limits, over
100 times better as compared to conventional hydride generation AA, but also may
reduce potential interferences by the complete removal of matrix components.
Stanley Greenfield of Birmingham, England is credited with the first published report
(1964) on the use of an atmospheric pressure inductively coupled plasma (ICP) for
elemental analysis via optical emission spectrometry (OES) [1]. The conclusions
from this landmark paper summarize what Greenfield identified as the advantages
of plasma emission sources over flames, ac sparks and dc arcs:
The plasma source has a high degree of stability, has the ability to
overcome depressive interference effects caused by formation of
stable compounds, is capable of exciting several elements that are
not excited in orthodox chemical flames, and gives increased sensitivity
of detection [over flame photometry].

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第3楼2005/03/30

The plasma source is far simpler to operate than the conventional
arc and spark methods, especially in solution and liquid analysis,
and gives the high degree of stability associated with the a.c. spark
combined with the sensitivity of the d.c. arc. Particular advantages
of the high-frequency plasma torch are the lack of electrodes, which
gives freedom from contamination, and the extremely low background
produced.
As with most new techniques, the original optical emission results using ICP sources
were not spectacular. The technique was better than flame atomic absorption for
only a few of the most refractory elements. Along with Greenfield, Velmer Fassel
and his colleagues at Iowa State University are generally credited with the early
refinements in the ICP that made it practical for analysis of nebulized solutions by
OES. The technique continued to be refined as sources of noise were tracked down
and eliminated, and gas flows, torch designs and plasma settings were optimized.
By 1973, the low detection limits, freedom from interferences and long linear working
ranges obtained with the ICP proved that it was clearly an emission source superior
to those used previously in analytical optical emission spectrometry. Since that time,
an ever-increasing number of academic, governmental and industrial researchers
have joined in the development of the ICP.
It has been mentioned that an affiliated technique to atomic spectroscopy for
elemental analysis is ICP-MS. Though ICP-MS is not the subject of this handbook
on the concepts, instrumentation and techniques in optical emission spectrometry,
ICP-MS has become an important tool for the analyst since its commercial introduction
in 1983. ICP-MS was pioneered by just a few prominent laboratories: Iowa State
University, Ames, Iowa; Sciex, a manufacturer’s laboratory in Toronto, Canada; and
several facilities in the U.K. including the University of Surrey, the British Geological
Survey and a U.K. manufacturer. The technique features high sensitivity and
excellent detection limits, equal to or better than GFAAS for most elements. The
mass spectra are considerably simpler than the atomic emission spectra from the
ICP but the mass spectra are complicated by mass interferences from molecular
ions originating in the ICP. However, ICP-MS allows for the routine use of isotopic
ratio and isotopic dilution measurements to assist in the solution of analytical
problems. Furthermore, qualitative analysis can be rapidly performed by ICP-MS
techniques.

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