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خاص.........معامل قسم-الثروة المعدنية
والصخور
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AN OVERVIEW OF ELEMENTAL
ANALYSIS VIA ATOMIC
SPECTROSCOPY TECHNIQUES |
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One of the simplest questions that an analyst can ask about the chemical
composition of a sample is "which elements are present and at what concentrations?"Since there are only 92 naturally occurring elements and millions of different molecules, differentiating among the elements is a much easier task than differentiating among the molecules.Nonetheless,the elemental composition of a sample is often an important part of the information needed to assess its properties. For example,consider a water sample which is determined to contain 88.2%oxygen and 11.0%hydrogen by mass,meaning that only 99.2%of the sample could be made up of water molecules.Whether the water from which this sample was taken is useful for a particular purpose may well depend on the remaining 0.8%.If this water sample contained as much as a microgram of boron per gram of sample (0.0001%of the mass),the water would be perfectly useful for most
purposes.If, however,you wanted to use that water in the fabrication of ceramic turbine blades for jet engines,purification would be required.When water containing as much as one part per million boron is used in the manufacture of the ceramics for these turbine blades,their failure rate rises dramatically.Research has shown that boron collects on the grain boundaries of the ceramic turbine blades,causing fractures that have been implicated in catastrophic failures of jet engines. There are many other examples of the need for determining the trace level concentrations of elements within samples.For example,the United States Envi-ronmental Protection Agency has strict rules concerning trace levels of dangerous metals allowed in wastewaters.Some of these limits are in the parts per billion range. Determination of elemental concentrations at these trace levels requires the use of sensitive scientific instrumentation.
The most commonly used techniques for the determination of trace concentrations
of elements in sample are based on atomic spectrometry.As the name atomic
spectrometry implies,these techniques involve electromagnetic radiation
(light)that
is absorbed by and/or emitted from atoms of a sample.(Not implicit in the term."atomic spectrometry",however,is that we generally include emission and absorp-tion
of electromagnetic radiation by charged atoms,or ions,also under the heading of atomic spectrometry.)By using atomic spectrometry techniques,meaningful quantitative and qualitative information about a sample can be obtained.In general, quantitative information
(concentration)is related to the amount of electromagnetic radiation that is emitted or absorbed while qualitative information (what elements are present)is related to the wavelengths at which the radiation is absorbed or emitted.
An affiliated technique to atomic emission or absorption spectrometry is atomic mass spectrometry.In mass spectrometry,instead of obtaining analytical informa-tion from the radiation of atoms or ions,ions introduced into a mass spectrometer are separated according to their mass to charge ratio and are either qualitatively or quantitatively detected.
Nature of Atomic or Ionic Spectra
The measurement of absorption and emission of electromagnetic radiation can be more easily described once the nature of atomic and ionic spectra is understood. Consider the Bohr model of an atom shown in Figure 1-1.The atom is depicted as a nucleus surrounded by electrons which travel around the nucleus in discrete مدارات.Every atom has a number of مدارات in which it is possible for electrons to travel.Each of these electron مدارات has an energy level associated with it.In general,the further away from the nucleus an orbital,the higher its energy level. When the electrons of an atom are in the مدارات closest to the nucleus and lowest in energy,the atom is in its most preferred and stable state,known as its ground state.When energy is added to the atom as the result of absorption of electromag-netic radiation or a collision with another particle
(electron,atom,ion,or molecule), one or more of several possible phenomena take place.The two most probable events are for the energy to be used to increase the kinetic energy of the atom (i.e., increase the velocity of the atom)or for the atom to absorb the energy and become excited.This latter process is known as excitation.
Figure 1-1.Bohr model of an atom.As energy is absorbed by an atom,an elec-tron jumps to an orbital with a higher energy level.The atom may decay to a lower energy state by emitting a
photon,hn.
When an atom becomes excited,an electron from that atom is promoted from its ground state orbital into an orbital further from the nucleus and with a higher energy level.Such an atom is said to be in an excited state.An atom is less stable in its excited state and will thus decay back to a less excited state by losing energy through a collision with another particle or by emission of a
"particle"of electromag-netic radiation,known as a photon.As a result of this energy loss,the electron returns to an orbital closer to the nucleus.
If the energy absorbed by an atom is high enough,an electron may be completely dissociated from the atom,leaving an ion with a net positive charge.The energy required for this process,known as ionization,is called the ionization potential and is different for each element.Ions also have ground and excited states through which they can absorb and emit energy by the same excitation and decay processes as an atom.
Figure 1-2 shows the excitation,ionization and emission processes schematically.
The horizontal lines of this simplified diagram represent the energy levels of an atom. The vertical arrows represent energy transitions,or changes in the amount of energy of an electron.The energy transitions in an atom or ion can be either radiational (involving absorption or emission of electromagnetic radiation)or thermal (involving energy transfer through collisions with other particles).
The difference in energy between the upper and lower energy levels of a radiative transition defines the wavelength of the radiation that is involved in that transition. Figure 1-2.Energy level diagram depicting energy transitions where a and b rep-resent excitation,c is ionization,d is
ionization/excitation,e is ion emission,and f,g and h are atom emission.
The relationship between this energy difference and wavelength can be derived
through Planck's equation
E =hn
where E is the energy difference between two levels,h is Planck's constant,and n
is the frequency of the radiation.Substituting c/l for n,where c is the speed of light
and l is wavelength,we get
E =hc/l
This equation shows that energy and wavelength are inversely related,i.e.,as the energy increases,the wavelength decreases,and vice versa.Using Figure 1-2 as an example,the wavelength for emission transition f is longer than the wavelength for emission transition g since the energy difference for f is less than for transition g.
Every element has its own characteristic set of energy levels and thus its own unique set of absorption and emission wavelengths.It is this property that makes atomic spectrometry useful for element-specific analytical techniques. The
ultraviolet(UV)/visible region (160 -800 nm)of the electromagnetic spectrum is the region most commonly used for analytical atomic spectrometry.This is also the region of the electromagnetic spectrum that we generally refer to as "light", although technically,all electromagnetic radiation can be considered as light.For further discussions in this book,the term
"light"will often be used in place of "UV/visible electromagnetic radiation".
The principal reasons for the popularity of analytical techniques that use the UV/visible region are that these techniques are accurate,precise,flexible,and relatively inexpensive compared to techniques which use other regions,such as gamma ray spectrometry and X-ray spectrometry.Many of the devices used in UV/visible atomic spectrometry,such as photomultipliers and gratings,are relatively inexpensive since they were developed for and are commonly used in high-volume applications.Also,unlike gamma rays and
X-rays,UV/visible radiation is not ionizing radiation.This lessens the expenses associated with shielding and licensing of the laboratory and with disposal of analyzed samples.
Analytical Techniques Based on Atomic Spectrometry
In the atomic spectrometry techniques most commonly used for trace element analysis,the sample is decomposed by intense heat into a cloud of hot gases containing free atoms and ions of the element of interest.Figure 1-3 shows the instrumental arrangements for four different techniques used to detect these atoms or ions.
In atomic absorption spectrometry (AAS),light of a wavelength characteristic of the element of interest is shone through this atomic vapor.Some of this light is then absorbed by the atoms of that element.The amount of light that is absorbed by these atoms is then measured and used to determine the concentration of that element in the sample.
In optical emission spectrometry (OES),the sample is subjected to temperatures high enough to cause not only dissociation into atoms but to cause significant amounts of collisional excitation (and ionization)of the sample atoms to take place. Once the atoms or ions are in their excited states,they can decay to lower states through thermal or radiative
(emission)energy transitions.In OES,the intensity of the light emitted at specific wavelengths is measured and used to determine the concentrations of the elements of interest.
One of the most important advantages of OES results from the excitation proper-ties
of the high temperature sources used in OES.These thermal excitation sources
can populate a large number of different energy levels for several different elements
at the same time.All of the excited atoms and ions can then emit their characteristic
radiation at nearly the same time.This results in the flexibility to choose from several
different emission wavelengths for an element and in the ability to measure emission
from several different elements concurrently.However,a disadvantage associated
with this feature is that as the number of emission wavelengths increases,the
Figure 1-3.Atomic spectrometry systems.
probability also increases for interferences that may arise from emission lines that are too close in wavelength to be measured separately. In atomic fluorescence spectrometry
(AFS),a light source,such as that used for AAS,is used to excite atoms only of the element of interest through radiative absorption transitions.When these selectively excited atoms decay through radia-tive transitions to lower levels,their emission is measured to determine
concentra-tion, much the same as in OES.The selective excitation of the AFS technique can lead to fewer spectral interferences than in OES.However,it is difficult to detect a large number of elements in a single run using AFS,as the number of spectral excitation sources and detectors that can be used at one time is limited by the instrument.
Another technique,called atomic mass spectrometry,is related to three atomic spectroscopy techniques described above.Instead of measuring the absorption, emission or fluorescence of radiation from a high temperature source,such as a flame or plasma,mass spectrometry measures the number of singly charged ions from the elemental species within a sample.Similar to the function of a monochro-mator in emission/absorption spectrometry that separates light according to wave-length, a quadrupole mass spectrometer separates the ions of various elements according to their mass-to-charge ratio in atomic mass spectrometry.
Atomization and Excitation Sources
In general,there are three types of thermal sources normally used in analytical atomic spectrometry to dissociate sample molecules into free
atoms:flames, furnaces and electrical discharges.High-power lasers have also been used for this purpose but tend to be better suited for other uses such as solids sampling for other atomization sources.
The first two types of sources,flames and furnaces,are hot enough to dissociate most types of molecules into free atoms.The main exceptions are refractory carbides and oxides,which can exist as molecules at the upper flame and furnace temperatures of 3000 -4000
°K.When configured properly,flames and furnaces can also be used to excite many elements for emission spectrometry.Because most of the free atoms in typical flames and furnaces are in their ground
states,however, absorption spectrometry is the preferred method to detect the presence of elements of interest.The exceptions are those elements whose lowest excited state is low enough in energy that it can be easily populated by a flame or furnace.Examples of such elements are lithium,sodium and potassium.In fact,flame emission spectrometry is still widely regarded as the preferred method for detecting the alkali elements.
Electrical discharges are the third type of atomization sources used in analytical optical emission spectrometry.For many years,dc arcs and ac sparks were the mainstay of OES.These electrical discharges are created by applied currents or potentials across an electrode in an inert gas and typically produce higher
tempera-tures than traditional flame systems.
More recently,other types of discharges,namely plasmas,have been used as atomization/excitation sources for OES.Strictly speaking,a plasma is any form of matter that contains an appreciable fraction (>1%)of electrons and positive ions in addition to neutral atoms,radicals and molecules.Two characteristics of plasmas are that they can conduct electricity and are affected by a magnetic field. The electrical plasmas used for analytical OES are highly energetic,ionized gases. They are usually produced in inert gases,although some work has also been done using reactive gases such as oxygen.These plasma discharges are considerably hotter than flames and furnaces and,thus,are used not only to dissociate almost any type of sample but also to excite and/or ionize the atoms for atomic and ionic emission.The present state-of-the-art in plasma sources for analytical optical emission spectrometry is the argon-supported inductively coupled plasma
(ICP). Other plasmas currently being used include the direct current plasma
(DCP)and the microwave induced plasma (MIP).
Because the argon ICP can efficiently generate singly charged ions from the elemental species within a sample,it makes an ideal ion source to use synergisti-cally with mass spectrometers.This combination of an ICP and mass spectrometer is called
ICP-MS.
A Short History of Optical Emission Spectroscopy
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,spectrochemi-cal 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
spectro-chemical 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.
While arc/spark emission techniques enjoyed widespread popularity for the
deter-mination
of metals,flame emission spectrometry,also known as flame photometry,
was used extensively for determination of the alkalis and other easily excited
elements.A Swedish
agronomist named Lun-degهrdh
is credited with be-ginning
the modern era of
flame photometry in the late
1920's.His apparatus for
elemental analyses of
plants,shown in Figure 1-4,
used pneumatic nebuliza-tion
and a premixed air-acetylene
flame and is re-markably
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 limita-tion
of the technique was
that the flames were not hot
enough to cause emission
for many elements.Despite
that limitation,several suc-cessful
commercial instru-ments
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 interfer-ences which plagued arc/spark emission techniques were also greatly reduced by atomic absorption techniques.
Figure 1-4.Spray chamber,nebulizer and burner
such as those used by Lundegهrdh for flame emis-sion
spectrography.(Used with permission from the
Division of Analytical Chemistry of the American
Chemical Society.)
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 interfer-ences, 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-injec-tion 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 sen-sitivity of detection [over flame photometry].
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 back-ground 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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