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GENERAL CHARACTERISTICS OF
ICP-OES |
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A well-known analytical chemist in the 1930's once facetiously suggested that the ideal analytical laboratory would have a shelf with 92 bottles and in each bottle a reagent which specifically reacts with one of the then-known 92 elements.Although it is not quite what the chemist had in mind when he made that suggestion,modern ICP-OES instruments can nearly achieve that chemist's wish.Routine determinations of 70 elements can be made by ICP-OES at concentration levels below one milligram per liter.Figure 2-1 contains a periodic table representation of the elements that can be determined by ICP-OES along with their detection limits. In this chapter,some of the general characteristics of ICP-OES are described with the goal of providing an overview of the ICP and how it is used as a source for optical emission spectrometry.Also included is information regarding the typical performance characteristics and figures of merit that can be expected of the technique. Finally,the role of ICP-OES instrumentation in the modern analytical laboratory is discussed briefly.More detailed information about ICP-OES instrumentation,methodology and applications is covered in subsequent chapters.
The ICP Discharge
The inductively coupled plasma discharge used today for optical emission spectrometry is very much the same in appearance as the one described by Velmer Fassel in the early 1970's.Argon gas is directed through a torch consisting of three concentric tubes made of quartz or some other suitable material,as shown in Figure 2-2.A copper coil,called the load coil,surrounds the top end of the torch and is connected to a radio frequency (RF)generator.
When RF power (typically 700 -1500 watts)is applied to the load coil,an alternating
current moves back and forth within the coil,or oscillates,at a rate corresponding
to the frequency of the generator.In most ICP instruments this frequency is either
27 or 40 megahertz (MHz).This RF oscillation of the current in the coil causes RF
electric and magnetic fields to be set up in the area at the top of the torch.With
argon gas being swirled through the torch,a spark is applied to the gas causing
some electrons to be stripped from their argon atoms.These electrons are then.caught up in the magnetic field and accelerated by them.Adding energy to the
electrons by the use of a coil in this manner is known as inductive coupling.These high-energy electrons in turn collide with other argon atoms,stripping off still more electrons.This collisional ionization of the argon gas continues in a chain reaction, breaking down the gas into a plasma consisting of argon atoms,electrons,and argon ions,forming what is known as an inductively coupled plasma (ICP)discharge. The ICP discharge is then sustained within the torch and load coil as RF energy is continually transferred to it through the inductive coupling process. Figure 2-1.Periodic table with ICP-OES detection limits (side-on viewing).All detection limits are reported as 3s and were obtained on a Perkin-Elmer Optima 3000 under simultaneous multielement conditions with a side-viewed plasma. Detection limits using an axially-viewed plasma are typically improved by 5--10 times.
The ICP discharge appears as a very intense,brilliant white,teardrop-shaped discharge.Figure 2-3 shows a cross-sectional representation of the discharge along with the nomenclature for different regions of the plasma as suggested by Koirtyohann et al.[2].At the base,the discharge is toroidal,or "doughnut-shaped" because the sample-carrying nebulizer flow literally punches a hole through the center of the discharge.The body of the "doughnut"is called the induction region (IR)because this is the region in which the inductive energy transfer from the load coil to the plasma takes place.This is also the area from which most of the white light,called the argon continuum,is emitted.Allowing the sample to be introduced through the induction region and into the center of the plasma gives the ICP many of its unique analytical capabilities.
Figure 2-2.Cross section of an ICP torch and load coil depicting an ignition sequence. A -Argon gas is swirled through the torch.B -RF power is applied to the load coil.C -A spark produces some free electrons in the argon.D -The free electrons are accelerated by the RF fields causing further ionization and forming a plasma.E -The sample aerosol-carrying nebulizer flow punches a hole in the plasma.
Most samples begin as liquids
that are nebulized into an
aerosol,a very fine mist of
sample droplets,in order to be
introduced into the ICP.The
sample aerosol is then carried
into the center of the plasma
by the inner (or nebulizer)argon
flow.The functions of the
ICP discharge (hereafter referred
to as the ICP or "the
plasma")at this point are several
fold.Figure 2-4 depicts
the processes that take place
when a sample droplet is introduced
into an ICP.
The first function of the high
temperature plasma is to re-move
the solvent from,or
desolvate,the aerosol,usually
leaving the sample as microscopic
salt particles.The
next steps involve decomposing
the salt particles into a gas
of individual molecules (vaporization)
that are then dissociated into atoms (atomization).These processes, which occur predominantly in the preheating zone (PHZ)shown in Figure 2-3,are the same processes that take place in flames and furnaces used for atomic absorption spectrometry.
Once the sample aerosol has been desolvated,vaporized and atomized,the plasma has one,or possibly two,functions remaining.These functions are excitation and ionization.As explained in Chapter 1,in order for an atom or ion to emit its characteristic radiation,one of its electrons must be promoted to a higher energy level through an excitation process.Since many elements have their strongest emission lines emitted from the ICP by excited ions,the ionization process may also be necessary for some elements.The excitation and ionization processes occur predominantly in the initial radiation zone (IRZ)and the normal analytical zone (NAZ).The NAZ is the region of the plasma from which analyte emission is typically measured.
Figure 2-3.Zones of the ICP.IR -Induction Region,
PHZ -Preheating Zone,IRZ -Initial Radiation
Zone,NAZ -Normal Analytical Zone.
While the exact mechanisms for excitation and ionization in the ICP are not yet fully understood,it is believed that most of the excitation and ionization in the ICP takes place as a result of collisions of analyte atoms with energetic electrons [3].There is also some speculation about the role of argon ions in these processes.In any case,the chief analytical advantage of the ICP over other emission sources are derived from the ICP's ability to vaporize,atomize,excite,and ionize efficiently and reproducibly a wide range of elements present in many different sample types.
One of the important reasons for the superiority of the ICP over flames and furnaces
for the above is in the high temperature within the plasma.Figure 2-5 shows
approximate temperatures for different regions of the ICP.While flames and
furnaces have upper temperature ranges in the area of 3300 K,the gas temperature
in the center of the ICP is about 6800 K [4].Besides improving excitation and
Figure 2-4.Process that takes place when a sample droplet is introduced into
an ICP discharge ionization efficiencies,the higher temperature of the ICP also reduces or eliminates
many of the chemical interferences found in flames and furnaces. Other electrical discharge emission sources,such as arcs,sparks,direct current plasmas,and microwave induced plasmas,also have high temperature and thus may be as efficient at excitation and ionization as the ICP.However,it is largely the ICP's combination of stability and freedom from sample matrix interferences that makes the ICP a better source for atomic emission spectrometry than these other electrical discharge sources.
An important feature of the ICP that is not common to most other emission sources is that since the sample aerosol is introduced through the center of the ICP,it can be surrounded by the high temperature plasma for a comparatively long time, approximately 2 milliseconds.It is this long residence time of the analyte particles in the center of the plasma that is largely responsible for the lack of matrix interferences in the ICP.In addition,because the aerosol is in the center of the discharge and the energy-supplying load coil surrounds the outside of the plasma, the aerosol does not interfere with the transfer of the energy from the load coil to the discharge.In some other sources,such as the direct current plasma,the sample travels around the outside of the discharge where it does not experience uniform high temperature for as long.In the arcs and sparks,the sample may commingle with the entire electrical discharge and interfere with the production and sustain-ment of the discharge.These situations lead to the higher levels of matrix effects and poorer stability that are often characteristic of non-ICP discharges.
Detection of Emission
In ICP-OES,the light emitted by the excited atoms and ions in the plasma is measured to obtain information about the sample.Because the excited species in the plasma emit light at several different wavelengths,the emission from the plasma is polychromatic.This polychromatic radiation must be separated into individual wavelengths so the emission from each excited species can be identified and its intensity can be measured without interference from emission at other wavelengths. The separation of light according to wavelength is generally done using a mono-chromator, which is used to measure light at one wavelength at a time,or a polychromator,which can be used to measure light at several different wavelengths at once.The actual detection of the light,once it has been separated from other wavelengths,is done using a photosensitive detector such as a photo-multiplier tube (PMT)or advanced detector techniques such as a charge-injection device (CID)or a charge-coupled device (CCD).Further details about monochromators, polychromators and detectors are included in Chapter 3.
Extracting qualitative and
quantitative information about
a sample using ICP-OES is
generally straightforward.Ob-taining
qualitative information,
i.e.,what elements are present
in the sample,involves identi-fying
the presence of emission
at the wavelengths charac-teristic
of the elements of inter-est.
In general,at least three
spectral lines of the element
are examined to be sure that
the observed emission can be
indeed classified as that be-longing
to the element of inter-est.
Occasional spectral line
interferences from other ele-ments
may make one uncer-tain
about the presence of an
element in the plasma.Fortu-nately,
the relatively large
number of emission lines avail-able
for most elements allows
one to overcome such interfer-ences
by choosing between several different emission lines for the element of interest.(Note:Qualitative analysis should only be attempted with ICPs utilizing a monochromator or advanced detector technology.Polychromators with photomul-tiplier detectors should not be used for qualitative analyses.Monochromators, polychromators and detectors are discussed in some detail in Chapter 3.) Obtaining quantitative information,i.e.,how much of an element is in the sample, can be accomplished using plots of emission intensity versus concentration called calibration curves (Figure 2-6).Solutions with known concentrations of the ele-ments of interest,called standard solutions,are introduced into the ICP and the intensity of the characteristic emission for each element,or analyte,is measured. These intensities can then be plotted against the concentrations of the standards to form a calibration curve for each element.When the emission intensity from an analyte is measured,the intensity is checked against that element's calibration curve to determine the concentration corresponding to that intensity.
Figure 2-5.Temperature regions of a
typical ICP discharge.
The computers and software used with ICP-OES instruments represent these calibration curves mathematically within the computer's memory.Thus,it is not necessary for the analyst to construct these curves manually for quantitation of the elements in the sample.Because calibration curves are generally linear over four to six orders of magnitude in ICP-OES,it is usually necessary to measure only one or two standard solutions,plus a blank solution,to calibrate the ICP instrument.In contrast to ICP-OES,arc and spark sources require five or more standards per element because of nonlinear calibration curves.The nonlinearity in these sources is a direct result of self-absorption which is the process by which some of the emitted radiation of the analyte is absorbed by ground state atoms in the plasma.In conventional ICPs,nonlinearity in the calibration curves is usually only observed for high analyte concentrations;i.e.,greater than 5 to 6 orders of magnitude above the detection limit.(The effect of self-absorption with regard to an axial or end-on ICP is discussed in Chapter 3.)
Performance Characteristics
As indicated in Figure 2-1,the ICP-OES technique is applicable to the determination of a large number of elements.The detection limits for these elements are generally in the mg/L (ppb)range.As in many techniques,the detection limit is regarded as the lowest concentration at which the analyst can be relatively certain that an Figure 2-6.Calibration curve used for ICP-OES. element is present in a sample.Measurements made at or near the detection limit, however,are not considered to be quantitative.For purposes of rough quantitation (±10%),it is recommended that an element's concentration should be at least five times higher than the detection limit.For accurate quantitation (± 2%),the concen-tration should be greater than 100 times the detection limit.
While most of the over 70 elements that can be determined by ICP-OES have low detection limits,it is worthwhile to discuss the elements that are usually not determined at trace levels by ICP-OES.These elements fall into three basic categories.The first category includes those elements that are naturally entrained into the plasma from sources other than the original sample.For example,in an argon ICP,it would be hopeless to try to determine traces of argon in a sample.A similar limitation might be encountered because of the CO2 contamination often found in argon gas.When water is used as a solvent,H and O would be inappro-priate elements,as would C if organic solvents were used.Entrainment of air into the plasma makes H,N,O and C determinations quite difficult,although not impossible.
Another category of elements generally not determined at trace levels by ICP-OES includes those elements whose atoms have very high excitation energy require-ments such as the halogens,Cl,Br and I.Though these elements may be determined,the detection limits are quite poor compared to most ICP elements. The remaining category includes the man-made elements which are typically so radioactive or short-lived that gamma ray spectrometry is preferable for their determination.
The upper limit of linear calibration for ICP-OES is usually 10 4 to 10 6 times the detection limit for a particular emission line.For example,the maximum linear concentration for the Mn 257.610 nm emission line is about 50 mg/L or about 10 5 times its 0.0004 mg/L detection limit.The range of concentrations from the detection limit to this upper limit is known as the linear dynamic range (LDR)of the emission line.
The advantages of long LDRs are basically twofold.Firstly,it makes calibration of the instrument simpler.Atomic absorption,arc and spark techniques have LDRs of only one or two orders of magnitude and require the use of nonlinear calibration curves to extend the working range (i.e.,the actual calibration range used)for an element.While the techniques for calculating nonlinear curves have improved over the years,they still require multiple data points,i.e.,multiple standards must be run in order to define the nonlinear curve.In ICP-OES,where linear calibration curves are the norm,only two solutions,the blank and a high standard,need to be analyzed to produce a calibration curve.
The other advantage of long LDRs is that less sample dilution is required.Even when nonlinear curves are used,the techniques that have shorter LDR's tend to require more sample dilution to keep the analyte concentrations within the working range for the element of interest.
Besides being able to determine a large number of elements over a wide range of concentrations,a major advantage of the ICP-OES technique is that many elements can be determined easily in the same analytical run.This multielement capability arises from the fact that all of the emission signals needed to obtain qualitative and quantitative information are emitted from the plasma at the same time. The precision and accuracy of the ICP-OES analyses are considered sufficient for most trace elemental analyses.Even in the presence of interferences,modern signal compensation techniques allow the analyst to perform analyses with remark-able accuracy.Precision of analysis is usually in the 1%or less RSD (relative standard deviation)range when the concentration is greater than 100 times the detection limit.Better precision can be obtained,but often with trade-offs in speed and/or flexibility,or through the use of longer measurement times and special signal compensation techniques.
When ICP-OES was first introduced as a technique for trace elemental analysis,it was claimed by some experts that the technique was free from interferences.While this was somewhat optimistic,the ICP-OES technique probably experiences the fewest interferences of any of the commonly used analytical atomic spectrometry techniques.Chemical interferences are largely eliminated by the high temperature of the plasma.Physical interferences can be compensated for easily by taking advantage of the ICP's multielement capability.While spectral interferences have the potential for causing the most inaccuracies with ICP-OES analyses,the use of high-resolution spectrometers and advanced background correction techniques, coupled with the flexibility to choose from many possible emission lines,allows for interference-free analyses of the majority of samples.The interferences mentioned above and methods for overcoming them will be described in greater detail in Chapter 4.
Role of the ICP in an Analytical Laboratory
The obvious role of the ICP-OES instrument is to determine the concentrations of certain elements in the samples of interest.There are,however,several instrumen-tal alternatives to ICP-OES for trace elemental analysis.The analyst must,there-fore, define their analytical goals,taking into careful consideration factors such as sample type,elements to be determined,sensitivity and speed of analysis required, sample volume,sample loads,cost,and a host of other factors before selecting a technique or combination of techniques.
In many laboratories,ICP-OES is used to complement the other techniques available in the lab.For example,many labs are equipped with an ICP-OES instrument to perform moderate sensitivity,high sample throughput,multielement analyses and a graphite furnace AAS instrument to perform single element deter-minations which require higher sensitivity.(In Chapter 3,the possibilities of replac-ing the single element GFAAS system with the axially viewed ICP is discussed.) To provide a better understanding of the role of ICP-OES instruments in the modern analytical laboratory,the ICP-OES technique is compared to three other trace elemental analysis techniques----flame (FAAS)and graphite furnace (GFAAS) atomic absorption spectrometry and ICP-mass spectrometry (ICP-MS). Flame and Furnace Atomic Absorption Spectrometries.The flame and furnace AAS techniques are both excellent choices for many analytical laboratories.Two princi-pal advantages of FAAS are initial instrument cost and simplicity of operation.FAAS detection limits for many elements are comparable to those obtained by ICP-OES, although the ICP-OES technique is typically preferred for refractory compound-forming elements.The prices of the least expensive ICP-OES instruments are now approaching the prices of the top-of-the-line FAAS.
A principal advantage of the graphite furnace AAS technique over FAAS and ICP-OES is its greater sensitivity,which results in significantly lower detection limits for most elements.Another advantage of GFAAS is the ability to analyze very small amounts (mL)of sample easily.A low-cost GFAAS instrument is less expensive than ICP-OES instruments while top-of-the-line GFAAS instruments are in the same price range as the lower-to mid-priced ICP-OES instruments. The main advantage of ICP-OES over the AAS techniques in general are its multielement capabilities,longer linear dynamic ranges,and fewer condensed phase interferences.In addition,besides the refractory compound-forming ele-ments, elements such as I,P and S are detected with more sensitivity by the ICP-OES technique.
Inductively Coupled Plasma -Mass Spectrometry is one of the most recently developed techniques for trace elemental analysis.ICP-MS uses the same type of ICP source as is used for ICP-OES.In the ICP-MS technique,the analyte ions formed in the ICP are sent through a mass spectrometer where they are separated according to their mass/charge ratios (m/e).The number of ions at the m/e's of interest are then measured and the results used for qualitative and quantitative purposes.
Since its commercial introduction in 1983,ICP-MS has been demonstrated to be a powerful trace elemental analysis technique.It has the sensitivity and detection limits typical of GFAAS,combined with the multielement capability of ICP-OES. While mass spectral interferences are few,ones that do exist generally can be overcome by using alternate masses or mathematical correction techniques.Early ICP-MS systems,in addition to being much more expensive than ICP-OES instru-ments, have experienced more severe sample matrix interferences than are experienced in ICP-OES.However,these interferences,along with the cost of the instruments,have been considerably reduced through refinements in the instru-mentation, particularly in the sample introduction system.
Because of their respective advantages and disadvantages,selecting between the ICP-OES,FAAS,GFAAS,and ICP-MS techniques for a given set of circumstances is generally not a difficult task.For example,if an application requires single element trace analyses for relatively few samples or if initial cost is the most important factor, then FAAS is quite often the technique of choice.If an application requires very low detection limits for just a few elements,the GFAAS technique would probably be selected.If however,an application required very low detection limits for forty elements per sample,then ICP-MS would be a likely candidate.Likewise,if the application called for multielement analyses of samples in a complicated matrix or if a high sample throughput rate with moderate sensitivity was required,then ICP-OES might be the best choice.
Of course,there are many more criteria for selection of techniques than those mentioned in the examples given.In many labs,all four types of instruments,FAAS, GFAAS,ICP-OES,and ICP-MS are all used,each with its own set of applications.
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