Atomic Emission Spectroscopy. Atomic Emission Spectroscopy.
Atomic Emission Spectroscopy
Atomic emission spectroscopy (AES), in contrast to AAS, uses the very high temperatures of atomization sources to excite atoms, thus excluding the need for lamp sources. Emission sources, which are routinely used in AES, include plasma, arcs and sparks, as well as flames. We will study the different types of emission sources, their operational principles, features, and operational characteristics. Finally, instrumental designs and applications of emission methods will be discussed.
The term “plasma” is defined as a homogeneous mixture of gaseous atoms, ions and electrons at very high temperatures. Two types of plasma atomic emission sources are frequently used:
A typical ICP consists of three concentric quartz tubes through which streams of argon gas flow at a rate in the range from 5-20 L/min. The outer tube is about 2.5 cm in diameter and the top of this tube is surrounded by a radiofrequency powered induction coil producing a power of about 2 kW at a frequency in the range from 27-41 MHz. This coil produces a strong magnetic field as well.
Ionization of flowing argon is achieved by a spark where ionized argon interacts with the strong magnetic field and is thus forced to move within the vicinity of the induction coil at a very high speed. A very high temperature is obtained as a result of the very high resistance experienced by circulating argon (ohmic heating). The top of the quartz tube will experience very high temperatures and should, therefore, be isolated and cooled.
This can be accomplished by passing argon tangentially around the walls of the tube. A schematic of an ICP (usually called a torch plasma) is shown below:
The torch is formed as a result of the argon emission at the very high temperature of the plasma. The temperature gradients in the ICP torch can be pictured in the following graphics:
The viewing region used in elemental analysis is usually about 6000 oC, which is about 1.5-2.5 cm above the top of the tube. It should also be indicated that argon consumption is relatively high which makes the running cost of the ICP torch high as well. Argon is a unique inert gas for plasma torches since it has few emission lines. This decreases possibility of interferences with other analyte lines.
There are several methods for sample introduction; the most widely used is, of course, the nebulization of an analyte solution into the plasma. However, other methods, as described earlier, are fine where vapors of analyte molecules or atom from electrothermal or ablation devices can be driven into the torch for complete atomization and excitation. For your convenience, sample introduction methods are summarized here again:
1. Pneumatic Nebulizers
Samples in solution are usually easily introduced into the atomizer by a simple nebulization, aspiration, process. Nebulization converts the solution into an aerosol of very fine droplets using a jet of compressed gas. The flow of gas carries the aerosol droplets to the atomization chamber or region.
In this case samples are pumped onto the surface of a piezoelectric crystal that vibrates in the kHz to MHz range. Such vibrations convert samples into homogeneous aerosols that can be driven into atomizers. Ultrasonic nebulization is preferred over pneumatic nebulization since finer droplets and more homogeneous aerosols are usually achieved. However, most instruments use pneumatic nebulization for convenience.
An accurately measured quantity of sample (few mL) is introduced into an electrically heated cylindrical chamber through which an inert gas flows. Usually, the cylinder is made of pyrolytic carbon but tungsten cylinders are now available. The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation.
Hydride Generation Techniques
Samples that contain arsenic, antimony, tin, selenium, bismuth, and lead can be vaporized by converting them to volatile hydrides by addition of sodium borohydride. Volatile hydrides are then swept into the plasma by a stream of an inert gas.
A variety of techniques were used to introduce solid samples into atomizers. These include:
1. Conductive Samples
If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. Upon drying, this solid composite can be used as an electrode. The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume of very fine particulates and atoms that are swept into the plasma by argon flow.
Sufficient energy from a focused intense laser will interact with the surface of samples (in a similar manner like arcs and sparks) resulting in ablation. The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation. Laser ablation is becoming increasingly used since it is applicable to conductive and nonconductive samples.
The technique is used for sample introduction and atomization as well. The electrodes are kept at a 250 to 1000 V DC. This high potential is sufficient to cause ionization of argon, which will be accelerated to the cathode where the sample is introduced. Collision of the fast moving energetic argon ions with the sample (cathode) causes atomization by a process called sputtering. Samples should thus be conductive to use the technique of glow discharge. The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation by flowing argon. However, nonconductive samples were reported to be atomized by this technique where they were mixed with a conductor material like graphite or powdered copper.
A plasma torch looks very much like a flame but with a very intense nontransparent brilliant white color at the core (less than 1 cm above the top). In the region from 1-3 cm above the top of the tube, the plasma becomes transparent. The temperatures used are at least two to three orders of magnitude higher than that achieved by flames which may suggest efficient atomization and fewer chemical interferences.
Ionization in plasma may be thought to be a problem due to the very high temperatures, but fortunately the large electron flux from the ionization of argon will suppress ionization of all species.
The DCP is composed of three electrodes arranged in an inverted Y configuration. A tungsten cathode resides at the top arm of the inverted Y while the lower two arms are occupied by two graphite anodes. Argon flows from the two anode blocks and plasma is obtained by momentarily bringing the cathode in contact with the anodes. Argon ionizes and a high current passes through the cathode and anodes.
It is this current which ionizes more argon and sustains the current indefinitely. Samples are aspirated into the vicinity of the electrodes (at the center of the inverted Y) where the temperature is about 5000 oC. DCP sources usually have fewer lines than ICP sources, require less argon/hour, and have lower sensitivities than ICP sources. In addition, the graphite electrodes tend to decay with continuous use and should thus be frequently exchanged. A schematic of a DCP source is shown below:
A DCP has the advantage of less argon consumption, simpler instrumental requirements, and less spectral line interference. However, ICP sources are more convenient to work with, free from frequent consumables (like the anodes in DCP’s which need to be frequently changed), and are more sensitive than DCP sources.
2. Minimum chemical interferences
3. Minimum spectral interferences except for higher possibility of spectral line interference due to exceedingly large number of emission lines (because of high temperature)
4. Uniform temperature which results in precise determinations
5. No self-absorption is observed which extends the linear dynamic range to higher concentrations
6. No need for a separate lamp for each element
7. Easily adaptable to multichannel analysis
Three classes of plasma emission instruments can be presented including:
1. Sequential instruments
In this class of instruments a single channel detector is used where the signal for each element is read using the specific wavelength for each element sequentially. Two types of sequential instruments are available:
b. Slew scan instruments where the monochromator is preset to provide specific wavelengths; moving very fast in between wavelengths while moving slowly at the specific wavelengths. Therefore, a two-speed motor driving the grating is thus used.
Radial vs. Axial Viewing
Radial – traditional side view, better for concentrated samples.
Axial – direct view into plasma, lower sensitivity, shifts detection range lower.
Sequential vs. multichannel
Slew scan spectrometer
This class of instruments is also referred to as simultaneous instruments in which all signals are reported at the same time using two types of configurations:
Multiple detectors, usually photomultiplier tubes are used. Beams of radiation emerging from the grating are guided to exit slits (each representing the wavelength of a specific element) are focused at several PMTs for detection. Detection, thus, takes place simultaneously
This multichannel type instrument uses a multichannel detector like a charge injection device or a charge-coupled device. Diffracted beams from a grating pass through a prism where further resolution of diffracted beams takes place by a prism. The prism will disperse the orders of each diffracted beam. The multichannel detector can also be a linear photodiode array as in the figure below:
Diode Array Detector
CCD or CID Detector
Instruments in which the signal is coded will need a decoding mechanism in order to see the signal. FT is a very common technique for decoding time domain spectra. In such instruments, the detector records the change of signal with time, which is practically not useful. However, Fourier transformation of the time domain signal yield a frequency domain spectrum, which is the usual signal, obtained by conventional methods. Instruments that rely on decoding a coded signal is also said to have a multiplex design.
1. Since plasma sources result in a very large number of emission lines, these sources can be used for both qualitative and quantitative analysis.
2. The signal obtained from plasma sources is stable, has a low noise and background, as well as freedom from interferences.
3. Requires sample preparation similar to AAS
4. Plasma sources are usually best suited for operation in the ultraviolet region, therefore, elements having emission lines below 180 nm (like B, P, S, N, and C) can be only analyzed under vacuum since air components absorb under 180 nm. Also, alkali metals are difficult to analyze since their best lines under plasma conditions occur in the near infrared.
5. An analytical emission line can easily be located but will depend on the other elements present since spectral line interferences are encountered in plasma sources due to the very high temperatures used.
6. Linear calibration plots are usually obtained but departure from linearity is observed at high concentrations; due to self absorption as well as other instrumental reasons. An internal standard is often used in emission methods to correct for fluctuations in temperature as well as other factors. The calibration plot in this case is a plot between the concentration of analyte and the ratio of the analyte to internal standard signal. The internal standard is a substance that is added in a constant amount to all samples, blanks, and standards; therefore it must be absent from initial sample matrix. The internal standard should have very close characteristics (both chemically and physically) to analyte.
Elements by ICP-AES
Different elements have different emission intensities. Alkalis (Na, K, Rb, Cs) are weakly emitting. Alkaline Earths (Be, Mg, Ca, Sr, Ba ) are strongly emitting.
Concepts, Instrumentation, and Techniques in Inductively Coupled Plasma Optical Emission Spectrometry, Boss and Freeden, Perkin Elmer
Physical interferences of ICP
Interferences of ICP
include molecular compound formation, ionization effects, and solute vaporization effects.
Normally, these effects are not significant with the ICP technique.
Chemical interferences are highly dependent on matrix type and the specific analyte element.
When analytes in a previous sample contribute to the signals measured in a new sample.
Memory effects can result
from sample deposition on the uptake tubing to the nebulizer
from the build up of sample material in the plasma torch and spray chamber.
The site where these effects occur is dependent on the element and can be minimized
by flushing the system with a rinse blank between samples.
High salt concentrations can cause analyte signal suppressions and confuse interference tests.
INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY
- Very sensitive and good for trace analysis
- Plasma produces analyte ions
- Ions are directed to a mass spectrometer
- Ions are separated on the basis of their mass-to-charge ratio
- A very sensitive detector measures ions
- Very low detection limits
Inductively Coupled Plasma Emission
- High cost
- No lamp required
- Low background signals
- Low interference
- Moderate sensitivity
Inductively Coupled Plasma-Mass Spectrometry
- Very high cost
- No lamp required
- Least background signals
- Least interference
- Very high sensitivity
Techniques for elemental analysisICP-MSICP-AESFAAS GFAAS
Detection LimitsExcellentGoodGood Excellent
ProductivityExcellentVery goodGood Low
LDR10 5 10 6 /10 10 HDD10 3 10 2
Precision1-3 %0.3-2 %0.1-1 % 1-5 %
Spectral interferenceFewCommonAlmost none Very few
Chemical interferenceModerateFewMany Many
Mass efffectsHigh on lownonenone none
Dissolved solids0.1-0.4 %up to 30 %0.5-3 % up to 30 %
No. of elements~75~73~68 ~50
Sample usagelowmediumhigh very low
Isotope analysisyesnono no
routine operationSkill requiredeasyeasy skill required
Method developmentskill requiredskill requiredeasy skill required
Running costshighhighlow medium
Capital costsvery highhighlow medium
Samples are excited in the gap between a pair of electrodes connected to a high potential power supply (200 VDC or 2200-4400 VAC). The high potential applied forces a discharge between the two electrodes to occur where current passes between the two separated electrodes (temperature rises due to very high resistance).
The very high temperature (4000-5000 oC) realized in the vicinity between the two electrodes provide enough energy for atomization and excitation of the samples in this region or when the sample is, or a part of, one of the electrodes.
Arc and spark methods are mainly used as qualitative techniques and can also be used as semiquantitative techniques.
10B. Arc and Spark AES
If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. Upon drying, this solid composite can be used as an electrode. The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume of very fine particulates and atoms that are excited and emission is collected. The figure below shows some common shapes of graphite electrodes used in arc and spark sources.
Sample pressed into electrode or mixed with Cu powder and pressed - Briquetting (pelleting)
Cyanogen bands (CN) 350-420 nm occur with C electrodes in air -He, Ar atmosphere
each line measured >20 s
needs multichannel detection
In most cases, emission from atoms in an arc or spark is directed to a monochromator with a long focal length and the diffracted beams are allowed to hit a photographic film. This typical instrument is called a spectrograph since it uses a photographic film as the detector.
Long integration times
Difficult to develop/analyze
Non-linearity of line "darkness“
The blackness of the lines on the photographic film is an indication of the intensity of the atomic line and thus the concentration of the analyte. The location of emission lines as compared to standard lines on a film serves to identify the wavelengths of emission lines of analyte and thus its identity. The use of spectrographs is not very convenient since a lot of time and precautions must be spent on processing and calibrating the photographic film.
Qualitative analysis is accomplished by comparison of the wavelengths of some emission lines to standards while the line blackness serves as the tool for semiquantitative analysis.
Polychromators are also available as multichannel arc and spark instruments. However, these have fixed slits at certain wavelengths in order to do certain elements and thus they are not versatile.
Recently, arc and spark instruments based on charge injection and charge coupled devices became available. These have extraordinarily high efficiency and performance in terms of easier calibration, short analysis time, as well as superior quantitative results.
CCD or CID Detector
1. Typical temperatures between 4000-5000 oC are high enough to cause atomization and excitation of sample and electrode materials.
2. Usually, cyanogens compounds are formed due to reaction of graphite electrodes with atmospheric nitrogen. Emission bands from cyanogens compounds occur in the region from 350-420 nm. Unfortunately, several elements have their most sensitive lines in this same region which limits the technique. However, use of controlled atmosphere around the arc (using CO2, Helium, or argon) very much decreases the effect of cyanogens emission.
3. The emission signal should be integrated over a minute or so since volatilization and excitation of atoms of different species differ widely. While some species give maximum signal, others may still be in the molecular state.
4. Arc sources are very good for qualitative analysis of elements while only semiquantitative analysis is possible. It is mandatory to compare the emission spectrum of a sample with the emission spectrum of a standard. In some cases, a few milligrams of a standard is added to the sample in order to locate the emission lines of the standard and thus identify the emission wavelengths of the different elements in the sample. A comparator densitometer can be used to exactly locate the wavelengths of the standard and the sample components.
The lines from the standard are projected on the lines of the combined sample/standard emission spectra in order to identify sample components. Only few lines are shown in the figure.
We have previously seen the use of graphite in electrothermal AAS as well as arc and spark AES, even though molecular spectra are real problems in both techniques due to cyanogens compounds absorption and emission. The reasons after graphite common use in atomic spectroscopy can be summarized below:
Most of the instruments in this category are arc based instruments. Spark based instruments are of the same idea except for a spark source substituting an arc source. The spark source is constructed as in the figure below where an AC potential in the order of 10-50 KV is discharged through a capacitor which is charged and discharged through the graphite electrodes about 120 times/s; resulting in a discharge current of about 1000 A.
This very high current will suffer a great deal of resistance, which increase the temperature to an estimated 40000 oC. Therefore, ionic spectra are more pronounced.