Inductively Coupled Plasma Absorption Emission Spectrometry (ICP-AES)
ICP-AES : Introduction
ICP-AES stands for : Inductively Coupled Plasma Absorption Emission Spectrometry (ICP-AES)
ICP-OES stands for : Inductively Coupled Plasma Optical Emission Spectrometry (ICP-AES)
•To understand the concept of ICP we need to understand the following :
- Absorption / Emission Spectrometry
- Inductively Coupled Plasma
Basics of ICP - AES
Atomic spectroscopy is the study of the electromagnetic radiation absorbed and emitted by atoms
A plasma is a hot ionized gas consisting of approximately equal numbers of positively charged ions and negatively charged electrons
The characteristics of plasmas are significantly different from those of ordinary neutral gases so that plasmas are considered a distinct "fourth state of matter." Plasma conduct electricity and is strongly influenced by electric and magnetic fields
Inductively Coupled Plasma
Adding energy to the electrons by the use of a coil in this manner is known as inductive coupling
Atomic /Ionic Spectra
If the energy is high enough the electron will completely dissociate leaving an ion with net positive charge, this energy is called Ionization Energy or Potential
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 ultraviolet(UV)/visible region (160 - 800 nm) of the electromagnetic spectrum is the region most commonly used for analytical atomic spectrometry.
Atomic Spectrometry Systems
Atomic Spectrometry Systems
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.
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 radiative transitions to lower levels, their emission is measured to determine concentration, much the same as in OES.
The selective excitation of the AFS technique can lead to fewer spectral interferences than in OES.
Atomic mass spectrometry - 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 monochromator in emission/absorption spectrometry that separates light according to wavelength, a quadrupole (is the component of the instrument responsible for filtering sample ions, based on their mass-to-charge ratio(m/z)) mass spectrometer separates the ions of various elements according to their mass-to-charge ratio in atomic mass spectrometry
Atomisation & Excitation Sources
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.
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.
Electric Discharge - 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 temperatures than traditional flame systems
Plasma - More recently, other types of discharges, namely plasmas, have been used as atomization/excitation sources for OES.
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 exceptional characteristics of plasmas are that they can conduct electricity and are affected by a magnetic field
Plasma and ICP Discharge
The electrical plasmas used for analytical OES are highly energetic, ionized gases. They are usually produced in inert gases, These plasma not only to dissociate almost any type of sample but also to excite and/or ionize the atoms for atomic and ionic emission.
- Argon supported induced couple plasma ICP
- Direct current plasma DCP
- Microwave induced Plasma MIP
The sample is typically introduced into the ICP plasma as an aerosol, either by aspirating a liquid or dissolved solid sample into a nebulizer or using a laser to directly convert solid samples into an aerosol
Once the sample aerosol is introduced into the ICP torch, it is completely desolvated and the elements in the aerosol are converted first into gaseous atoms and then ionized towards the end of the plasma
The argon discharge, with a temperature of around 6000-10000°K, is an excellent ion source. The ions formed by the ICP discharge are typically positive ions, M+ or M+², therefore, elements that prefer to form negative ions, such as Cl, I, F, etc., are very difficult to determine via ICP-MS
The detection capabilities of the technique can vary with the sample introduction technique and sample matrix, which may affect the degree of ionization that will occur in the plasma or allow the formation of species that may interfere
Parts of ICP-AES
Flame /Furnace = Temp 3300K
ICP Temperature = 6800K
High temp also eliminates chemical interferences found in flame/furnace
Features of ICP Torch
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
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
Detection of Emission
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 monochromator, 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 detection 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).
Extraction of Information
Qualitative information - what elements are present in the sample, involves identifying the presence of emission at the wavelengths characteristic of the elements of interest. 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 belonging to the element of interest
Quantitative information - how much of an element is in the sample, can be accomplished using plots of emission intensity versus concentration called calibration curves . Solutions with known concentrations of the elements 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
Not determined in ICP-OES
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 inappropriate 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
Second Category is those elements whose atoms have very high excitation energy requirements 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
Remaining category includes the man-made elements which are typically so radioactive or short-lived that gamma ray spectrometry is preferable for their determination
In ICP-OES technique, many elements can be determined easily in the same analytical run. The detection limits for these elements are generally in the µg/L (ppb) range.
Sample Introduction in ICP
Nebulizers are devices that convert a liquid into an aerosol that can be transported to the plasma. The low pressure and high-speed gas combine to break up the solution into an aerosol
The sample is usually transported into the instrument as a stream of liquid sample. Inside the instrument, the liquid is converted into an aerosol through a process known as nebulization.
The sample aerosol is then transported to the plasma where it is desolvated, vaporized, atomized, and excited and/or ionized by the plasma. The excited atoms and ions emit their characteristic radiation which is collected by a device that sorts the radiation by wavelength. The radiation is detected and turned into electronic signals that are converted into concentration information for the analyst
Once the sample aerosol is created by the nebulizer, it must be transported to the torch so it can be injected into the plasma. Because only very small droplets in the aerosol are suitable for injection into the plasma, a spray chamber is placed between the nebulizer and the torch
The primary function of the spray chamber is to remove large droplets from the aerosol. A secondary purpose of the spray chamber is to smooth out pulses that occur during nebulization, often due to pumping of the solution. spray chambers for the ICP are designed to allow droplets with diameters of about 10 µm or smaller to pass to the plasma. With typical nebulizers, this droplet range constitutes about 1 - 5% of the sample that is introduced to the nebulizer. The remaining 95 - 99% of the sample is drained into a waste container
Drains - the drain that carries excess sample from the spray chamber to a waste container can have an impact on the performance of the ICP instrument. Besides carrying away excess sample, the drain system provides the backpressure necessary to force the sample aerosol-carrying nebulizer gas flow through the torch’s injector tube and into the plasma discharge.
If the drain system does not drain evenly or if it allows bubbles to pass through it, the injection of sample into the plasma may be disrupted and noisy emission signals can result
Alternative Sample Introduction Techniques
The most widely used alternative technique is hydride generation. With this technique, the sample, in dilute acid, is mixed with a reducing agent, usually a solution of sodium borohydride in dilute sodium hydroxide.
The reaction of the sodium borohydride with the acid produces atomic hydrogen. The atomic hydrogen then reacts with the Hg, Sb, As, Bi, Ge, Pb, Se, Te, and Sn in the solution to form volatile hydrides of these elements. These gaseous com- pounds are then separated from the rest of the reaction mixture and transported to the plasma
Production of emission
The torches contain three concentric tubes for the flow of argon gas and aerosol injection. The spacing between the two outer tubes is kept narrow so that the gas introduced between them emerges at high velocity.
This outside chamber is also designed to make the gas spiral tangentially around the chamber as it proceeds upward. One of the functions of this gas is to keep the quartz walls of the torch cool and thus this gas flow was originally called the coolant flow or plasma flow but is now called the "outer" gas flow
The plasma torch consists of three concentric tubes, which are usually made from quartz.
The gas (usually argon) used to form the plasma (plasma gas) is passed between the outer and middle tubes at a flow rate of ;12–17 L/min.
A second gas flow, the auxiliary gas, passes between the middle tube and the sample injector at ;1 L/min
A third gas flow, the nebulizer gas/inner flow, also flowing at ;1 L/min carries the sample, in the form of a fine-droplet aerosol, from the sample introduction system and physically punches a channel through the center of the plasma.
The plasma torch is mounted horizontally and positioned centrally in the RF coil
Radio Frequency Generators
The radio frequency (RF) generator is the device that provides the power for the generation and sustainment of the plasma discharge. This power, typically ranging from about 700 to 1500 watts, is transferred to the plasma gas through a load coil surrounding the top of the torch.
The load coil, which acts as an antenna to transfer the RF power to the plasma, is usually made from copper tubing and is cooled by water or gas during operation
Collection and Detection
The emission radiation from the region of the plasma known as the normal analytical zone (NAZ) is sampled for the spectrometric measurement
Until recently, the analytical zone was observed from the side of the plasma operating in a vertical orientation. This classical approach to ICP spectroscopy is referred to as a radial or side-on viewing of the plasma.
Instruments that combine both radial and axial viewing, called dual view, have been introduced.
Wavelength Dispersive Devices
No matter whether the ICP is a side-on or end-on viewing type configuration, the radiation is usually collected by a focusing optic such as a convex lens or a concave mirror. This optic then focuses the image of the plasma onto the entrance slit of the wavelength dispersing device or spectrometer.
When multiple exit slits and detectors are used in the same spectrometer, the device is called a polychromator. Each exit slit in a polychromator is aligned to an atomic or ionic emission line for a specific element to allow simultaneous multi-element analyses
A monochromator, on the other hand, normally uses only one exit slit and detector. Monochromators are used in multielement analyses by scanning rapidly from one emission line to another
It is used to seperate wavelengths
Once the proper emission line has been isolated by the spectrometer, the detector and its associated electronics are used to measure the intensity of theemission line. By far the most widely used detector for ICP-OES is the photomultiplier tube or PMT.
The PMT is a vacuum tube that contains a photosensitive material, called the photocathode, that ejects electrons when it is struck by light. These ejected electrons are accelerated towards a dynode which ejects two to five secondary electrons for every one electron which strikes its surface. The secondary electrons strike another dynode, ejecting more electrons which strike yet another dynode, causing a multiplicative effect along the way.
PMT (Photo Multiplier)
Typical PMTs contain 9 to 16 dynode stages.
The final step is the collection of the secondary electrons from the last dynode by the anode. The electrical current measured at the anode is then used as a relative measure of the intensity of the radiation reaching the PMT
Charge Injection Device (CID)
Charge-injection Device (CID) - A two-dimensional array detector that uses a charge-injection readout of the photoelectrons generated by light absorption
Interferences in ICP-AES
Spectral interferences (also referred to as background interferences) encountered in ICP-OES fall into four different categories. These categories are
simple background shift, sloping background shift, direct spectral overlap, and complex background shift
Environmental Application of ICP-AES
Analyses of sewage sludge, domestic and industrial refuge, coal and coal fly ash, and dust and other airborne particulates
Analyses of sewage sludge, various refuses, and coal and coal fly ash require more rigorous sample preparation while collection of airborne particulates requires use of air filtering techniques
Examples of environmental ICP-OES applications include various water quality
analyses as required by the U.S. Environmental Protection Agency; determination of Fe, Cd, Cu, Mo, Ni, V, and Zn in seawater; determination of phosphorus in municipal wastewater; determination of heavy metals in inner-city dust samples; and trace metal analysis of coal fly ash