ATOMIC ABSORPTION SPECTROMETRY

INTRODUCTION

Atomic Absorption Spectrometry (AAS) is a technique which is used for the analysis of quantities of elements present in a sample by measuring the absorbed radiation by the chemical element of interest.

This is done by measuring the spectra produced when the sample is excited by radiation. The atoms absorb ultraviolet or visible light and get excited to higher energy levels. Atomic absorption technique measures the amount of energy in the form of photons of light that are absorbed by the sample.

A detector measures the wavelengths of light transmitted by the sample, and compares them to the wavelengths which originally passed through the sample. A signal processor then integrates the changes in wavelength absorbed, which appear in the readout as peaks of energy absorption at discrete wavelengths.

The energy required for an electron to leave an atom is known as ionization energy and is specific to each and every element. When an electron moves from one energy level to another within the atom, a photon is emitted with energy E. Atoms of an element emit a characteristic spectral line. Every atom has its own distinct pattern of wavelengths at which it will absorb energy, due to the unique configuration of electrons in its outer shell.

This enables the qualitative analysis of a sample. The concentration is calculated based on the Beer-Lambert law. Absorbance is directly proportional to the concentration of the analyte absorbed for the existing set of conditions. The concentration is usually determined from a calibration curve, obtained using standards of known concentration or certified reference materials (CRMs). However, applying the Beer-Lambert law directly in AAS is difficult due to:

·       variations in atomization efficiency from the sample matrix non-uniformity of concentration and path length of analyte atoms (in graphite furnace AA).

The chemical methods used are based on matter interactions, i.e. chemical reactions. For a long period of time these methods were essentially empirical, involving, in most cases, great experimental skills. In analytical chemistry, AAS is a technique used mostly for determining the concentration of a particular metal element within a sample. AAS can be used to analyse the concentration of over 62 different metals in a solution. Typically, the technique makes use of a flame to atomize the sample, but other atomizers, such as a graphite furnace, are also used. Three steps are involved in turning a liquid sample into an atomic gas:

1. Desolvation – the liquid solvent is evaporated, and the dry sample remains;

2. Vaporization – the solid sample vaporizes to a gas; and

3. Volatilization – the compounds that compose the sample are broken into free atoms.

To measure how much of a given element is present in a sample, first of all , we must establish a basis for comparison using certified reference materials or known quantities of that element to produce a calibration curve.

To generate this curve, a specific wavelength is selected, and the detector (Usually Photomultiplier tube detectors are used) is set to measure only the energy transmitted at that wavelength. As the concentration of the target atom in the sample increases, the absorption will also increase proportionally.

A series of samples containing known concentrations of the element to be measured are analysed, and the corresponding absorbance, which is the inverse percentage of light transmitted, is recorded.

The measured absorption at each concentration is then plotted, so that a straight line can then be drawn between the resulting points. From this line, the concentration of the substance under investigation is extrapolated from the substance’s absorbance. The use of special light sources and the selection of specific wavelengths allow for the quantitative determination of individual components in a multi-element mixture.

BASIC PRINCIPLE

The selectivity in AAS is very important, since each element has a different set of energy levels and gives rise to very narrow absorption lines. Hence, the selection of the monochromator is vital to obtain a linear calibration curve (Beers' Law), the bandwidth of the absorbing species must be broader than that of the light source; which is difficult to achieve with ordinary monochromators. The monochromator is a very important part of an AA spectrometer because it is used to separate the thousands of lines generated by all of the elements in a sample.

Without a good monochromator, detection limits are severely compromised. A monochromator is used to select the specific wavelength of light that is absorbed by the sample and to exclude other wavelengths. The selection of the specific wavelength of light allows for the determination of the specific element of interest when it is in the presence of other elements. The light selected by the monochromator is directed onto a detector,typically a photomultiplier tube, whose function is to convert the light signal into an electrical signal proportional to the light intensity. The challenge of requiring the bandwidth of the absorbing species to be broader than that of the light source is solved with radiation sources with very narrow lines.

The study of trace metals in wet and dry precipitation has increased in recent decades because trace metals have adverse environmental and human health effects. Some metals, such as Pb, Cd and Hg, accumulate in the biosphere and can be toxic to living systems.

Anthropogenic activities have substantially increased trace metal concentrations in the atmosphere. In addition, acid precipitation promotes the dissolution of many trace metals, which enhances their bioavailability. In recent decades, heavy metal concentrations have increased not only in the atmosphere but also in pluvial precipitation. Metals, such as Pb, Cd, As, and Hg, are known to accumulate in the biosphere and to be dangerous for living organisms, even at very low levels. Many human activities play a major role in global and regional trace element budgets. Additionally, when present above certain concentration levels, trace metals are potentially toxic to marine and terrestrial life. Thus, biogeochemical

perturbations are a matter of crucial interest in science.

The atmospheric input of metals exhibits strong temporal and spatial variability due to short atmospheric residence times and meteorological factors. As in oceanic chemistry, the impact of trace metals in atmospheric deposition cannot be determined from a simple consideration of global mass balance; rather, accurate data on net air or sea fluxes for specific regions are needed.

Particles in urban areas represent one of the most significant atmospheric pollution problems, and are responsible for decreased visibility and other effects on public health, particularly when their aerodynamic diameters are smaller than 10 μm, because these small particles can penetrate deep into the human respiratory tract. There have been many studies measuring concentrations of toxic metals such as Ag, As, Cd, Cr, Cu, Hg, Ni, Pb in rainwater and their deposition into surface waters and on soils. Natural sources of aerosols include terrestrial dust, marine aerosols, volcanic emissions and forest fires. Anthropogenic particles, on the other hand, are created by industrial processes, fossil fuel combustion, automobile mufflers, worn engine parts, and corrosion of metallic parts. The presence of metals in atmospheric particles are directly associated with health risks of these metals. Anthropogenic sources have substantially increased trace metal concentrations in atmospheric deposition.

The instrument used for atomic absorption spectrometry can have either of two atomizers. One attachment is a flame burner, which uses acetylene and air fuels. The second attachment consists of a graphite furnace that is used for trace metal analysis. Figure 1 depicts a diagram of an atomic absorption spectrometer.

Fig. 1. The spectral, or wavelength, range captures the dispersion of the grating across the linear array.

           

 Flame and furnace spectroscopy has been used for years for the analysis of metals. Today these procedures are used more than ever in materials and environmental applications. This is due to the need for lower detection limits and for trace analysis in a wide range ofsamples. Because of the scientific advances of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), have left Atomic Absorption (AA) behind. This technique, however, is excellent and has a larger specificity that ICP does not have.

Figure 2 shows a diagram of an atomic absorption spectrometer with a graphite furnace.

 AAS is a reliable chemical technique to analyze almost any type of material. This post describes the basic principles of atomic absorption spectroscopy in the analysis of trace metals, such as Ag, As, Cd, Cr, Cu, and Hg, in environmental samples. For example, the study of trace metals in wet and dry precipitation has increased in recent decades because trace metals have adverse environmental and human health effects. Anthropogenic activities have substantially increased trace metal concentrations in the atmosphere. In recent decades, heavy metal concentrations have increased not only in the atmosphere but also in pluvial precipitation.

Many human activities play a major role in global and regional trace element budgets. Additionally, when present above certain concentration levels, trace metals are potentially toxic to marine and terrestrial life. Thus, biogeochemical perturbations are a matter of crucial interest in science.

The atmospheric input of metals exhibits strong temporal and spatial variability due to short atmospheric residence times and meteorological factors. As in oceanic chemistry, the impact of trace metals in atmospheric deposition cannot be determined from a simple consideration of global mass balance; rather, accurate data on net air or sea fluxes for specific regions are needed.

Particles in urban areas represent one of the most significant atmospheric pollution

problems, and are responsible for decreased visibility and other effects on public health, particularly when their aerodynamic diameters are smaller than 10 μm, because these small particles can penetrate deep into the human respiratory tract. There have been many studies measuring concentrations of toxic metals such as Ag, As, Cd, Cr, Cu, Hg, Ni, Pb in rainwater and their deposition into surface waters and on soils. Natural sources of aerosols include terrestrial dust, marine aerosols, volcanic emissions and forest fires. Anthropogenic particles, on the other hand, are created by industrial processes, fossil fuel combustion, automobile mufflers, worn engine parts, and corrosion of metallic parts. The presence of metals in atmospheric particles and the associated health risks of these metals.

Anthropogenic sources have substantially increased trace metal concentrations in

atmospheric deposition. In addition, acid precipitation favors the dissolution of many trace metals, which enhances their bioavailability. Trace metals from the atmosphere are deposited by rain, snow and dry fallout. The predominant processes of deposition by rain are rainout and washout (scavenging). Generally, in over 80 % of wet precipitation, heavy metals are dissolved in rainwater and can thus reach and be taken up by the vegetation blanket and soils. Light of a specific wavelength, selected appropriately for the element being analyzed, is given off when the metal is ionized in the flame; the absorption of this light by the element of interest is proportional to the concentration of that element.

Quantification is achieved by preparing standards of the element.

• AAS intrinsically more sensitive than Atomic Emission Spectrometry (AES)
• Similar atomization techniques to AES
• Addition of radiation source
• High temperature for atomization necessary
• Flame and electrothermal atomization
• Very high temperature for excitation not necessary; generally no plasma/arc/spark in AAS

We will discuss the Flame AAS technique and AAS with Graphite Furnace (GFAA) in the upcoming posts.

COMPARISON BETWEEN ED-XRF AND WD-XRF

We have discussed about  ED-XRF & WD-XRF in the earlier posts . Now I would like to highlight the major differences between the two X-ray techniques . The most important point of comparison are listed below ::

1. RESOLUTION :

It describes the width of the spectra peaks. The lower the resolution number the more easily an elemental line is distinguished from the nearby X-ray line intensities.

a) The resolution of the WD-XRF system is dependent on the crystal and optics design,particularly collimation, spacing and positional reproducibility. The effective resolution of a WD-XRF system may vary from 20 eV in an inexpensive bench top to 5 eV or less in a laboratory instrument. The resolution is not detector dependant.

Advantage of WD-XRF: High resolution means fewer spectral overlaps and lower

background intensities

b)  The resolution of ED-XRF system is dependent on the resolution of the detector. This can vary from 150 V or less for a liquid nitrogen cooled Si(Li) detector, 150 – 220 eV for various solid state detectors, or 600 eV or more for gas filled proportional counter.

Advantage of ED-WRF: WD-XRF crystal and optics are expensive, and are one more failure mode.

2. SPECTRAL OVERLAPS:

Spectral deconvolutions are necessary for determining net intensities when two spectral lines overlap because the resolution is too high for them to be measured independently.

a) With a WD-XRF instrument with very high resolution (low number of eV) spectral

overlap corrections are not required for a vast majority of elements and applications.

The gross intensities for each element can be determined in a single acquisition.

Advantage WD-XRF: Spectral deconvolutions routines introduce error due to counting statistics for every overlap correction onto every other element being corrected for. This can double or triple the error

b) The ED-XRF analyzer is designed to detect a group of elements all at once. The some type of deconvolutions method must b used to correct for spectral overlaps. Overlaps are less of a problem with 150 eV resolution systems, but are significant when compared to WD-XRF. Spectral overlaps become more problematic at lower resolutions.

3. BACKGROUND: 

The background radiation is one limiting factor for determining detection limits, repeatability, and reproducibility.

a) Since a WD-XRF instrument usually uses direct radiation flux the background in the region of interest is directly related to the amount of continuum radiation within the region of interest the width is determined by the resolution.

b) The ED-XRF instrument uses filters and/or targets to reduce the amount of continuum radiation in the region of interest which is also resolution dependant, while producing a higher intensity X-ray peak to excite the element of interest.

Even, WD-XRF has the advantage due to the resolution. If a peak is one tenth as wide it has one tenth the background. ED-XRF counters with filters and targets that can reduce the background intensities by a factor of ten or more.

4. EXCITATION EFFICIENCY:

Usually expressed in PPM per count-per-second (cps) or similar units, this is the other main factor for determining detection limits, repeatability, and reproducibility. The relative excitation efficiency is improved by having more source x-rays closer to but above the absorption edge energy for the element of interest.

a. WDXRF generally uses direct unaltered x-ray excitation, which contains a continuum of energies with most of them not optimal for exciting the element of interest.

b. EDXRF analyzers may use filter to reduce the continuum energies at the elemental

lines, and effectively increasing the percentage of X-rays above the element absorption edge. Filters may also be used to give a filter fluorescence line immediately above the absorption edge, to further improve excitation efficiency. Secondary targets provide an almost monochromatic line source that can be optimized for the element of interest to achieve optimal excitation efficiency.

ENERGY DISPERSIVE X-RAY FLUORESCENCE (ED-XRF)

INTRODUCTION

In Energy Dispersive X-Ray Fluorescence spectrometry (ED-XRF), the identification of characteristic lines is performed using detectors that directly measure the energy of the photons. In energy dispersive X-ray fluorescence analysis (EDXRF), a solid-state detector is used to count the photons, simultaneously sorting them according to energy and storing the result in a multichannel memory. The result is an X-ray energy vs. intensity spectrum. The range of detectable elements ranges from Be (Z = 4) for the light elements and goes up to U (Z = 92) on the high atomic number Z side. In principle, XRF analysis is a multielement analytical technique and in particular, the simultaneous determination of all the detectable elements present in the sample is inherently possible with EDXRF. In WDXRF both the sequential and the simultaneous detection modes are possible. Although energy dispersive detectors generally exhibit poorer energy resolution than wavelength dispersive analyzers, they are capable of detecting simultaneously a wide range of energies. The most frequently used detector in EDXRF is the silicon semiconductor detector, which nowadays can have excellent energy resolution.

INSTRUMNTATION

An ED-XRF system consists of several basic functional components, as shown in

Figure The major components are as follows :

1. X- Ray excitation source
2. Sample Chamber
3. Si (Li) detector
4. Preamplifier
5. Main Amplifier
6. Multichannel Pulse Height Analyzer

The properties and performances of an EDXRF system differ upon the electronics and the enhancements from the computer software.   

Typical ED-XRF detection arrangement.

We will discuss in detail for every component :

1. Excitation Mode

A) Direct Tube Excitation .

Because of the simplicity of the instrument and the availability of a high photon output flux by using direct tube excitation, the X-ray fluorescence spectrometer equipped with an Xray tube as direct excitation source is gaining more and more attention from manufactures. The spectrometer is more compact and cheaper compared to secondary target systems. Of course, the drawback is still the less flexible selection of excitation energy. However, by using an appropriate filter between tube and sample, one can obtain an optimal excitation.

The most popular X-ray tube used in direct excitation ED spectrometer is the side window tube for reasons of simplicity and safety. With direct tube excitation, low powered X-ray tubes (< 100 W) can be used. These air cooled tubes are very compact, less expensive, and only require compact, light, inexpensive, highly regulated solid state power supplies. In a WD spectrometer, on the other hand, high-power tubes (3-4 kW) are essential to compensate for the losses in the crystal and collimator. With the low-power tubes used in ED spectrometer, better excitation of light elements (i.e. low-Z element), analysis of smaller samples, small spot analysis, and compact systems can be obtained.

B) Secondary Target Excitation.

The principle of secondary target excitation was developed to avoid the intense

Bremsstrahlung continuum from the X-ray tube by using a target between tube and sample. 

Schematic illustration of secondary target excitation

The ratio of the intensity of the characteristic lines to that the continuum in secondary target excitation is much higher than that in direct tube excitation because the continuum part of the excitation spectrum of the secondary target is generated only by scattering. One can excite various elements efficiently by selecting a secondary target that has characteristic lines just above the absorption edges of the elements of interest in the sample. Therefore, secondary target excitation has some obvious advantages over direct tube excitation: its flexibility for getting an optimized and near monochromatic excitation providing a better selectivity and an improved sensitivity. However, to compensate for the intensity losses that occur at the secondary scatterer, a high-powered tube as used in WD spectrometers is required; making the whole system more sophisticated and expensive compared to direct tube excitation setups.

C) Radio Isotopic Excitation.

A variety of about 30 commercially available radio-isotopic materials can be chosen for an optimal excitation. The X-rays and/or γ-rays emitted from these radio-isotopic sources cover a wide range (10 – 60 keV) of excitation energies. With a high energy source like 241 Am, K lines instead L lines can be used for quantification in the case of analyzing high-Z rare earth elements, with considerably less matrix effects and spectrum overlaps. Sometimes the same idea as in the secondary target excitation is used to avoid non-photon radiation. A proper design of excitation-detection geometry can improve greatly the sensitivity and accuracy of the XRF analysis with such excitation source. The disadvantages of using radioisotopic sources however lie in their low photon output, intensity decay and storage problems.

2. Detectors

Energy dispersive X-ray spectrometry is based upon the ability of the detector to create signals proportional to the X-ray photon energy, therefore, mechanical devices, such as analyzing crystals, are not required as in wdxrf . Several types of detectors have been employed, including silicon, germanium and mercuric iodide .

Cross section of an Si(Li) detector crystal with p-i-n structure and the

production of electron-hole pair.

The solid state, lithium-drifted silicon detector, Si(Li), was developed and applied to Xray detection in the 1960’s. Early 1970’s, this detector was firmly established in the field of X-ray spectrometry, and was applied as an X-ray detection system for scanning Electron Microscopy (SEM) as well as X-ray spectrometry. The principal advantage of the Si(Li) detector is its excellent resolution.

Si(Li) detector can be considered as a layered structure in which a lithium-drifted active region separates a p-type entry side from an 

n-type side. Under reversed bias of approximately 600 V, the active region acts as an insulator with an electric field gradient throughout its volume. When an X-ray photon enters the active region of the detector, photoionization occurs with an electron-hole pair created for each 3.8 eV of photon energy. Ideally, the detector should completely collect the charge created by each photon entry, and result in a response for only that energy. In reality, some background counts appear because of the energy loss in the detector. Although these are kept to a minimum by engineering, incomplete charge collection in the detector is a contributor to background counts. In the X-ray spectrometric, important region of 1 – 20 keV, silicon detectors have excellent efficiency for conversion of X-ray photon energy into charge. Some of the photon energy may be lost by photoelectric absorption of the incident X-ray, creating an excited Si atom which relaxes to yield an Si Kα X-ray. This X-ray may escape from the detector, resulting in an energy loss equivalent to the photon energy; in the case of Si Kα, this is 1.74 keV. Therefore, an escape peak 1.74 keV lower than the true photon energy of the detected X-ray may be observed for intense peaks. For Si(Li) detectors, these are usually a few tenths of one percent, and never more than 2%, of the intensity of the main peak.

 The Si(Li) detector schematic

Resolution of an energy dispersive X-ray spectrometer is normally expressed as the Full Width at Half Maximum amplitude (FWHM) of the Mn X-ray at 5.9 keV. The resolution will be somewhat count rate dependent. Commercial spectrometers are supplied routinely with detectors which display approximately 145 eV (FWHM @ 5.9 keV). The resolution of the system is a result of both electronic noise and statistical variations in conversion of the photon energy. Electronic noise is minimized by cooling the detector, and the associated preamplifier with liquid nitrogen (Figure). In many cases, half of the peak width is a result of electronic noise.

3. Pulse Height Analysis

The X-ray spectrum of the sample is obtained by processing the energy distribution of X-ray photons which enter the detector. A single event of one X-ray photon entering the detector causes photoionization and produces a charge proportional to the photon energy. Numerous electrical sequences must take place before this charge can be converted to a data point in the spectrum.

When an X-ray photons enters the Si(Li) detector, it is converted into an electrical charge which is coupled to a Field Effect Transistor (FET). The FET, and the rest of the associated electronics which make up the preamplifier, produce an output proportional to the energy of the X-ray photon. Using a pulsed optical preamplifier, this output is in the form of a step signal. Because photons vary in both energy and number per unit time, the output signal, due to successive photons being emitted by a multielement sample, resembles a staircase with various step heights and time spacing. When the output reaches a predetermined level, the detector and the FET circuitry is reset to its starting level, and the process repeated.

The preamplifier stage integrates each detector charge signal to generate a voltage step proportional to the charge. This is then amplified and shaped in a series of integrating and differentiating stages. Owing to the finite pulse-shaping time, in the range of microseconds, the system will not accept any other incoming signals in the meanwhile (dead time), but extend its measuring time instead. In a further step the height of these signals is digitized as a channel number (analog-to-digital converter, ADC), stored to a memory (multichannel analyzed, MCA) and finally displayed as a spectrum, where the number of counts reflects the respective intensity. In a more modern approach, the output signals of the preamplifier are digitized directly, which can increase the throughput of the system significantly.

4. Energy Resolution

Mn-Kα spectrum and calibrated pulser

The energy resolution of the EDXRF spectrometer determines the ability of a given system to resolve characteristic X-rays from multiple-element samples and is normally defined as the full width at half maximum (FWHM) of the pulse-height distribution measured for a monoenergetic X-ray. A conventional choise of X-ray energy is 5.9 keV, corresponding to the Kα energy of Mn. Figure II.6 shows a typical pulse-height spectrum of Mn-Kα X-rays simultaneously with a calibrated pulser. The purpose of the pulser measurement is to monitor the resolution of the electronic system independent of any peak broadening due to the detector itself. Typical state-of the art detectors Si(Li) and Ge(HP) achieve 130 to 170 eV, but depends strongly on the size of the crystal. The smaller the crystal, the better is the resolution.

INDUCTIVE COUPLED PLASMA OPTICAL EMISSION SPECTROMETER

INTRODUCTION

ICP/OES is one of the most powerful and popular analytical tools for the determination of trace elements in a sample types. The technique is based upon the spontaneous emission of photons from atoms and ions that have been excited in a RF discharge. Liquid and gas samples may be injected directly into the instrument, while solid samples require extraction or acid digestion so that the analytes will be present in a solution. The sample solution is converted to an aerosol and directed into the central channel of the plasma. At its core the inductively coupled plasma (ICP) sustains a temperature of approximately 10 000 K, so the aerosol is quickly vaporized. Analyte elements are liberated as free atoms in the gaseous state. Further collisional excitation within the plasma imparts additional energy to the atoms, promoting them to excited states. Sufficient energy is often available to convert the atoms to ions and subsequently promote the ions to excited states. Both the atomic and ionic excited state species may then relax to the ground state via the emission of a photon. These photons have characteristic energies that are determined by the quantized energy level structure for the atoms or ions. Thus the wavelength of the photons can be used to identify the elements from which they originated. The total number of photons is directly proportional to the concentration of the originating element in the sample.The instrumentation associated with an ICP/OES system is relatively simple. A portion of the photons emitted by the ICP is collected with a lens or a concave mirror. This focusing optic forms an image of the ICP on the entrance aperture of a wavelength selection device such as a monochromator. The particular wavelength exiting the monochromator is converted to an electrical signal by a photo detector. The signal is amplified and processed by the detector electronics, then displayed and stored by a computer.

ICP OES OPERATION 

As shown in Figure, the so-called ICP ‘‘torch’’ is usually an assembly of three concentric fused-silica tubes. These are frequently referred to as the outer, intermediate, and inner gas tubes. The diameter of the outer tube ranges from 9 to 27 mm. A water-cooled, two- or three-turn copper coil, called the load coil, surrounds the top section of the torch, and is connected to a RF generator. The outer argon flow (10–15 L min) sustains the high temperature plasma, and positions the plasma relative to the outer walls and the induction coil, preventing the walls from melting and facilitating the observation of emission signals. The plasma under these conditions has an annular shape. The sample aerosol carried by the inner argon flow (0.5–1.5 Lmin) enters the central channel of the plasma and helps to sustain the shape. The intermediate argon flow (0–1.5 Lmin) is optional and has the function of lifting the plasma slightly and diluting the inner gas flow in the presence of organic solvents.

The ICP is generated as follows. RF power, typically 700–1500 W, is applied to the load coil and an alternating current oscillates inside the coil at a rate corresponding to the frequency of the RF generator. For most ICP/OES instruments, the RF generator has a frequency of either 27 or 40 MHz. The oscillation of the current at this high frequency causes the same high-frequency oscillation of  electric and magnetic fields to be set up inside the top of the torch. With argon gas flowing through the torch, a spark from a Tesla coil is used to produce ‘‘seed’’ electrons and ions in the argon gas inside the load coil region. These ions and electrons are then accelerated by the magnetic field, and collide with other argon atoms, causing further ionization in a chain reaction manner. This process continues until a very intense, brilliant white, tear drop shaped, high-temperature plasma is formed. Adding energy to the plasma viaRF-induced collision is known as inductive coupling, and thus the plasma is called an ICP. The ICP is sustained within the torch as long as sufficient RF energy is applied in a cruder sense, the coupling of RF power to the plasma can be visualized as positively charged Ar ions in the plasma gas attempting to follow the negatively charged electrons flowing in the load coil as the flow changes direction 27 million times per second. Figure shows the temperature gradient within the ICP with respect to height above the load coil. It also gives the nomenclature for the different zones of the plasma .The induction region at the base of the plasma is ‘‘doughnut-shaped’’ as described above, and it is the region where the inductive energy transfer occurs. This is also the region of highest temperature and it is characterized by a bright continuum emission. From the IR upward towards to the tail plume, the temperature decreases. An aerosol, or very fine mist of liquid droplets, is generated from a liquid sample by the use of a nebulizer. The aerosol is carried into the center of the plasma by the argon gas flow through the induction region. Upon entering the plasma, the droplets undergo three processes. The first step is desolvation, or the removal of the solvent from the droplets, resulting in microscopic solid particulates, or a dry aerosol. The second step is vaporization, or the decomposition of the particles into gaseous state molecules. The third step is atomization, or the breaking of the gaseous molecules into atoms. These steps occur predominantly in the preheating zone . Finally, excitation and ionization of the atoms occur, followed by the emission of radiation from these excited species. These excitation and ionization processes occur predominantly in the initial radiation zone, and the normal analytical zone from which analytical emission is usually collected.

ICP OES INSTRUMENTATION

In inductively coupled plasma-optical emission spectrometry, 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. A representation of the layout of a typical ICP-OES instrument is shown in Figure.

Major components and ICP-OES instrument

SAMPLE INTRODUCTION

A sample introduction system is used to transport a sample into the central channel of the ICP as either a  gas, vapor, aerosol of fine droplets, or solid particles. The general requirements for an ideal sample introduction  system include amenity to samples in all phases (solid, liquid, or gas), tolerance to complex matrices, the ability to analyse very small amount of samples (<1mL or <50 mg), excellent stability and reproducibility, high  transport efficiency, simplicity, and low cost. A wide variety of sample introduction methods have been developed, such as nebulization, hydride generation (HG), electro thermal vaporization (ETV), and laser  ablation.

• NEBULIZERS

Nebulizers are the most commonly used devices for solution sample introduction in ICP/OES. With a nebulizer, the sample liquid is converted into an aerosol and transported to the plasma. Both pneumatic and ultrasonic nebulizers (USNs) have been successfully used in ICP/OES. Pneumatic nebulizers make use of high-speed gas flows to create an aerosol, while the USN breaks liquid samples into a fine aerosol by the ultrasonic oscillations of a piezoelectric crystal. The formation of aerosol by the USN is therefore independent of the gas flow rate. Only very fine droplets (about 8 mm in diameter) in the aerosol are suitable for injection into the plasma. A spray chamber is placed between the nebulizer and the ICP torch to remove large droplets from the aerosol and to dampen pulses that may occur during nebulization. Thermally stabilized spray chambers are sometimes adopted to decrease the amount of liquid introduced into the plasma, thus providing stability especially when organic solvents are involved. Pneumatic nebulization is very inefficient, however, because only a very small fraction (less than 5%) of the aspirated sample solution actually reaches the plasma. Most of the liquid is lost down the drain in the spray chamber. However, the pneumatic nebulizer retains its popularity owing to its convenience, reasonable stability, and ease of use. Efficiency may only be a concern when sample volumes are limited, or measurements must be performed at or near the detection limit. Three types of pneumatic nebulizers are commonly employed in ICP/OES: the concentric nebulizer, the cross-flow nebulizer, and the Babington nebulizer .

 1.   CONCENTRIC NEBULIZER

The concentric nebulizer is fashioned from fused silica. Sample solution is pumped into the back end of the nebulizer by a peristaltic pump. Liquid uptake rates may be as high as 4mLmin, but lower flows are more common. The sample solution flows through the inner capillary of the nebulizer. This capillary is tapered so that flexible tubing from the pump is attached at the entrance (4mm outer diameter) and the exit has a narrow orifice approaching 100 mm or less in inner diameter. Ar gas (0.5–1.5 Lmin) is supplied at a right angle into the outer tube. This tube is also tapered so that the exit internal diameter approaches the outer diameter for the sample capillary. As the Ar passes through this narrow orifice, its velocity is greatly increased, resulting in the shearing of the sample stream into tiny droplets. Concentric nebulizers have the advantages of excellent sensitivity and stability, but the small fragile fused-silica orifices are prone to clogging, especially when aspirating samples of high salt content. Concentric nebulizers also require a fairly large volume of sample, given the high uptake rate. The microconcentric nebulizer (MCN) is designed to solve this problem. The sample uptake rate for the MCN is less than 0.1mLmin. The compact MCN employs a smaller diameter capillary (polyimide or TeflonÒ) and poly(vinylidine difluoride) body to minimize the formation of large droplets.

 2.   CROSS-FLOW NEBULIZER

A second type of pneumatic nebulizer is the cross-flow nebulizer, shown in Figure . The operation of cross-flow nebulizers is often compared to that of a perfume atomizer. Here a high speed stream of argon gas is directed perpendicular to the tip of a capillary tube (in contrast to the concentric or micro-concentric nebulizers where the high-speed gas is parallel to the capillary). The solution is either drawn up through the capillary tube by the low-pressure region created by the high-speed gas or forced up the tube with a pump. In either case, contact between the high-speed gas and the liquid stream causes the liquid to break up into an aerosol. Cross-flow nebulizers are generally not as efficient as concentric nebulizers at creating the small droplets needed for ICP analyses. However, the larger diameter liquid capillary and longer distance between liquid and gas injectors minimize clogging problems. Many analysts feel that the small penalty paid in analytical sensitivity is more than compensated for by the freedom from clogging. Another advantage of cross-flow nebulizers is that they are generally more rugged and corrosion-resistant than glass concentric nebulizers. In fact, this nebulizer is available with a Ryton body, a clear sapphire liquid capillary tip and a red ruby gas injector tip both contained in a polyetheretherketone (PEEK) body, all which provide chemical resistance to samples .

 3. BABINGTON NEBULIZER 

The third type of pneumatic nebulizer used for ICP/OES is the Babington nebulizer that allows a film of the sample solution to flow over a smooth surface having a small orifice . High-speed argon gas emanating from the hole shears the sheet of liquid into small droplets. The essential feature of this type of nebulizer is that the sample solution flows freely over a small aperture, rather than passing through a fine capillary, resulting in a high tolerance to dissolved solids In fact, even slurries can be nebulized with a Babington nebulizer. This type of nebulizer is the least susceptible to clogging and it can nebulize very viscous liquids.


SPRAY CHAMBER

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. Some typical ICP spray chamber designs are shown in Figure 3-10. 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. In general, spray chambers for the ICP are designed to allow droplets with diameters of about 10 mm 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.

The material from which a spray chamber is constructed can be an important characteristic of a spray chamber. Spray chambers made from corrosion-resistant materials allow the analyst to introduce samples containing hydrofluoric acid which could damage glass spray chambers.


TORCH

The torches used in ICP-OES are very similar in design and function to those in the early days of ICP-OES.As shown schematically in Figure, the torches contain three concentric tubes for argon flow 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. For argon ICPs, the outer gas flow is usually about 7 - 15 liters per minute. The chamber between the outer flow and the inner flow sends gas directly under the plasma toroid. This flow keeps the plasma discharge away from the intermediate and injector tubes and makes sample aerosol introduction into the plasma easier. In normal operation of the torch, this flow, formerly called the auxiliary flow but now the intermediate gas flow, is about 1.0 L/min. The intermediate flow is usually introduced to reduce carbon formation on the tip of the injector tube when organic samples are being analyzed. However, it may also improve performance with aqueous samples as well. With some torch and sample introduction configurations, the intermediate flow may be as high as 2 or 3 L/min or not used at all. The gas flow that carries the sample aerosol is injected into the plasma through the central tube or injector. Due to the small diameter at the end of the injector, the gas velocity is such that even the 1 L/min of argon used for nebulization can punch a hole through the plasma. Since this flow carries the sample to the plasma, it is often called the sample or nebulizer flow but in present terminology, this flow is known as the inner gas flow. Furthermore, this flow acts as the carrier gas for solid aerosols from spark ablation and laser ablation sample introduction techniques.


RADIO FREQUENCY GENERATOR  

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.Most RF generators used for ICP-OES operate at a frequency between 27 and 56 MHz, most ICP generators were operated at 27.12 MHz. However, an increasing number of instruments now operate at 40.68 MHz because of improvements in coupling efficiency and reductions in background emission intensity realized at this frequency. Frequencies greater than 40 MHz also have been used but have not been as successful commercially. There are two general types of RF generators used in ICP instruments. Crystal-controlled generators use a piezoelectric quartz crystal to produce an RF oscillating signal that is amplified by the generator before it is applied at the load coil. The proper electrical parameters, such as output impedance, needed to keep the generator operating efficiently are controlled by a matching network that utilizes manual or automatic (servo mechanical) components. The speed and accuracy of this matching network are critical to the operation of this type of generator.


 DETECTION OF EMISSION

Most of the analytically useful emission lines for ICP-OES are in the 190 - 450 nm region; thus, spectrometers used for ICP-OES are usually optimized for operation in this wavelength region. However, there are also some important ICP emission lines between 160 and 190 nm and above 450 nm. Unfortunately, electromagnetic radiation in the 160 - 190 nm wavelength region is readily absorbed by oxygen molecules, and instruments must be specially designed to remove the air from the spectrometer to observe emission in this wavelength region. Removing oxygen from the spectrometer is done either by purging the spectrometer with a gas, usually nitrogen or argon, that doesn’t absorb the emission, or by removing the air from the spectrometer with a vacuum system. Recently, nitrogen filled optics maintained at atmospheric pressure and incorporating a catalyst for scrubbing the recycled nitrogen have been introduced.

1.  PMT DETECTOR

Once the proper emission line has been isolated by the spectrometer, the detector and its associated electronics are used to measure the intensity of the emission line. By far the most widely used detector for ICP-OES is the photomultiplier tube or PMT. To know more about the PMT read the topic Photomultiplier Tube.

 2. ARRAY CCD DETECTOR

Charge transfer devices  include a broad range of solid-state silicon-based array detectors. They include the charge injection device (CID) and the charge-coupled device (CCD). The CCD has found extensive use in nonspectroscopic devices such as video cameras, bar code scanners, and photocopiers. With the CTDs, photons falling on a silicon substrate produce electron–hole pairs.The positive electron holes migrate freely through the p type silicon semiconductor material, while the electrons are collected and stored temporarily by an array of metal oxide semiconductor (MOS) capacitors The CCD differs from the CID mainly in the readout scheme. The CCD is read out in a sequential charge shifting manner towards the output amplifier. The CID on the other hand may be read out in a non-destructive manner by shifting charge between adjacent electrodes, and then shifting it back again. The CID thus benefits from quick random access, even during long integration periods. Spectroscopic  applications of CTD’s has been hampered by the physical mismatch between the relatively small surface area of the detector and the large sometimes two dimensional focal plane associated with polychromators. This mismatch may be overcome, however, and one  commercial ICP spectrometer employs a CID detector having more than 250 000 pixels positioned upon an echelle focal plane. Alternative approaches have been successful with the CCD detector. In one case, a group of several CCD arrays are arranged around a Circular Optical System (CIROS) based upon a Rowland circle design. Rather than monitoring discrete wavelengths as is the case with the multiple PMT Rowland circle systems, the CIROS system provides total wavelength coverage from 120 to 800 nm, with resolution on the order of 0.009 nm

References

1. Inductively Coupled Plasma/Optical Emission Spectrometry by Xiandeng Hou and Bradley T. Jones 
2. Concepts, Instrumentation and Techniques in Inductively Coupled Plasma Optical Emission Spectrometry by Charles B. Boss and Kenneth J. Fredeen (Second edition)