X RAY FLUORESCENCE

INTRODUCTION

X-ray fluorescence (XRF) analysis is one of the most common non-destructive methods for  qualitative as well as quantitative determination of elemental composition of materials. It is suitable for solids, liquids as well as powders. There are two main methodological techniques that are wavelength dispersive analysis (WD-XRF) and energy dispersive analysis (ED-XRF) (In the next post we will briefly discuss about WDXRF & EDXRF ,this post will only explain the basics of x-ray fluorescence which is required to understand the upcoming posts about WDXRF & EDXRF ). The spectra are collected simultaneously in a wide energy range. The range of detectable materials covers all elements from Sodium (Na) to Uranium (U) and the concentration can range from 100% down to ppm. Detection limit depends upon the specific element and the sample matrix but in general heavier elements have higher detection limit.

X-ray Fluorescence (XRF) Spectroscopy involves measuring the intensity of X-rays emitted from a specimen as a function of energy or wavelength. The energies of large intensity lines are characteristic of atoms of the specimen. The intensities of observed lines for a given atom vary as the amount of that atom present in the specimen. Qualitative analysis involves identifying atoms present in a specimen by associating observed characteristic lines with their atoms. Quantitative analysis involves determining the amount of each atom present in the specimen from the intensity of measured characteristic X-ray lines. The emission of characteristic atomic X-ray photons occurs when a vacancy in an inner electron state is formed, and an outer orbit electron makes a transition to that vacant state. The  energy of the emitted photon is equal to the difference in electron energy levels of the transition. As the electron energy levels are characteristic of the atom, the energy of the emitted photon is characteristic of the atom. Molecular bonds generally occur between outer electrons of a molecule leaving inner electron states unperturbed. As X-ray fluorescence involves transitions to inner electron states, the energy of characteristic X-ray radiation is usually unaffected by molecular chemistry. This makes XRF a powerful tool of chemical analysis in all kinds of materials. In a liquid, fluoresced X-rays are usually little affected by other atoms in the liquid and line intensities are usually directly proportional to the amount of that atom present in the liquid. In a solid, atoms of the specimen both absorb and enhance characteristic X-ray radiation. These interactions are termed 'matrix effects' and much of quantitative analysis with XRF spectroscopy is concerned with correcting for these effects.

X rays are electromagnetic radiation. All X-rays represent a very energetic portion of

the electromagnetic spectrum (Table 1) and have short wavelengths of about 0.1 to 100 angstroms (Å). They are bounded by ultraviolet light at long wavelengths and gamma rays at short wavelengths X-rays in the range from 50 to 100 Å are termed soft X-rays because they have lower energies and are easily absorbed.The range of interest for X-ray is approximately from 0.1 to 100 Å. Although,angstroms are used throughout these notes, they are not accepted as SI unit. Wavelengths should be expressed in nanometers (nm), which are 10-9 meters (1 Å = 10-10 m), but most texts and articles on micro probe analysis retain the use of the angstroms. Another commonly used unit is the micron, which more correctly should be termed  micrometer  (μm), a micrometer is 104 Å. The relationship between the wavelength of electromagnetic radiation and its corpuscular energy (E) is derived as follows. 

For all electromagnetic radiation:

E = h ν ;

where:

h is the Planck constant (6.62 10-24 J.s);

ν is the frequency expressed in Hertz.

For all wavelengths,

ν = c / λ ;

where:

c = speed of light (2.99782 108 m/s);

λ= wavelength (Å).

Thus:

E = hc / λ = 1.9863610−24 /λ ;

where E is in Joule and λ in meters.

The conversion to angstroms and electron volts (1 eV = 1.6021 10-19 Joule) yields the

Duane-Hunt equation:

E(eV) 12.396/ (A)

= λ . 

Note the inversion relationship. Short wavelengths correspond to high energies and long wavelengths to low energies. Energies for the range of X-ray wavelengths are 124 keV (0.1 Å) to 124 eV (100 Å). The magnitudes of X-ray energies suggested to early workers that Xrays are produced from within an atom. Those produced from a material consist of two distinct superimposed components: continuum (or white) radiation, which has a continuous distribution of intensities over all wavelengths, and characteristic radiation, which occurs as a peak of variable intensity at discrete wavelengths.

PROPERTIES OF X-RAYS

A general summary of the properties of X-rays is presented below:

• Invisible.
• Propagate with velocity of light (3.10^8 m/s).
• Unaffected by electrical and magnetic fields.
• Differentially absorbed in passing through matter of varying composition, density and thickness.
• Reflected, diffracted, refracted and polarized.
• Capable of ionizing gases.
• Capable of affecting electrical properties of solids and liquids.
• Capable of blackening a photographic plate.
• Able to liberate photo electron. And recoils electrons.
• Emitted in a continuous spectrum.
• Emitted also with a line spectrum characteristic of the chemical element.
• Found to have absorption spectra characteristic of the chemical element.

THE ORIGIN OF X-RAYS

An electron can be ejected from its atomic orbital by the absorption of a light wave

(photon) of sufficient energy. The energy of the photon (hν) must be greater than the energy with which the electron is bound to the nucleus of the atom. When an inner orbital electron is ejected from an atom, an electron from a higher energy level orbital will transfer into the vacant lower energy orbital (Figure). During this transition a photon may be emitted from the atom. To understand the processes in the atomic shell, we must take a look at the Bohr’s atomic model. The energy of the emitted photon will be equal to the difference in energies between the two orbitals occupied by the electron making the transition. Due to the fact that the energy difference between two specific orbital shells, in a given element, is always the same (i.e., characteristic of a particular element), the photon emitted when an electron moves between these two levels will always have the same energy. Therefore, by determining the energy (wavelength) of the X-ray light (photons) emitted by a particular element, it is possible to determine the identity of that element.

PRINCIPLE OF THE X-RAY FLUORESCENCE PROCESS

If the primary energy of X-rays is equal to or is larger than the binding energy of an inner shell electron it is likely that electrons will be ejected and consequently vacancies are created. The hole state has certain life time and becomes refilled again. The transition of the excited atom into a state with lower energy occurs via two competitive processes, the above mentioned photoelectric and Auger effects. In the photoelectric effect, the recombination is accompanied by a transfer of electrons from the outer shells with energy Em into the inner shells with energy En filling the vacancies. This process induces the emission of a characteristic X-ray (fluorescence) photon with energy

                                                                   hV = Em - En

Therefore the energy of these secondary X-rays is the difference between the binding energies of the corresponding shells in the figure below. The excited atom can also recombine by emission of Auger electrons, instead of characteristic X-rays, via the Auger effect.

The probability that characteristic X-rays will be emitted - and not an Auger electron- varies from one element to another and is described as the fluorescence yield. For elements of low atomic numbers, the Auger effect dominates, whereas emission of characteristic X-rays is more likely for heavy elements.

Each element has its unique characteristic energy spectrum (Fluorescence spectrum) composed by the allowed transitions of the specific atom in the result of X-ray excitation. XRF technique consists on the study of the produced characteristic spectrum. The XRF emission induced by photoelectron effect is shown in figure below for an atom of titanium (Z=22), whose K-shell electron acquires sufficient energy to escape from the atom.

Photoelectric effect on the K-shell

An electron in the K-shell absorbs a

photon of the primary x-ray beam and

becomes free, while the atom gains a

vacancy in the K-shell.

The K lines production

An electron from the L or M shell “jumps in” to fill the vacancy and in  turn, produces a vacancy in the L or M shell. In the process, the atom emits a characteristic photon from the x-ray  range of electromagnetic spectrum, unique to this chemical element

The L lines production

After a vacancy is created in the L shell by either the primary beam photon or by the previous event, an electron from the M or N shell “jumps in” to occupy the vacancy. In this process, the atom emits a characteristic photon, unique to this chemical element, and a vacancy in the M or N shell is produced

•  Ionization of the K-shell electron in the atom of Ti by photoelectric effect and emission of characteristic photons of different spectral series as a result of electron transitions in the atom.

              Electron transitions and emitted spectral lines in the atom after the K-shell ionization

X-ray fluorescence provides a rapid non-destructive means for both qualitative and quantitative analysis. A wide range of materials varying in size and shape can be studied with minimal requirements for sample preparation. Detection sensitivities as low as one part in a million can be obtained with this technique.The two types of X-ray fluorescence i.e EDXRF and WDXRF will be discussed and explained in the next post .

Some basic terms and definitions related to X- ray fluorescence, which can be useful in the upcoming post .

• Attenuation coefficient – a natural logarithm of the ratio of the emergent and incident radiation intensities I / I0 divided by either the depth of the radiation penetration (linear attenuation coefficient) or the surface density (mass absorption coefficient).
• Bremsstrahlung – a continuous spectrum produced by a charged particle moving with deceleration.
• Continuous spectrum – a spectrum formed by photons with non-quantized energies in a wide range.
• Detection limit – a lowest amount of chemical element that can be found with probability of 99%.
• Detector resolution – possibility to distinguish two overlapping peaks in the spectrum; depends on the ratio of the distance between the two peaks and FWHM; usually accepted as a value of FWHM.
• Efficiency of a detector – the ratio of the number of photons participated in creation of a useful signal in the detector to the total number of photons incident on the detector surface.
• Energy-dispersive technique – the technique used to simultaneously detect the photons of the line spectrum in a wide range of energies.
• Fluorescence – emission of photons by a substance that has absorbed photons with higher energy.
• FWHM – full width at half maximum of the peak usually measured in electronvolts.
• Ionizing radiation – the particles or electromagnetic waves whose energy is sufficient to ionize a neutral atom or a molecule.
• Line spectrum – a spectrum formed by photons with specific quantized energies only.
• Matrix effects – The combined effect of all components of the sample other than the analyte on the measurement of the quantity of the analyte. The two main matrix effects are::

                           -(a) The attenuation of characteristic peak intensity due to inelastic scattering of photons, emitted by atoms of one chemical element, on atoms and electrons of other components

                            -(b)The enhancement of characteristic peak intensity due to additional excitation of atoms of one element by photons, emitted by other components.

• Peak intensity – the value proportional to the total number of photons with same energy registered by a spectrometer and exposed as a bell-shaped curve called the peak.
• Quantitative analysis – determination of amount of each component (chemical element) of a sample.
• Spectral series – series of spectral peaks produced by electron transitions from different energy levels to one specific energy level; K-series corresponds to all transitions to the lowest possible energy level.
• Spectrum – a function of a number of photons versus their energy, or versus their wavelength.
• Spectrum background – A component of a spectrum which does not belong to the peak of interest, may be formed by bremsstrahlung radiation or by the tails of adjacent peaks.
• X-ray tube – A kind of a vacuum tube with a filament as a cathode, emitting electrons, and a pure metal plate as an anode, producing radiation in the x-ray range of electromagnetic spectrum.

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)

CARBON MONOXIDE ANALYZER (CO ANALYZER)

INTRODUCTION

As we know that the cost of fuels are increasing day by day ,the high cost of fuels makes an economic necessity to increase the efficiency of the fuel , harness it to fullest and to minimize excess air levels and thermal air stack losses . Efforts toward combustion efficiency optimization, however, must be aimed at reducing total energy loss. This requires achieving minimum unburned combustible, as well as thermal stack losses. More precise control of air/fuel ratio, optimized for minimum total energy loss, can yield significant gains in efficiency and result in substantial savings in reduced fuel consumption.

 Flue gas concentration of carbon monoxide is a reliable and accurate indication of burner flame stoichiometry and the completeness of combustion. It is the most sensitive indicator of unburned combustibles losses. Used as a primary combustion efficiency parameter, in conjunction with oxygen analysis, carbon monoxide offers significant advantages in controlling combustion at optimum levels of excess air. Controlling air/fuel ratio to an optimum level of carbon monoxide assures minimum total energy loss, and maximum efficiency, independent of variations in burner load, fuel type and fuel quality. The measurement is relatively unaffected by air in-leakage, and burner maintenance requirements are immediately identified .

OPERATING PRINCIPLE

The CO analyser  utilizes infrared absorption spectroscopy to continuously measure CO concentration in combustion flue gases. The infrared source is mounted directly on the flue gas duct or stack on the side opposite from the receiver. Infrared energy is radiated by the source, through the flue gas, to the receiver. The receiver employs gas filter correlation and narrow band pass optical filtration with a solid state detector to determine the absorption of radiation by CO in the flue gas. These principles are illustrated in block diagram form in below given Figure . Infrared energy, radiated by the source, passes through the flue gas, where a portion of the energy is absorbed by any CO present. The remaining energy passes through the receiver window, focusing lens and, alternately, through two gas cells. One of the two cells is filled with CO,the other, nitrogen. These are inserted alternately in the optical path at a fixed frequency. Energy at the wavelengths of interest is, effectively, fully absorbed in the CO reference cell; however, energy is transmitted through the nitrogen cell without further absorption. After passing through the narrow bandpass filter, the remaining energy impinges upon the detector. Two energy levels are sensed alternately by the detector: source radiation reduced by the flue gas and reference cell CO and source radiation reduced by flue gas CO only. The resulting signals are rationed and compared with the rationed signals developed under zero CO calibration conditions. The comparative difference in ratios is used to compute flue gas CO concentration. The calibration source and span calibration cell are inserted into the optical path during automatic zero and span calibration of the instrument.

INFRARED SOURCE

The infrared source module emits broadband infrared radiation, including the waveband of interest, from 4.5 to 4.9 microns. The source consists of a stainless steel body with a conical surface for uniformity of surface temperature and maximum emissivity. The source is heated to a temperature of 1,112°F (600°C) and is controlled at this temperature to assure constant intensity. The source is fully insulated and enclosed in a carbon steel mounting sleeve designed for welding directly to the duct.

Since the IR source module is installed such that the source surface is flush with the inner wall of the duct, the source is not subject to coating or particulate buildup in most applications. Consequently, there is no purge air requirement to maintain source cleanliness. Due to the large diameter of the source surface, focusing is not required, and the source contains no focusing optics whatsoever. An added benefit of the large diameter source is insensitivity to duct vibration and elimination of the need for constant realignment otherwise required of focused systems. Maintaining the source temperature at 1,112°F (600°C) requires powering the heater at a nominal 50% duty cycle, extending the heater element life considerably. The operating life of the infrared source is approximately four

times that of conventional infrared sources

INFRARED RECEIVER MODEL

The infrared receiver module is designed to house the optics, detector and necessary electronics and hardware to determine absorption of infrared radiation emitted by the infrared source module. The infrared receiver module is designed to house the optics, detector and necessary electronics and hardware to determine absorption of infrared radiation emitted by the infrared source module.

In situ CO instrumentation employing thermoelectrically cooled, photoconductive detectors, the receiver employs a non-cooled pyro electric detector. Not only does this provide reliable, stable performance at high ambient temperature, it completely eliminates the maintenance associated with thermoelectric cooling systems.

FLUE GAS TEMPERATURE MEASUREMENT

The absorption of infrared radiation by carbon monoxide in combustion flue gases is a function of flue gas temperature. The temperature affects the density of the gas and, therefore, the number of molecules encountered by the radiation. In addition, temperature variations induce variations in the infrared absorption characteristics of carbon monoxide. To account for these variations, flue gas temperature must be measured continuously. Temperature data is input to the receiver module and communicated to the control module. The control module software is fully characterized to provide accurate temperature compensation over the full flue gas temperature range of 200°F to 600°F (93°C to 316°C).

References :

Rosemount Analytical-Product Data Sheet PDS 106510A