Paper No. : Atomic Spectroscopy
Module : 24 Flame Atomic Emission Spectroscopy
Principal Investigator: Dr.NutanKaushik, Senior Fellow
The Energy and Resouurces Institute (TERI), New Delhi Co-Principal Investigator: Dr. Mohammad Amir, Professor of Pharm. Chemistry,
JamiaHamdard University, New Delhi
Paper Coordinator: Dr. MymoonaAkhtar, Associate professor, Dept. of Pharm.
Chemistry, JamiaHamdard, New Delhi.
Content Writer: Dr. MymoonaAkhtar, Associate professor, Dept. of Pharm.
Chemistry, JamiaHamdard, New Delhi.
Content Reviwer: Dr. Gita Chawla, Associate Professor of Pharm.
Chemistry, JamiaHamdard University, New Delhi
Description of Module
Subject Name Analytical Chemistry / Instrumentation Paper Name Atomic Spectroscopy
Module Name/Title Flame Atomic Emission Spectroscopy
Module Id 24
Pre-requisites
Objectives Basic introduction of Flame Photometry and working of a flame photometer and events that are involved.
Types of nebulizers and their use in particular condition and conditions for best use.
How different flame temperatures can be obtained by using different fuels and oxidants.
Types of monochromators and detectors and their advantage and disadvantage Keywords Flame Photometry, grating, atomizers, nebulizers, Total consumption burner
1. Introduction
Flame photometry is a branch of atomic spectroscopy, more accurately it is flame atomic emission spectroscopy. It is quantitative application of the well known flame colouring probes used for determining the metal ions in different types of samples both organic and non organic. sodium, potassium, calcium, lithium, Cesium are some of the elements very frequently detected by this technique .
In other words it is a method of inorganic chemical analysis used for measuring the concentration of upto 10 different elements usually from the species of alkali metals (Group 1) and alkaline earth metals (Group II) by using interchangeable filters or diffraction gratings.
1.1 Basis of flame photometric working :
The elements are dissociated into atoms by the thermal energy provided by the flame .These atoms get excited to the higher energy levels where they highly unstable. These atoms tend to move to lower energy ground state to stablise them and in doing so they emit some radiations which can be visualized in the visible region of the spectrum. These wavelengths are specific for specific elements.
Fig 1: Brief overview of the Flame emission spectroscopy process.
1.2 Events that take place in FES
The solution containing metallic salt is sprayed as a fine solution into the burner. The droplets in the solution dry due to the heat leaving the fine residue of salt. The flame converts the salt into gaseous atoms. These free atoms are then transformed into excited electronic states by the absorption of additional thermal energy from the flame where they are not stable and and to
return to the ground state .The radiation emitted is characteristic of the element and its intensity of is related to the concentration of the element. The wavelength used for measurement of an element depends on the selection of a line with sufficient intensity to provide adequate sensitivity. It Should also be free from interfering lines near the selected wavelengths .g. Sodium – yellow; lithium – red; Potassium – Violet; Magnesium – blue.
These colours are characteristic to metal as cations.
Fig 2: Schematic diagram of flame photometer. (Adapted from Source:
2. Parts of a flame photometer
The flame spectrometer or flame photometer consists of the following components
Pressure regulators and flow meters or the fuel gases and Source of flame:
Flame atomizer
Optical system (Monochromators):
Photo detector:
Recorder
2.1 Pressure Regulators and Flow Meters—
It is necessary to keep gas pressure and gas flows constant in order to achieve a steady emission reading while the flame photometer is in use. Double diaphragm pressure regulators—a 10 lb/in2 for the fuel and a 20 lb/in2 gauge for the oxygen or air supply followed by a rotameter should be installed in the line from the gas cylinders to the burners.
2.2 Flame Source —
The source system in flame photometry consists of a gas flow regulators, an atomizer and a burner. The actual analysis depends upon the various variables which include the flow rates of the fuel and oxidant, the rate of introduction of the sample, and the drop let size of the atomized solution.
3. NEBULIZERS:
The general term nebulizer refers to an apparatus that converts liquids into a fine mist.
Nebulization is a process in which the sample is converted into a fine mist of finely divided droplets by using jet of compressed gas. The flow of the gas carries the sample into the atomization region.
3.1 Types of nebulizers: Most common types of nebulizers used are Pneumatic nebulizers (most common) and ultrasonic nebulizer
3.2
Fig 3: various types of Nebulizers. Source
3.2 Pneumatic nebulizers.
Pneumatic nebulizers are further divided into three types:
Fig 4: Types of Pneumatic nebulizer.
3.2.1 Concentric tube. This is the most common type of Pneumatic nebulizer used. In this type of Pneumatic nebulizers, the liquid sample is drawn up by the pressure drop generated as the nebulizer gas passes through the orifice. This is also called “free running” or “self aspiration” Due to high velocity the sample breaks down into a mist and is carried to the atomization region.
Advantage- – Generally, the ion signal produced is much more stable. However this type of nebulizers have som limitations. They are as it cannot handle the sample with high total dissolved salts (TDS - 0.25% m/v solids); i.e. 250 mg sample dissolved in 100 g of solution.
Fig 5: Concentric tube Nebulizer. source
3.2.2 Cross-flow In this case, the flow of jet stream is at right angles to the capillary tip.
Sometimes the sample is pumped through the capillary.
Fig 6: cross flow nebulizer
3.2.3 Fritted disk the gas jet is flowed through the fritled disk in which the sample is pumped. With this the aerosols obtained are finer than the others.
Fig 7: Fritted disk
3.3 ULTRASONIC NEBULIZER
The sample is fed to the surface of a vibrating piezoelectric transducer operated at a frequency of between 0.2 and 10 MHz. The mist obtained is more homogeneous and denser than pneumatic nebulizers. The production of aerosols is also very efficient and independent of gas flow rate unlike pneumatic nebulizers.
The efficiency and detection limits of ultrasonic nebulizer are better than the pneumatic nebulizers. However Long wash-out times and lots of glassware required, bad memory effects and cost are some of the limitations for its use.
Fig 8: Ultrasonic nebulizer 3.4 Electrothermal Vaporizers:
An electrothermal vaporizer (ETV) is a device which is heated, to the temperature required for analyte vaporization, by the passage of electrical current through its body. It is used as a sample introduction device for liquid and solid samples into a spectroscopic excitation or ionization source such as a flame or plasma discharge.
Fig 9: Electrothermal Vaporizers 4. Atomization
After nebulization, the sample is carried into a flame where the process of atomization takes place. Once the sample reaches the flame, three more steps occur, desolvation, volatilization, and dissociation.
Desolvation is a process in which the solvent is evaporated to produce molecular aerosol.
Volatilization is in which gaseous molecules are produced from the aerosol and
Dissociation is in which atomic gas is formed by the dissociation of molecules. In this process of ionization of atomic gases some cations and electrons are also formed.
4.1 Fuels and Oxidants used in Flame Atomizer.
The most common fuels and oxidants used to produce flames for AAS are listed in the table 1. To achieve specific temperature range a mixture of different oxidants and fuels can be used.
Table 1. Common fuels and oxidants used to produce flames for AAS.
.
Flame Structure
Fig:10 Flame Structure Adopted from blogs/Maryville.com
The different locations of flame are not equal in temperature and in fuel to oxidant ratio.
The three important regions of Flame are primary combustion zone, interzonal region and secondary combustion zone (Fig 5). The interzonal region is the hottest area of the flame and is prevalent in free atoms. Therefore it is, the most widely part of the flame used for spectroscopic analysis. Above the burner tip the flame usually rises about 5 cm with max temperature point at 2.5 cm. The portion of the flame used for AAS is specific as to what element is being analyzed. For different elements the max absorbance is achieved at different distances (cm) above the burner due to the formation of oxides.
REQUIREMENTS OF A FLAME Should have proper temperature.
Temperature should remain throughout the operation Functions of flame
To convert liquid sample into vapour state
To decompose sample into atoms and simple molecules To excite atoms to emit radiant energy
Performance
For all the liquid- sample introductions Flame atomic atomization is the most reproducible, however it has many limitations.
Sensitivity of flame atomization is lower than electrothermal atomization The absorbance of samples are reduced due to formation of oxides . Fluctuations of flame affect the absorbance of samples.
5. BURNERS.
There are two types of burners, Total consumption burner and Premix or laminar flow Burner
5.1 Total consumption burner:
In the total consumption burner, the fuel, oxidant, and sample all meet at the base of the flame. under pressure the fuel (usually acetylene) and oxidant (usually air) are forced into the flame where the Sample is drawn for aspiration through a capillary by high pressure.
The vacuum is created in the sample line by the rush of the fuel and oxidant through the burner head and draws the sample from the sample container into the flame with a
"nebulizing" or mixing effect.
The main advantages of this burner are whole sample is consumed, fractionation of samples do not takes place, is relatively inexpensive and free from explosion hazards
Fig 11 : Total consumption burner
5.2Premix or laminar flow Burner:
The sample is nebulized and mixed with the fuel and oxidant prior to introduction into the flame, with the use of a series of baffles. Here also the sample is drawn from the sample container via the vacuum created by the rushing fuel and oxidant (aspiration). A drain line is required in this design in order to remove sample solution droplets that do not make it all the way to the flame.
Fig 12: Premix burner Advantages
High turndown ratio: up to 10:1
Efficient combustion
Low CO and NOx
Low noise generation
Uniform surface temperature
Low running cost
Low capital cost
However major drawback In these type of burners is that the loss of sample is high as compared to total combustion
6. Monochromators
The name monochromator is derived from the Greek roots mono meaning Single and Chroma meaning Colour and the latin suffix –ator denoting an agent. A monochromator is an optical device used to transmit a selectable narrow band of wavelengths of light or other radiation chosen from a broad range of wavelength at input or output. A monochromator either uses the phenomenon of optical dispersion in a prism or differaction from differaction gratings to spatially
6.1 Components of Monochromators
An entrance slit that provides a rectangular optical image,
a collimating lens or mirror that produces a parallel beam of radiation,
a prism or a grating that disperses me radiation into its component wavelengths,
a focusing element that reforms the image of the entrance slit and focuses it on a planar surface called a focal plane, and
an exit slit in the focal plane that isolates the desired spectral band.
In addition, most monochromators have entrance and exit windows, which are designed to protect the components from dust and corrosive laboratory fumes 6.2 Types of monochromators
In flame emission spectroscopy two basic types of monochromators are used which are as
Grating Monochromators
Prism Monochromators 6.2.1 Grating Monochromators
Based on diffraction & interference Transmission Gratings & Reflection Gratings consist of a series of grooves in glass or quartz or a mirror (usual kind)
A transmission grating consists of the lines are ruled on glass and the unruled portions acts as the slits
Reflection grating: the lines are ruled on a polished metal surface and the incident light is reflected from the unruled areas, producing by reflection the same result as that by transmission through the transmission grating.
Fig 13: Czerney-Turner grating monochromator
Depending on the shape of the ruled surface, one can also distinguish between plane and curved gratings.
Curved reflection gratings have a focusing capability, depending on the radius of the curvature of the surface, which will be superimposed on the diffraction effects of the grating itself.
6.2.3 Prism Monochromators
Prisms create angular dispersion by refracting the light at the surface of two interfaces and can be used to disperse all the three types of radiations viz ultraviolet, visible, and infrared radiation. Despite low dispersion a prism has an advantage of having wide spectrums.
However it has limitation for focusing a desired wavelength through the exit slit as the method used by prisms is non-linear dispersion.
The wavelength and dispersion are inversely proportional, where increased dispersion is caused
by shorter wavelengths. The figure below depicts the nonlinear dispersion of a prism.The material used for their construction differs, however, depending upon the wavelength region.
Fig 14: Bunsen Prism Monochromator
7. Detectors
Important characteristics:
Wavelength response Sensitivity
Frequency of response (response time) Stability
Cost
convenience
7.1 Types of detectors 7.1.1 Photovoltic Detector:
Photovoltic Detector are made up of two types of semi-conductor material, p- type(acceptor, Al, B, In, Ga) and n-type (donor, As, Sb, P).Both types contain silicon crystal which is commonly used in semiconductor.In a silicon crystal, each silicon atom is bonded to its neighboring atom by four electrons, forming covalent bonds.
The p-type material is differentiated from the n-type material by alteration in the silicon
Prism
Enterance Slit
Collimating
Lens
Focusing
Lens
Exit Slit
intrinsic semiconductor silicon crystal. In case of p-type material , boron atoms are added as a doping agent which have only three valence electrons. This results in the formations of
“electron holes” in the silicon lattice of the p-type material. In case of n-type material phosphorus atoms are doped which have five valence electrons, an extra valence electron which results in the formation of an extra electron free from the covalent bonds in the silicon lattice. The free mobile electrons of the n-type material diffuse in the electron holes of the p- type material, making the atom positively charged with newly filled holes. The mobile electron holes also diffuse to the electrons of the n-type material, making the atom negatively charged.
This formation of negatively and positively charged ions creates a region absent of mobile charge carriers known as the depletion region. Further diffusion of charge carriers is prevented by the potential difference, when it reaches at its peak in the depletion region. This results in the reduction of conductance nearly to zero in this region. When the radiation is applied across the diode in this region, it swept through the device to produce a current that is proportional to the radiant power.
7.1.2 Photomultiplier Tube: Among all the present photosensitive devices used, photomultipier tube (PMT) is a versatile device. It has extremely high sensitivity and is used for the measurement of low radiant power. The schematic of a PMT is similar to that of a traditional phototube. The traditional phototube consists of two electrodes, a cathode and an anode. When the voltage is applied to the electrodes, the electrons are generated at the cathode and moves towards the anode. This flow of electrons generates photocurrent in the anode which is measured. The diagram of a traditional phototube is shown in the fig 15.
PMT contains series of electrodes called dynodes, each dynode is given slightly more positive potential than that of the neighboring nearer to the photocathode. The anode is kept more photopositive than any electrode. When the photon impinges the cathode, the electrons are accelerated towards the dynodes because of the increasing positive charge.
The electrons gather at the anode where they are collected in the form of a current. This current is then converted to a voltage and measured. However, these tubes are limited to measure low power radiation because if the source of radiation is intense it can cause
limitation’s associated with the PMTs is the noise resulted from the thermal dark currents. These dark currents usually result from thermal emission which can be reduced by cooling the transducer to ~-30 degrees Celsius. A coolant can be circulated around the PMT to achieve this. With the proper set-up and care, PMTs can be used to detect individual photons at the cathode.
8. Readout Devices.
To enable to read the signal, the system requires a type of display that can process the information that the instrument is sending. This is achieved through an electronic component that displays the information in a format the researcher can use effectively. The data collected by the instrument is analog based, and needs to be converted into a digital format for the display. This is accomplished by a transducer. The transducer sends the digital energy to the processor, which allows for the processing of discrete times, frequencies, and domains of the signal. This signal is put into a sequence of numbers or symbols that can be displayed on a readout. Several types of readout devices are used in modern instruments.
These devices include Digital Meters, Recorders, Cathode-Ray Tubes, LCD panels, and Computer Displays.
Summary
In this module we discussed about basic introduction of Flame Photometry and its instrumentation. The working of a flame photometer and events that are involved were explained. The instrumentation for determination of emission from various elements is discussed in detail including various parts. Types of nebulizers and their use in particular condition and conditions for best use. How different flame temperatures can be achieved by using different fuels and oxidants in different proportions. Types of monochromators and detectors and their advantage and disadvantage were also discussed.
