TOMICBSORPTIONPECTROSCOPY

ATOMSNucleus- protons (+ve) and neutrons (neutral).Electrons- (-ve) charged particle.Shells- consists of subshells.

NucleusBohrs shell model: SODIUM atomShellsElectrons

Shell, Subshells, ElectronsElectrons are arranged according to their energy levels. They are arrange in subshells, the subshells are arrange in shells and shells are arrange around the nucleus. Shells Atom (around the nucleus)SubshellsElectrons!Note:Electrons that are near the nucleus has a lower energy level than the electrons that are much far away. However, they experience stronger attraction in the nucleus than those ones that are further away.

Exciting atomsground state: is a status where the atoms electrons are in their lowest possible energy level. (stable)

excited state: another status where the atoms electrons absorb enough energy to be promoted to a higher level. Therefore, they are not in their lowest energy level. (unstable)

Excited atomGenerally, atoms are in their ground state but when an atom receives enough input of energy that their electrons requires to be promoted to a higher energy level. They will then turn to their excited state.Ground state: sodium atomExcited state: sodium atomSince, an atoms excited state is very unstable it rapidly jump back down to its ground state. This jump then causes the atom to release the energy it absorbed in the form of photons of light.!Note:Take note that the electron can go back down to its original place in more than one jump.

*Technique Flame Test

The Copper Flame

The Potassium Flame

The Manganese Flame

The Cobalt Flame

The Calcium Flame The calcium flame is red. This is intensely red because the calcium content is high.

*Flame Atomizers (Continuous Atomizers)Flame Atomizers (Continuous Atomizers)There are several types of flame atomizers available. The simplest is a turbulent flow burner that is very similar to conventional Bunsen burner. This type of burner suffers from fluctuations in temperature since there is no good mechanism for homogeneous mixing of fuel and oxidant. The drop size of nebulized sample is also inhomogeneous which adds to fluctuations in signal. The path length of radiation through the flame is small which suggests a lower sensitivity of the technique.

*Turbulent flow burners are also susceptible to flashback. These drawbacks were overcome using the most widely used laminar flow burner where quite flames and long path length are obtained. Flashback is avoided and very homogeneous mixing between fuel, oxidant, and droplets take place. Larger droplets are excluded and directed to a waste container. A schematic representation of the burner is shown below:

*

*

*

*

Elements detectable by atomic absorption are highlighted in pink in this periodic table

What is AAS?(AAS) Atomic Absorption Spectroscopy is:Quantitative technique

Typical samples: low viscosity samples

Generally uses to determine the amount of several metals (e.g. Cu, Fe, Zn, Mg) in the soil, blood, air, water, and food.

Occasionally can be use as a qualitative technique (can indentify chemicals that are present) 68 elements

In the electromagnetic spectrum, it uses the visible part to detect the presence of metals (p.77 chem book)Foundation of the technique: the absorption of the light energy that has the right wavelength causes the electrons from the sample to be promoted from a lower energy level to a higher energy level.

A block diagram of the AA spectrometer appears below.The IB does not require the inclusion of the photomultiplier tube (PMT), but it none the less is an important part of the instrumentation.

Overview of AA spectrometer.Light SourceDetectorSampleCompartment

Dr. Maha Daghestani

Dr. Maha Daghestani

*

*1. Flame AtomizationFlames are regarded as continuous atomizers since samples are continuously introduced and a constant or continuous signal is obtained. Samples in solution form are nebulized by one of the described nebulization techniques discussed previously. The most common nebulization technique is the pneumatic nebulization. Nebulized solutions are carried into a flame where atomization takes place.

*Several processes occur during atomization including: a. Nebulized samples are sprayed into a flame as a spray of very fine dropletsb. Droplets will lose their solvent content due to very high flame temperatures in a process called desolvation and will thus be converted into a solid aerosol. c. The solid aerosol is volatilized to form gaseous molecules

*d. Gaseous molecules will then be atomized and neutral atoms are obtained which can be excited by absorption of enough energy. If energy is not enough for atomization, gaseous molecules will not be atomized and we may see molecular absorption or emissione. Atoms in the gaseous state can absorb energy and are excited. If energy is too much, we may observe ionization.

*

*

*Processes occurring during atomizationFlame atomization

*Nebulizer - burnerA typical premix burnerFlame atomization

*Nebuliser - burnerTo convert the test solution to gaseous atomsNebuliser --- to produce a mist or aerosol of the test solutionBurner head --- The flame path is about 10 12 cmVaporising chamber --- Fine mist is mixed with the fuel gas and the carrier gas Larger droplets of liquid fall out from the gas stream and discharged to waste

*Fuel and oxidant flameb Air acetyleneAir- propaneAir- hydrogenb Nitrous oxide acetyleneAuxiliary oxidantFuel

*Common fuels and oxidants used in flame spectroscopy

*Disadvantages of flame atomizationOnly 5 15 % of the nebulized sample reaches the flameA minimum sample volume of 0.5 1.0 mL is needed to give a reliable readingSamples which are viscous require dilution with a solvent

*Graphite furnace techniqueEletrothermal atomization

*Plateau Graphite Tube

*Graphite furnace techniqueprocessdryingashingatomization

*Graphite furnace techniqueAdvantagesSmall sample sizes ( as low as 0.5 uL)Very little or no sample preparation is neededSensitivity is enhanced ( 10 -10 10-13 g , 100- 1000 folds)Direct analysis of solid samples

*Graphite furnace techniqueDisadvantagesBackground absorption effects Analyte may be lost at the ashing stageThe sample may not be completely atomizedThe precision was poor than the flame method(5%-10% vs 1%)The analytical range is relatively narrow(less than two orders of magnitude)

*Cold vapour techniqueHg2+ + Sn2+ = Hg + Sn (IV)

*Hydride generation methodsFor arsenic (As), antimony (Te) and selenium (Se)As (V) AsH3As0(gas) + H2NaBH4(sol)heatin flame[H+]

*

*--- diffraction gratingMonochromator

*Detector--- photomultiplier

*Read-out system--- meter --- chart recorder--- digital display

*Atomic absorption spectrophotometer

*Interferences Spectral interferences Chemical interferences Physical interferences

*Spectral interferences ----- spectral overlap +, positive analytical error Cu 324.754 nm, Eu 324.753 nmAl 308.215 nm , V 308.211nm,Al 309.27 nmAvoid the interference by observing the aluminum line at 309.27 nm

*Spectral interferences ----- non-absorption line Non- absorbable suture ----- molecular absorption + combustion products (the fuel and oxidant mixture)Correct by making absorption measurements while a blank is aspirated into the flame

*Spectral interferences ----- light scatter +Metal oxide particles with diameters greater than the wavelength of lightWhen sample contains organic species or when organic solvents are used to dissolve the sample, incomplete combustion of the organic matrix leaves carbonaceous particles that are capable of scattering light

*Spectral interferences ----- light scatter Light Scattering+The interference can be avoided by variation in analytical variables, such as flame temperature and fuel-to oxidant ratioStandard addition methodZeeman background correction

*Chemical interferences----- Formation of compound of low volatilityIncrease in flame temperaturebbbUse of releasing agents (La 3+ ) Separation Ca 2+ PO43- Mg2+, Al3+bUse of protective agents (EDTA)

*Chemical interferences----- IonizationbAdding an excess of an ionization suppressant (K)

*Physical interferences----- viscosity ----- density----- surface tension----- volatilitybMatrix matching

*Experimental preliminariesPreparation of sample solutionsOptimization of the operating conditions----- resonance line----- slit width----- current of HCL----- atomization conditionCalibration curve procedure

*The standard addition technique

*Sensitivity and detection limitSensitivity----- the concentration of an aqueous solution of the elements which absorbs 1% of the incident resonance radiation----- the concentration which gives an absorbance of 0.0044

*Detection limitSensitivity and detection limit----- the lowest concentration of an analyte that can be distinguished with reasonable confidence from a field blankD = c 3 / A

*Sensitivity and detection limit(ng/mL)

*Advantages and disadvantagesHigh sensitivity [10-10g (flame), 10-14g (non-flame)]Good accuracy (Relative error 0.1 ~ 0.5 % )High selectivityWidely usedA resonance line source is required for each element to be determined

*The different processes occurring in flames are complicated and are not closely controlled and predicted. Therefore, it can be fairly stated that the atomization process in flames may be one of the important parameters limiting the precision of the method. It is therefore justified that we have a closer look at flames and their characteristics and the different variables contributing to their performance.

*Types of FlamesFlames can be classified into several types depending on fuel/oxidant used. For example, the following table summarizes the features of most familiar flames.Therefore, it can be clearly seen that significant variations in flame temperatures can be obtained by changing the composition of fuel and oxidant.

*

*On the other hand, flames are only stable at certain flow rates and thus the flow rate of the gas is very important where at low flow rates (less than the maximum burning velocity) the flame propagates into the burner body causing flashback and, in some cases, an explosion. As the flow rate is increased, the flame starts to rise above the burner body. Best flames are obtained when the flow rate of the gas is equal to the maximum burning velocity. At this equity ratio the flame is most stable. At higher ratios, flames will reach a point where they will no longer form and blow off the burner.

Discovery of AAS1952, the Australian scientist Alan Walsh was working on the measurement of small concentrations of metals at the CSIRO using atomic emission spectroscopy.

The idea of AAS came into his mind as he was gardening at his Melbourne home. On the normal Sunday morning he had the idea about looking at the light absorbed by the atoms except than looking at the light they emits.

Alan Walsh did not just discover a process that has the ability to save lives but also proven that atoms will only absorb light that has the EXACT value requires to promote their electron to a higher level.

Sir Isaac Newton1643 - 1727Sir Isaac Newton was one of the first people to study light scientifically.

In 1672, Newton directed a beam of white light through a triangular bar of glass, called a prism. He discovered that the light coming out of the prism was separated into bands of colors.

The arrangement of colors produced by a prism is called a spectrum.

Prior to this it was believed that white light was equal to purity. A Quantitative Study of Light

.

When a narrow band of light from a white light source is sent through a prism, a continuous spectrum containing all wavelengths of visible light is formed.Original Studies Of Light Used Only One Prism

Newtons Contribution to SpectroscopyIn fact, his main contribution was to show that after the sunlight had been broken down into its components by one prism, if a narrow ray of the light from the first prism was passed through another prism there would be no further breakdown.Newton contributed more to spectroscopy than scientifically proving that sunlight traveling through a prism was always broken down into the components of the rainbow.

Classification of Electromagnetic RadiationThe color components of light are separated along the visible range of light. The visible range of light (400-700 nm) is merely a small portion of the entire electromagnetic spectrum.

How does AAS works?LampBurnermonochromatorslitdetectormeter

The source of light is a lamp whose cathode is composed of the element being measured.Each analyzed element requires a different lamp.For example, a hollow cathode lamp for Aluminum (Al) is shown below

The cathode lamps are stored in a compartment inside the AA spectrometer. The specific lamp needed for a given metal analysis is rotated into position for a specific experiment.

*Hollow cathode lamp (HCL)

Cathode--- in the form of a cylinder, made of the element being studied in the flameAnode---tungsten

*A hollow cathode lamp for Aluminum (Al)

*SpectrAA - AAS motorizedMirrorHCL

How can we obtain the data?The intensity of the light coming through the cathode lamp is measuredThe light can then be absorbed by the atoms from the sample that has been vaporized in the flame.This wavelength can then promote the electrons to a higher energy level = excited stateThe intensity of the light is then again measured and compared to the first result. = absorbanceThe higher the concentration of the metal that is being observe in the sample the greater the absorbance.

How do we analyze the data?By comparing the light intensity that has passed through the sample (refer to previous diagram) with that of the same light after it has passed through a blank, the absorbance is measured.

The absorbance of different standard solutions of a compound of the element are also measured and a calibration curve is constructed.

Absorbance is plotted against concentration. We then use the calibration curve to determine the unknown concentration.

Example of a Calibration curveAn AAS was used to determine the concentration of lead ions (in ppm) in fish. The AAS was set up with a lamp that emitted light with a wavelength that is absorbed by lead atoms. The AAS was calibrated using different solutions containing known concentrations of lead ions. The graph on the next slide shows the variation of absorbance with the concentration of lead. A 2.0g sample of the fish was ground up and heated on a hot plate with 10 ml of nitric acid. This mixture was filtered and then sprayed into the flame of the AAS. The absorbance reading was 6.0. Determine the concentration of lead ions in the fish.

Graph:

Solution:

AAS, Flame test, UV-Vis, ICP-AESFlame test: uses the basic principle of AAS. The flame test is basically done by exposing a sample that is in observation into a non-luminous Bunsen burner flame. UV-Vis spectroscopy: is also similar to AAS in number of ways:Have the similar basic principle which is promoting electrons from lower energy level to a higher energy level.Both techniques uses similar steps to interpret results.Dissimilarity:AAS uses visible part of the emission spectrumUV-Vis ultraviolet part of the emission spectrum

ICP-AESInductively coupled plasma (ICP) can produce very high temperatures (7 000 10 000 K) to create the plasma instead of a flame.

All the atoms in the sample are excited and able to emit electromagnetic radiation as they return to their unexcited ground state.

Some of its advantages over AAS:can be used to identify most elementsCan identify many elements at once-50 elements simultaneously (AAS can only determine one at a time)Is very fast (analysing 70 elements takes 2 minutes).

Common application of AASMostly used in: mines, food industries, environmental control, petroleum products as they:

detect deficiencies / excessive amounts of certain metals in our body fluids such as: our blood and urine

Tracks harmful metals in our food/drinks

analyze metal ions that are polluting the soil, air and water.

Investigating different locations to test for different elements present and how much of these elements are present.

Analyze metals present in engine oils.

ADVANTAGES & DISADVANTAGES:Precise and very sensitiveaccurate results can be obtained.

Moderately expensiveCan only process one element at a time.Slower than ICP-AESCan only identify limited types of elements

A life-saving techniqueCanada: AAS was used to determine unsafe levels of lead in children who was lives nearby a lead smelter. Japan: From 1932 to 1968, AAS was used to identify the reason why over 3,000 residents who lives near the Minimata Bay started showing neurogical problems and pregnant women starts giving birth to impaired children. Scientist starts taking samples and performing AAS process; AAS results shows a very high concentration of mercury in their blood. This result on stopping the company, Chisso corporation who dumped approximately 27 tones of mercury in the bay.

Safety Precautions Exhaust System: AAS flames produce large amounts of heat & the resultant fumes & vapours may be toxic.Gas Cylinders: should be located outside of the laboratory in a cool well-ventilated area.Flammable Solvents: The combination of flame & solvent is a hazardous situation. Always use a solvent with the highest flashpoint consistent with the analysis being conducted. Use covered containers & the smallest practical volume.Burners: Keep burners clear & do not allow them to block.UV Radiation: Hazardous UV radiation is emitted by flames, hollow cathode lamps, analytical furnaces. Never look directly at any of these. Operate the AAS with the door or flame shield closed and wear appropriate safety glasses.

*********1515151515*www.valemount.com/joel/ lightoptics/spectrum.htm and web.mit.edu/.../research/ A-Vision/A10-1.html****1010101010******