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CHAPTER 4 :Atomic Spectroscopy

Basic Principles & Instruments

CONTENTS:Principle of AAS.Instrumentation.Applications.Experiments.

Atomic SpectroscopyAbsorption Spectroscopy: AAS Emission Spectroscopy: FES, ICP-AES (OES)Mass Spectrometry

PRINCIPLEAtomic Absorption Spectroscopy utilizes the phenomenon that atoms in ground state absorbs radiation from the light source and goes up into their excited state. The amount of light absorbed is proportional to concentration of the element.

Cu*

DETECTORA=Log P0/PA Process called Atomization

A Process called Excitation

Hallow Cathode Lamp

Cu(aq)

Cu(g)

Cu*

Elements detectable by AA are highlighted in pink

INSTRUMENTATIONThe basic design in AAS includes a light source, chopper, atomizer (burner), monochromator and detector.Single and double beam instruments are available, however, double beam AAS are more sensitive and accurate.FIT has a double beam AAS

Hallow Cathode Lamp

MonochromatorDetector and readout

ChopperAspirationBurnerA=Log P0/PFunctionsAtomizerChopperMonochromator

Atomic Absorption Spectrophotometer

Flame AAS9

Flame AA uses a slot type burner to increase the path length, and therefore to increase the total absorbance (seeBeer-Lambert law). Sample solutions are usually aspirated with the gas flow into a nebulizing/mixing chamber to form small droplets before entering the flame.

Flame AA can only analyze solutions9

Graphite Furnace AAS

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The graphite furnace has several advantages over a flame. It is a much more efficient atomizer than a flame and it can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms.

Graphite furnace AA can accept solutions, slurries, or solid samples.10

Schematic diagram of AAS:

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Atomic Spectroscopy

Nebulizer converts the solution into a sprayFlame (or Plasma) causes the solvent to evaporate, leaving dry aerosol particles, then volatilizes the particles, producing atomic, molecular and ionic species

Atomic absorption spectroscopy (AAS) Gases mixture flame (1800 4500 C): air-propane, air-acetylene etc. ; Atomic absorption spectrometry quantifies the absorption of ground state atoms in the gaseous state ; The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels . The analyte concentration is determined from the amount of absorption.

Atomic absorption spectroscopy (AAS)

Operation principle of AASLight source hollow cathode lamp. Each element has its own unique lamp.Atomic cell flame (gas mixture) or graphite furnance (accepts solutions, slurries, or even solids).Detector photomultiplier.

LIGHT SOURCE:Hollow Cathode Lamp are the most common radiation source in AAS.

It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined.

These are sealed in a glass tube filled with an inert gas (neon or argon ) .

Each element has its own unique lamp which must be used for that analysis .

Quartz windowPyrex bodyAnodeCathode

Hollow Cathode Lamp:

cathode

NEBULIZER:

Suck up liquid samples at controlled rate.

Create a fine aerosol spray for introduction into flame.

Mix the aerosol and fuel and oxidant thoroughly forintroduction into flame.

Atomizers:Elements to be analyzed needs to be in atomic state.

Atomization is separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing the analyte to high temperatures in a flame or graphite furnace .

ATOMIZERS:

FLAME ATOMIZER: To create flame, we need to mix an oxidant gas and a fuel gas.

In most of the cases air-acetylene flame or nitrous oxide-acetylene flame is used.

Liquid or dissolved samples are typically used with flame atomizer.

BURNERSThere are two basic types of aspirator*-burners that are used in AAS; Total Consumption Burner and Premix Chamber Burner.

The fuel and oxidant (support) gases are mixed and combust at the tip of the burner. [The sample is drawn up into the flame by the venture effect (suction effect) by the support gas, finally it is broken into a fine spray (aerosol) at the tip of the burner where it is mixed with the fuel and oxidant and burned].

It is called a total consumption burner because the entire aspirated sample enters the flame

FuelOxidantSample

PREMIX BURNERPremix burners have a long, narrow burner head that serves as a sample path (b). Sample introduced via aspiration. The nebulizer controls sample flow by producing a mist.The mixing chamber assures that the sample mixes with the fuel and oxidant before to entry into the flame.Premix burners are largely used for AA analysis.

Capillary

b

Can analyze viscous samples example undiluted serum and urine.Uses entire aspirated sample.Can be used for most types of flames both high velocity and low velocity.

Advantage of using a Total Combustion BurnerDisadvantage Shorter path length thus larger droplets are not completely vaporized. This partially vaporized droplets produce solids in the light path hence causing light scattering giving inaccurate absorbances. Combustion is noisy.

Larger droplets condense and drains out only allowing the fine droplets to enter in the flame.Longer path length hence very sensitive/accurate absorbances.Quiet combustion.Disadvantage Out of the 100% sample that enters in and only 10% of it goes into the flame and 90% is lost. Limited to relatively low velocity flames.Advantage of using a Premix Burner

GRAPHITE TUBE ATOMIZER: Uses a graphite coated furnace to vaporize the sample.

ln GFAAS sample, samples are deposited in a small graphite coated tube which can then be heated to vaporize and atomize the analyte.

The graphite tubes are heated using a high current power supply.

MONOCHROMATOR: This is a very important part in an AA spectrometer. It is used to separate out all of the thousands of lines.

A monochromator is used to select the specific wavelength of light which is absorbed by the sample, and to exclude other wavelengths.

The selection of the specific light allows the determination of the selected element in the presence of others.

DETECTOR: The light selected by the monochromator is directed onto a detector that is typically a photomultiplier tube , whose function is to convert the light signal into an electrical signal proportional to the light intensity.

The processing of electrical signal is fulfilled by a signal amplifier . The signal could be displayed for readout , or further fed into a data station for printout by the requested format.

Photomultiplier Tube (PMT)

Calibration CurveA calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve of concentration vs absorbance is plotted.

The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured .The unknown concentration of the element is then calculated from the calibration curve

Example 1A 3.9877g of fish sample is charred, ashed and dissolved in nitric acid. After filtering and diluting the filtrate to the mark of a 100 ml volumetric flask, a further diluting was performed, 10 ml diluted to 100 ml. The following absorbance data was obtained from the atomic absorption spectrometer.

C (ppm) Absorbance 5.0 0.097 10.0 0.201 15.0 0.314 20.0 0.411 25.0 0.520 Sample 0.395i. Plot the data and calculate the concentration of potassium in the original fish sample.ii. Calculate the amount of potassium in milligrams in the fish sample. [K: Mr 39.09g/mol]

Solution

* Always get the concentration from the graph and not the equation in exams, concentration of K is very close to 19ppm* By substituting the absorbance into the equation the concentration is 19.1ppm: 0.395=0.0211x-0.0082; x= (0.395+0.0082)/0.0211; x= 19.1ppmA. Concentration of K= 19.1 ppmB. Mass= 19.1 mg/L x 0.1L = 1.9 milligram

APPLICATIONS: Determination of even small amounts of metals (lead, mercury, calcium, magnesium, etc) as follows:

Environmental studies: drinking water, ocean water, soil.

Food industry.

Pharmaceutical industry.

EXPERIMENTS DETERMINATION OF VANDIUM IN LUBRICATING OIL

DETERMINATION OF TRACE ELEMENTS IN CONTAMINATED SOIL

VANADIUM IN LUBRICATING OIL:THEORY:

High temperature corrosion and fouling can be attributed to vanadium in the fuel. During combustion, the element oxidize and form semi-liquid and low melting salts (vanadium pentoxide), which adhere to exhaust valves and turbochargers. In practice, the extent of hot corrosion and fouling are generally maintained at an acceptable level through temperature control, an operational solution, and material selection.

The oil is dissolved in white spirit and the absorption of this solution is compared with the absorption of standard.

STANDARD SOLUTION: the standard solutions are made up from vanadium naphthenate in white spirit which contain about 3% of vanadium.

(weigh out 0.6 g of vanadium naphthenate into a 100 ml flask and made up to mark with white spirit. dilute portions of this stock solution to obtain a series of working standards containing 10-40 mg ml-1 of vanadium).

PROCEDURE: Weigh out accurately about 5 g of the oil sample, dissolve in small volume of white spirit and transfer to 50 ml flask.

Using same solvent, make up the sol. to the mark.

Set up a vanadium hollow cathode lamp selecting a resonance line of wavelength 318.5 nm.

Adjust gas controls to give a fuel rich acetylene-nitrous oxide flame.

Aspirate successfully into the flame the solvent blank, standard solutions and finally the test solution

In each case recording the absorbance reading .plot the calibration curve and ascertain the vanadium content of the oil.

LEAD IN CONTAMINATED SOIL:SAMPLING: samples of approx. 50g should be taken from specified sampling points on the site.

The sampling point should include surface soil and two further samples taken at depth, at 0.5 and 1.0m.

The exact location of these points should be noted, for it may be necessary to take further samples.

PROCEDURE:Weight out about 1g of seived soil and transfer to a 100ml beaker. add 20 ml of 1:1 nitric acid .

Boil gently on a hot plate until the volume of nitric acid is reduced to 5ml.

Add 20ml of deionised water and boil gently again until the volume is 10ml.

Cool the suspension and filter through a whatman filter paper, washing the beaker and filter paper with deionised water until a volume of about 25ml is obtained.

Transfer the filtrate to a 50ml flask and make up to the mark with deionised water.

Setup acetylene-air flame with resonance line 217.0 nm.

Standard lead solutions containing 1-10 mg ml-1 are suitable for measurement

REFERENCES:Vogels Textbook of Quantitative Analysis, G. Svehla, Pearson.Principles of Instrumental Analysis, Skoog.Basic Concepts Of Analytical chemistry, S M Khopkar.

Chart10.0970.2010.3140.4110.52

Concentration (ppm)AbsorbanceGraph of concentration v/s Absorbance

DES

DES0.1640.4250.6280.9511.261.582

ConcentrationAbsorbanceGraph: Absorbance v/s Concentration

DILTB0.12.00E-040.24.00E-040.36.00E-040.48.00E-04

DILTB0000

AbsorbanceConcentrationGraph: Absorbance v/s Concentration

DILTA0.1041.00E-0650.0970.1982.00E-06100.2010.313.00E-06150.3140.4024.00E-06200.4110.55.00E-06250.52

DILTA00000

AbsorbanceConcentrationGraph: Absorbance v/s Concentration

00000

Concentration (ppm)AbsorbanceGraph of concentration v/s Absorbance