Department of Chemistry

Analytical and Environmental Chemistry (CH-204)

Atomic SpectroscopyMichael J. Hynes

Atomic SpectroscopyLecturerMichael J. HynesTexts(1) Quantitative Chemical Analysis , by Daniel C. Harris Pub. Freeman. Chapter 21 (4th Edition)(2) Atomic Absorption and Emission Spectroscopy by E. Metcalfe & F.E. Prichard, Pub. Wiley (ACOL series).(3) Most Analytical Chemistry Textbooks.(4) Handout

Principle of Atomic Absorption pectroscopyAtomic Absorption (AA) is based on the principle that a ground state atom is capable of absorbing light of the same characteristic wavelength as it would emit if excited to a higher energy level. In flame AA, a cloud of ground state atoms is formed by aspirating a solution of the sample into a flame of a temperature sufficient to convert the element to its atomic state.The degree of absorption of characteristic radiation produced by a suitable source will be proportional to the population of ground state atoms in the flame, and hence to the concentration of the element in the analyte.

Spectra of atoms consist of SHARP LINES.Each element has a characteristic spectrum.Due to sharpness of lines, there is little overlap between the spectral lines of different elements.Therefore, there is little interference.HeatCompoundAtomsAtomic SpectroscopySampleHighTemperatureVapourMeasure absorbance or emission of the atomic vapour.Atomic spectroscopy deals with atoms.Fe2+ and Fe3+ will not be distinguished.

SensitivityAtomic spectroscopy is very sensitive for most elements.Concentrations at the ppm level may be routinely determined using flame atomisation.Using electrothermal atomisation, concentrations at the ppb may be determined.1 ppm = 10-6g/g or 1g/gThe density of dilute aqueous solutions is approximately 1.00 so that: 1 g/g of aqueous solution = 1g/ml = 1 ppm 1 ppm Fe = 1 x 10-6 g Fe/ml = 1.79 x 10-5 mol dm-31 ppm = 1 second in 11.6 days1 ppb = 1 second in 31.7 years.

Atomic Absorption SpectroscopyAbsorbance = -log(It/I0)Io = incident radiation (on sample)It = transmitted radiation.Atomic Emission SpectroscopyAbsorbance = -log(It/I0)I0 =intensity of radiation reaching detector in the absence of sample.It = intensity of radiation reaching detector when sample is being aspirated.

Both methods are used to determine the concentration of an element in solution.Both methods use a standard curve.Difference between UV and IR spectroscopy is that sample must be atomised.Sample may be atomised by:(1) A flame(2) Electrically heated furnace(3) A Plasma

xxW1/2AbsBand Width = W1/2 = width of band at half the height25 nm+AbsMolecular Absorption BandW1/2 =0.003 nmAtomic VapourAbsorption Band

E3(Excited state)E2(Excited state)E1(Excited state)Eo(Ground state)AbsorptionEmissionResonance LinesMost Intense Line3 Absorption Lines6 Emission linesAtomic Absorption and Emission LinesE = E1 - E0 = h = hc/E

Comparison of the atomic emission spectrum of silicon compared with the molecular absorptionspectrum of ethanal.

Laminar Flow Burner

Acetylene flow rate: 1000 ml/minuteAir flow rate: 5000 ml/minuteSample uptake: 2 ml/minute 80% of sample goes to waste.Therefore, dilution factor = 15,000Highly inefficient!EXAMPLES OF FLAME TEMPERATURES

Fuel

Oxidant

Temperature

(K)

Acetylene

Air

2400-2700

Acetylene

Nitrous Oxide

2900-3100

Acetylene

Oxygen

3300-3400

Hydrogen

Air

2300-2400

Effect of temperature in atomic spectroscopyE1 (g = 2)E0 (g = 1)Boltzman Distribution(Ground state)(Excited state)E1Na E1 = 3.37 x 10-19 J/atomAt 2600 K At 2610 K N1/No = 1.74 x 10-4 g is the multiplicity

The effect of a 10 K temperature rise on the ground state population is negligible (0.02 %).In the excited state the fractional change isEffect of Temperature on Sodium AtomsSo!

Temp

K

% Ground

State

% Excited State

2600

99.9833

0.0167

2610

99.9826

0.0174

Effect of TemperatureSmall changes in flame temperature (10 K) have little effect in atomic absorption but have significant effects in atomic emission spectroscopy.Must have good flame control in atomic emission spectroscopy.

Radiation SourceBeers Law only applies to monochromatic radiation.In practice, monochromatic implies that the linewidth of the radiation being measured is less than the bandwidth of the absorbing species.Atomic absorption lines very sharp with an inherent linewidth of 0.0001 nm.Due to Doppler effect and pressure broadening, linewidths of atoms in a flame are typically 0.001 - 0.01 nm.Therefore, we require a source having a linewidth of less than 0.01 nm.Typical monochromator has a bandwidth of 1 nm i.e. x100 greater than the linewidth of the atom in a flame.

Hollow Cathode LampUsed because of the requirement for a source of narrow lines of the correct frequency.Hollow Cathode lampfilled with argon or neon at a pressure of 130 - 170 Pa (1 - 5 torr)

Monochromator bandwidth(~100x greater than atomic lines

Hollow Cathode Lamp

Reactions in the Hollow Cathode LampApply sufficiently high voltage between the cathode and the anode:(1) Ionization of the filler gas: Ne + e- = Ne+ + 2e-(2) Sputtering of the cathode element (M):M(s) + Ne+ = M(g) + Ne(3) Excitation of the cathode element (M) M(g) + Ne+ = M*(g) + Ne(4) Emission of radiation M*(g) (M(g) + h

Hollow Cathode LampThe cathode of the hollow cathode lamp (HCL) contains the element being analysed.Therefore the atomic radiation emitted by the HCL has the same frequency as that absorbed by the analyte atoms in the flame or furnace.The linewidth from the HCL is relatively narrow (compared to linewidths of atoms in the flame or furnace) because of low pressure in lamp and lower temperature in lamp (less Doppler broadening).Thus the linewidth from the HCL is nearly monochromatic (vs sample).Different lamp required for each element although some are mulit-element.

Hollow Cathode Lamp - The Filler Gas

Electrothermal Atomisation - Graphite Furnace Sample holder consists of a graphite tube.Tube is heated electricallyBeam of light passes through the tube.Offers greater sensitivity than flames.Uses smaller volume of sampletypically 5 - 50 l (0.005 - 0.05 ml).All of sample is atomised in the graphite tube.Atomised sample is confined to the optical path for several seconds (residence time in flame is very short).Uses a number of stages as shown on next slide.

Stages in a Graphite FurnaceTypical conditions for Fe:Drying stage:125o for 20 secAshing stage1200o for 60 secAtomisation2700o for 10 sec

Requires high level of operator skill.Method development difficult.

Schematic Diagram of a Graphite FurnaceHCLh

Advantages and Disadvantages of Flame AASAdvantagesequipment relatively cheapeasy to use (training easy compared to furnace)good precisionhigh sample throughputrelatively facile method developmentcheap to runDisadvantageslack of sensitivity (compared to furnace)problems with refractory elementsrequire large sample sizesample must be in solution

Advantages and Disadvantages of Electrothermal AtomisationAdvantagesvery sensitive for many elementssmall sample sizeDisadvantagespoor precisionlong cycle times means a low sample throughputexpensive to purchase and run (argon, tubes)requires background correctionmethod development lengthy and complicatedrequires a high degree of operator skill (compared to flame AAS)

MonochromatorThe operation and sensitivity of the atomic absorption spectrometer spectrometer depends on the spectral band width of the resonance line emitted by the primary radiation source (the hollow cathode lamp).The function of the monochromator is to isolate the resonance line from non-absorbing lines close to it in the source spectrum and from background continua and molecular emissions originating in the flame.Hollow cathode lamps emit a number of lines, in the case of multi-element lamps, the number can be quite large and it is necessary to isolate the line of interest.

Detector/Measuring SystemThe intensity of the line source is measured with a photomultiplier, which produces an electrical signal proportional to the intensity of the incident light.This signal is amplified and processed electronically to produce an output which on older instruments was read on a digital or analogue meter in either absorbance or concentration mode.In modern instruments, the output is usually displayed on the computer screen of the PC controlling the instrument.In order to eliminate the unwanted emissions from the flame, the light source is modulated by a chopper which is located between the hollow cathode lamp and the flame. The amplifier which modifies the signal from the photomultiplier is tuned in to the same frequency .(Alternatively, the hollow cathode lamp may be modulated by applying an AC voltage at say 50 Hz.).

AB

InterferencesInterference is any effect that changes the signal when analyte concentrations remain unchanged.While atomic absorption spectroscopy is relatively free from interferences, there are a number of interferences which must be dealt with.

Spectral InterferenceThis refers to overlap of analyte signals with signals originating from other elements in the sample or with signals due to the flame or furnace.Example:Al308.216 nmV308.211 nmSolution:Separate elements or use a different line (which may be less sensitive).

Chemical InterferenceFormation of Stable or Refractory Compounds Elements that form very stable compounds are said to be refractory because they are not completely atomised at the temperature of the flame or furnace.SolutionUse a higher flame temperature (nitrous oxide/acetylene)Use a release agentUse protective chelation

ExamplesDetermination of calcium in the presence of sulfate or phosphate (e.g. in natural waters) 3Ca2+ + 2PO43- = Ca3(PO4)2 (stable compound)Release agent Add 1000 ppm of LaCl32LaCl3 + Ca3(PO4)2 = 3CaCl2 + 2La(PO4) CaCl2 readily dissociatesProtective chelationCa3(PO4)2+3EDTA = 3Ca(EDTA) + 2PO43-Ca(EDTA) dissociates readily.

Ionisation Interference M(g) M+(g) + e-A problem in the analysis of alkali metal ions at low flame temperatures and other elements at higher temperatures. Because alkali metals have the lowest ionisation potentials, they are most extensively ionised in flames.At 2450 K and a pressure of 0.1 Pa, sodium is 5% ionised.Potassium is 33% ionised under the same conditions.Ionised atoms have energy levels which are different to the parent atomstherefore the analytical signal is reduced.

SolutionAdd an ionisation suppressor

Add an easily ionised element such as Cs.Add 1000 ppm of CsCl when analysing Na or K.

Cs is more readily ionised than either Na or K.This produces a high concentration of electrons in the flame.

Matrix effectsThe amount of sample reaching the flame is dependent on the physical properties of the solution:viscositysurface tensiondensitysolvent vapour pressure.To avoid differences in the amount of sample and standard reaching the flame, it is necessary that the physical properties of both be matched as closely as possible.Example:Analysis of blood

Non-Atomic AbsorptionNon-atomic absorption is caused by molecular absorption or light scattering by solid particles in the flame.The absorption measurement obtained with a hollow cathode lamp is the sum of the atomic absorption and the non-atomic absorption. The interference is corrected for by making a simultaneous measurement of the non-atomic absorption using a continuum source (usually deuterium)this is called background correction

Operational ParametersSensitivitythe concentration of an element which will reduce the transmission by 1%

This corresponds to an absorption of 0.00436

CalculationsFor an absorbance of 1.0 we require1.0/0.00436 = 230 times the sensitivity.For Cu, sensitivity = 0.05 ppmFor an absorbance of 1 we require a concentration of 11.5 ppm.Using scale expansion of 10 we can usually obtain an absorbance of 1 using a 1 ppm solution of copper.

Detection LimitThe concentration of an element that gives a signal equal to three times the peak to peak noise level of the baseline.Measure the baseline while aspirating a blank solution.

Terms to understand in atomic spectroscopy (1)Atomic absorption spectroscopyAtomic emission spectroscopyAtomisationBackground correctionBoltzman distributionChemical interferenceDetection limitGraphite furnaceHollow-cathode lamp

Terms to understand in atomic spectroscopy (2)Inductively coupled plasmaIonisation interferenceIonisation suppressorMatrixMatrix modifierNebulisationPremix burnerReleasing agentSpectral interference

Applications of AASAgricultural analysissoilsplantsClinical and biochemistrywhole blood, plasma and serum Ca, Mg, Li, Na, K, Cu, Zn, Fe etc.Metallurgyores, metals and alloys

Applications of AASLubricating oilsBa, Ca, Mg and Zn additivesGreasesLi, Na, Ca

Applications of AASWater and effluentsmany elements e.g. Ca, Mg, Fe, Si, Al, BaFoodwide range of elementsAnimal feedstuffsMn, Fe, Co, Cu, Zn, Cr, SeMedicinesrange of elements