1 department of chemistry analytical and environmental chemistry (ch-204) atomic spectroscopy...

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Department of Chemistry

Analytical and Environmental Chemistry (CH-204)

Atomic Spectroscopy

Michael J. Hynes

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Atomic Spectroscopy• Lecturer

– Michael J. Hynes

• Texts

(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

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Principle of Atomic Absorption pectroscopy

• Atomic 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.

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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.

HeatCompound Atoms

Atomic Spectroscopy

SampleHigh

TemperatureVapour

Measure absorbance or emission of the atomic vapour.Atomic spectroscopy deals with atoms.Fe2+ and Fe3+ will not be distinguished.

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Sensitivity

• Atomic 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/g

• The 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-3

1 ppm = 1 second in 11.6 days

1 ppb = 1 second in 31.7 years.

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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.

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• 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

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x

xW1/2Abs

Band Width = W1/2 = width of band at half the height

25 nm+Abs

Molecular Absorption Band

W1/2 =0.003 nm

Atomic VapourAbsorption Band

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E3(Excited state)

E2(Excited state)

E1(Excited state)

Eo(Ground state)Absorption Emission

Resonance Lines

Most Intense Line

3 Absorption Lines6 Emission lines

Atomic Absorption and Emission Lines

E = E1 - E0 = h = hc/

E

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

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HCL h

Chopper

CloudChamberFuel

Nebuliser

Sample uptake

Oxidant(Air)

Monochromatorh Detectorh Amplifier

Outpute.g. PC/Printer

Source

Figure 1. Schematic Diagram of an Atomic Absorption Spectrophotometer

Waste

HCL = Hollow-cathode Lamp

Io It

i

V

Sample (Atoms in flame)

Burner

Flame

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Laminar Flow Burner

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Fuel Oxidant Temperature

(K)

Acetylene Air 2400-2700

Acetylene Nitrous Oxide 2900-3100

Acetylene Oxygen 3300-3400

Hydrogen Air 2300-2400

Acetylene flow rate: 1000 ml/minute

Air flow rate: 5000 ml/minute

Sample uptake: 2 ml/minute

80% of sample goes to waste.

Therefore, dilution factor = 15,000

Highly inefficient!

EXAMPLES OF FLAME TEMPERATURES

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Effect of temperature in atomic spectroscopy

E1 (g = 2)

E0 (g = 1)

Boltzman Distribution

(Ground state)

(Excited state)

kT

oN

N 1E

o

11 e g

g

E1

Na E1 = 3.37 x 10-19 J/atom

4)(2600)10x 1.381

10x -3.37

NN 10x 1.67 e

1

2

23-

-19

o

1

At 2600 K

At 2610 K N1/No = 1.74 x 10-4

g is the multiplicity

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Temp

K

% Ground

State

% ExcitedState

2600 99.9833 0.0167

2610 99.9826 0.0174

The effect of a 10 K temperature rise on the ground state population is negligible (0.02 %).

In the excited state the fractional change is

% 4

0167.0

1000.0167 - 0.0174

Effect of Temperature on Sodium Atoms

So!

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Effect of Temperature

• Small 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.

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M+X- M+X-

Solution Mist

EvaporationMX

Solid

Vapourisation

MX

Gas

DissociationM (gas)

X (gas)

Thermal

excitationM*(g)

Flameemission

hMeasure forflame emissionspectroscopy

Absorption of radiant energy h

Measure for atomic absorption spectroscopy

M*(g)

Re-emit radiation at 90o

(Atomic fluorescence)

Process by which gaseous atoms are produced in flames

Measure for atomicfluorescence spectroscopy

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HCL h

Chopper

CloudChamberFuel

Nebuliser

Sample uptake

Oxidant(Air)

Monochromatorh Detectorh Amplifier

Outpute.g. PC/Printer

Source

Figure 1. Schematic Diagram of an Atomic Absorption Spectrophotometer

Waste

HCL = Hollow-cathode Lamp

Io It

i

V

Sample (Atoms in flame)

Burner

Flame

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Radiation Source• Beer’s 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.

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Hollow Cathode Lamp

• Used because of the requirement for a source of narrow lines of the correct frequency.

• Hollow Cathode lamp

– filled with argon or neon at a pressure of 130 - 170 Pa (1 - 5 torr)

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Monochromator bandwidth(~100x greater than atomic lines

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Hollow Cathode Lamp

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Reactions in the Hollow Cathode Lamp

• Apply 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

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Hollow Cathode Lamp• The 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.

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Hollow Cathode Lamp - The Filler Gas

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Electrothermal Atomisation - Graphite Furnace

• Sample holder consists of a graphite tube.

• Tube is heated electrically

• Beam of light passes through the tube.

• Offers greater sensitivity than flames.

• Uses smaller volume of sample

– typically 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.

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Stages in a Graphite Furnace

• Typical conditions for Fe:

– Drying stage: 125o for 20 sec

– Ashing stage 1200o for 60 sec

– Atomisation 2700o for 10 sec

• Requires high level of operator skill.

• Method development difficult.

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Schematic Diagram of a Graphite Furnace

HCLh

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Advantages and Disadvantages of Flame AAS

• Advantages– equipment relatively cheap

– easy to use (training easy compared to furnace)

– good precision

– high sample throughput

– relatively facile method development

– cheap to run

• Disadvantages– lack of sensitivity (compared to furnace)

– problems with refractory elements

– require large sample size

– sample must be in solution

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Advantages and Disadvantages of Electrothermal Atomisation

• Advantages

– very sensitive for many elements

– small sample size

• Disadvantages

– poor precision

– long cycle times means a low sample throughput

– expensive to purchase and run (argon, tubes)

– requires background correction

– method development lengthy and complicated

– requires a high degree of operator skill (compared to flame AAS)

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HCL h

Chopper

CloudChamberFuel

Nebuliser

Sample uptake

Oxidant(Air)

Monochromatorh Detectorh Amplifier

Outpute.g. PC/Printer

Source

Figure 1. Schematic Diagram of an Atomic Absorption Spectrophotometer

Waste

HCL = Hollow-cathode Lamp

Io It

i

V

Sample (Atoms in flame)

Burner

Flame

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Monochromator• The 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.

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Detector/Measuring System• The 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.).

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A B

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Interferences

• Interference 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.

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Spectral Interference

• This 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:–Al 308.216 nm

–V 308.211 nm

• Solution:–Separate elements or use a different line

(which may be less sensitive).

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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.

• Solution–Use a higher flame temperature (nitrous

oxide/acetylene)

–Use a release agent

–Use protective chelation

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Examples

• Determination 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 LaCl3

2LaCl3 + Ca3(PO4)2 = 3CaCl2 + 2La(PO4)

CaCl2 readily dissociates

• Protective chelation

Ca3(PO4)2+3EDTA = 3Ca(EDTA) + 2PO43-

Ca(EDTA) dissociates readily.

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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 atoms

– therefore the analytical signal is reduced.

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Solution

• Add an ionisation suppressor

K

]][e[M [M]

[M]

]][e[M K

-

-

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.

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Matrix effects

• The amount of sample reaching the flame is dependent on the physical properties of the solution:

– viscosity

– surface tension

– density

– solvent 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

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Non-Atomic Absorption

• Non-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

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Operational Parameters• Sensitivity

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

0.00436 100

99log-

I

Ilog- Abs

0

t

This corresponds to an absorption of 0.00436

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Calculations

• For an absorbance of 1.0 we require

1.0/0.00436 = 230 times the sensitivity.

• For Cu, sensitivity = 0.05 ppm

• For 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.

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Detection Limit

• The 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.

Absorbance

Peak to peak noise level

Time

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Terms to understand in atomic spectroscopy (1)

• Atomic absorption spectroscopy

• Atomic emission spectroscopy

• Atomisation

• Background correction

• Boltzman distribution

• Chemical interference

• Detection limit

• Graphite furnace

• Hollow-cathode lamp

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Terms to understand in atomic spectroscopy (2)

• Inductively coupled plasma

• Ionisation interference

• Ionisation suppressor

• Matrix

• Matrix modifier

• Nebulisation

• Premix burner

• Releasing agent

• Spectral interference

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Applications of AAS• Agricultural analysis

–soils

–plants

• Clinical and biochemistry–whole blood, plasma and serum Ca, Mg, Li,

Na, K, Cu, Zn, Fe etc.

• Metallurgy–ores, metals and alloys

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Applications of AAS

• Lubricating oils–Ba, Ca, Mg and Zn additives

• Greases–Li, Na, Ca

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Applications of AAS• Water and effluents

–many elements e.g. Ca, Mg, Fe, Si, Al, Ba

• Food–wide range of elements

• Animal feedstuffs–Mn, Fe, Co, Cu, Zn, Cr, Se

• Medicines– range of elements