Atomic Spectroscopy based on Flame Atomization;

Theory, Instrumentation and Application

Nurul Auni binti Zainal AbidinFaculty of Applied Science

UiTM Negeri Sembilan

ATOMIC SPECTROSCOPY

• Three techniques (methods) included in atomic spectroscopy:

1) atomic absorption spectroscopy2) atomic emisssion spectroscopy3) atomic fluorescene spectroscopy

Atomic spectroscopy

• In order to perform atomic spectroscopy, atoms of the analyte must first be formed, usually in the form of an atomic vapor.

• Atomization – the process by which a sample is converted to an atomic vapor.

• Atomizer – a device used to converted a sample to an atomic vapor.

Types of atomizer

1) flame atomizer

2) plasma atomizer3) electrothermal atomizer

Atomization process

• Solution of the analyte is evaporated rapidly at an elevated temperature to yield a finely divided solid.

• Further heating will break down into gaseous atoms.

6

Atomic Absorption Spectroscopy

• Uses absorption of light to measure the concentration of gas-phase atoms.

• Since samples are usually liquids or solids, the analyte atoms must be vapourised in a flame (or graphite furnace).

7

Atomic Absorption Spectroscopy

• The analyte concentration is determined from the amount of absorption.

8

Atomic Absorption Spectroscopy• The analyte concentration is determined

from the amount of absorption.

Atomization process

• Solution of the analyte is evaporated rapidly at an elevated temperature to yield a finely divided solid.

• Further heating will break down into gaseous atoms.

Sample Atomization

• Nebulization – Conversion of the liquid sample to a fine spray.

• Desolvation - Solid atoms are mixed with the gaseous fuel.

• Volatilization – Solid atoms are converted to a vapor in the flame.

• There are three types of particles that exist in the flame:1) Atoms2) Ions3) Molecules

Flame Atomization

1. Types of FlamesFuel / Oxidant Temperature

H–CC-H acetylene / air 2100 – 2400 C (most common) acetylene / N2O 2600 – 2800 C

acetylene / O2 3050 – 3150 C

• Selection of flame type depends on the volatilization of interest.

2. Flame Structure• Interzonal region is the hottest part of the flame

and best for atomic absorption.

• Oxidation of the atoms occurs in the secondarycombustion zone where the atoms will form molecular oxides and are dispersed into the surroundings.

FUELS/OXIDANTS• Low T flames : easily reduced

elements (Cu, Pb, Zn, Cd) • High T flames: difficult to reduce

elements (e.g. alkaline earths).• Fuels: natural gas, propane, butane,

H2, and acetylene; • Oxidants- Air and O2 (low temperature

flames). N2O (high temperature flames).

• Flame characteristics: • Sample enters flame, is vaporized,

reduced and eventually oxidized.• Exact region of flame in which each of

these occurs depends upon: • flow rate, • drop size, and • oxidizability of sample.• Optimum position for flame for many

metals.

Temperature Profile

Temperature profile in degrees Celsius for a natural gas-air flame

3. Burners

• Two types of burners in flame spectroscopy

i. Turbulent flow (total consumption burner)ii. Laminar flow (premix burner)

Turbulent Flow Burner

• Nebulizer & burner are combined into a single unit.

• Sample is drawn up the capillary & nebulized

• Sample flow rate: 1 to 3 mL/min

Turbulent Flow Burner

• Advantage:– introduce relatively large & representative

sample into the flame.

• Disadvantages:– A relatively short path length through flame– Problems with clogging of the tip– Burners noisy from electronic and auditory

stand point

• Sample is nebulized by the flow of oxidant past a capillary tip.

• Resulting aerosol then mixed with fuel & flows past a series of baffles that remove all but the finest droplets.

• Majority of the sample collects in the bottom of mixing chamber, drained to a waste container.

• Aerosol, oxidant & fuel are burned in a slotted burner that provided a flame, 5 – 10 cm in length.

Laminar Flow Burner

Laminar Flow Burner

• Advantages:i. Provide quiet flameii. Longer path length, enhance sensitivity & reproducibility

• Disadvantages:i. Lower rate of sample introductionii. Possibility of selective evaporation of mixed

solvents in the mixing chamber, create analytical uncertainties.iii. Mixing chamber contains a potentially explosive mixture that can flash back if the flow rates are too low.

Atomic Absorption Spectroscopy (based on flame)

• Currently the most widely used of all the atomic methods.

• Simplicity, effectiveness, relatively low cost.

• Flame atomization best for reproducibility (precision)(<1%)

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Atomic Absorption Spectroscopy

• AAS is commonly used for metal analysis

• A solution of a metal compound is sprayed into a flame and vaporises

• The metal atoms absorb light of a specific frequency, and the amount of light absorbed is a direct measure of the number of atoms of the metal in the solution

Atomic Absorption Spectroscopy (based on flame)

• The spectra result form the atomized sample absorbing photons of radiation of the appropriate energy (wavelength).

• Energy of radiation absorbed by a vaporized atom is identical with that needed to bring about excitation to a higher electronic state.

• Transitions: excitation of an electron from ground state to a higher energy level E

1

E

0

INSTRUMENTATION

Spectrophotometers

• Contain same basic components as an instrument designed for absorption analysis of solution.

• Source sample container (flame or hot surface) selector detector readout system

AA Spectrophotometer Design1. Single Beam Instrument

• The modulated power source can be replaced by a chopper

AA Spectrophotometer Design

2. Double-Beam Instrument

Double-Beam Instrument

• Radiation from hollow cathode lamp is split into 2 beams– One passes through the flame– The other around the flame

• A half-silvered mirror returns both beams to a single path then pass through the monochromator then detector.

• Note: monochromator placed between sample and detector– Eliminates most of the radiation originating from

the flame.

A. Line Sources

LINE SOURCE – is required because width of the absorption line is very narrow

Examples of line source used in AAS:

Hollow cathode lamp (HCL)

Electrodeless discharge lamp

(EDL)

done

1. Hollow Cathode Lamp (HCL)

• Light source in AA instrument

• Light from this lamp exactly light required for the analysis, even no monochromator is used.

• Atoms of metal tested are present within the lamp.

How does the Hollow Cathode Lamp works?

• When lamp on, atoms are supplied with energy

• Causes atoms elevate to excited states

• Upon returning to ground state, exactly the same wavelength that are useful in the analysis are emitted

• Since the analyzed metal with exactly the same energy levels that undergoes excitation

• Hollow cathode lamp MUST contain the element to be determined

Mechanism of excitation process in Hollow Cathode Lamp

• Lamp is a sealed glass envelope filled with argon (or Neon) gas

• When lamp is ON, Ne atoms are ionized with electrons drawn to anode (+ charge electrode)

• The Ne ions, Ne+ bombard the surface of the cathode (- charge electrode)

• Metal atoms, M, in the cathode are elevated to the excited state and are rejected from the surface as a result of this bombardment

Mechanism of excitation process in Hollow Cathode Lamp

• When atoms return to ground state, characteristic line spectrum of that atom is emitted

• This light is directed at the flame, where unexcited atoms of the same element absorb the radiation and are themselves raised to the excited state

• Absorbance is measured, and related to the concentration

Illustration of mechanism of excitation process in HCL

1. Electrodeless Discharge Lamp (EDL)• Constructed of a metal or salt of interest sealed in a quartz tube

filled with a noble gas (Ne or Ar) at low pressure (1 – 5 torr).

• The noble gas is ionized and accelerated by a strong radio-frequency (RF) or microwave field and in turn excite the metal or salt of interest.

• Provide radiant intensities usually one to two orders of magnitude greater than HCL.

Source Modulation

• Function:Employed in AAS to distinguish

between the component of radiation arising from the source and the component of arising from the flame background

• Device: light chopper

Source ModulationLight chopper

• A chopper or a modulated power supply is used to modulate the source radiation that passes through the atomizer (flame).

• The chopper rotates resulting in an alternating atomic absorption and atomic emission signal.

• The signal from emission of radiation will be continuous and can be substracted from the modulated AA signal.

Light chopper

• Eliminate the effects of radiation from the flame• Light is “chopped” with a rotating half-mirror so

that detector sees alternating light intensities• One moment, only light emitted by flame is read

since the light from the source is cut off• Next moment, light from both the flame emission

& transmission of the source’s light is measured since the source’s light is allowed to pass

• The elements of the detector is such that the emission signal is substracted from the total signal & this difference is what we measured.

Questions

1. In flame atomic absorption spectroscopy, briefly describe the ‘atomization’ process which the analyte undergoes.

2. Why is source modulation employed in atomic absorption spectroscopy? Name the device used for this purpose.

3. Name the common line source used in AAS.

INTERFERENCES

Spectral interference Chemical

interference

Ionization interference

Spectral interference

• Arise when particulate matter from atomization scatters the incident radiation from the source.

• Or when absorption or emission of an interfering species either overlaps or lies so close to the analyte absorption that resolution by the monochromator becomes impossible.

Spectral interference (simple words)

• Is one in which the spectral line of the elements being determined overlaps with a spectral line (or band) from another element present in the sample.

• The effect of the element will also be measured & thus the results will be incorrect.

Spectral interference

The most common method of solving this problem

i) Tune the monochromator to a different spectral line for the element of interest so that there is no overlap.

ii) secondary lines (can be found in the literature)

Chemical interferences

• 2 common types of chemical interferences (that reduce the concentration of free gaseous atoms)

(i) ionization(ii) formation of molecular species

Chemical interference – Ionization

• Sample (liquid/solid) must be vaporized & atomized in a high temperature source such as flame.

• This high temperature environment lead to ionization of the analyte atoms.

• Analyte ionization can be supressed by adding a source of electrons, which shifts the equilibrium of the analyte from the ionic to the atomic form.

• Cesium and potassium are common ionization supressors that are added to analyte solutions.

Analyte Analyte+ + e-

Chemical interference – Ionization

• Cesium and potassium are common ionization suppressors that are added to analyte solutions

• These atoms (Cs & K) are easily ionized & produce a high concentration of free electrons in the flame.

Chemical interference – Refractory Formation

• Some elements can form refractory compounds that are not atomized in flames.

• E.g. in the presence of phosphates, which interferes with calcium measurements due to formation of refractory calcium phosphate.

• Formation of refractory compounds can be prevented or reduced by adding a releasing agent.

Chemical interference – Refractory Formation

• For calcium measurement:– Adding lanthanum to the sample (and

standard) solutions binds the phosphates as LaPO4.

– LaPO4 has a very high formation constant, Kf and effectively ties up the phosphate interferent.

QUANTITATIVE ANALYSIS BY ATOMIC ABSORPTION SPECTROSCOPY

– Calibration Curve– Standard Addition Method

Absorption behavior follows Beer’s Law & concentration of unknown are determined the same way.

All atomic species have an absorptivity, a

Pathlength, b is width of the flame

A = abc

Calibration Curve Method

• A general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of std sample of known concentration.

• Plot is linear over a significant concentration range (the dynamic range).

• Analysis should never be based on the measurement of a single standard with assumption that Beer’s law is being followed.

Standard Addition Method

• Extensively used in AAS• Compensate for variation caused by physical &

chemical interferences in the analyte solution• The most common way involves adding one or

more increments of a standard solution to sample aliquots of the same size.

• This process is often called spiking the sample.• Each solution is then diluted to a fixed volume

before measurement.

Example: Standard Addition MethodThe chromium in an aqueous sample was determined by pipetting 10.0 mL of the unknown into each of five 50.0 mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cr were added to the flasks, following which the solutions were diluted to the volume.

1. Plot the data.2. Calculate the concentration of Cr in the sample.

Volume of unknown (mL)

Volume of standard (mL)

Absorbance

10.0 0.0 0.201

10.0 10.0 0.292

10.0 20.0 0.378

10.0 30.0 0.467

10.0 40.0 0.554

Standard Addition- if Beer’s law is obeyed,

A = bVstdCstd+ bVxCx

Vt Vt

= kVstdCstd + kVxCx

k is a constant equal to b Vt

• Standard Addition- Plot a graph: A vs Vstd (should yield a straight line)

A = mVstd + b slope; m = kCstd

intercept; b = kVxCx

Concentration of unknown:

Cx = bCstd

mVx

• Detection Limits (ng/mL)

Element AAS Flame Element AAS Flame

Al 30 Cu 2

As 100 Fe 5

Ca 1 Hg 500

Cd 1 Pb 10

Cr 3 Zn 2

Nanogram/mililiter = 10-3 g/mL = 10-3 ppm

Accuracy

– Relative error associated with a flame absorption analysis is the order of 1% to 2%

– With special precautions, this figure can be lowered to a few tenths of one percent.

Standard addition method

- used to overcome matrix effect- involves adding one or more increments of a standard solution to sample aliquots of the same size.- Each solution is diluted to a fixed volume before measuring its absorbance.

Abs

orba

nce

Standard Addition Plot

How to produce standard addition curve?

1. Add same quantity of unknown sample to a series of flasks.

2. Add varying amounts of standard (made in solvent) to each flasks, e.g. 0, 5, 10, 15 mL).

3. Fill each flask to line, mix and measure.

Standard Addition Methods

Single-point standard addition

method

Multiple standard addition method

Standard addition- if Beer’s Law is obeyed,

A = bVstdCstd + bVxCx

Vt Vt

= kVstdCstd + kVxCx

k is a constant equal to b Vt

Standard Addition- Plot a graph: A vs Vstd

A = mVstd + b

where the slope m and intercept b are:

m = kCstd ; b = kVxCx

• Cx can be obtained from the ratio of these two quantities: m and b

b = kVxCx

m kCstd

Cx = bCstd

mVx

• 10 ml aliquots of raw-water sample were pipetted into 50.0 ml volumetric flasks. Then, 0.00, 5.00, 10.00, 15.00 and 20.00 ml respectively of a standard solution containing 10 ppm of Fe3+ were added to the flasks, followed by an excess of aqueous potassium thiocyanate in order to produce the red iron-thiocyanate complex. All the resultant solutions were diluted to volume and the absorbance of each solution was measured at the same.

Example:

Vol. of std added(ml)

Absorbance(A)

0 0.215

5.00 0.424

10.00 0.625

15.00 0.836

20.00 1.040

The results obtained:

Calculate the concentration of Fe3+ (in ppm) in the raw-water sample

-10 -5 0 5 10 15 20 250

0.2

0.4

0.6

0.8

1

1.2

Absorbance vs Vol. of std added

Vol. of std

Ab

sorb

ance

Slope, m = 0.0382

b = 0.24

(Vstd)0 = -6.31 ml

Note: From the graph, extrapolated value represents the volume of reagent corresponding to zero instrument response.

• The unknown concentration of the analyte in the solution is then calculated:

Csample = -(Vstd)0Cstd

Vsample

Cx = bCstd

mVx

The chromium in an aqueous sample was determined by pipetting 10.0 ml of the unknown into each of 50.0 mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cr were added to the flasks, following which the solutions were diluted to the mark.

SELF-EXERCISE

Volume of unknown (mL)

Volume of standard (mL)

Absorbance

10.0 0.0 0.201

10.0 10.0 0.292

10.0 20.0 0.378

10.0 30.0 0.467

10.0 40.0 0.554

i) Plot a suitable graph to determine the concentration of Cr in the aqueous sample.