ATOMIC SPECTROSCOPY

Lecture 5

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.

ATOMIC SPECTROSCOPY Three types of atomizer:

1) flame atomizer2) 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.

FLAME 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) Atoms

2) Ions

3) Molecules

Process occuring during 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 secondary

combustion zone where the atoms will form

molecular oxides and are dispersed into the

surroundings.

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 arecombined into a single unit.

Sample is drawn up thecapillary & nebulized

Sample flow rate: 1 to 3mL/min

Turbulent Flow Burner

Advantage: introduce relatively large & representative

sample into the flame.

Disadvantages:A relatively short path length through flameProblems with clogging of the tipBurners 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 flame

ii. Longer path length, enhance sensitivity & reproducibility

Disadvantages:

i. Lower rate of sample introduction

ii. 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%)

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 E1

E0

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 Design

1. 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. Radiation Sources for AAS

LINE SOURCE – narrow emission bands.

Examples of line source used in AAS:

Hollow cathode lamp (HCL)

Electrodeless discharge lamp

(EDL)

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 of Modulation

Function: Employed in AAS to distinguish between

the component of radiation arising from the source and the component of radiation 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 interferenc

e

Chemical interferenc

e

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 interefrence

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

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.

AAS AND AES Both methods use atomization of a sample and

therefore determine the concentrations of elements.

For AAS, absorption of radiation of a defined wavelength is passed through a sample and the absorption of the radiation is determined. The absorption is defined by the electronic transition for a given element and is specific for a given element. The concentration is proportional to the absorbed radiation.

In AES, the element is excited. A rapid relaxation is accompanied by emission of UV or visible radiation is used to identify the element. The intensity of the emitted photon is proportional to element concentration.

ATOMIC EMISSION SPECTROMETRY

INTRODUCTION Basic Schematic

Scanning instruments can detect multiple elements.

Many lines detected so sometimes it is a quantitatively difficult method.

Source can be flame, but more commonly plasma because it is much hotter.

1. Plasma SourcesPlasma – Highly ionized, electrically neutral gaseous mixture of cations and electrons that approaches temperature 10,000 K.

There are three types of plasma sources:A) Inductively coupled plasma (ICP)B) Direct current plasma (DCP)C) Microwave induced plasma (MIP)

ICP is the most common plasma source.

Inductively Coupled Plasma (ICP) Constructed of three concentric

quartz tube.

RF current passes through the water-cooled Cu coil, which induces a magnetic field.

A spark generates argon ions which are held in the magnetic field and collide with other argon atoms to produce more ions.

Argon in outer tube swirls to keep plasma above the tube.

The heat is produced due to the formation of argon ions.

Inductively Coupled Plasma (ICP)1. Sample Introduction

a. Liquid Sample

- Nebulizer similar to FAAS

- Sample nebulized in a stream of argon with a flow rate of 0.3 – 1.5 L/min.

- Sample aerosol enters the plasma at the base

through the central tube.

b. Solid Samples

- Sample atomized by electrothermal atomization

a and carried into the plasma by a flow of argon gas.

Inductively Coupled Plasma (ICP)Analyte Atomization and Ionization

By the time the analyte reaches the observation zone it has resided in the excitation zone for 2 ms.

Advantages of ICP over flame:a) Temperature is two to three times higher than in a flame or furnace, which results in higher atomization and excitation efficiencies.

b) There is little chemical interference.c) Atomization in the inert (argon) atmosphere minimizes oxidation of the analyte.d) Short optical path length minimizes the probability of self-absorption by argon atoms in the plasma.e) Linear calibration curves can cover up to five orders of magnitude.

Inductively Coupled Plasma (ICP)

Plasma Appearance

a. Excitation Region The bright, white, donut

shapedregion at the top of the torch.

Radiation from this region is a continuum with the argon line spectrum superimposed.

Temperature: 8000 – 10 000 K

b. Observation Region The flame shaped region

above the torch with temperatures 1000 – 8000 K.

The spectrum consists of emission lines from the analyte along with many lines from ions in the torch.

2. Flame Emission Spectroscopy

Measure the intensity of emitted radiation

INSTRUMENTATION Consists of:

1. Nebulizer2. Burner3. Monochromator4. Detector5. Readout device / computer

FLAME EMISSION SPECTROSCOPY

PROCESS Sample is sprayed by the nebulizer into burner

Carried into the flame

Atomized & excited

The emission from the excited atoms passes into the monochromator where the selected wavelength is passed through for measurement

Intensity of the emitted wavelength is measured by the detection system & indicated on the readout/computer

Relationship Between Atomic Absorption and Flame Emission Spectroscopy

Flame Emission -> it measures the radiation emitted by the excited atoms that is related to concentration

Atomic Absorption -> it measures the radiation absorbed by the unexcited atoms that are determined.

Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame.

The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations.

Flame Emission Spectroscopy

Flame Emission Spectroscopy is based upon those particles that are electronically excited in the medium.

The Function of Flame1. To convert the constituents of liquid sample into the vapor state.

2. To decompose the constituents into atoms or simple molecules: M+ + e- (from flame) M + hv

3. To electronically excite a fraction of the resulting atomic or molecular species:

M M*

SUMMARYFlame Atomic Absorption

Flame Atomic Emission

Process measured Absorption (light absorbed by unexcited atoms in flame)

Emission (light emitted by excited atoms in a flame)

Use of flame Atomization Atomization & excitation

Instrumentation Light source No light source

Beer’s Law Applicable Not applicable

Data obtained A vs c I vs c