part 2. flame emission: it measures the radiation emitted by the excited atoms that is related to...

Part 2

• 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.

Relationship Between Atomic Absorption and Flame Emission

2Prof. Dr. Hisham Ezzat Abdellatef

Atomizers in emission techniques

Type Method of Atomization Radiation SourceArc sample heated in an sample

electric arc (4000-5000oC)

Spark sample excited in a sample high voltage spark

Flame sample solution sample aspirated into a flame

(1700 – 3200 oC)

Argon sample heated in an sampleplasma argon plasma (4000-6000oC)


Atomizers in absorption techniques

Type Method of Atomization Radiation SourceAtomic sample solution aspirated HCL(flame) into a flame

atomic sample solution evaporated HCL(nonflame) & ignited (2000 -3000 oC)(Electrothermal)

Hydride Vapor hydride generated HCLgeneration

Cold vapor Cold vapor generated (Hg) HCL


Atomizers in fluorescence techniques Type Method of Atomization Radiation Sourceatomic sample aspirated sample(flame) into a flame atomic sample evaporated sample(nonflame) & ignitedx-ray not required samplefluorescence


Flame Atomization: In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. The following processes then occur in the flame.

• Desolvation (produce a solid molecular aerosol) • Dissociation (leads to an atomic gas) • Ionization (to give cations and electrons) • Excitation (giving atomic, ionic, and molecular emission)


Processes that take place in flame or plasma

T 1 7

Sample AtomizationFor techniques samples need to be atomizedTechniques are useful for element identification

Molecular information destroyed by atomization

Flame AtomizationSample nebulizedMixed with fuelCarried to flame for atomization

8

The Atomization Process

[M+,X-]aq [M+,X-]aqnebulization

solution mist[MX]solid

vaporizationdesolvation

[X0]gas[M0]gas

[MX]gas

atomiza

tion

[M+]gas [X+]gas

atom

izati

on

[M*]gas[M0]gasemission

excitation or absorption

(via heat or light)ground state excited state

Prof. Dr. Hisham Ezzat Abdellatef

Types of Flames:Several common fuels and oxidants can be employed in flame spectroscopy depending on temperature needed. Temperatures of 1700oC to 2400oC are obtained with the various fuels when air serves as the oxidant. At these temperature, only easily decomposed samples are atomized. For more refractory samples, oxygen or nitrous oxide must be employed as the oxidant. With the common fuels these oxidants produce temperatures of 2500oC to 3100oC.


Burning Velocity:The burning velocities are of considerable importance because flames are stable in certain ranges of gas flow rates only. If the gas flow rate does not exceed the burning velocity, the flame propagates itself back in to the burner, giving flashback. As the flow rate increases, the flame rises until it reaches a point above the burner where the flow velocity and the burning velocity are equal. This region is where the flame is stable. At higher flow rates, the flame rises and eventually reaches a point where it blows off of the burner.


• Flame Structure:Important regions of a flame include:

1. primary combustion zone 2. interzonal region 3. secondary combustion zone

1. Primary combustion zone: Thermal

equilibrium is ordinarily not reached in this region, and it is, therefore, seldom used for flame spectroscopy.


2. Interzonal region: This area is relatively narrow in stoichiometric hydrocarbon flames, is often rich in free atoms and is the most widely used part of the flame for spectroscopy.

3. Secondary combustion zone: In the

secondary reaction zone, the products of the inner core are converted to stable molecular oxides that are then dispersed into the surroundings.


Temperature Profiles:

A temperature profile of a typical flame for atomic spectroscopy is shown in Fig. 9-3. The maximum temperature is located in the flame about 1 cm above the primary combustion zone. It is important– particularly for emission methods – to focus the same part of the flame on the entrance slit for all calibrations and analytical measurements.


secondary combustion zone

interzonal region

primary combustion (reaction) zonepre heating zone

appearance depends on fuel/oxidantlength of zones depends on gas flow rateDANGER gas flow too high- lifts flame off burner gas flow too low- flash back

pre heating zone gases heated rapidlyprimary combustion region contains free radicals but not in thermodynamic equilibrium interzonal region used for spectrometry has free radicalssecondary combustion zone combustion products formed i.e. CO and H2O


Types of fuel/oxidantair/acetylene2300oC most widely used.

nitrous oxide/acetylene 2750oC hot and reducing red feather zone -

due to CN very reactive free radical scavenger for 02 → lowers partial pressure of 02 in zone reducing atmosphere

C2H2 + 2.502 + 10N2 → 2CO2 + H2O + 10N2

stoichiometric reaction

C2H2 + 5N2O → 2 CO2 + H2O + 5N2


Why do you need a different burner for different oxidants?

because to prevent flash back linear gas flow rate

needs to 3 x speed of which flame can travel,

burning velocity).


Role of Chemistry in the Flame

sample atomised by thermal and chemical dissociationH2 + Q → H + H O2 + Q → O + O H + O2 → OH + O O+ H2 → OH + H

equilibrium achieved by 3rd body collision (B) i.e. N2, O2

H + H + B → H2 + B + QH + OH + B → H2O + B + QFree reductions may react with sample to produce atoms i.e. H + HO + NaCl → H2O + Na + Cl

Na + Q → Na*

10 20Prof. Dr. Hisham Ezzat Abdellatef

Flame Atomisation Process

Sample must be in the form of a fine mist so as not to put out flame.

Breaks down sample into very fine drops to formliquid aerosol or mist.This assist atomisation as sample only in flame ≈ 0.025sSample drawn up capillary tube at high velocity

Sample

oxidant 21Prof. Dr. Hisham Ezzat Abdellatef

Suction caused by high flows of oxidant gas and Venturi effect.

The high gas flow rate at the end of the capillary creates a pressure drop in the capillary – the pressure in capillary is below atmospheric pressure and sample solution is pulled up.

The high speed gas breaks the solution into a fine mist by turbulence as it emerges from capillary.

How do we get a better aerosol?• use impact bead (glass or alloy) to

encourage aerosol formation and remove large droplets.


Flame absorbance Profiles:Fig. 9-4 shows typical absorption profiles for three elements. Magnesium exhibits a maximum in absorbance at the middle of the flame. The behavior of silver, which is not readily oxidized, is quite different, a continuous increase in the number of atoms, and thus the absorbance, is observed from the base to the periphery of the flame. Chromium, which forms very stable oxides, shows a continuous decrease in absorbance beginning close to the burner tip.



Flame absorbance profile for three elements

Flame Atomizers:

Figure 9-5 is a diagram of a typical commercial laminar flow burner that employs a concentric tube nebulizer. The aerosol is mixed with fuel. The aerosol, oxidant, and fuel are then burned in a slotted burner that provides a flame that is usually 5 or 10 cm in length.



Laminar-Flow Burner

Advantages:1. Uniform dropsize2. Homogeneous flame3. Quiet flame and a long path

length

Disadvantages:1. Flash back if Vburning > Vflow

2. ~90% of sample is lost3. Large mixing volume 27Prof. Dr. Hisham Ezzat Abdellatef

Sample introduction techniques


Methods of Sample Introduction in Atomic Spectroscopy


Nebulization

• Nebulization is conversion of a sample to a fine mist of finely divided droplets using a jet of compressed gas.– The flow carries the sample into the atomization region.

• Pneumatic Nebulizers (most common)Four types of pneumatic nebulizers:

• Concentric tube - the liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary (Bennoulli effect). – This is also referred to aspiration. The high velocity breaks the

sample into a mist and carries it to the atomization region.


Concentric tube; Cross flowConcentric tube; Cross flow

Fritted diskBabington

Types of pneumatic nebulizers


• Cross-flow The jet stream flows at right angles to the capillary tip. The sample is

sometimes pumped through the capillary. • Fritted disk

The sample is pumped onto a fritted disk through which the gas jet is flowing. Gives a finer aerosol than the others.

• Babington Jet is pumped through a small orifice in a sphere on which a thin film of

sample flows. This type is less prone to clogging and used for high salthigh salt content samples.

• Ultrasonic Nebulizer• The sample is pumped onto the surface of a vibrating piezoelectric

crystal. • The resulting mist is denser and more homogeneous than pneumatic

nebulizers.• Electro-thermal Vaporizers (Etv)

An electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer.


Liquid samples introduced to atomizer through a nebulizer

Pneumatic nebulizer Ultrasonic-Shear Nebulizer


AtomizationAtomizers

Flame

Electrothermal

SpecialGlow DischargeHydride GenerationCold-Vapor


Flame ChemistryFlames are used in atomic emission spectrometry for excitation (emission spectrometry) but in atomic absorption flames are used as Atom Cells to produce gaseous atoms.Why must the atoms not be excited for atomic absorption spectrometry?

If the atom is already in the excited state it cannot absorb the light.


Different Atomization Sources for Atomic Spectroscopy

Source Type Typical Source Temperature

Combustion Flame 1700 - 3150 C.

Electrothermal Vaporization (ETV) on graphite platform

1200 - 3000 C

Inductively coupled plasma (ICP)

5000 -8000 C

Direct-current plasma (DCP)

6000-1000 C

Microwave induced plasma (MI))

2000-3000 C

Glow Discharge plasma (GDP)

non-thermal

Spark Sources (dc or ac Arc)

~ 40,000 C(?)


Flame Atomizers

• Superior method for reproducible liquid sample introduction for atomic absorption and fluorescence spectroscopy.

• Other methods better in terms of sampling efficiency and sensitivity.



Laminar-Flow Burner

39

Atomizers

• Atomization occurs in a flame created by mixing a fuel with an oxidant

• Analyte and background ions are atomized simultaneously

• Only a small percentage of the aqueous sample is atomized – much of the sample goes to waste

Flame Atomic Absorption SpectrometryFlame Atomic Absorption Spectrometry


Laminar flame atomizer


Performance Characteristics Of Flame Atomizers

In terms of reproducible behavior, flame atomization appears to be superior to all other methods for liquid sample introduction. In terms of sampling efficiency and thus sensitivity, however, other atomization methods are markedly better. A large portion of the sample flows down the drain and the residence time of individual atoms in the optical path in the flame is brief (~10-4s).


Electrothermal Atomization

• Atomization of entire sample in short period• Average sample time in optical path is seconds

– Evaporation of sample• Microliter volume• Low temperature

– Sample ashed at higher temperature– Increase current

• Sample temperature goes to 2000-3000 °C– Sample measured above heated surface

• High sensitivity for small samples


Electrothermal AtomizationIt provides enhanced sensitivity because the entire sample is atomized in a short period, and the average residence time of the atoms in the optical path is a second or more. A few microliters of sample are first evaporated at a low temperature and then ashed at a somewhat higher temperature in an electrically heated graphite tube or in a graphite cup. Then the current is rapidly increased to several hundred amperes, which caused the temperature to soar to perhaps 2000oC to 3000oC; atomization of the sample occurs in a period of a few milliseconds to seconds.


47

Atomizers

• Atomization occurs in an electrically heated graphite tube

• The graphite tube is flushed with an inert gas (Ar) to prevent the formation of (non-absorbing) metal oxides

Electrothermal or Graphite Furnace AtomizerElectrothermal or Graphite Furnace Atomizer

graphite tubeProf. Dr. Hisham Ezzat Abdellatef

Performance Characteristics: Electrothermal atomizers offer the advantage of unusually high sensitivity for small volumes of sample. Typically, sample volumes between 0.5 and 10 L are used; absolute detection limits lie in the range of 10-10 to 10-13 g of analyte. Furnace methods are slow-typically requiring several minutes per element. A final disadvantage is that the analytical range is low, being usually less than two orders of magnitude.


Electrothermal atomizer

Sample concentration


50

Atomization

From Skoog et al. (2004); Table 28-1, p.840Prof. Dr. Hisham Ezzat Abdellatef

51

Atomization and Excitation

Atomic Emission Spectroscopy• The heat from a flame or an

electrical discharge promotes an electron to a higher energy level

• As the electron falls back to ground state, it emits a wavelength characteristic of the excited atom or ion

From Skoog et al. (2004); Figure 28-1, p.840Prof. Dr. Hisham Ezzat Abdellatef

52

Atomic Line Spectra

Each spectral line is characteristic of an individual energy transition

E = h

nc

h


Flame atomic absorption spectrometry

Beer Lamberts Law A = log (Po/P)

A = b c where is the molar absorptivity coefficient in units of mol-1 dm3 cm-1

b is the pathlength in cm

and c is the concentration in mol dm-3

In limits (below 0.8 Absorbance)

A vs. concentration

samplePo P

b


part 2. flame emission: it measures the radiation emitted by the excited atoms that is related to...

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