5- ATOMIC ABSORPTION SPECTROSCOPY (AAS)

PRINCIPLE OF AAS

• Atoms of a metal are volatilized in a flame and their absorption of a narrow band of radiation produced by a hollow cathode lamp (light source) , coated with the particular metal being determined, is measured.  

Atoms of a metal are volatilized in a flame and their absorption of a narrow band of radiation produced by a

hollow cathode lamp (light source) , coated with the particular metal being determined, is measured.  

BEER-LAMBERT LAW (BEER’S LAW)

Relates the _

concentration of an analyte in a sample to the sample’s absorption of electromagnetic radiation

BEER-LAMBERT LAW (BEER’S LAW)
states that

the optical absorbance of a chromophore in a transparent solvent is linearly proportional to the chromophore’s concentration and also to the sample cell path length

BEER-LAMBERT LAW (BEER’S LAW)
formula

A=Ebc

TYPES OF AAS

• Single-Beam AAS

• Double-Beam AAS

Single-Beam AAS

• The beam from the line source is electrically modulated

• An amplifier is placed after the detector and turned only to this modulation frequency

• Noise from the radiation emitted at all frequencies, except for the resonance frequency, is rejected, and the signal-to-noise ratio is improved

Single-Beam AAS

• The beam from the line source is

electrically modulated

Single-Beam AAS

An amplifier is placed _

after the detector and turned only to this modulation frequency

Single-Beam AAS

Noise from the radiation emitted at all frequencies, except for the

resonance frequency, is rejected, and the signal-to-noise ratio is improved

Double-Beam AAS

• Beam from the line source is mechanically modulated (divide) by the chopper into a reference beam and sample beam

• Then recombined by a half-silvered mirror and are directed into a monochromator where the photons at the characteristic wavelength are measured by the detector

Double-Beam AAS

• Beam from the line source is

mechanically modulated (divide) by the chopper into a reference beam and sample beam

Double-Beam AAS

• Beam from the line source is mechanically modulated (divide) by the chopper into a reference beam and sample beam

• Then recombined by a

half-silvered mirror and are directed into a monochromator where the photons at the characteristic wavelength are measured by the detector

The stability of a double-beam spectrophotometer is

superior to that of a single-beam spectrophotometer

SAMPLE CELL DESIGNS

• flame AAS 

• flameless AAS 

Flame AAS

• The flame, through which the sample passes, is considered to be the sample cell 

• Liquid sample is aspirated into a flame via a nebulizer

• The sample is converted (by the nebulizer) to a mist that is composed of uniform droplets 

• Flame desolvates (remove the solvent from the drops = what remains is the analyte)  and atomizes the sample providing a source of neutral atoms or molecules for analysis

Flame AAS

• The flame, through which the sample passes, is

considered to be the sample cell 

Flame AAS

• Liquid sample is aspirated into a flame via

• a nebulizer

Flame AAS

• The sample is converted (by the nebulizer) to a

• mist that is composed of uniform droplets 

Flame AAS

Flame desolvates (remove the solvent from the drops = what remains is the analyte)  and atomizes the sample providing a

source of neutral atoms or molecules for analysis

Air-Acetylene Flame

burns within a temperature range of 2125°C – 2400°C

Nitrous Oxide-Acetylene Flame

burns within a temperature range of 2650°C – 2800°C

Flameless AAS

1. Electrothermal Vaporization Aas (ETVAAS) / Graphite Furnace Aas (GFAAS)

2. Cold Vapor Aas (CVAAS)

3. Hydride Generation Aas (HGAAS)

ELECTROTHERMAL VAPORIZATION AAS (ETVAAS) / GRAPHITE FURNACE AAS (GFAAS)

• for ultra-trace analyses of metals

ELECTROTHERMAL VAPORIZATION AAS (ETVAAS) / GRAPHITE FURNACE AAS (GFAAS)

1. liquid sample is deposited through a small opening into a graphite tube called  mini-Massmann furnace

2. sample is heated at increasing temperatures until the solvent is evaporated, solid residue is ashed or pyrolyzed, and neutral atoms are atomized in their ground states

3. atoms are then excited by absorption of radiation at characteristic wavelength

4. samples can be deposited either directly onto the wall of the graphite furnace or onto a small graphite platform, known as a L’vov platform, which sits inside of the graphite furnace

ELECTROTHERMAL VAPORIZATION AAS (ETVAAS) / GRAPHITE FURNACE AAS (GFAAS)

1. liquid sample is deposited through a

small opening into a graphite tube called  mini-Massmann furnace

ELECTROTHERMAL VAPORIZATION AAS (ETVAAS) / GRAPHITE FURNACE AAS (GFAAS)

2. sample is heated at increasing temperatures until the

• solvent is evaporated

• solid residue is ashed or pyrolyzed

• neutral atoms are atomized in their ground states

ELECTROTHERMAL VAPORIZATION AAS (ETVAAS) / GRAPHITE FURNACE AAS (GFAAS)
3. atoms are then excited by

absorption of radiation at characteristic wavelength

ELECTROTHERMAL VAPORIZATION AAS (ETVAAS) / GRAPHITE FURNACE AAS (GFAAS)

4. samples can be deposited either

• directly onto the wall of the graphite furnace or

• onto a small graphite platform, known as a L’vov platform, which sits inside of the graphite furnace

COLD VAPOR AAS (CVAAS)

• for analysis of mercury

COLD VAPOR AAS (CVAAS)

• a chemical reduction generates atoms, and a stream of inert gas sweeps the cold vapor into a cold quartz cell in the optical path of the instrument

• very sensitive and has detection limits that range from parts per billion (ppb) to parts per trillion (ppt) (very small amount) depending on the sample and the laboratory environment

COLD VAPOR AAS (CVAAS)

• a chemical reduction generates atoms, and a stream of inert gas

sweeps the cold vapor into a cold quartz cell in the optical path of the instrument

COLD VAPOR AAS (CVAAS)

• very sensitive and has detection limits that range from

parts per billion (ppb) to parts per trillion (ppt) (very small amount) depending on the sample and the laboratory environment

HYDRIDE GENERATION AAS (HGAAS)

• for the analysis of arsenic, bismuth, germanium, lead, antimony, tin, and tellurium

HYDRIDE GENERATION AAS (HGAAS)

• a reaction with sodium borohydride (NaBH₄) and hydrochloric acid generates the hydride of the analyte of interest.

• resulting gas is swept into an inert quartz cell (quartz absorption tube) that is positioned on top of the burner

• cells (quartz tube) can be externally heated, or heated by an air-acetylene flame

• heat of the flame breaks down the hydride and creates the elemental form of the analyte - direct transfer mode of hydride generation

• very sensitive and has detection limits in the ppb or ppt range.

HYDRIDE GENERATION AAS (HGAAS)

a reaction with sodium borohydride (NaBH₄) and hydrochloric acid

generates the hydride of the analyte of interest.

HYDRIDE GENERATION AAS (HGAAS)

resulting gas is swept into an

inert quartz cell (quartz absorption tube) that is positioned on top of the burner

HYDRIDE GENERATION AAS (HGAAS)
cells (quartz tube) can be

externally heated, or heated by an air-acetylene flame

HYDRIDE GENERATION AAS (HGAAS)

heat of the flame breaks down the hydride and

creates the elemental form of the analyte - direct transfer mode of hydride generation

HYDRIDE GENERATION AAS (HGAAS)

• very sensitive and has detection limits in the

ppb or ppt range.

LINE SOURCE

• emit spectral lines corresponding to the energy required to elicit the electronic transition from the ground state to an excited state in the sample

LINE SOURCE

most commonly used 

hollow cathode lamp & electrodeless discharge lamp

CHARACTERISTICS OF LINE SOURCES

• Produce lines of sufficiently narrow bandwidths specific to a particular atomic absorption peak

• Produce a beam of radiation of sufficient intensity to allow high signal-to-noise absorption measurements

• Produce a beam of radiation that is stable for extended periods of time

• Easy to start, have a short warm-up time and an extended shelf life

Hollow Cathode Lamp (HCL)

1. gas (e.g., Ne or Ar) is ionized when an electrical potential is applied across the electrodes (to anode and cathode)

2. from their neutral form, gases are ionized to gaseous cations, then acquire sufficient kinetic energy (they’re moving) to dislodge some of the metal atoms from the cathode surface, a process known as sputtering

3. portion of the resulting cloud of metal ions is excited 

4. upon relaxation to the ground state, the ions emit photons (of light or energy) at the characteristic wavelengths for that metal

Electrodeless Discharge Lamps (EDL)

• or continuum electrodeless discharge lamps

• produce much more intense radiation beams than HCLs 

Electrodeless Discharge Lamps (EDL)

limited for certain metals only (for the analysis of: 

Sb	Antimony	Hg	Mercury
As	Arsenic	P	Phosphorus
Bi	Bismuth	Se	Selenium
Cd	Cadmium	Te	Tellurium
Cs	Cesium	Tl	Thallium
Ge	Germanium	Sn	Tin
Pb	Lead	Zn	Zinc

Electrodeless Discharge Lamps (EDL)

• ionize the inert gas by

means of an intense radio-frequency (RF) field from the RF coil

WAVELENGTH SELECTOR

monochromator 

Ebert and Czerny–Turner monochromators

WAVELENGTH SELECTOR

polychromator 

Echelle polychromators

• for simultaneous measurements of multiple elements

DETECTOR

• Converts radiant energy, photons (analytical signal),  into a concentration-proportionate electronic signal.

Photomultiplier tubes (PMTs)

are widely used detector to convert photons passed through the monochromator into voltages.

ANALYTICAL CONSIDERATIONS

Sample Preparation: FAAS

requires the introduction of a liquid sample into the nebulizer.

ANALYTICAL CONSIDERATIONS

Sample Preparation: ETV/GFAAS

• normally performed using liquid samples, but analyses can be performed using slurries and solid samples.

• Skip liquifying or incorporating a liquid (if solid or slurry), perform the operation immediately.

Interferences

1. Spectral Interferences

2. Ionization of the Analyte

3. Matrix Effects

4. Spectral Line Broadening

5. Compounds that do NOT dissociate in the flame

SPECTRAL INTERFERENCES

• arise when there is an overlapping signal from another element (e.g., when it’s near the analyte & can absorb light) that is a component of the sample or sample matrix. 

IONIZATION OF THE ANALYTE

• Some elements such as sodium, potassium, calcium, and cesium are easily ionized, and ionization of the analyte reduces the analytical signal. 

MATRIX EFFECTS

• arise from differences between sample, standard, and blank viscosities or can be introduced by surface tension. 

SPECTRAL LINE BROADENING

• occur as a result of several factors, including:

  • Self-absorption

  • lorentz effect

  • doppler effect

  • quenching 

self-absorption

re-absorbed before exiting the source and eventually reaching the detector

Lorentz effect

Broadening due to moving particles

Doppler effect

due to the thermal motion of the emitting atoms or ion

quenching

quencher causes de-excitation or collision

COMPOUNDS THAT DO NOT DISSOCIATE IN THE FLAME

compounds must dissociate for them to atomize (neutral form)

Matrix Modifiers

• can improve results

INCREASING THE VOLATILITY OF THE SAMPLE MATRIX

• so that matrix components are removed during the ashing or pyrolysis step. 

REDUCING THE VOLATILITY OF THE ANALYTE

• helps to eliminate loss of the analyte during the ashing or pyrolysis step.

REDUCING BACKGROUND ABSORPTION

• by eliminating matrix components so that they do not interfere with the analyte signal during atomization.

Application in Pharmaceutical Analysis

• Determination of metal residues (e.g., dialysis solutions that may have Ca & Mg) remaining from the manufacturing process in drugs.

Strengths

• More sensitive (detect smaller amounts) than AES. 

• A highly specific(emit light of specific metal only) method of analysis useful in some aspects of quality control.

Limitations

• Only applicable to metallic elements. 

• Each element requires a different hollow cathode lamp for its determination. (related to specificity)