ATOMIC EMISSION SPECTROSCOPY (AES)

PRINCIPLE OF AES

• Atoms are thermally excited so that they emit light and the radiation emitted is measured 

ATOMIC EMISSION SPECTROSCOPY (AES)
aka

• flame photometry or optical emission spectroscopy (OES)

Line Emission Spectrum

• The emitted light can be viewed through a spectroscope to produce a line emission spectrum, a series of colored lines on a dark background. 

• The line emission (atomic) spectrum of an element is unique and thus, an element can be identified based on the emission spectrum.

The emitted light can be viewed through a

spectroscope to produce a line emission spectrum, a series of colored lines on a dark background. 

The line emission (atomic) spectrum of an element is

unique and thus, an element can be identified based on the emission spectrum.

FLAME 

• The sample containing the metal is volatilised in a natural gas / compressed air flame at 2000 C. 

• A higher temperature (2500 C) may be obtained using air/acetylene and is required for analysis of Mg by AES.

The sample containing the metal is volatilised in a

natural gas / compressed air flame at 2000 C. 

A higher temperature (2500 C) may be obtained using

air/acetylene and is required for analysis of Mg by AES.

MONOCHROMATOR / FILTER

The radiation emitted by the excited atoms is passed through a filter, or a monochromator in more expensive instruments. Thus a narrow band of emitted radiation is selected and interfering sources of radiation such as the flame and other components in the sample are screened out

The radiation emitted by the excited atoms is passed through a filter, or a monochromator in more expensive instruments.

Thus a narrow band of emitted radiation is selected and interfering sources of radiation such as the flame and other components in the sample are screened out

DETECTOR

The intensity of the selected radiation is then measured using a photosensitive cell. (usually photomultiplier tube)

INTERFERENCES

• ionisation 

• viscosity 

• anions 

IONISATION

• may reduce the observed emission from the sample.

IONISATION

• In the volatilization or atomization of the sample, it must be

atomized to its neutral form, not in its ionized form since ionization reduces its observed emission.

VISCOSITY

• can either increase or decrease the rate at which the sample is drawn into the flame relative to the standard solution thus giving a false high or low reading 

• Sample and standard solution must have the same viscosity.

False High Reading:

less viscous sample = faster aspiration

False Low Reading:

more viscous sample = slower aspiration

ANIONS

• Anions such as sulphate and phosphate form involatile (will not volatilize into the flame) salt with metal ions and reduce the reading of the sample solution.

The sample (containing metals) in the solution and not atomized yet

can react with anions (if present).

Application in Pharmaceutical Analysis

• Quantification of alkali metals in:

  • alkali metal salts

  • infusion 

  • dialysis solutions

• Determination (identification) of metallic impurities in some of the other inorganic salts used in preparing these solutions

Application in Pharmaceutical Analysis

• Quantification of alkali metals in:

• alkali metal salts

• infusion 

• dialysis solutions

Application in Pharmaceutical Analysis

• Determination (identification) of

• metallic impurities in some of the other inorganic salts used in preparing these solutions

Strengths

Provides a robust, cheap and selective method based on relatively simple instrumentation for quantitative analysis of some metals

Limitations

Only applicable to the determination of alkali and some alkaline earth metals

INDUCTIVELY-COUPLED PLASMA AES (ICP-AES)

• If high enough temperatures can be reached, ANY element can be excited to a level where it will produce emission of radiation

• Such high temperatures can be achieved by using plasma emission (heating inert gas e.g., argon with high intensity RF radiation at 7000 °C)

• Provides rapid analysis of active ingredients because of its high sensitivity and specificity which is advantageous during formulation development

INDUCTIVELY-COUPLED PLASMA AES (ICP-AES)

• If high enough temperatures can be reached,

ANY element can be excited to a level where it will produce emission of radiation

INDUCTIVELY-COUPLED PLASMA AES (ICP-AES)

• Such high temperatures can be achieved by using

• plasma emission (heating inert gas e.g., argon with high intensity RF radiation at 7000 °C)

INDUCTIVELY-COUPLED PLASMA AES (ICP-AES)

• Provides rapid analysis of active ingredients because

of its high sensitivity and specificity which is advantageous during formulation development

ICP Torch

• Once the sample aerosol is heated in Argon plasma, it will be excited.