Atomic Emission Spectroscopy - Instrumental Methods of Analysis B. Pharma 7th Semester

Atomic Emission Spectroscopy

Objectives

After this session students will be able to

       Differentiate between Flame emission spectroscopy and atomic emission spectroscopy

       Explain the instrumentation of atomic absorption spectrophotometers

Atomic Emission Spectroscopy Using Non-Flame excitation sources

  1. There is no single  excitation source can excite all elements
  2. The emitted radiation usually consists of sharp well defined lines, which fall in UV or visible region
  3. Identification of the l of these lines permits qualitative analysis of these elements, whereas measurements of their intensities permits quantitative analysis

Advantages

  1. Excellent method for trace element analysis at ppm level
  2. Used nearly for all elements in periodic table
  3. Used for very small samples, even less than 1 mg
  4. There is no need for prior separation
  5. Relatively rapid technique

Disadvantages

  1. Expensive
  2. Low precision and accuracy
  3. Destroying the sample
  4. Used mainly for metals

High energy excitation sources

Plasma excitation sources

Laser

Arc and spark emission spectrometry (Spectrography)

 Microwave and x-ray

Plasma excitation sources

  1. A plasma is a cloud of highly ionized gas containing significant numbers of positive and negative ions, free electrons and neutral particles.
  2. Plasma sources operate at high temperatures between 7000 and 15000 K. Thus, it produces a greater number of excited emitted atoms, especially in the UV region, than that produced by flame.
  3. Using this technique, excitation operates through a plasma produced electrically in a carrier gas such as nitrogen or argon.

The main types of argon plasma sources

  1. Inductively coupled plasma; ICP
  2. Direct current plasma; DCP
  3. A microwave-induced plasma is recently introduced to spectro-chemical analysis methods.

Inductively coupled plasma; ICP

Argon gas flows upward through a quartz tube, around which is wrapped with a copper or selenoid induction coil.

The coil is energized by a radio frequency AC generator creating a changeable magnetic field on the flowing gas inside. This induces a circulating current in the gas, which in turn heats it.

Argon is not a conductor at low temperatures, but becomes electrically conducting by heating it. The induction is initiated by arc or a heated graphite rod.

It is used for multi-element determination

Direct current, DCP

It consists of a high-voltage discharge between two graphite electrodes. The recent design employs a third electrode arranged in an inverted Y-shaped which improves the stability of discharge.

The sample is nebulised at a flow rate of 1 ml/min. Argon is used as carrier gas. The argon ionized by the high-voltage is able to sustain a current.

DCP generally has lower detection limits than ICP.  However, DCP is less expensive than ICP.

Advantages of plasma excitation source:

1- The sample could be introduced in solution form through a nebulizer (easy for quantitative analysis).

2- It is suitable for quantitative multielement determinations

3- The high temperature of plasma eleminates many chemical interferences present in a flame

4- It is well suited for refractory (oxide forming) elements e.g. P, Ur and tungeston and for difficult-to-excite elements such as Zn and Cd.

5- The emission intensity-versus-cencentration range is linear over a very wide dynamic ranges of analytes.

Laser excitation source

Laser beam is used to vaporize the sample, which is then excited electrically.

The sample is loaded just beneath the two electrodes that will be used to generate the electrical discharge.

A ruby laser is then focused through a microscope onto the surface. The energy from the laser causes an intense local hot spot which vaporizes a small quantity of sample.

The vapor circuits the electrodes and electrical discharge occurs which excites the metals in vapor. The excited metals emit typical emission spectra which are collected and measured as usual.

Advantages of laser excitation source

  1. Laser excitation produces a high density plasma and is used for the spectrochemical analysis of solid materials.
  2. The localization effect permits examination of areas as small as 50 ┬Ám in diameter, providing the biological researcher with a tool capable of examining the insides of individual cells without destruction of organic materials.
  3. In laser excitation, the sample needs not to be electrically conducting.

Quantitative analysis

Use of an internal standard

If the composition of sample and matrix is unknown. The internal standard is added to both unknown and calibration standards.

The internal standard should

1. Resemble the element to be determined in rate of volatilization and chemical reactivity.

2. Have a measurable emission line in the same spectral vicinity as the sample emission line.

3. It must not also present in the original sample.

Then, by plotting the ratio of intensities of the element to the internal-standard element vs. concentration of the element, any fluctuations should be compensated for.

Standard Addition Method

In order to partially or wholly counteract the chemical and spectral interferences introduced by the sample matrix.

Applications of AES (using non-flame excitation sources)

AES is rapid method for qualitative and quantitative determination of most metals.

It is superior than flame and atomic absorption methods. Flame emission spectroscopy has the limitations of being only good for few elements while atomic absorption techniques need a separate source lamp for each element. AES methods; being very sensitive, have numerious applications in analysis of biological samples.

For examples:

  1. evaluation of platinum in body fluids and tissues after administration of platinum containing anticancer drugs
  2. determination of organic and inorganic Se compounds in biological fluids and environmental samples
  3. determination of trace elements such as Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni and Pb
  4. Silicon is recogonized as an essential trace element participating in normal body metabolism.

SUMMARY

       AAS makes use of non-flame energy sources

       The energy sources are Inductively Coupled Plasma, Direct Current Plasma  and laser excitation sources

       The emitted radiation usually consists of sharp well defined lines, which fall in UV or visible region

       Identification of the l of these lines permits qualitative analysis of these elements, whereas measurements of their intensities permits quantitative analysis

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