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The Working Principle of Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) for Elemental Analysis

ICP-AES, also known as ICP-OES (inductively coupled plasma-optical emission spectrometry), is a widely used analytical technique for determining elemental compositions of samples.

ICP-AES utilizes high-temperature argon plasma to excite electrons in sample atoms/ions to quantitatively measure characteristic electromagnetic emissions for elemental identification and quantification.

In an ICP-AES instrument, the sample to be analyzed is first converted into an aerosol via nebulization with argon gas. This aerosol is then transported to the plasma torch – the core of the instrument.

The Working Principle and Phases of ICP-AES

Phase 1: Generating the Plasma

The plasma is formed by partially ionizing argon gas. This is achieved by initially igniting a Tesla coil to form a spark. This spark provides energy to free electrons present in the flowing argon gas, creating more ions and electrons – leading to a self-sustaining plasma.

The plasma torch consists of 3 concentric quartz glass tubes. The outermost tube transports the argon gas tangentially to form the bulk plasma. The middle tube introduces the nebulized sample aerosol into the plasma. The innermost tube allows for the insertion of an induction coil carrying high-frequency radio waves at frequencies ~27 MHz or 40 MHz.

Phase 2: How the Radio Frequency Induction Coil Sustains the Plasma

When the oscillating current in the induction coil is switched on, it creates an oscillating magnetic field according to the principle of electromagnetic induction. This time-varying magnetic field in turn induces eddy currents in the flowing bulk plasma. This heats the plasma further, as the eddy currents encounter resistance in the plasma.

This external energy supplied to the plasma compensates for the energy lost during collisions, thereby maintaining the population of ions and electrons – in other words, sustaining the plasma.

Phase 3: Atomization, Excitation, and Emission

As the sample aerosol enters the plasma, the extreme temperature of 6000-10,000 K causes instantaneous desolvation, vaporization, atomization, and finally excitation of the atoms.

The 3 prominent mechanisms by which the atomic species are excited in the plasma are:

  1. Collisional excitation by the kinetic energy of electrons, ions, and neutrals
  2. Penning ionization
  3. Charge transfer reactions

These excitation mechanisms promote electrons of the atomic species from lower energy levels to higher energy levels. As the electrons drop back down to lower energy levels, electromagnetic rays characteristic to the element are emitted.

Phase 4: Sorting the Emission Spectrum

The emitted light from the plasma passes through a diffraction grating which acts like a prism to split it into its constituent wavelengths. The intensity of light at each wavelength is then measured using a charged couple device (CCD) detector.

As every element emits light at specific wavelengths according to its unique electronic configuration, the emission spectrum acts like a fingerprint to identify the element. The intensity of light at the element’s characteristic wavelengths is directly proportional to its concentration, allowing quantitative determination.

Phase 5: Minimizing Interferences

However, interferences can affect the emission intensity recorded at an analyte wavelength. Spectral interferences from overlapping wavelengths and background emission are mitigated by careful selection of the most appropriate emission lines.

Matrix effects and ionization interference are minimized by optimizing experimental parameters like radio frequency power, plasma gas flow rate, and sample introduction flow rate to ensure complete atomization and excitation. Internal standards can also help compensate for residual interferences.

Frequently Asked Questions

What type of samples can be analyzed using ICP-AES?

ICP-AES can analyze solutions, suspensions, slurries, soils, powders, metals, and other solid samples. Samples may require acid digestion before analysis. The technique is suitable for both trace and major/minor elemental analysis.

How does ICP-AES compare to other elemental analysis techniques like AAS?

ICP-AES has lower detection limits than flame atomic absorption spectroscopy (AAS). It offers higher sample throughput and simultaneous multi-element capability, unlike graphite furnace AAS. However, the running costs are higher than AAS.

What are the limitations of ICP-AES?

  1. High initial investment and ongoing argon gas costs
  2. Trained expertise required for method development and operation
  3. Spectral and matrix interferences require careful optimization
  4. Not all elements can be analyzed easily (e.g. H, C, S, O, N)