Inductively Coupled Plasma

Question 1

What is plasma?

Answer 1

An electrically conducting gaseous mixture containing significant concentrations of cations and electrons.

Question 2

Name the plasma sources

Answer 2

• Inductively coupled plasma (ICP)
• Direct Current Plasma (DCP)
• Microwave induced Plasma (MIP)

Question 3

Describe the function of Inductively Coupled Plasma (ICP)

Answer 3

• Plasma generated in a device called a Torch
• Ions and electrons interact with magnetic field and begin to flow in a circular motion. Resistance to movement (collisions of elections and cations with ambient gas) leads to ohmic heating. Rapid tangential flow of argon cools outer quartz and centers plasma. Sample introduction is analogous to atomic absorption.

Question 4

Describe inductively coupled plasma (ICP)

Answer 4

• Sample atoms reside in plasma for ~2 msec and reach temperature of 4000 to 8000 K.
• Ionization interference small due to high density of electron
• Plasma chemically inert, little oxide formation

Question 5

Describe the types of interferences that can occur in ICP

Answer 5

Spectral interference: caused by background emission form continuous or recombination phenomena, stray light from the line emission of high concentration elements, overlap of a spectra line from another element, or unresolved overlap of molecular band spectra.

Background emission and stray light compensated for by subtracting background emission determined by measurements adjacent to the analyte wavelength peak.
Correction factors can be applied if interference is well characterized

Inter-element corrections will vary for the same emission line among instruments because of differences in resolution, as determined by the grating, the entrance and exit slit widths, and by the order of dispersion

Chemical interferences: include molecular compound formation, ionization effects, and solute vaporization effects. Normally, these effects are not significant with the ICP technique. Chemical interferences are highly dependent on matrix type and the specific analyte element.

Memory interferences: result when analytes in a previous sample contribute to the signals measured in a new sample. Memory effects can result from sample deposition on the uptake tubing to the nebulizer and from the buildup of sample material in the plasma torch and spray chamber. The site where these effects occur is dependent on the element and can be minimized by flushing the system with a rinse blank between samples.
High salt concentrations can cause analyte signal suppressions and confuse interference tests.