Inductively Coupled Plasma

Plasma A Plasma consists of a collection of free-moving electrons and ions and is very hot. Energy must be continually applied to sustain the plasma

For an ICP torch, the plasma is initiated by application of sparks from a Tesla coil. The plasma is propagated by means of an induction coil. The applied rf produces a fluctuating magnetic field. Ions flow and their resistance to flow generates heat. Temperataures can reach 6,000 k – 10,000 K. Argon cooling is absolutely imperative.

Three Ar flows are introduced to the torch: 1) Cool Gas - the outer flow ~ 14 L min-1 keeps the sides of thetorch from melting; 2) Auxilliary Flow - this is the intermediate flow through the torch that keeps the plasma away from the end of the torch at a rate of 0.5-1.5 L min-1; 3) Sample Flow - this central flow introduces the sample to the plasma at ~ 0.7-1.0 L min-1. The cool sample injected through the center of the plasma cools it to ~ 7,000 K which reduces the abundance of Ar+ but still maximizes sample ionization.

ICP is used for Emission Spectroscopy Occasionally used for fluorescence Not used for absorption because the high temperatures means the atoms are in excited states Also coupled with Mass spectrometry

ICP Torch Plasma forms in and above inductive coil in a shape like a flame Temperature of Plasma varies with height PREHEATING ZONE – where sample first enters plasma evaporation of solvent melting and vaporization of salt INITIAL RADIATIVE ZONE (IRZ) atoms formed and excited atomic emission takes place

NORMAL ANALYTICAL ZONE (NAZ) ion formation occurs usually +1 and +2 TAIL OR PLUME atoms can recombine to polyatomics An yttrium salt can be used to visually locate these regions of the plasma IRZ – red NAZ – blue tail – red

Optimal sensitivity for different elements occurs at different heights But similar enough that you can successfully do simultaneous analysis using an ‘average’ height Usually measure emission from 15-25 mm above coil-low background where there are few Ar lines Below this height the plasma is brilliant white and transparent – a continuum with the atomic spectrum for Ar superimposed

Advantages of ICP 6000 – 10000 K – gives better atomization Long residence times (~2 ms) ~2 × flame Chemically inert environment (no radicals as found in flames) prevents oxide formation Get a lot of lines – helps identify (also a disadvantage because spectra get so complicated)

Temperature cross-section of plasma is relatively uniform no self absorption problems can get linear calibration curves over several orders of magnitude One set of conditions is close to optimum for many elements – so can do multielement analysis

ICP-AES One can obtain a simple Li spectrum at low temperatures, for example with an air-propane flame (2000 K) Other elements such as Uranium require higher temperatures At higher temperatures you get a complex spectrum and you need a high resolution spectrometer to separate the lines Interference corrections are essential

Spectrum of Mixture of Elements 200 300 400 500 600 700 800 Pb Hg Mg Cu Zr Ca Ba Na U K