Plasma A Plasma consists of a collection of
- Slides: 20
Plasma • A Plasma consists of a collection of free-moving electrons 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 airpropane 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 t 300 400 Pb. Hg Mg Cu Zr Ca 500 600 Ba Na 700 U 800 K
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