Inductively Coupled Plasma

1. Inductively Coupled Plasma

2. 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

6. 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.

8. 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.

9. ICP is used for EmissionSpectroscopy • Occasionally used for fluorescence • Not used for absorption because the high temperatures means the atoms are in excited states • Also coupled with Mass spectrometry

10. 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

13. 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

15. 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

16. 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)

17. 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

19. 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

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