Plasma A Plasma consists of a collection of free-moving electrons and ions and is very hot. Energy...

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 stillmaximizes sample ionization.

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

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 t 400 500 600 700 800300

PbHg Mg Cu Zr Ca Ba Na U K