plasma a plasma consists of a collection of free-moving electrons and ions and is very hot. energy...
TRANSCRIPT
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