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Inductively Coupled Plasma

Inductively Coupled Plasma. Mass Spectrometry. Mass spectrometry. Ions are analyzed on the basis of their m/z Chlorine has 2 isotopes, 35 Cl and 37 Cl, in the approximate ratio of 3 :1. Electrons are knocked off Cl 2 in the beam and a molecular ion, Cl 2 + is formed.

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Inductively Coupled Plasma

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  1. Inductively Coupled Plasma Mass Spectrometry

  2. Mass spectrometry • Ions are analyzed on the basis of their m/z • Chlorine has 2isotopes, 35Cl and 37Cl, in the approximate ratio of 3 :1. • Electrons are knocked off Cl2 in the beam and a molecular ion, Cl2+ is formed. • Cl2+ → Cl + Cl+ • We just determine the +ve ions (sometimes the –ve)

  3. For the Cl2+ ion There are three possible masses: • 35 + 35 = 70 • 35 + 37 = 72 • 37 + 37 = 74

  4. ICP-MS • A sample’s mass spectrum is generally much simpler than its emission spectrum • Elements with isotopes produce a peak for each mass eg. Cl: 35, 37 • For a mass analyzer – needs ions • You measure m/Z where often z = 1 eg. F- or Cl- • Few interferences-masses differ by at least 1

  5. Characteristics of an ICP-MS • Analyses are fast • >30 elements/min can be determined at concentrations ~10 × LOD • all masses accessible • 3 – 300 amu • background noise is small • Spectra are simple and mostly free of interferences

  6. ICP-MS Instrumentation • ICP torch and nebulizer system • Interface • Vacuum system • Quadrupole MS • Powerful data management and control system

  7. Interface • ICP operates at high temperature and with 10-20 L/min Ar and atmospheric pressure • MS – 10-5 torr or less • Plasma is operated in horizontal position • One interface is made of two cones/orifices • Expansion through the first orifice (the sampler) (~1 mm) produces a supersonic beam

  8. Interface • Mechanically pumped to ~1 torr • Only a small amount of plasma gas enters through the sampler • Some of this continues through the second orifice, the skimmer

  9. One commercial design uses cryogenic pumping ~3 × 10-5 torr • Any non-ionized gas atoms entering skimmer are cooled rapidly to 14 K and frozen out on cryoshell • Ions do not condense and go into the quadrupole

  10. Quadrupole Spectrometer is used • Compact design • Moderate resolution • High Stability • Separates mass according to stability of ions in an oscillating electric field • Scan by ramping dc and ac voltages • Can peak-hop also

  11. ICP-MS • Use plasma as ion source • Nebulize aqueous samples into Ar plasma • See H2O and Ar peaks • 0 16, OH 17, NO 30, O2 32, Ar 40, ArH 41 • Noise – random – not set by interfering ions

  12. Low LOD in ICP-MS • Ionization efficiency is high and similar for most elements • 90% of elements give LOD’s 0.01-0.09 ng/mL • limits are fairly similar for all elements – not the case for ICP-OES

  13. Comparison with ElectrothermalAA (furnace) • Lower detection limits • Faster, more convenient • Can do multielement analyses • Can determine isotope ratio • MORE EXPENSIVE

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