mass spectrometry ions are analyzed on the basis of their m/z chlorine has 2 isotopes, 35 cl and 37...
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Mass spectrometry
• Ions are analyzed on the basis of their m/z
• Chlorine has 2 isotopes, 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)
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For the Cl2+ ion There are three possible masses:
•35 + 35 = 70
•35 + 37 = 72
•37 + 37 = 74
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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
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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
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ICP-MS Instrumentation
• ICP torch and nebulizer system
• Interface
• Vacuum system
• Quadrupole MS
• Powerful data management and control system
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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
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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
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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
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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
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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
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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
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Comparison with ElectrothermalAA (furnace)
• Lower detection limits
• Faster, more convenient
• Can do multielement analyses
• Can determine isotope ratio
• MORE EXPENSIVE