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. Cl 2 + → Cl + Cl + We just determine the +ve ions (sometimes the –ve)

For the Cl 2 + ion There are three possible masses: = = = 74

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

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

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

Interface ICP operates at high temperature and with L/min Ar and atmospheric pressure MS – 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

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

One commercial design uses cryogenic pumping ~3 × 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

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

ICP-MS Use plasma as ion source Nebulize aqueous samples into Ar plasma See H 2 O and Ar peaks 0 16, OH 17, NO 30, O 2 32, Ar 40, ArH 41 Noise – random – not set by interfering ions

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

Comparison with Electrothermal AA (furnace) Lower detection limits Faster, more convenient Can do multielement analyses Can determine isotope ratio MORE EXPENSIVE