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)

For the Cl2+ ion There are three possible masses:

•35 + 35 = 70

•35 + 37 = 72

•37 + 37 = 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 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

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 × 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

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 H2O and Ar peaks

• 0 16, OH 17, NO 30, O2 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 0.01-0.09 ng/mL

• limits are fairly similar for all elements – not the case for ICP-OES

Comparison with ElectrothermalAA (furnace)

• Lower detection limits

• Faster, more convenient

• Can do multielement analyses

• Can determine isotope ratio

• MORE EXPENSIVE