Instrumental Chemistry
Chapter 11Atomic Mass Spectrometry
Atomic mass
The mass of a single atom, usually expressed in atomic mass units (amu)
Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus
Each proton or neutron weighs about 1 amu, and thus the atomic mass is always very close to the mass number (total number of protons and neutrons in the nucleus)
Mass Spectrometers
• Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other
• In general a mass spectrometer consists of an ion source, a mass-selective analyzer, and an ion detector
Diagram of a Mass Spectrometer
Stage 1: Ionization
• The atom is ionized by knocking one or more electrons off to give a positive ion
• These positive ions are persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge
Ionization (cont.)
Stage 2: Acceleration
The ions are accelerated so that they all have the same kinetic energy
All the ions are accelerated into a finely focused beam
Acceleration (cont.)
Stage 3: Deflection
• The ions are then deflected by a magnetic field according to their masses
The lighter they are, the more they are deflected
• The more the ion is charged, the more it gets deflected
Deflection (cont.)
Stage 4: Detection
The beam of ions passing through the machine is detected electrically
When an ion hits the metal box, its charge is neutralized by an electron jumping from the metal on to the ion
That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it
Detection (cont.)
Fourier-Transform MS
Magnetic-sector MS
Quadrupole MS
Time-of-flight MS
ICP-MS
Inductively Coupled Plasma Mass Spectrometry
In ICP-MS, the plasma is formed from Argon gas
Plasma is defined as a gas consisting of ions, electrons, and neutral particles
SSMS
Spark Source Mass Spectrometry Semi-quantitative trace element technique for
the analysis of solids and liquid materials Advantages include total simultaneous
elemental coverage, low detection limits, semi-quantitative accuracy (+2-3x), and high resolution capabilities to eliminate many spectral interferences
GDMS
Glow-Discharge Mass Spectrometry Analytical technique for the bulk elemental
analysis of inorganic solid samples The most comprehensive and sensitive
technique available for the analysis of solids
Glow Discharge Ion Source
Useful Websites Dealing With Mass Spectrometry
• http://www.anachem.umu.se/jumpstation.htm• http://userwww.service.emory.edu/~kmurray/mslist.html• http://www.chemcenter/org• http://www.sciencemag.org• http://reference.allrefer.com/encyclopedia/A/atomMas.html• http://www.webref.org/geology/a/atomic_mass.htm• http://www.chemguide.co.uk/analysis/masspec/howitworks.html• http://www.northernanalyticallab.com/tech2.htm• http://www.shivatec.com/new/gdmsdesc.php4