Instrumental Chemistry

Chapter 11

Atomic 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