CH415 Chapter 11: Atomic Mass Spectrometry

Atomic Spectroscopy Systems

1. Atomic Absorption

2. Atomic Emission

3. Atomic Fluorescence

4. Atomic Mass Spectroscopy

Atomic Mass Spectrometry

• Nearly all elements in the periodic table can be determined by mass spectrometry

• More selective and sensitive than optical instruments

• Simple spectra

• Isotope ratios

• Much more expensive instrumentation

Processes:

1. Atomization

2. Ionization (production of ions)

3. Analyzer (Separation of ions based on m/z ratio)

4. Transducer (Counting number of ions reaching detector)

4 main components of an atomic mass spectrometer:

1. inlet

2. ionization source/ion accelerator

3. mass analyzer

4. detector

Understanding your mass spectrum:

• signal abundance vs. m/z

• mass resolution - ability to separate mass

• mass accuracy - measurement of closeness to true mass

• Mass range - difference b/t highest and lowest measurable mass

Atomic mass (in amu) of Carbon, Chlorine, Hydrogen

• Atomic mass of Carbon

– 12.000000000000000000000000000 amu • Atomic mass of Chlorine

– 35.4527 amu (atomic mass units)
• Atomic mass of Hydrogen

– 1.00794 amu
1amu = 1 dalton (Da) (use kD for macromolecules)

What about isotopes?

• Atomic mass of Carbon

– 12.000 amu for 12C but 13.3355 for 13C • Atomic mass of Chlorine

– 34.9688 amu for 35Cl and 36.9659 for 37Cl • Atomic mass of Hydrogen

– 1.00794 amu for H and 2.0141 for D!

• 1 amu = 1.66056*10-27 kg.

• proton, mp = 1.67265*10-27 kg,

• neutron, mn = 1.67495*10-27 kg.

Ways to define and calc. the mass of an atom, molecule, or ion

• Average mass: calculated using the atomic weight, which is the weighted average of the atomic masses of the different isotopes of each element in the molecule (find it in periodic table).

• Nominal mass: calculated using the mass of the predominant isotopes of each element rounded to the nearest integer value that corresponds to the mass number.

• Monoisotopic mass: calculated using the exact mass of the most abundance isotope for each constituent element.

  *Use monoisotopic mass if possible in MS