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What’s in my Vanilla?Cocaine in Money? An introduction to Atomic
(Molecular) Masses and Mass Spectrometry (or GC/MS)
OC
H
H
H
H
OH
O H
H
C
H
CC
CC
C
C
CH3
O
O
O
OCH3
N
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Every atom has a certain number of protons, neutrons, and electrons.
• Z = atomic number (number of protons in an element, also determines which element you have, X)
• X = abbreviation for the element• A = mass number (total number of protons and
neutrons in a particular isotope of that element)• For a neutral atom, the number of electrons is
equal to Z. If the atom is has a charge of +n, the number of electrons is equal to Z-n, if a charge of –n, the number of electrons is equal to Z+n
XAZ
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Complete the following table.
Symbol Z N A # elec.
12C-
15 31 13
6 11 5
Bi3+ 126
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Isotopes of an element contain the same number of protons (i.e. the same element) but
different numbers of neutrons
• Examples:
C126 C14
6C136
H11 T H3
1 D H21
• Isotopes do not have to be radioactive• Many elements contain more than one naturally occurring
isotope, a few have only one
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Problem: if we know the number of protons and electrons in an atom, can we get the mass of an atom?
• Possible solution: can we add up the masses of the individual nucleons?
(mp) = 1.673 X 10-27 kg(me) = 9.11 X 10-31 kg(mn) = 1.675 X 10-27 kg
Let’s try this for H and D
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Problem: The masses of H and D are known to be: 1.674 X 10-27 kg
and 3.344 X 10-27kg• We can’t add up the individual masses
because when the nucleons combine to form atoms some of their mass is converted into energy to hold the nucleus together (binding energy)
• What do we do?
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Let’s choose a standard atomic mass unit (amu). Then we can measure the masses of all the
atoms relative to that.• Definition: 1 atom of carbon-12 has a mass
of exactly 12 amu or 1 amu is exactly ½ the mass of a carbon-12 atom.
• Using this definition, 1 amu ( 1u) = 1.6605387 X 10-27 kg, H has a mass of 1.007825 amu , D has a mass of 2.0140 amu
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Problem: Now that we’ve defined a standard, how do we measure masses of all the other
atoms relative to that standard?• One solution: use a mass spectrometer that
has been calibrated to the carbon-12 standard.
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A mass spectrometer can measure mass to charge ratio of particles (single atomsor groups of atoms)
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The mass spectrum is isotope specific
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The atomic mass of an atom on the periodic table is really an
average massaverage relative atomic mass = A1p1 + A2p2 + … + AnPn
An = exact mass of specific isotopepn = fractional abundance of isotope
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Example
Chlorine contains two naturally occurring isotopes: chlorine-35 (mass 34.968852 amu, 75.77% natural abundance) and chlorine-37 (mass 36.965903, 24.23% natural abundance). What is the average mass of chlorine?
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OK, so what does this have to do with vanilla?• Natural vanilla is extracted from the vanilla orchid
– Natural vanilla may contain more than one flavoring component
– Sometimes natural vanilla can be adulterated with coumarin; this reduced costs, but is illegal because coumarin is hepatotoxic.
• Artificial vanilla is produced from wood pulp• The chief constituent of both forms of vanilla is vanillin, C8H8O3
OC
H
H
H
H
OH
O H
H
C
H
CC
CC
C
C
O H
OH
OCH3
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Problem: What is the molecular mass of a molecule of vanillin containing only 12C? one 13C?
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OK, so what does this have to do with vanilla?
• The metabolic pathway by which the vanilla orchid fixes carbon is different than that by which trees (used for wood pulp) do. This gives a slightly different 12C to 13C ratio.
• Using a high resolution mass spectrometer, we could measure the abundances of the 12C and 13C peak and determine the ratio to determine whether or not a sample is natural.
• We probably won’t be able to make this distinction on our mass spectrometer, but...
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Practice Sheet Objectives
• Understand mass spectroscopy• Understand chromatography• Use GC/MS to examine differences
between artificial vanilla extract, real vanilla extract, and vanilla beans
• Identify minor flavoring components, preservatives, etc.
• Identify coumarin as an illegal additive.
O H
OH
OCH3
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Gas Chromatography Mass Spectroscopy
• Determine MM of a compound (mass of a molecule)• Determine structure of a compound• Determine ID of unknown• Used in conjunction with GC to separate, determine, and quantify mixtures
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A gas chromatograph will separate mixtures into components. This will lead to a chromatogram.
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Each peak in a chromatogram has its own mass spectrum.
mesitylene, C9H12
Other peaks are due to fragmentation of the molecule in the mass spec.
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What Else Can We do With Mass Spectroscopy?
• Arson analysis• Drug analysis• Trace analysis