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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
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
Complete the following table.
Symbol Z N A # elec.
12C-
15 31 13
6 11 5
Bi3+ 126
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
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
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?
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
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.
A mass spectrometer can measure mass to charge ratio of particles (single atomsor groups of atoms)
The mass spectrum is isotope specific
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
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?
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
Problem: What is the molecular mass of a molecule of vanillin containing only 12C? one 13C?
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...
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
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
A gas chromatograph will separate mixtures into components. This will lead to a chromatogram.
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.
What Else Can We do With Mass Spectroscopy?
• Arson analysis• Drug analysis• Trace analysis