What*s in my Vanilla? * An introduction to Atomic (Molecular

Report
What’s in my Vanilla?
Cocaine in Money?
An introduction to Atomic
(Molecular) Masses and Mass
Spectrometry (or GC/MS)
O
H
N
CH3 O
C
OCH3
H
C
C
H
C
O
H
C
H
C
C
O
O
C
H
H
H
O
Every atom has a certain number of
protons, neutrons, and electrons.
A
X
Z
• 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
Complete the following table.
Symbol Z
N
A
# elec.
31
13
11
5
12C-
15
6
Bi3+
126
Isotopes of an element contain the same
number of protons (i.e. the same element) but
different numbers of neutrons
• Examples:
12
6 C
1
1H
13
6 C
14
6 C
2
1HD
3
1HT
•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
-27
are known to be: 1.674 X 10 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 atoms
or groups of atoms)
The mass spectrum is isotope specific
The atomic mass of an atom on
the periodic table is really an
average mass
average relative atomic mass = A1p1 + A2p2 + … + AnPn
An = exact mass of specific isotope
pn = 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
O
O
H
C
of both forms of vanilla
H
C
H
is vanillin, C8H8O3
C
C
H
H
C
H
C
C
O
O
C
H
H
OCH3
H
OH
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...
O
H
Practice Sheet Objectives
OCH3
OH
• 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.
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

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