Mass Spectrometery
Mass spectrometry gives information on the molecular or atomic weight of compound or atom.
Mass spectrometry is used on molecules as well as elements. In molecules, information about the
molecular formula and structure can also be derived based on fragmentation patterns that may
occur. In elements, the information can be used to calculate the average atomic mass and
relative abundance(s) of each isotope of an element.
The Mass Spectrometer has FOUR major stages:
1. Ionization:
2. Acceleration:
3. Deflection:
4. Detection:
Example: The overall mass spectrum for chlorine looks like this:
What are the isotopes of chlorine? Cl-35 and Cl-37
Are there peaks corresponding to these two values? ______________
What are the ratios of two isotopes in nature? _____________________________________
What about the 3 lines at the end of the spectrum? ________________________________
Various isotopes exist for different elements:
M+ + 1 can be found for elements: carbon, hydrogen and nitrogen
M+ + 2 can be found for elements: oxygen, sulfur, bromine and chlorine
Other recognizable features of some elements are:
For bromine the M+ + 2 peak is as large as the M+ peak
For chlorine the M+ + 2 peak is one-third as large as the M+ peak
For iodine there is a large gap between a cluster of fragments and an M + peak at 127
For nitrogen the M+ peak will have an odd number value
Ultraviolet-Visible Spectroscopy
Based on electronic transitions of conjugated systems (systems that have unsaturation(s) and
their electrons are delocalized). Since pi-bond electrons are more easily excited than sigma bond
electrons, the energies required for the transitions are in the ultraviolet region but often extend into
the visible region. Thus it has known as UV-VIS spectroscopy.
Compounds that contain unsaturated groups that absorb in the UV-VIS region are called
_____________________. For example, _______________ is a chromophore due the six
delocalized electrons. However, the amount of energy needed to excite these pi-bond electrons is
also affected by the substitutents attached to the chromophores. These substituents are called
_______________________.
Most organic compounds absorb in the UV range however molecules with an extensive number of
pi-electrons will absorb in the visible region. Example include: Beta-carotene, chlorophyll and
phenolphthalein in a basic
medium.
The rule: the higher the number of conjugated double bonds the longer the wavelength (toward
visible) of which the compound absorbs light.
Beta-carotene
How does a UV-VIS spectrometer work? The spectrometer compares the amount of light
transmitted through the sample (sample beam) with the amount of light transmitted in a reference
beam. The information is then recorded as %transmittance (A) to the wavelength.
This concept can be extended to UV-VIS Spectrophotometry as well. The information from this
technique can be used to determine the concentration of a solution.
The absorbance (A) of a sample at a particular wavelength is governed by Beer’s Law:
Where,
Ir:
Is :
c:
l:
:
Since, the path length is fixed, and the molar absorptivity coefficient () is constant, this means
that absorbance is directly proportional to the concentration. At low concentrations a linear
relationship exists between concentration and absorbance. The amount of light absorbed is
related to the concentration.
By preparing solutions of varying concentrations, at a certain wavelength the absorbance data can
be generated for each dilution. This can be plotted to create a standard curve or a calibration
curve. This standard curve can be used to
determine the concentration of an unknown
solution.
Chromatography
Purpose: To separate mixtures into components. All chromatography techniques operate on the
principle of partition or adsorption.
Stationary
phase
A solid or liquid on a solid surface.
Particle do NOT move.
Mobile phase
A liquid or gas that flows through the stationary phase and carries components
with it
Partition
Adsorption
Two liquids that do not mix or are ____________________.
Sometimes applied to a gas/liquid separation (GLC).
Separating types of molecules based on how a compound distributes itself
between two immiscible phases. Based on solubility of substance.
A substance forming some sort of a __________ to the surface of another one.
mobile phase
And remember: polar molecules are miscible in a polar solvent, while non-polar molecules are
miscible in non-polar solvents.
Example 1:
Stationary phase: polar
Mobile phase: non-polar solvent.
Component to be separated is a mixture of a polar molecule (A)
and a non-polar molecule (B).
Which molecule will travel the furthest? ___________
Why?
Example 2:
Stationary phase: polar
Mobile phase: polar solvent.
Component to be separated is a mixture of a polar molecule (C)
and a non-polar molecule (D).
Which molecule will travel the furthest? ___________
Why?
Types of Chromatography
Type
Stationary
Phase
Mobile Phase
Applications
Type
Stationary
Phase
Mobile Phase
Applications
The difference between paper chromatography and TLC? Sample can be isolated for further
analysis.
Type
Stationary
Phase
Mobile Phase
Applications
Used for separation on a larger scale
Eluent: solution that passes through the column.
Elution: process of washing solution through column.
Another type of column chromatography is Ion-Exchange Chromatography: this has beads of ionexchange resin for the stationary column. A ion-exchange resin is a polymer with ions bonded to
it. The process of separating the ions is based on desirable ions from sample displace ions
already bonded to the resin.
Type
Stationary
Phase
Mobile Phase
Applications
Benefit: sample size required is very small (microlitre)
Retention time: time the sample is injected to the point at which the display shows as a maximum
peak or how long it takes for a substance to work its way through the column.
Type
Stationary
Phase
Mobile Phase
Applications