FORS-2107EL Lecture Notes - Lecture 8: Impact Ionization, Chemical Ionization, Derivatization
27 Jun 2018
School
Department
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Forensic Science - Day 8 2018.02.08
Liquid Chromatography (LC)
-In LC, analyte partition is usually based upon
relative solubility between stationary and mobile
phase
-Separation of compound may be based upon
relative affinity/interactions between two phases
-Analyte retention time in the column is therefore
controlled by mobile phase composition (the
solvent), and temperature (to a lesser extent)
-Here temperature will be less controlling to the
retention time, and solvent will control it more
-Resolution is controlled primarily through mobile phase composition gradients
(adjustment of mobile phase composition over time)
Thin layer Chromatography (TLC)
-This is a planar phase of liquid chromatography since it is on a plate like stationary
phase
-In TLC the extract is spotted on a
plate coated with stationary phase
particles (silica, alumina, or
hydrocarbon-functionalized silica)
-The plate is placed vertically in a
shallow bath of mobile phase
solvent, which is carried upwards
through the stationary phase by
capillary action
-Separation is based on differences
in intermolecular interactions
(solubility and absorption) of the
solutes with the mobile and
stationary phases
Detectors in Chromatography
-Once the constituents in a mixture have been separated, they
leave the column and enter a detector
-When the detector interacts with the constituent, it will yield a
response which may vary in magnitude as a function of the
constituents concentration, so each peak in the graphs are
related to the concentration of the sample and how much of it
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is present
-The larger and higher the peak, the larger the concentration
-Detectors may be selective for certain types of compounds (or non-selective) and
may or may not prove spectral information that can help identify the components
eluted
-Spectral detector: will measure the spectrum of a substance, therefore this is another
descriptor that will improve the power of discrimination for the sample (Ex. Mass
spectrum)
-The detectors that are used depend on the different application as well as the budget
-Mass Spectrometric Detection is used in both liquid and gas chromatography
Mass Spectrometric Detection
-MS detection is the gold
standard in many forensic
application because it can
provide both retention time data
(GC or LC) as well as structural
information through mass
spectral data
-In MS, a compound (that has
been separated through GC or LC) is first introduced into the spectrometer, where it
undergoes ionization and subsequent fragmentation
-Then the charged fragments are separated (based on mass to charge ratio, where
the charge is usually +1/-1, so basically measured by mass) in the mass selection
process, and are subsequently detected and this gives you a mass spectral data in
the form of a mass spectrometric graph
-You can ionize particles many ways, and the choice
depends on the separation method, usually in GC-
MS compounds are ionized through
-Electron impact ionization
-Here compounds are exposed to an electron
beam, which ionizes the compounds to M+,
which then disperse energy through
fragmentation
-Chemical ionization
-How this occurs:
-In GC-MS (through electron impact ionization (EI-MS)) the ion must be subjected to
a really strong vacuum (about 10-4 or 10-8 torr), in our environment we feel 1
atm=750 torr, therefore very low pressor, and this high strength vacuum will prevent
electrons from getting sucked away
-At the same time, there will be a filament that will be releasing electrons,and due to
the very low pressure due to the vacuum, the electrons are just flowing around
open space
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Document Summary
In lc, analyte partition is usually based upon relative solubility between stationary and mobile phase. Separation of compound may be based upon relative af nity/interactions between two phases. Analyte retention time in the column is therefore controlled by mobile phase composition (the solvent), and temperature (to a lesser extent) Here temperature will be less controlling to the retention time, and solvent will control it more. Resolution is controlled primarily through mobile phase composition gradients (adjustment of mobile phase composition over time) This is a planar phase of liquid chromatography since it is on a plate like stationary phase. In tlc the extract is spotted on a plate coated with stationary phase particles (silica, alumina, or hydrocarbon-functionalized silica) The plate is placed vertically in a shallow bath of mobile phase solvent, which is carried upwards through the stationary phase by capillary action.
