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PLOT Columns
Porous
Layer
Open
Tubular
PLOT Columns
Porous layer open tubular (PLOT) columns are defined as
capillary columns where the inner surface is coated with a
layer of solid porous material.
This layer is usually about 5-50 mm thick as opposed to
liquid coated columns where the stationary phase is
commonly 0.25- 2.0 mm thick.
PLOT Columns
Retention time of compounds in PLOT columns depends
partly on diffusion into the thin porous layer and partly on
the interaction of the compound with the solid surface.
In other words, the main mechanism of separation is
gas-solid adsorption rather than gas-liquid partitioning
which is the mechanism for columns with a liquid coating.
The difference between PLOT column chromatography
and the other major forms of gas chromatography is
similar to the difference between what 2 types of HPLC.
PLOT Columns
The stationary phase in PLOT columns is much more retentive
than liquid stationary phases which makes these columns ideal
for analysis of low molecular weight hydrocarbons, sulfur gases,
permanent gases or other very volatile compounds.
However, since PLOT columns are so retentive they are
unsuitable for many compounds. Analytes that are not gases
at room temperature or are not highly volatile liquids will
take a very long time to elute, a may actually be
permanently bound to the column.
PLOT Columns
Plot columns are relatively new. Previously gases had been
separated using packed GC columns. However, Plot columns
have much higher efficiency than packed columns and are
becoming more popular. The main problem with the
adoption of PLOT columns was the difficulty in
manufacturing stable, uniform coatings with reproducible
retention times.
PLOT Columns
1. Alumina
2. Molecular Sieve
3. Carbon Based
4. Porous Polymer
PLOT Columns (Alumina)
Some of the first PLOT columns
developed used a coating of alumina
(Al2O3) modified with various
additives to give different selectivity.
PLOT Columns (Molecular Sieve)
What detector would you use?
Separation is based on
molecular size and the
ease of diffusion through
the porous structure.
-separate gases such as
hydrogen, helium,
argon, and oxygen
Will not separate polar
gases such as carbon
dioxide, water vapor,
sulfur gases
Carbon Based Stationary Phases.
A coating of graphitized carbon black is one such coating
that can be used to separate both polar and non-polar
compounds. An advantage of some of the carbon based
columns is that they can be less affected by water in the
sample, which can cause problems such as changes in
retention, selectivity, efficiency, and peak area in the
alumina or molecular sieve columns.
Porous Polymers
PoraPLOT Q Styrene-divinylbenzene
alcohols, water, polar volatiles, hydrocarbons
PoraPLOT S Divinylbenzene-vinylpyridine
ketones, esters, halogenated compounds, hydrocarbons
PoraPLOT U Divinylbenzene-ethyleneglycol-dimethylacrylate
polar volatiles, nitriles, nitro-compounds, sulfur compounds
PoraPLOT amines - ????
ammonia; primary, secondary, and tertiary amines
Bonded Phases
A recent advance in PLOT
column technology is bonded
phases. Unlike earlier columns,
the stationary phase is “grown”
in-situ on the interior of the
column. This coating is
corsslinked, providing much
higher mechanical and thermal
stability than columns where
pre-formed particles were
coated on the inside of the
column.
PLOT Columns
PLOT columns are a fairly recent development in capillary
gas chromatography. This is mostly due to the difficulty in
producing a stable uniform stationary phase with highly
reproducible chromatography. There are many possible
adsorbants that could be used as stationary phases, and
new developments in PLOT columns are likely.