PLOT Columns

Porous

Layer

Open

Tubular

Porous layer open tubular (PLOT) columns are defined as capillary columns where the inner surface is coated with a layer of solid porous material.

PLOT Columns

This layer is usually about 5-50 m thick as opposed to liquid coated columns where the stationary phase is commonly 0.25- 2.0 m thick.

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 chromatographyand the other major forms of gas chromatography is similar to the difference between what 2 types of HPLC.

PLOT Columns

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 columnshave much higher efficiency than packed columns and arebecoming more popular. The main problem with the adoption of PLOT columns was the difficulty in manufacturing stable, uniform coatings with reproducibleretention times.

PLOT Columns

1. Alumina2. Molecular Sieve3. Carbon Based4. 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)

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

What detector would you use?

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-divinylbenzenealcohols, water, polar volatiles, hydrocarbons

PoraPLOT S Divinylbenzene-vinylpyridineketones, esters, halogenated compounds, hydrocarbons

PoraPLOT U Divinylbenzene-ethyleneglycol-dimethylacrylatepolar volatiles, nitriles, nitro-compounds, sulfur compounds

PoraPLOT amines - ????ammonia; primary, secondary, and tertiary amines

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.

Bonded Phases

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.

PLOT Columns