Question

What are open tubular columns and give some differences between open tubular column and packed column

Answer 1

There are two general types of column: packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.

Capillary columns have an internal diameter of a few tenths of a millimeter. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns. Both types of capillary column are more efficient than packed columns

Packed column: filled with particles of stationary phase.

Open tubular: narrow, hollow capillary with stationary phase coated on the inside walls.

In open tubular or capillary columns, the stationary phase is held on the inner surface of a capillary, whereas in packed columns, the stationary phase is supported on particles that are contained in a glass or metal tube. Open tubular columns contain an enormous number of plates that permit rapid separations of closely related species. They suffer from small sample capacities.

The main differences is that:

Packed columns offer high sample capacity, while open tubular columns give better separation efficiency (smaller plate height), shorter analysis time, and increased sensitivity to small quantities of analyte.

Open tubular columns eliminate the multiple path term (A) from the van Deemter equation, decreasing plate height. Also the lower resistance to gas flow allows longer columns to be used with the same elution time.

Compared to packed columns, open tubular columns can provide:

1. Higher Resolution
2. Shorter analysis time
3. Increased sensitivity
4. Lower sample capacity

Compared to packed columns, open tubular columns allow:

1. Increased linear flow rate or a longer column or both
2. Decreased plate height, which means higher resolution

Some of the main advantages of open tubular columns over packed columns are:

1. faster analysis (high flow of carrier possible)
2. shorter retention times
3. more inert
4. longer life
5. less bleed of coating material
6. higher efficiencies
7. greater reproducibility