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What are Capillary GC Columns?
In this first of our series of articles about capillary columns for gas chromatography, we talk about what capillary columns are, their general specifications, construction, advantages, and types. We hope the reader gains a general sense of what capillary columns are, how they work and how they can be properly integrated to maximize their own utilization.
Capillary columns for gas chromatography (GC) are long, narrow tubes which are coated or chemically bonded with a liquid stationary phase or adsorbent on their inner walls. These columns are typically constructed out of fused-silica tubes with a 0.1-0.8 mm internal diameter and length of 5-100 meters which is coiled to fit inside ovens of gas chromatography machines.
The hollow construction of capillary columns is commonly called an open tubular design. This design allows low resistance to the flow of gaseous and volatile components through the length of the column as they are separated by their degree of interaction with the stationary phase. Capillary columns provide superior performance to older packed columns with less flow path diffusion leading to sharper peaks and lower column pressure allowing for much longer column lengths to be used making capillary columns the preferred columns for very efficient and highly sensitivity analysis today.
Table 1. Comparison of Capillary and Packed GC columns
| Capillary Column |
Packed Column |
|
| I.D. (mm) |
0.1-0.8 | 2.0 - 4.0 |
| Length (m) |
5.0 - 100.0 | 1.0 - 5.0 |
| Thickness (µm) |
0.1 - 5.0 | 3 - 10 |
| theoretical plate number, N |
7,500 - 300,000 | 2,000 - 100,000 |
| N／m |
1,500 - 5,000 | 2,000 - 2,500 |
| Material |
Fused silica・stainless steel etc | Glass・Stainless Steel etc |
| Inert |
High | Low |
| Sample Load (µg) |
0.05 - 3.0 | 10.0 - 20.0 |
Fused-silica is the preferred material to construct capillary columns. Fused-silica is desirable chemically because it is quite unreactive to most substances, with a ≤ 1 ppm metal content and <120 ppm oxide content. The surface of fused-silica allows is also very smooth allowing for efficient binding of the stationary phase for much higher counts of theoretical plates, up to 5,000, compared to packed columns, which are up to 2,500. Fused-silica columns can have much thinner walls than glass columns while also being flexible which is advantageous for production and handling. Polyimide resin is coated on the outer wall for added durability.
Stationary phases used for capillary columns have two types: solid phases and liquid phases. Liquid stationary phases are much more common for general analysis. Capillary columns can be grouped into three types based on the method the stationary phase has immobilized on the inner walls: Wall-Coat Open Tubular (WCOT) column, Support-Coated Open Tubular (SCOT) column, Porous-Layer Open Tubular (PLOT) column. The different types are described below:
Wall-Coat Open Tubular (WCOT) Column
WCOT columns are coated or chemically bonded with a layer of stationary phase with a thickness of about 0.05-3μm on their inner wall. After application, the liquid phase is conventionally cross-linked or chemically bonded to further increase efficiency. WCOT columns typically provide the highest performance among capillary column types in most applications and are the most widely used type
Support-Coated Open Tubular (SCOT) Column
In SCOT columns, an inert solid carrier is first impregnated with the liquid phase before being fixed to the inner wall to serve as support. A common support material is diatomaceous earth. SCOT columns have a greater capacity for volume than WCOT columns but are typically less efficient
Porous-Layer Open Tubular (PLOT) Column
PLOT columns are similar to SCOT columns in that solid particulates are fixed to the inner wall but, in contrast to SCOT columns, they are no longer impregnated and instead sorbent materials are used such as porous polymer, alumina, and molecular sieves. PLOT columns excel for specific applications such as analysis of light or specific compounds.
Summary
We hope you've gained an understanding of the nature and operation of capillary GC columns. The use of capillary columns is essential for analyzing many volatile compounds in the laboratory.
Check back for our next article on capillary columns for gas chromatography, where we will be looking at the different types of liquid stationary phases.
To learn more, take a look at GL Sciences' selection of capillary GC columns.
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