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Specifications of GC Capillary columns
In our last installment, we took a look at the various types of liquid stationary phases for gas chromatography (GC) capillary columns which is the most important feature affecting the a column's ability to separate components. In our third installment of our series for GC capillary columns, we take a look at the specifications for capillary columns which have an effect on their performance. These specifications can greatly influence a column's usability and convenience for a given method. After reading this article, we hope users will have a better idea on how these specifications matter towards choosing the right column.
Column Length
Column length affects efficiency, resolution, and analysis time. A longer column provides samples greater exposure to the stationary phase allowing greater separation of components. The obvious disadvantage is that a longer column (with the same flow rate) amounts to a longer runtime.
Given the same flow rate, a longer column will have better efficiency than a shorter column. With a well-behaved (that is, very close or similar, peak shape) a longer column will have more theoretical plates than a shorter column as given by:
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where:
N: Number of theoretical plates
W: Peak width at baseline (see Figure 1)
tR : Retention time
Figure 1. Chromatogram of sample with relevant measurements labeled (tR, Retention time, t0, Dead time, tR', Adjusted Retention time, W, Peak width at baseline, h, Peak height). The peak width at baseline is considered as ± 2 standard deviations from peak center, the same as a Gaussian curve.
The improvement in resolution is less than direct scaling and only proportional to the square root of the number of plates as given by:
where:
R: Resolution
α: Separation factor
k: Retention factor of the component with higher tR
The choice of column length should be made with regards to meeting a certain separation performance while using the shortest column possible to minimize the runtime. Common lengths of capillary columns range from 5-100 meters where 30-60 m columns are usually employed.
Column Inner Diameter (I.D.)
The primary influence of column inner diameter is on the flow rate. As an open tubular structure, capillary columns have full loading capacity of their inner volume and more sample can be flowed through a column with a bigger diameter. The volumetric flow rate (cm^3/min) is given by:
where:
F: Volumetric flow rate
r: Column radius (diameter/2)
v : Linear flow rate
GC capillary columns have ranges of column I.D. from 0.1-0.53 mm. As the stationary phase is only present in the circumference of the cross-section of the capillary column, less percentage of the sample interacts with the stationary phase as the column ID increases. Hence, narrower ID columns have higher efficiency. The column I.D. dictates the type of analysis that can be performed.
At the same linear flow rate (v), narrower IDs have a higher theoretical plate count but lower volumetric flow rate (F) and, consequently, sample load. Injection technique, such as split injection or other compensation may be necessary for the introduction of samples for narrow-bore and micro-bore columns.
Table 1. Classification of GC capillary columns based on column ID
| Column ID (mm) |
Classification |
Qualities |
||||||
| 0.53 | Megabore | Lower resolution, mostly useful for qualitative analysis | ||||||
| 0.32 | Wide-bore | High efficiency, decent resolution | ||||||
| 0.25 | Narrow-bore | High efficiency, decent resolution | ||||||
| 0.18 | Mini-bore | Highest efficiency, excellent resolution | ||||||
Film Thickness of the Liquid Phase
The liquid stationary phase on the inner walls of the capillary column is the other region wherein samples may be found and, hence, also impact sample load. Thicker films allow for larger sample loading.
Film thickness influences retention of components which can allow better separation of components. However, dispersion in the stationary phase is a concern which can produce unwanted peak broadening. In higher temperatures, baseline drift also becomes a concern with thicker phases. Similar to column length, film thickness also largely contributes to analysis time.
Figure 2. A thicker film allows better separation of organic compounds. The baseline drift is shown for thicker films compared to thinner films.
Film thickness of GC capillary columns can range from 0.10 to 5.00 µm. Film thickness should be chosen as thin as possible while producing desired separation.
Summary
The physical specifications influencing column performance have been provided. In summary, column specifications should be chosen based on:
| Liquid Phase | Chemically close to the target compound |
| Column Length | The shortest length allowing the required degree of separation |
| Column ID | The type of analysis required |
| Film Thickness | The thinnest film allowing the required degree of separation |
Check back for our next article on parametric factors affecting column performance. To learn more, take a look at GL Sciences' selection of capillary GC columns.
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