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Parameters of GC columns
In our fourth installment of our series for GC capillary columns, we discuss parameters for analysis using capillary columns and their impact towards results.
Flow velocity of the Carrier Gas
The flow velocity or flow rate of the carrier gas is probably the most important parameter affecting how well components are separated. The flow velocity determines the runtime, the distances and shape of the peaks.
Diffusion of components, which is the process of species migrating from a high concentration space to low concentration space is the major concern involving the flow velocity. Longitudinal (along the path) diffusion causes significant peak broadening, lower height equivalent to theoretical plates (HETP), efficiency and resolution. Diffusion is paramount with gas chromatography as gases have the highest velocities among all types of mobile phases used.
As diffusion is a time dependent process, increasing flow velocity can be utilized to suppress it. However, a faster velocity, itself, can produce peak broadening. There is an optimum rate with the shortest height of HETPs along the range of the flow rate which is dependent on the column inner diameter, column length the type of carrier gas used. Columns with smaller I.D. have lower HETPs and are more sensitive. A shorter column lengths widens the range of optimum flow velocity (given components are sufficiently separated).
Figure 1. Left: Effects of flow velocity (v) on HETP for various sizes of column I.D.s. Right: Effects of flow velocity (v) on HETP for various column lengths.
As the objective of chromatography is to achieve sufficient separation in an acceptable run time, the flow rate can be adjusted dependent on user requirements
Types of Carrier Gases
The type of carrier gas used has an impact on the efficiency and retention time. Among run parameters, the type of carrier gas used is the most easily controlled as gas chromatography is the one chromatography method that does not rely on solvent interaction with the mobile phase; instead, the carrier gas is just mostly used to push components along the column. The size and speed of the gas particles influences its ability to flow the sample through the column. Commonly used carrier gases are inert gases which are available based on their cost and ease of handling.
Table 1. Commonly available carrier gases
| Nitrogen | Inexpensive and safe but has low linear velocity. Nitrogen usually has long analysis time with a narrow range of optimum flow velocity. |
| Hydrogen | Inexpensive with a wide range of optimum flow velocities but has safety concerns (flammability) requiring quite a bit of safety precautions in handling. Hydrogen is the most reactive out of all commonly used carrier gases. |
| Helium | Excellent flow velocity and stable but expensive. |
Figure 2. Average linear velocity vs. height equivalent to theoretical plates (HETP) for various types of carrier gases. Hydrogen and helium maintain low HETP for a good range of velocities but nitrogen escalates quickly.
Column Temperature
Control of column temperature is critical for two reasons: vaporization of components and changing retention time. For the former, this is important as components need to be vaporized to be able to flow through the column. Control over the temperature allows control of the time at which components are vaporized to which evaporation itself becomes a separation method similar to other forms of separation techniques such as distillation. However, components with similar boiling points will still remain mixed. Temperature also changes retention time. At higher temperatures, samples are not retained as well on the stationary phases, as more energy is introduced into the system, and the retention factor is changed. A higher temperature reduces the retention ratio as well as the analysis time. Best practice is to use a column temperature or just slightly above boiling point of the components being analyzed.
There are two methods for managing temperature. The isothermal temperature method uses a constant temperature to perform the entire analysis and is best for situations where the number of components being analyzed is relatively small. This method has the advantage of allowing the next sample can be introduced immediately after the previous analysis is completed and avoiding column bleed. However, the isothermal method cannot easily perform simultaneous analyses of components with vastly different boiling points. The programmed temperature method uses a controlled increase in the temperature to separate and analyze components based on different boiling points. The lengths of time to which certain temperatures are held are determined. The linear velocity is also affected by temperature as the diffusion of the carrier gas changes with the temperature; hence adjustment to the linear velocity of the carrier gas is also required for this method. However, sufficiently controlled, this method allows for efficient separation of multi-component samples.
Figure 3. The separation of a multi-component sample, kerosene, at different temperatures.
Summary
Controlling several parameters for gas chromatography runs is critical to achieve efficient separation of components. These parameters can affect the shape, degree of separation and run time of an analysis. When analyzing complex multi-component systems, it might be necessary to compromise on the results of some components during a certain run to be able to isolate a particular component (the general elution problem) to which multiple runs may be necessary to fully separate and analyze all the components.
We thank you for reading through our article on Parameters for Gas Chromatography using Capillary Columns. Check back for our next article on our next piece for handling GC capillary columns. To learn more, take a look at GL Sciences' selection of capillary GC columns.
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