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When selecting a GC column for your analysis, it can be difficult to choose the most appropriate column since there is a wide range of options. However, considering a number of questions about the planned separation makes the choice a lot easier. Read the following information to find the most suitable column for your analysis.
When first developing a method, you should consider these column characteristics to determine the best column for the separation:
1) Column phase
2) Internal diameter
3) Film thickness
4) Column length
In GC, the separation of two analytes occurs due to differences in their interaction with the stationary phase, therefore a phase must be chosen that matches the properties of the sample. For example, if the components have different boiling points (greater than 2°C), a non-polar column such as the TG-1MS is recommended. If the products differ primarily in their polarities, then a polar column such as the TG-WaxMS will be ideal. Always select the least polar column which will perform the separation.
The choice of inner diameter is often determined by the instrument or detection method. Most modern GC instruments are suitable for most column sizes. With a larger inner diameter, the sample capacity of the column increases, but resolution and sensitivity decrease. Vice versa, a column with a smaller inner diameter can improve resolution and sensitivity, but with the drawback of lower sample capacity and higher sample preparation requirements. It is therefore a good idea to find a similar application that allows separation of the desired components and use this as a guide.
The film thickness of the column has an influence on retention, dissolution, column bleed, inertness and capacity. Increasing the film thickness increases the sample capacity of the column and slows down the elution of the peaks, which can be helpful when analyzing volatile compounds. A thicker film also reduces the risk of overloading the column, thus improving the resolution. However, a thicker film may also be more sensitive to degradation. The same component will elute at a higher temperature on a thick film than on a thin film.
Compounds with high boiling points or high molecular weight should be analyzed with a thin film to improve resolution and avoid unnecessarily long analysis times.
Another essential factor is the phase ratio, which can be calculated based on the inner diameter and the film thickness. The phase ratio is considered 1) to characterize the best dimensions for an application and 2) when an analysis is to be transferred from one column with a certain inner diameter to another without changing the method significantly.
A longer column length leads to higher efficiency and resolution, but it is not a linear relationship. The resolution is proportional to the square root of the column length, so doubling the column length increases the resolution by about 40%. However, increasing the column length also increases the retention time. Double column length, double analysis time. In general, it is recommended to use the shortest column with which the desired separation can be performed.
It is useful to know that 95% of all used GC columns are either TG-1MS, TG-5MS, or TG-WaxMS type columns. A good starting column is a 30m x 0.25mm ID, 5% phenyl column with a film thickness of 0.25μm, such as e.g. the TG-5MS. This is a non-polar column that separates primarily by boiling point, but also has some polar characteristics.