GC column selection
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.
Method development considerations
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
1) Column phase
The polarity oft he stationary phase is chosen according to the compounds to analyze. 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.
Non polar phases separate compounds by their boiling points. For example, if the components have different boiling points (greater than 2°C), a non-polar column such as the TG-1MS or DB-1 / DB-5 is recommended.
Intermediate polarity phases such as DB-WAX or DB-1701, combine boiling point with the more selective hydrogen bridge or dipolar moment interactions and thus provide a higher selectivity.
If the products differ primarily in their polarities, then a polar column such as the TG-WaxMS will be ideal.
In general, non polar phases are more thermally stable than the polar phases. In other words, the higher the column polarity, the lower ist thermal stability. Hence, always select the least polar column which will perform the separation.
2) Internal diameter
The choice of inner diameter is often determined by the instrument or detection method. Most modern GC instruments are suitable for most column sizes. The internal diameter (ID) of a column is inversely proportional to ist separation power. 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.
For samples containing a large number of substances where you may need a high resolution, small internal diameter columns (0.20-0.25 mm) are recommended. For samples with a high range of concentrations wider columns should be used since these larger diameters allow fort he injection of a higher sample amount.
Columns of 0.53 mm ID (semicapillary) have a loading capacity similar to that of packed columns, but with better resolution, improved chemical inertness and lower analysis times.
The 0.32-0.53 mm ID columns can be used with either a standard injector (that is used for capillary columns) or with a packed column injector, as they can operate at high flow rates. For GC-MS systems it is recommended to work with small ID columns (0.10 mm, 0.15 mm, 0.18 mm, 0.20 mm, and 0.22 mm) so as not to exceed the capacity oft he MS vacuum system.
Capillary columns of 0.1 mm ID can generate high plates numbers and reduce analysis time without losing resolution. The high efficiency of these columns (7000-10,000 plates/meter) allows the resolution of complex samples using shorter column lengths. However their loading capacity is a limiting factorr and in order to obtain the best performance from these columns instrumental factors (injector/detector) have to be taken into account.
3) Film thickness
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. As a general rule, thin films (0.1 µm) must be used for compounds with a high molecular weight such as rilycerides, antioxitants etc., which have elution temperatures over 300 °C.
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 film thickness of 0.25-0.32 µm ist he standard thickness allowing for a compromise between capacity and resolution; and fort he injection of samples with a wide volatility range.
Thick films increase retention of the most volatile components whereas thin films provide faster elution at lower temperatures. Thick films must be used for low boiling substances. Specifically, 3-5 µm films are used to separate solvents, gases, and very volatile substances at room temperature or lower. When the thickness oft he stationary phase increases, thermal stability decreases. Therefore the bleed level is higher and this can limit the maximum operating temperature oft he column.
4) Column length
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.
- Columns with a length of 25 to 30 meters are suitable for the beginning of method development. This provides a high efficiency with relatively shot analysis times.
- Columns 10 to 15 meters long are suitable for samples containing very few or easily separable analytes. Shorter columns are used for very small diameters to reduce initial pressure.
- Columns 50 to 60 meters long should only be used when resolution is not possible by other means (smaller diameter, different stationary phase, temperature change of the column). However, they are ideal for complex samples with many substances. Long columns require a long analysis time and cause higher costs.
Some additional considerations
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.