The column and its associated chemistry forms not only the physical center of GC system hardware, but is also at the heart of your methodology. Choices you make here influence efficiency, reproducibility, separation time, pressure, flow rate, and capacity, for example. Both new and experienced chromatographers are always looking to narrow their choices, save time, and reduce trial and error when developing new methods. This article summarizes these fundamentals.
Making informed GC column choices
There are four major factors to consider when selecting a GC column – the choice of stationary phase, plus three column-related parameters: diameter, length, and film thickness. Read on for some top tips on what to consider from each of these selection criteria.
Choice of stationary phase
Choosing the best stationary phase is the most important decision when selecting a capillary column. To make the right choice, here are some top tips:
Look back: Check out Agilent’s extensive applications library. While an exact example application may not be available, information can be obtained to simplify the decision or reduce the number of column choices
Start here: Where no information or example is available, start with a DB-1ms or DB 5ms. They give the lowest column bleed and highest column inertness for a wide range of analytes, including active compounds and trace level samples
Think like for like: Polar compounds are more strongly retained by a polar stationary phase than a less polar stationary phase, and vice versa
Column diameter has an influence over five parameters of primary concern – efficiency, retention, pressure, carrier gas flow rate, and capacity. Here’s some advice on what to look out for:
- Use 0.15 or 0.18 mm id when higher column efficiencies are needed. The efficiency of a capillary column, measured as number of theoretical plates (N), increases as the id of the column decreases. Smaller id columns however, have the lowest capacities and require the highest head pressures
- Use 0.25 mm id for a compromise between good efficiency and sample capacity. This id is the most popular column diameter in gas chromatography, and is used in a wide range of applications while allowing for adequate sample capacity
- Use 0.32 mm id columns when higher sample capacity is needed. They often provide better resolution of earlier eluting solutes for splitless injections or large injection volumes (> 2 µL) than 0.25 mm id columns
- Use 0.45 or 0.53 mm id columns when only a Megabore direct injector is available and higher column efficiency is desired. These columns offer the highest sample capacities. The columns are also well suited for high carrier gas flow rate situations, such as with purge and trap, headspace samplers, and valve injection applications. 0.53 mm id columns have the highest sample capacities at constant df
Column length influences three parameters of major concern – efficiency, retention (analysis time), and carrier gas pressure.
Agilent experts recommend the following:
- Start with 25 to 30 m columns when the optimum length is unknown
- Go short – use 10 to 15 m columns for samples containing well separated solutes or few solutes. Shorter lengths are used for small diameter columns to reduce head pressures
- Go long – use 50 to 60 m columns when resolution is not possible by other means (smaller diameter, different stationary phase, or change in column temperature). These columns are best suited for complex samples containing many solutes, have extended analysis times and are a higher cost
Column film thickness
Column film thickness influences five major parameters – retention, resolution, bleed, inertness, and capacity.
Thick film columns retain and resolve volatile solutes (e.g., light solvents, gases). They are more inert and have higher capacities. Thick film columns exhibit higher column bleed and decreased upper temperature limits. Thin film columns minimize the retention of high boiling, high molecular weight solutes (e.g. triglycerides). They are less inert, have lower capacities, and exhibit lower column bleed.
Agilent experts recommend:
- For 0.18 to 0.32 mm id columns, a film thickness of 0.18 to 0.25 µm is average and can be used for most analyses
- For 0.45 to 0.53 mm id columns, a film thickness of 0.8 to 1.5 µm is average and can be used for most analyses
As the world’s leading provider of GC capillary columns, Agilent is uniquely positioned to offer you superior quality and unmatched service and support. For more column recommendations, chromatograms, and method parameters, go to http://www.agilent.com/en-us/products/gas-chromatography/gp54838