Tech Tip 2: Separation - standard column or something more specialised?

Obtaining the best resolution for the method is critical for accurate qualitative and quantitative analysis, especially where chromatographic resolution is relied upon with standard GC detectors, as there is no MSD to take advantage of analytical resolution.

By far the most commonly used column is an X-5 with 5% phenyl functional groups on a methylpolysiloxane phase, usually with the dimensions of 30 m length, 0.25 mm i.d. and a 0.25 mm film thickness. This is a robust phase, has a high temperature limit of around 350°C, gives a good resolution for many samples and analytes and survives most unknown samples being injected into it. So why do we need more polar, unstable phases with a lower temperature limit?

Over the years I've seen many methods with multiple oven temperature ramps, with ramp rates ranging from 40°C/min to 2°C/min in a single run, making the method quite complicated. In addition, the run time increases and in some circumstances slowing the ramp rate decreases the resolution due to longitudinal diffusion because the analytes have been in the column so long! Programming the column flow or head pressure are also common ways to try to improve the separation in a method. Often these types of analysis involve the use of the 'standard' column, making the method more complicated and the run time longer than it really needs to be.

When developing a method, separation of the target analytes needs to be adequate for the questions to be answered, the matrix and the detector used. It may be the case that the 'standard' column achieves all of this with a simple method, however this may not be the case. The column phase is one of the most powerful parameters to achieve the best separation, so before making your method more complicated and spending lots of time trying to get the ramp rate from time A to time B just right using the 'standard' column, think about trying a new phase which could result in a more robust, faster method.

To learn more about GC columns, attend Day 2 of our Complete GC & GC-MS course and Module 4 of our Virtual Classroom Complete GC & GC-MS course.

Don't forget that you can also subscribe to our newsletters and receive useful tips straight to your inbox!