At first glance developing a GCxGC method can be intimidating. There are two columns, two ovens, and a modulator all with variables that can impact method performance. Fortunately, there are a few tried and true tips to help take the guesswork out of GCxGC method development. Start by maximizing the first dimension separation, match the first and second column dimensions, and finally keep the modulation time short. Following these three simple tips can make a big difference in GCxGC performance.
1. Maximize resolution in the FIRST dimension
When jumping to a GCxGC analysis, don’t forget about the first dimension. Choose an appropriate stationary phase and column dimensions that will optimize the efficiency (i.e. resolution) of the first dimension separation. A good place to start is a 30 m x 0.25 mm id column, or jump to 60 m for really tricky separations, or if you want to super-charge your separation! Once you have a good first dimension separation, then choose a second dimension column phase that is different (i.e. orthogonal) from the primary column that will exploit the differences in closely eluting (or coeluting) first dimension peaks (Figure 1 and 2).
Figure 1: Resolution in first dimension is maximized using an efficient PAH-specific column.
2. Match the first and second column dimensions
If the first dimension column is 0.25 mm id × 0.25 µm, it is best if the second dimension column is also 0.25 mm × 0.25 µm (Figure 2). This will give the best sample loading capacity and reduce the chances of overloading the second dimension column. This is also the easiest way to keep a consistent flow throughout the analysis. The exception to the rule is for atmospheric pressure detectors (e.g. ECD, FID). In that case, reducing the internal diameter of the second dimension column helps to maintain the linear velocity through the column and into the detector.
Figure 2: After the first dimension is optimized, the second dimension is used to exploit differences in closely eluting 1D peaks. The second dimension column id and film thickness match the first dimension column.
3. Keep the second dimension separation time SHORT
The second dimension separation time is commonly referred to as the “modulation time”. It is the duration in which we sample the first dimension column effluent in the second dimension. In practice, we want to sample the first dimension effluent faster than the first dimension peak width (this is called “slicing”). We want to do this quickly so that we maintain the first dimension column separation. Ideally we want to slice the first dimension peak 3 to 5 times. So if our first dimension peak width is 6 seconds, our second dimension separation time (i.e. modulation time) should be no longer than 2 seconds (Figure 3). If you have a 10 second modulation time that means your first dimension peak width should be at least 30 seconds wide (I hope you don’t have this wide of a peak)!
Figure 3: The first dimension peak width is 9 seconds. Therefore, we want a maximum of a 3 second 2D separation time. This will give 3 slices across each peak and preserve the first dimension resolution.
Want to learn more about the benefits of GCxGC?
You may have heard about GCxGC, but do you fully understand how it can transform your lab? Some shy away from GCxGC due to its highly technical aspects; others believe that its capabilities are over-hyped to the point that it does not have a place in a routine laboratory setting. Yet GCxGC is a powerful tool for a wide variety of applications and has a number of benefits to save you time, increase your productivity, and improve confidence in your analyte identification.
Download our free white paper, authored by Michelle Misselwitz, and learn more about how you can put GCxGC to work in your laboratory today!
Michelle Misselwitz, firstname.lastname@example.org; Bellefonte, PA, USAMichelle Misselwitz is an experienced analytical chemist with expertise in gas chromatography (GC), comprehensive two-dimensional gas chromatography (GCxGC), mass spectrometry (MS), and sample preparation. As an applications chemist, she developed hot topic applications for environmental, food safety, environmental forensics, and botanical markets. Misselwitz has successfully combined her scientific and communication skills to present and write technical papers and training seminars for customers worldwide. With a decade of experience at Restek, a chromatography consumables company, and a B.S. in Chemistry from The Pennsylvania State University, Michelle is currently an independent consultant specializing in technical writing, presentations, and GC method development.