Developing a GCxGC technique can be daunting at first glance. There are two ovens, two columns, and a modulator all with variables that can affect method performance. Luckily, a few tried and true tips are available to help take the guesswork out of GCxGC method development. To start with, the first dimension separation should be maximized, then the first and second column dimensions should be matched, and finally, the modulation time should be kept short. GCxGC performance can be considerably optimized by following these three simple tips.
Maximize Resolution in the FIRST Dimension
When starting a GCxGC analysis, the first dimension has to be always kept in mind. An appropriate stationary phase and column dimensions must be selected to optimize the efficiency (i.e. resolution) of the first dimension separation. A 30 m x 0.25 mm id column is a good place to start but for really tricky separations or if you need to super-charge their separation, 60 m can be chosen. After selecting a good first dimension separation, users can select 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 (Figures 1 and 2).
Figure 1. Resolution in first dimension is maximized using an efficient PAH-specific column.
Match the First and Second Column Dimensions
In case 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 provide the best sample loading capacity and also reduce the chances of overloading the second dimension column. Moreover, this is the easiest way to maintain a consistent flow throughout the analysis. The exception to the rule is for atmospheric pressure detectors such as FID and ECD. In that case, the linear velocity can be maintained through the column and into the detector by reducing the internal diameter of the second dimension column.
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.
Keep the Second Dimension Separation Time SHORT
The second dimension separation time is generally known as the “modulation time”. It is the period during which the first dimension column effluent is sampled in the second dimension. In practice, Michelle Misselwitz, an experienced analytical chemist, and others wanted to sample the first dimension effluent more rapidly than the first dimension peak width (this is known as “slicing”). They wanted to do this quickly so that they can maintain the first dimension column separation. Ideally, Misselwitz and others wanted to slice first dimension peak 3 to 5 times. So, if the first dimension peak width is 6 seconds, the second dimension separation time (i.e. modulation time) should not exceed 2 seconds (Figure 3). If the modulation time is 10 seconds, then the first dimension peak width should be at least 30 seconds wide (Misselwitz hopes one doesn’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?
Many might have heard about GCxGC, but no one completely understands how it can transform a laboratory. Some avoid using GCxGC owing to its highly technical aspects; others assume that its capabilities are over-hyped to the point that it no longer has a place in a routine laboratory setting. However, GCxGC is a powerful tool for a broad range of applications and has several advantages to increase productivity, save time, and improve confidence in analyte identification.
This information has been sourced, reviewed and adapted from materials provided by LECO Corporation.
For more information on this source, please visit LECO Corporation.