Gas Chromatography

Gas chromatography continues to hold its own as a prominent analytical technique. Participants in this discussion are Bob Wiedemer from Thermo Fisher Scientific and Margit Geissler from Shimadzu.

There has been a noticeable increase in the number of capillary GC stationary phases on the market designed for a specific application. Can these columns be used for an application other than the one for which the column was designed?

Wiedemer: Column manufacturers “tune” the selectivity of specialty columns to give optimum separation of a specific sample by adjusting the type and amounts of functional groups in the phase. A column designed for a specific application may work for the analysis of a different type of sample just as a conventional stationary phase may work for the application for which the specialty column was designed. Capillary GC columns have the ability to separate many compounds. A 30 m x 0.25 mm column will have approximately 110,000 theoretical plates, although separation based on “horsepower” alone may not be the best technique for speed and resolution. Also consider column dimensions and temperature limits. Only experimental results will tell for sure.

Geissler: Yes, in general columns can be used for compounds according to their polarity and boiling point. If a column is promoted for something specific, such as PAH analysis, it can also be used for the analysis of other compounds with similar boiling point ranges and polarities.The increase in the number of columns designed for specific applications is due to the fact that many customers are looking not only for a column, but for a ready-to-use application. With a dedicated column and, in many cases, a dedicated method, the customer can start working immediately and does not need to spend time on method development.

What are the limiting factors to the use of fast GC with high pressure drop approaches (i.e., with vacuum conditions at the column outlet)?

Wiedemer: One limiting factor is the pumping capability of the vacuum system. In a GC system interfaced to a mass spectrometer a certain level of vacuum is needed for the MS system to operate properly. Using small-diameter columns for which the optimum flow-rate is low assures adequate MS vacuum. Columns with a 0.25-mm internal diameter are commonly used, but columns as small as 0.10 mm i.d. are practical. Smaller-diameter columns have more theoretical plates per meter than larger diameter columns, which facilitates fast GC because this higher efficiency means a shorter column can be used to achieve the desired separation.

Geissler: If fast GC columns with small inner diameters are used, the head pressure of the column needs to be set to rather high values. This is not possible with some specific GC systems; so a high pressure drop with columns of larger inner diameters is used. In general, it can be said that the smaller the inner diameter of a column, the better its separation capability (i.e., the higher the chromatographic resolution that can be obtained on a column). Another limitation is that linear velocity in the column is increased using the high pressure drop approach and one does not work in the range of optimum linear velocity in the column, although this is an important factor in obtaining a good separation.

Why do we still heat and cool columns with a big box of air? Surely this isn't the most efficient way to achieve the required temperature change.

Wiedemer: Although heating a GC column by changing the oven air temperature is not the most efficient method, it is reliable and reproducible within commonly used temperature ranges. It has been done this way for so long that innovative thinking is needed to find an alternative.There are now commercially available gas chromatographs that operate in the ultrafast mode, where direct column heating allows linear heating rates up to 1200 °C /min. The oven is not appreciably heated with ultrafast, so fast cooling rates are also possible, taking about 1 min to return to 50 °C from 350 °C.

Geissler: Many GC users are interested in using not only a single column and flow line, but also a second flow line and a second column, respectively. This approach has many advantages, for example, simultaneous confirmation analysis or the simple fact that manual modifications are not necessary if the user needs to choose a different column for another application. For this reason, a large column oven is needed. If the pumping capacity of the mass spectrometer is large enough, this approach can even be used for the coupling of GC with MS. However, if the GC system is dedicated to a specific application, a small column oven volume is, of course, desirable and possible.

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