GC061 Film Thickness

 Thin films (less than 0.2 µm) provide high efficiencies and resolutions. This imply that shorter columns and reduced operating temperatures can be used, reducing column bleed. Bleeding is the process of losing stationary phase due to high temperatures and flow inside the column.

A film thickness of 0.25 µm represents a compromise with high resolutions attainable with thin films and the high capacity of thick films. When capacity is higher, larger samples can be injected and the injection technique itself is simpler. Practical operating temperatures can be used and bleeding is not a big problem, because bleeding is proportional to the amount of the liquid phase in the column. With this film thickness and using fast flow rates, the column can be optimized for speed. Low flow rates allow to obtain high resolutions.

Thick films (1.00 µm or greater) show increased retention of sample components. This feature is essential to separate volatile compounds. Their capacity is greater, and bigger samples can be injected. This can be important when the GC is connected with subsequent analysis devices, like Fourier transform infrared spectrometers or mass spectrometers (MS). Disadvantages of thick films are they decreased efficiencies and higher bleed rates. Thus, longer columns should be used to compensate for efficiency, and lower temperatures to avoid excessive bleeding. Thicker films are more common today due to improved cross-linking techniques for the liquid phases and more inert fused silica surface.