Topics - Vacuum
Other than for general purposes, vacuum has been used in two ways in chromatography procedures. Firstly, it has been used for degassing solvents in liquid chromatography. Dissolved gasses, usually nitrogen and oxygen from the air, tend to be evolved in the mobile phase as the pressure is reduced when the mobile phase leaves the liquid chromatography column and enters the detector. Gasses in the mobile phase in the detector can produce completely unacceptable noise and, thus, must be removed. The dissolved gasses were originally removed under vacuum but, unfortunately, are soon replaced if the solvent is left in contact with air at atmospheric pressure. For this reason degassing is now usually carried out by bubbling helium through the mobile phase reservoirs. Secondly, vacuum is used in the thermionic detector. This consists of a device, very similar in design to the thermionic valve which is attached to a vacuum and a small quantity of the eluent from a gas chromatography colum allowed to bleed through it. Helium is used as the carrier gas. The presence of solute vapor causes the thermionic current to fall. This type of detector tends to become contaminated rather readily. In the very early gas chromatographs (the fIrst commercial chromatograph was manufactured by Griffin and George) the elution process was not fully understood and a vacuum was applied to the column outlet to assist in carrier gas flow. The vacuum was soon found to be unnecessary and eliminated.