Читать книгу Process Gas Chromatographs - Tony Waters - Страница 60

 It's not sufficient to say that some kinds of molecule travel faster in the column than others do.

 Identical molecules don't spend the same time in the column; some elute earlier than others do.

 Variation in the retention time of identical molecules is the root cause of the peak width and shape.

 Wide peaks are more difficult to separate (resolve) than narrow peaks are.

 Solid columns work by a different mechanism than liquid columns, but the end result is similar.

 Gases dissolve in liquids and rapidly reach dynamic equilibrium.

 Component solubility is affected by temperature, pressure, and the kind of liquid; nothing else.

 Component solubility in a liquid phase decreases with temperature increase or pressure decrease.

 At equilibrium, component molecules enter the liquid phase at the same rate as they are escaping.

 Although chromatography is continuous, it can be modeled as a succession of discrete equilibria.

 A succession of equilibria forms a symmetrical peak shape which narrows with more equilibria.

 Ideally, peaks would have perfect symmetry, but real peaks are often somewhat asymmetric.

 For every injected molecule there are only two speeds along the column; stop or go.

 All component molecules in the gas phase must move along the column at full carrier gas speed.

 The molecules that by random chance are stuck in the liquid phase cannot move with the carrier gas.

 Even identical molecules suffer random time in the liquid, which is the main cause of peak width.

 Narrower peaks are taller, and more easily separated from adjacent peaks.

 Plate number is the effective number of equilibria and is estimated from the chromatogram.

 Peaks become narrower as the plate number increases, making separation easier.

 Peaks become taller as the plate number increases, thereby increasing signal‐to‐noise ratio.

 The plate number affects only the width of a peak, not its position on the chromatogram.

 The peak apex is at the elution time for an average molecule, so is the best measure of retention time.