Читать книгу Process Gas Chromatographs - Tony Waters - Страница 76

 It's obviously true to say that peaks come out of the column at different times.

 It's not true to say that molecules move at different speeds inside the column.

 For all injected molecules, there are only two speeds possible inside the column; stop or go.

 When molecules are in the liquid phase, they cannot move along the column.

 When molecules are in the gas phase, they move at full carrier gas speed.

 The “air peak” doesn't dissolve in the liquid phase so it travels at the same speed as the carrier gas.

 It's impossible for any peak to travel faster than the carrier gas.

 No peak from the same injection can appear on the chromatogram earlier than the air peak.

 To get through the column, all injected molecules must travel for the same time as the air peak does.

 If the column had no liquid in it, all the peaks would elute together with the air peak.

 Any additional retention time beyond the air peak time is the time a peak stopped in the liquid phase.

 Separation is not caused by motion, it's due to peaks stopping for different times in the liquid.

 The time that a peak stops in the liquid phase is directly proportional to its solubility in that liquid.

 The spacing of peaks on the chromatogram is different than their spacing inside the column.

 Peaks that migrate slowly along the column come out very much later on the chromatogram.

 Retention time is the sum of time traveling in the gas phase and time stopped in the liquid phase.

 Holdup time is the time in the gas phase; adjusted retention time is the time in the liquid phase.

 Column operating performance can be evaluated from chromatogram measurements.

 Chromatogram measurements may be made in seconds or millimeters.

 You can measure the holdup time, peak retention time, and peak width on the chromatogram.

 When measuring base width, triangulate the peak and extend the baseline under the peak.

 Alternatively, measure the peak width at half the peak height.

 Plate number is calculated from measurements of retention time and peak width.

 Any change in operating conditions that increases plate number increases column separating power.