# Retention time and boiling point relationship help

Keywords: Lee retention index; Boiling points; Correlation; Gas retention time of the substance of in- quantitative structure–property relationship (QSPR) terest . Answer to What is the relationship between the boiling points and the retention times of the alkyl acetates. The boiling point of a compound is often related to its polarity (see also the higher the vapor pressure of the compound and the shorter retention time usually is spectrometer helps to identify them based on their fragmentation pattern (see Unfortunately, there is no direct relationship between the number of carbon.

Equilibrium Constant A components retention time is determined by the equilibrium constant K if all other factors are kept constant. In GC, specifically gas-liquid chromatography, there are two phases namely the: Mobile phase — usually a gas such as helium Stationary phase — a high boiling point liquid adsorbed onto a solid A vaporised sample is injected into the head of the GC column, which contains a liquid stationary phase, adsorbed onto the surface of an inert solid.

The inert solid support usually diatomaceous earth or clay is necessary to keep the liquid phase stationary in the column.

The speed with which a particular compound travels through the column depends on how much of its time is spent moving with the gas as opposed to being attached to the liquid. Materials that prefer the stationary phase have longer retention times than those that prefer the mobile phase.

### How Does Temperature Affect a Compound's Retention Time?

The equilibrium constant, K, is defined as the molar concentration of analyte in the stationary phase divided by the molar concentration of the analyte in the mobile phase. A high value of K means the compound is more soluble in the liquid phase than in the gas phase.

K is temperature dependent. Polar or Non-Polar Stationary Phase One of the key factors when setting up a GC method is to choose the polarity of the stationary phase. The polarity is chosen using knowledge of the sample matrix and what separation is required.

## What is Retention Time?

If the polarity of the target compound and the stationary phase are similar, then there is likely to be a greater interaction between the two. Chiral stationary phases that are based on amino acid derivatives, cyclodextrins and chiral silanes are capable of separating enantiomers because one enantiomer interacts slightly stronger than the other one with the stationary phase, often due to steric effects or other very specific interactions.

For instance, a modified -cyclodextrin column is used in the determination of the enantiomeric excess in the chiral epoxidation experiment Chem 30CL. Column temperature A excessively high column temperature results in very short retention time but also in a very poor separation because all components mainly stay in the gas phase.

However, in order for the separation to occur the components need to be able to interact with the stationary phase. If the compound does not interact with the stationary phase, the retention time will decrease. At the same time, the quality of the separation deteriorates, because the differences in retention times are not as pronounced anymore. The best separations are usually observed for temperature gradients, because the differences in polarity and in boiling points are used here.

Carrier gas flow rate A high flow rate reduces retention times, but a poor separation would be observed as well. Like above, the components have very little time to interact with the stationary phase and are just being pushed through the column.

## How Does Temperature Affect a Compound’s Retention Time?

Column length A longer column generally improves the separation. The trade-off is that the retention time increases proportionally to the column length and a significant peak broadening will be observed as well because of increased longitudinal diffusion inside the column.

One has to keep in mind that the gas molecules are not only traveling in one direction but also sideways and backwards. This broadening is inversely proportional to the flow rate.

Boiling point elevation and freezing point depression - Chemistry - Khan Academy

Broadening is also observed because of the finite rate of mass transfer between the phases and because the molecules are taking different paths through the column.

Amount of material injected Ideally, the peaks in the chromatogram display a symmetric shape Gaussian curve.

If too much of the sample is injected, the peaks show a significant tailing, which causes a poorer separation. Most detectors are relatively sensitive and do not need a lot of material in order to produce a detectable signal. The splitless mode will only be used if the sample is extremely low in concentration in terms of the analyte.

Conclusion High temperatures and high flow rates decrease the retention time, but also deteriorate the quality of the separation.

### Gas Chromatography Theory

Which detectors are used? The GC separates the compounds from each other, while the mass spectrometer helps to identify them based on their fragmentation pattern see Mass Spectrometry chapter.

If other carbon containing components, are introduced to this stream, cations will be produced in the effluent stream. The more carbon atoms are in the molecule, the more fragments are formed and the more sensitive the detector is for this compound. Unfortunately, there is no direct relationship between the number of carbon atoms and the size of the signal.

As a result, the individual response factors for each compound have to be experimentally determined for each instrument.