Column choice and optimization - Column choice and optimization - Chromedia
Answer to What is the relationship between the boiling points and the retention times of the alkyl acetates. analysis of complex samples with a wide range of boiling points. contribution on retention time, retention temperature and resolution. Results from statistically work in your laboratory and for all of your wonderful advice on work and life. I would .. relationships only hold true for isothermal separations. If a component has a low boiling point, then it is likely to spend more time in the gas phase. Therefore its retention time will be lower than a compound with a.
Well-known attempts to categorise phases are the Rohrschneider system, the McReynolds system and the Kovats retention index. In practice their applicability is limited. The Kovats index provides little information about the respective polarities of the solute and stationary phase, but by comparing values measured on two stationary phases a direct quantitative measurement of the relative polarity can be obtained. Increasing the polarity of the stationary phase will increase the selectivity of the column for polar components in the sample.
However, the actual selectivity of a stationary phase for a given mixture of compounds cannot be predicted from its polarity and has to be measured experimentally.
The figures below show the strong influence of stationary phase polarity on the selectivity for a specific test sample. As an example, the peak pair 8 and 9 is just baseline separated on the first column. On the second column, the resolution is much better, whereas on the third column the distance between the peaks 8 and 9 is very large.
Which column is the best will also clearly depend on the other compounds present in the sample. When looking at these 3 chromatograms above, compare the results for relative retention by observing how peaks shift relative to each other for the different conditions.
For example, check positions of peaks 8 and 9. Alkane 8 is much less retained on the more polar phase CP Sil 19 CBbecause it is less compatible with this phase.
We can interpret this as being due to lower stationary phase solubility distribution coefficient which reduces solute concentration in the carrier phase and elutes it earlier.
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Film thickness, temperature and retention In addition to the stationary phase characteristics and the nature of the sample components, the retention factor k is determined by 3 other parameters: The relation between the retention factor k and the phase ratio is described by: Film thickness and retention constant temperature Chromatographic selectivity depends on the characteristics of both the stationary phase and of the solute molecules.
Similar compounds, consisting of molecules of the same chemical type and having very similar boiling points, are difficult to separate on a non-polar phase unless very high efficiency columns are used.
In a chromatogram of aromatic and aliphatic hydrocarbons on a low polarity column CP-Sil 5 CB the components elute in strict order of boiling point and the separation is much as one would predict from Raoults law. A similar effect is shown on a different column containing the same phase for a separation of a mixture of solvents having different polarities.
The elution order is: The other solvents also elute mostly according to their polarities. The elution order now depends on both the relative vapour pressures and the differences in polarity.
What is Retention Time? Chromatography Today
Elution order of solvents In practice it is very difficult to predict the elution order of compounds on neutral, non-polar columns, only based on boiling points, if these compounds are different in polarity and physical properties.
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.Intermolecular Forces and Boiling Points
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.
What is the relationship between retention time and boiling point? | Yahoo Answers
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. 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.