GC017 Retention Factor

 

The retention factor, k, is the ratio of the mass of solute (not its concentration) in the stationary phase to the mass in the mobile phase:

The retention factor is measured experimentally as the ratio of the adjusted retention time, t′R, to the gas hold‐up time tM

The larger the value of k, the longer the solute will be retained in the column. The relevance of this factor is that can be obtained easily from the chromatogram.

The retention factor and the distribution constant are related through the phase ratio (beta). Kc can be broken down into two terms:

Kc equals the product of the retention factor and the phase ratio

In formula: Kc = k x beta

Beta is defined as the ratio between the volume of the mobile phase (VM) and the stationary phase (Vs). In formula

Beta = Vm / Vs

For capillary columns, with a known value of film thickness (df), beta is given by:

Beta equals the square difference between the radius of the capillary column (rc) and the film thickness (df) divided by 2 times the radius of the capillary column times the film thickness.

Most of the time, rc is much greater than df, and we can make:

Beta equals the radius of the capillary column (rc) divided by 2 times the film thickness (df).

Capillary columns have values of beta in the hundreds, and are about ten times higher than the values for packed columns. Beta cannot be easily calculated for packed columns. The parameter phase volume ratio is helpful in selecting a proper column.

Comments

Post a Comment

Popular posts from this blog

GC013 Acronyms for GC

LC - Liquid chromatography   LSC - Liquid‐solid chromatography   MDQ - Minimum detectable quantity   MP - Mobile phase MS - Mass spectrometry MSD - Mass selective detector OT - Open tubular PDMS - Polydimethyl siloxane PLOT - Porous layer open tubular PTV - Programmed temperature vaporization RSD - Relative standard deviation SCOT - Support coated open tubular SD - Standard deviation SEC - Size exclusion chromatography SP - Stationary phase TCD - Thermal conductivity detector TF - Tailing factor TOF - Time of flight (MS) TPGC - Temperature programmed gas chromatography WCOT - Wall coated open tubular
Read more

GC010 Chromatographic terms and symbols - Part 3

  In a typical chromatogram for a single solute A, we can see a small peak at the beginning, short time after the injection (which is performed at time zero). The solutes are characterized by their retention times or retention volumes (tr, or Vr). These quantities are depicted as the distance between the injection point and the peak maximum. The formula to calculate the retention volume Vr is: Retention volume equals (“retention time” multiplied by “gas flow rate”) Or we can express it in terms of the retention time as: retention time equals (“Retention volume” divided by “gas flow rate”) Or in formula Vr = tr x Fc   or   tr = Vr / Fc; where Fc is the gas flow rate The small initial peak corresponds to a component that does not is sorb into the stationary phase, thus it is an un-retained component. The IUPAC defines Vm, the holdup volume as: “the volume of the mobile phase (MP) required to elute the un‐retained compound from the chromatographic column and reported at column t
Read more

GC001 GC Introduction to gas chromatography

 Gas chromatography (GC) is a technique used for separation and analysis of volatile compounds. It can be used to analyze gases, liquids or solids, which are usually dissolved in volatile solvents. The molecular weight of the compounds analyze can vary in the molecular weight range from 2 to over 1000 Da. For example, it can be used to separate and analyze more than 450 components in coffee aroma.
Read more