Introduction

Band-broadening is a general term used to describe the overall dispersion or widening of a sample peak as it passes through a separation system. Band-broadening in Chromatography is a result of several effects. According to Robinson andFrame (2014), four factors generally affect peak broadening within chromatographic columns. These include:

1. Longitudinal diffusion
2. Eddy diffusion
3. Mass transport broadening in the stationary phase
4. Mass transport broadening in the mobile phase

Longitudinal diffusion

If we consider a peak of a compound in a chromatographic column with a concentration profile as demonstrated below, it is clear that the highest concentration is at the centre of the band:

If the flow in the band profile above were stopped and the column allowed to sit, some process will occur. First, from the random process of diffusion, there will be a statistical preference whereby more molecules will diffuse away from the highly concentrated region to a lowly concentrated region (Libretexts, 2018). This implies that if the band were allowed to sit stopped in the column, the particles would diffuseslowly out from the central region thus leading to a broadening of theconcentration profile as shown below.

Since the particles tend to diffuse horizontally away from the centre of the column the effect is thus known as longitudinal peak/band broadening

Eddy Diffusion

Consider a chromatographic column in which a group of molecules are flowing down a bed of particles as shown in the figure below.

The particles will offer paths of varying lengths through which the different compound molecules will follow. Some may end up taking longer tracks as compared to others depending on the channel taken, and the effect of currents as the particles bounce off obstacles(Robinson &Frame, 2014). Finally, the molecules taking the longest channel will take the longest time to travel through the column unlike those taking the shortest path.

If we have a distinction between the time it takes a set of molecules to move through the column based only on different path lengths, we have broadened the peak, and this is known as eddy diffusion.

Stationary Phase Mass Transport Broadening

Consider a compound whose molecules have been distributed between the fixed and the mobile phase in a chromatographic column as depicted below.

Since the mobile phase is mobile and the solute molecules in it are moving the profile for the mobile phase would move ahead a small amount as illustrated below:

From the above picture, it is observable that the solute molecules must spend a finite amount of time in the stationary phase.  Since the mobile phase solute molecules are moving away, particles stuck in the stationary phase lag behind and introduced a degree of broadening(Robinson &Frame, 2014). In summary, the finite time required for the molecules to move out of the stationary phase leads to an overall broadening of the concentration distribution and overall broadening of the peak(Robinson &Frame, 2014).

Mobile Phase Mass Transport Broadening

Just like in the stationary phase, similar mass transfer effect occurs within the mobile phase. Consider the concentration profile in the stationary and mobile phases below(Robinson &Frame, 2014).  If the molecules at the leading edge of the mobile phase profile fail to encounter and enter the stationary phase quickly, they will move further ahead and broaden the distribution thus causing band broadening (Robinson &Frame, 2014)

References

Libretexts, T. W. (2018, November 26). Broadening of Chromatographic Peaks. Retrieved February 17, 2019, from https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Analytical_Sciences_Digital_Library/JASDL/Courseware/Separation_Science/02_Text/03_Broadening_of_Chromatographic_Peaks

Robinson, J. W., Frame, E. M., & Frame, G. M. (2014). Undergraduate instrumental analysis. Boca Raton: CRC Press, Taylor & Francis Group.

Do you need high quality Custom Essay Writing Services?